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date: 13 December 2018

Human Exploration and Development in the Solar System

Summary and Keywords

Emergence of ballistic missile technology after World War II enabled human flight into the Earth’s orbit, fueling the imagination of those fascinated with science, technology, exploration, and adventure. The performance of astronauts in the early flights assuaged concerns about the functioning of “the human system” in the absence of the Earth’s gravity. However, researchers in space medicine have observed degradation of crews after longer exposure to the space environment and have developed countermeasures for most of them, although significant challenges remain. With the dawn of the 21st century, well-financed and technically competent commercial entities have begun to provide more affordable alternatives to historically expensive and risk-averse government-funded programs. The growing accessibility to space has encouraged entrepreneurs to pursue plans for potentially autarkic communities beyond the Earth, exploiting natural resources on other worlds. Should such dreams prove to be technically and economically feasible, a new era will open for humanity with concomitant societal issues of a revolutionary nature.

Keywords: spaceflight, exploration, solar system, space transportation, space environment, NASA

The Beginnings of Human Spaceflight

The launch of Sputnik I on October 4, 1957, is widely considered to mark the beginning of the Space Age. The launch was part of a written commitment by the Soviet Union to support and participate in a set of worldwide scientific investigations known as the International Geophysical Year (IGY). The United States had also pledged to do a series of projects, including the launch into orbit of a scientific satellite. The Soviets launched their satellite first. No one should have been surprised (McDougall, 1985).

Unfortunately, neither the general public nor the media was aware of the IGY or the planned satellite launches. The reaction in the United States bordered on panic. Citizens had been led to believe that the military technology of the Soviets was entirely derived from classified information stolen in the United States by a vast network of spies. That mythology was effectively skewered by the achievement of Earth orbit for the first time, using a rocket whose capability had clear implications for the delivery of warheads by intercontinental ballistic missiles (ICBM).

The impact of the event on the whole world in the context of the Cold War was not lost on Soviet Premier Nikita Khrushchev. He spurred his rocketeers to produce a string of space firsts, leading to the launch of the first human into orbit on April 12, 1961. At the time, the United States still had not launched an astronaut on a suborbital flight. Spaceflight enthusiasts around the world stage a “Yuri’s Night” celebration to commemorate that event as well as, coincidentally, the first flight of the U.S. Space Shuttle on April 12, 1981, exactly 20 years later.

President John F. Kennedy raised the stakes later in 1961, pledging that the United States would deliver a man to the surface of the Moon and return him safely to Earth by the end of the decade (Logsdon, 2010). Kennedy’s purpose was political—to beat the Soviets to the Moon as a demonstration of U.S. technological superiority. The fledgling National Aeronautics and Space Administration (NASA), established in 1958, was assigned the task. A group of engineers and technologists under the name Space Task Group (STG) was formed at the Langley Research Center to develop capabilities for crewed spaceflight (Swenson et al., 1989). They were transferred in 1962 to a new field center to be established in Houston, Texas: the Manned Spacecraft Center. Some years later that installation was renamed the Lyndon B. Johnson Space Center.

The project teams devised an extensive testing and demonstration program that included a number of spaceflights. Project Mercury included six orbital flights of a single-person spacecraft known as the Mercury Capsule. The project created procedures and teams for launch and recovery operations and established the capabilities of a human being in the space environment. Project Gemini included 10 crewed orbital flights of a two-person spacecraft known as the Gemini Spacecraft. These flights demonstrated orbital rendezvous techniques between two orbiting vehicles and exposed crews to the long duration in space required for the lunar missions. The early Project Apollo flights demonstrated the various operational capabilities of the spacecraft and its crew. Two of the test flights sent crews around the Moon.

The first landing itself was on the mission known as Apollo 11, and astronaut Neil Armstrong became the first human to set foot on the lunar surface. Saturn V launchers were built for nine more missions, but the last landing was the Apollo 17 mission in December 1972. President Nixon asked NASA for a roadmap to the future of human spaceflight. The Space Task Group report of 1969 laid out a plan heavily influenced by the writings of Wernher von Braun: there were to be 100 people in low Earth orbit space stations, bases on the Moon, and visits to Mars by 1984. The plan was filed away. After some political compromises, NASA was funded to develop and operate a reusable system, the Space Shuttle, capable of flying many times a year to reduce the high cost of transport to orbit, using airline operations as a model.

Many young people around the world saw the Apollo lunar landings as epic adventures, presaging eventual human exploration of the solar system (Chaikin, 2007). Few appreciated the purely political motivations of the program and were puzzled and disappointed by its abrupt end. Inside the NASA engineering culture, the missions to the Moon and beyond were assumed to be too expensive and too risky to be resumed; and focus shifted to the Space Transportation System with its Space Shuttle and, eventually, LEO (low Earth orbit) space station elements. Human space missions were designed to demonstrate routine operational capability, and exploration of space became lip service.

Visions of Solar System Exploration and Development

Prehistoric human migrations only required physical resources available to individuals, family groups, or tribes. Modes of transportation included walking, transport with pack animals, or ships. Motivation for relocation could be associated with searching for food and water or by escape from undesirable social settings. Migration of the earliest humans from Africa or the migrations in Polynesia—or even the expansion into the American West in the 19th century—produced historic cultural watersheds (Finney & Jones, 1986).

Should emigration from planet Earth become feasible, the resultant transformation of human civilization is difficult to imagine. However, the assets required for transportation to and sustenance of life on other worlds involves the utilization of our highest technology, demanding vast economic investment. Sixty years into the Space Age, resources of the appropriate scale for launching humans into space have been marshaled only by nation-states as the result of ad hoc political decisions. Initiation of a process of settlement of the solar system must begin with the transport of groups (e.g., space expedition crews) to interplanetary space. The resolve to work and live somewhere other than Earth must spring from a shared collective vision: economic, political, or ideological.

Such vision is always generated by inspirational accomplishments of individuals or by persuasive literature having a message resonant with a contemporary zeitgeist. Rudiments of a belief system do exist currently in some quarters, even though a collective societal will has not coalesced into a significant or effective political or economic force (Billings, 2007; Mendell, 2007). Nevertheless, hope for generating inspiration can be found in the work of certain individuals since the early 19th century.

Prior to the 20th-century, writings about visits to other worlds were speculative fantasy. From the Earth to the Moon by Jules Verne is perhaps the first popular work incorporating a transportation system for a human crew to the Moon that could be considered plausible within the contemporary understanding of physics and engineering. John Carter of Mars by Edgar Rice Burroughs emphasized adventure amid a setting of fantastical engineering.

Johannes Kepler’s formulation of the laws of orbital mechanics and Isaac Newton’s connection of those laws to gravitation are the theoretical foundations upon which space travel is built. However, Konstantin Tsiolkovsky is widely credited with deriving the Rocket Equation in 1897, a mathematical relationship used daily by aerospace engineers to determine the mass of a payload that can be delivered by a launch vehicle with a given thrust and fuel capacity. He used that equation in proving that spaceflight could be accomplished by a multistage rocket fueled by liquid hydrogen and liquid oxygen in his 1903 book Exploration of Outer Space by Means of Rocket Devices. Over the first three decades of the 20th century, he developed the principles of rocket-propelled spacecraft and produced designs for them as well as for dirigibles. Tsiolkovsky also penned the mantra used by advocates of human exploration of the solar system: “Earth is the cradle of mankind, but one cannot live in the cradle forever.” (Dick & Launius, 1985)

Hermann Oberth was a Romanian engineer who also studied medicine in Germany. He built his first model rocket at age 14 in 1910. While serving in the German Army during World War I on the Eastern Front, he found time to do experiments on weightlessness and subsequently resumed his rocket designs. Between the two world wars, he studied physics in Munich but had a dissertation on rocket principles disallowed for being “too utopian.” By 1929 he had expanded his writings into a tome entitled Ways to Spaceflight. He mentored young engineers in amateur rocket societies; a well-known photograph shows him working with a group of young people, including a teenage Wernher von Braun. From 1924 to 1938 he supported his family by teaching physics and mathematics in Romania. Along with Tsiolkovsky, he is considered one of the fathers of rocketry (von Braun & Ordway, 1975).

The father of rocketry in the United States is Robert H. Goddard. He became interested in science and technology as a boy but specifically developed an interest in space travel after reading War of the Worlds at age 16. His research on rocketry produced important technical advances, including two milestone patents on multistage rockets and rocket propulsion. However, his work early on failed to attract strong support. In fact, the New York Times ridiculed his claims of using rockets in space in an editorial. Noise from his engine tests elicited complaints from neighbors in New England, and Goddard moved his activities to New Mexico where he secured funding from the Guggenheim family with the help of Charles Lindbergh. Although his research was followed closely in Germany, he never made much of an impact on technology development in the United States. As a result, the United States entered World War II significantly behind Germany and the Soviet Union in rocketry (Gruntman, 2004).

Wernher von Braun led the team of engineers for Nazi Germany that developed the iconic V-2, the most advanced rocket-propelled missile of its day. In the days of the final German military collapse, von Braun gathered his team and surrendered to American troops. The Germans were brought to the United States where they formed a core of expertise for the modernization of missile technology. A decade later, he achieved public celebrity, teaming with Walt Disney to produce an animated film in which Donald Duck travels to space. A series of articles in the weekly magazine Colliers illustrated his vision of crews at LEO space stations. The designs strongly influenced the visual representations in Stanley Kubrick’s film 2001: A Space Odyssey. A book, The Mars Project, described a massive expedition of ships and crews to explore the planet Mars, giving much detail on the engineering and astrodynamics for the mission but lacking on issues associated with the crews and hazards of the space environment. Later in his career, von Braun would become the Director of the NASA Marshall Space Flight Center. The influence of von Braun can be seen in the strategic planning of the post-Apollo NASA human spaceflight program (Stuhlinger & Ordway, 1994).

Less well known but more comprehensive in its scope is the work of Krafft Ehricke on the human exploration and settlement of the Moon. During World War II, he was called from the German forces on the Eastern Front to report to Peenemünde and join von Braun’s team. He continued to work with his German colleagues in the United States but left for a position in private industry. At General Dynamics, he was the genius behind the Centaur, the world’s first rocket stage using liquid oxygen and liquid hydrogen for fuel. In the late 1960s he began formulating “The Extraterrestrial Imperative,” a philosophy for human settlement and economic development in the solar system. He published a three-part series under that title in the Journal of the British Interplanetary Society in 1977 and 1978. This work led to his book The Seventh Continent: Industrialization and Settlement of the Moon, the artwork for which consists of his own paintings (Freeman, 2008; Ehricke, 1985).

In 1969 Princeton University professor Gerard K. O’Neill, a prominent experimentalist in the field of high-energy physics, posed a question to his class in freshman physics as to whether a planetary surface was really the best location for future human habitation off of Earth. After reviewing the work of his students, he pursued further investigations into the possibility of giant autonomous habitats in space, possibly at the stable Lagrange points. He convinced himself that such structures were technically feasible but found it difficult to get his manuscript accepted in major scientific journals. His first paper in 1974 in the journal Physics Today stimulated a wellspring of interest among young people, which led to the initiation of a series of conferences on space colonization and on the various technologies needed for economic autarchy. In 1977, he and his wife founded the Space Studies Institute (SSI), a nonprofit organization at Princeton University. The success of SSI and the volumes of technical papers produced at its conferences formed the first substantial post-Apollo envisioning of human habitation in space. (O’Neill, 2001) One of the most effective pro-space advocacy groups, the L5 Society, was founded in 1975 to promote the concept of space colonies.

Congress in 1984 mandated the formation of a National Commission on Space to submit findings relevant to the formulation of future policy for the civilian space program. President Reagan did not appoint the members to the commission until 1985. After a year of hearings around the country and consultation with numerous experts, the report was ready to be submitted to the president in a public ceremony in January 1986. Unfortunately, the day before the presentation, the Space Shuttle Challenger exploded after launch, killing the entire crew. The report was shelved in the ensuing chaos in NASA and in the White House. It was officially submitted months later with little fanfare. No one can say what its impact may have been without the tragedy, but it does remain one of the most complete examinations of a vision for human settlement of the solar system.

The Report of the President’s National Commission on Space (NCOS), published as Pioneering the Space Frontier, never references Ehricke’s work and mentions space colonies only obliquely; but the report gives a thorough and complete road map for Ehricke’s Extraterrestrial Imperative. The NCOS put forth a vision within which the entire solar system is seen as the home for humanity. The commission emphasize the need for a long-range vision and commitment by government in a lead role with strong international cooperation. The report proposes to:

  • advance our understanding of our planet, our solar system, and the universe.

  • explore, prospect, and settle the solar system.

  • stimulate space enterprises for the direct benefit of the people on Earth.

To fulfill these goals, space policy must emphasize low-cost access to space, invest wisely in technology relevant to the nation’s vision, conduct an effective program of scientific exploration, and establish a legal environment in which private enterprise can freely innovate. The report goes on to discuss the opportunities available in the solar system for economic development and to outline the technical challenges to be overcome in the endeavor (Paine, 1986).

Space Transportation

It could be said that Isaac Newton is the father of space travel. Propulsion takes advantage of Newton’s third law, which is just a statement of conservation of momentum. A space vehicle ejects mass of some form in a given direction so that the vehicle will move in the opposite direction. After the ejection is complete, the vehicle will coast under the influence of gravity from bodies in the solar system. In general, the propulsion event places the vehicle in an orbit about some nearby body or about the sun, if it is far from other perturbing bodies. Almost all space travel consists of the movement from one orbit to another orbit. A propulsive event is required to depart an initial orbit, and a second event is required to enter a destination orbit. The trajectory connecting one orbit to the next is called a transfer orbit (Roy, 2005).

These propulsive events, called impulses, are carefully calculated to change the velocity of the spacecraft in a precise manner. Laymen often use the term “velocity” as synonymous with speed, but velocity as used in orbital mechanics has a direction as well as a magnitude. The change in velocity to initiate an orbital maneuver is called delta-v. The amount of fuel needed for a particular mission is often estimated from the sum of the delta-v maneuvers required.

The total mass of propellant available to be ejected is finite, and rarely do vehicles with chemical rockets fire their engines continuously. Consequently, space vehicles spend most of their time coasting in one orbit or another. The speed of a spacecraft traveling along an orbit is determined by the law of gravity (orbital mechanics); one does not “put the pedal to the metal” to shorten the time of the trip. Any applied propulsion changes the orbit and burns valuable fuel. The long coasting times in transfer orbits affect the planning of human missions to deep space.

The first stage of any space expedition is daunting. A cargo must be lifted off the Earth against the pull of gravity and against the aerodynamic resistance from the atmosphere. The launch vehicle and its payload must be guided on a trajectory that will place it into the proper orbit. Most commonly, that initial orbit is around the Earth. A subsequent brief propulsive impulse can cause the payload to depart for another orbit.

Note that trajectories in space are almost never straight lines, unlike roads on the Earth. All bodies in the solar system are in orbit about some other body, usually the sun, on paths that are elliptical, according to Kepler’s laws. While it is technically possible to travel from point to point, the amount of fuel required is so excessive as to make it impractical.

The orbits of the planets lie in a single plane called the ecliptic. The path from one orbit to another therefore can be visualized as a diagram on a piece of paper. If the elliptical orbit for one planet (e.g., Earth) lies inside the orbit of another planet (e.g., Mars), the most energy-efficient path from one to the other is another ellipse, connecting the two orbits and tangent to both. A chemical rocket will fire its engine briefly to leave the planetary orbit and proceed on to the transfer orbit. Later, it will fire its engine briefly to leave the transfer orbit and enter the solar orbit of the destination planet. The maneuver is called a Hohmann transfer. The timing of the departure from Earth must be such that Mars will be at the tangent point in its orbit when the spacecraft arrives. In the case of Earth and Mars, the appropriate alignments occur only once every 26 months.

All current Earth launch vehicles are chemical rockets. Inside the combustion chamber a fuel and an oxidizer are combined to create a highly exothermic reaction. The resultant hot gas expands, pressing against all sides of the chamber. At one end of the chamber is an opening through which the hot gas can escape. The rocket exhaust carries momentum away, causing the vehicle to move in the opposite direction, according to conservation of momentum. The more rapid the loss of mass out of the chamber and the faster the exhaust velocity, the more force or thrust is generated. The gas exits through a nozzle, whose shape has been designed to cause the gas to expand in such a pattern as to maximize the thrust.

A second important performance characteristic is specific impulse, a measure of the efficiency of a propulsion system. It is defined as the thrust divided by the mass flow rate. If two systems are on orbit and must change velocity to begin a different orbit, the system with the higher specific impulse will require less propellant to achieve the required delta-v. Ion propulsion systems exhibit specific impulses 20 or 30 times greater than chemical systems but generate low thrust, making them suitable only for in-space operations. They are useful for orbital transfers in space that require a large delta-v but do not need large thrust. They also can be fired continuously, resulting in non-Keplerian transfers.

Mission designers are asked to create a mission architecture to deliver a given payload to a given destination. They begin with the orbital mechanics. The mass of the payload dictates the smallest (and therefore cheapest) launch vehicle capable of delivering it to LEO. Occasionally the volume of the payload may dictate a launcher larger than that necessary to simply lift the payload. The total energy can be calculated from the payload mass and the trajectory. The designer then begins to determine various combinations of launchers, upper stages, in-space propulsion systems, and possible launch dates that might be dictated by interplanetary trajectories. The design goal is to find the least expensive set of vehicles and operational procedures that will result in mission success.

Choices of launchers may be limited by political considerations. Classified military satellites are usually launched only by the home country. Many launchers could be chosen for delivery of communication satellites to geostationary orbit, but a government may require that its satellites be placed on a domestically manufactured vehicle.

Reliability is another factor. Launch failures are always investigated, and mission designers must sometimes decide whether to choose a lower price with a higher risk. In particular, human missions receive extra scrutiny; and their launchers must be “man-rated,” at least in the United States. The extra analysis, oversight, and backup systems are expensive; and human mission launch costs are the highest of all.

Mission designers and project managers in general always prefer to use systems that have already been in space and whose reliability and performance characteristics are well understood, a requirement that is difficult to meet presently. Since 1972, no human being has gone beyond low Earth orbit. The systems used during the Apollo program no longer exist. No NASA system for launching humans has existed since the retirement of the Space Shuttle in 2011.

On what basis does a designer choose transportation systems and spacecraft for future human expedition into the solar system? Are existing launch vehicles capable of supporting a series of human expeditions to the surfaces of the Moon or Mars or to the resources of the asteroid belt? Are governments the agents of a new human migration; and, if so, which will take the lead?

Of the three space agencies that have launched humans into orbit, only NASA has published studies of potential mission architectures for human expeditions to destinations in deep space. In addition, there exists an extensive professional literature on various technical topics relevant to human exploration, including possible mission architectures.

To illustrate the classes of launchers needed for human expeditions, it is useful to review mission architectures designed for exploration of Mars. A crewed Mars mission is generally accepted as the most demanding project within the bounds of current technology; however, a complete review of human Mars mission architecture is quite complex. Here, we focus on scenarios requiring minimum energy, called “conjunction missions,” using chemical rockets. As discussed above, the major part of the trajectory will be a Hohmann transfer from the orbit of the Earth to the orbit of Mars.

In 1989, President George H. W. Bush laid out goals for the NASA human space flight enterprise (Stafford, 1991). Those goals included human expeditions to the Moon and to Mars. NASA responded to the president with candidate mission architectures to accomplish those goals. The mass of the Mars Transit Vehicle (MTV) to be assembled in LEO for the first human Mars mission was estimated to be between 1,100 and 1,500 metric tons. For comparison, the International Space Station, at completion, has a mass of about 420 metric tons and took 37 Space Shuttle missions to construct over 12 years.

Unlike the ISS, much of the mass of the MTV would be liquid rocket fuel. In addition, it would include a lander to take the crew to the surface of Mars and an ascent launcher to return the crew to the orbiting transfer vehicle for the trip back. Also included would be a habitat and provisions for the crew to survive 500 days on the surface of Mars. The total mission time from leaving Earth to return would be somewhat less than 1,000 days. Obviously, this mission architecture would require significant new developments of new space systems and a challenging logistics train for the assembly in LEO. Any significant delay in the assembly of the MTV could cause a shift to the next planetary alignment in 26 months, leading to large cost overruns. NASA postulated the development of new launch vehicle of the Saturn V class (100 to 120 metric tons to LEO). Even so, that launch rate and orbital activity would exceed that of the Apollo program.

The Space Exploration Initiative of 1989 was cancelled before any significant hardware was designed or constructed, but in 2004 President George W. Bush laid out the Vision for Space Exploration, containing more specific directions to NASA, including retirement of the Space Shuttle by 2010 (NASA, 2004). In the interim between the two Visions, NASA had rethought its architecture for Mars expeditions. The Design Reference Mission (DRM) 5.0 (Drake, 2009) for Mars incorporates new concepts not tried before in mission design with the goal of reducing the logistics burden in LEO and decreasing the risk of loss of crew.

The expedition elements would be broken into three parts. Two cargo transit ships would be sent from LEO to Mars during a launch window (i.e., planetary alignment). One cargo ship would carry primarily a Descent-Ascent Vehicle (DAV) that would land on Mars at a predetermined site. The second cargo ship would carry a Surface Habitat (SHAB), which would be inserted into a high orbit around Mars, where it would stay for two years. Once the DAV has landed, it will robotically deploy an automated processing plant to produce methane and oxygen from the thin martian atmosphere. This was to be used as fuel for the DAV to carry the crew to Mars orbit four years later. The DAV on the surface and the SHAB in orbit would be monitored from Earth to assure that all systems were functioning.

Twenty-six months after the cargo launched, the crew would leave Earth orbit in a Mars Transit Vehicle (MTV) to rendezvous with the SHAB in high Mars orbit. Once onboard the SHAB, they would land it near the DAV at the expedition site. At the end of martian surface operations, the crew boards the fully fueled DAV to launch into Mars orbit for rendezvous with the MTV. Once back in Earth orbit, the crew would use a capsule similar to the Orion, currently under development in NASA, to reenter the Earth’s atmosphere.

In a departure from previous NASA mission architectures, DRM 5.0 assumes nuclear thermal propulsion for the three transit vehicles and a fission reactor for the surface power source. The use of nuclear power makes perfect sense to engineers, but launching nuclear reactors has created political problems in the United States.

DRM 5.0 assumed the development of Saturn V class heavy-lift launcher named Ares V within the Constellation Program, as well as a crew launch vehicle called Ares I that would also carry crews to and from ISS after retirement of the Space Shuttle. The concepts for the Ares V and the Ares I were conceived early in the NASA Constellation Program. The two cargo missions require 5 Ares V launches, and the crew MTV mission requires 4 Ares V launches. The on-orbit assembly of a cargo ship requires only the docking and mating of two elements; assembly of the crew MTV requires mating three elements. In this way, the large logistics challenge of the 1989 architecture is avoided. The total mass leaving LEO among the three transfers is 850 metric tons, compared to 1,100 or more for the 1989 architecture. Mass savings arises from the production of fuel on the martian surface and from the use of nuclear propulsion.

The NASA Constellation Program (NASA, 2008), created to implement the Vision for Space Exploration, worked on a lunar architecture with a capability to land four astronauts anywhere on the lunar surface with two crew rovers and life support to last for two weeks of exploration (NASA, 2008). The space transportation assumed for each mission was one Ares 1 launch and one Ares 5 launch. When the program was cancelled in 2009, no viable lander had been designed. Some observers suggest that lunar missions of this scale would require double Ares 5 launches.

What becomes clear is that large-scale human exploration and settlement of the solar system will require heavy-lift launchers of the 120 metric ton class. The NASA Space Launch System Block 2 is advertised to have a capacity for 130 tons to LEO, but only the 70-ton Block 1 is currently in development: at the time of this writing it was at least three years behind schedule and estimated to cost more than $500 million per launch. Meanwhile, in early 2018 SpaceX carried out a successful test launch of its Falcon Heavy with a lift capacity of 64 tons and an estimated launch cost under $100 million; SpaceX also has announced the BFR rocket, which will lift 150 tons. China has under study a Long March 9 vehicle with an estimated lift capacity of 140 tons (Brigge, 2017; Wikipedia, 2017).

The Astronaut in the Space Environment

This Island Earth

From the perspective of humans, the solar system is mostly void. Among the objects in orbit around the sun is a remarkable and unique planet we call Earth, home to an array of complex life forms, including what we think of as sentient beings. The terrestrial biosphere could not have evolved and has continued to exist but for a set of planetary attributes that fortuitously shield the biota from various exogenic disruptive phenomena.

The dual planet aspect of the Earth-moon system has provided environmental stability over geologic time. The relatively massive moon has stabilized the spin axis of the Earth, preventing it from extreme excursions from the perpendicular to the ecliptic, unlike Mars whose extreme polar wander has resulted in a frozen, possibly lifeless, world (Laskar et al., 1993). While certain characteristics of planet Earth are seen in other solar system bodies, none exhibit the totality of attributes that have enabled the biosphere that is humanity’s native environment.

When life arose some 4 x 109 years ago, surface conditions were not those familiar to us today. Yet, the mass of the Earth was sufficient to allow geochemical differentiation, producing minerals and concentrations of key elements necessary for the carbon-based structures that evolved. Gravity was sufficient to allow vigorous convection to promote chemical reactions but not so great as to preclude three-dimensional carbonaceous tissue built upon calcic skeletons. The surface thermal regime permitted the existence of liquid water, which is the polar solvent so critical to organic chemistry. The internal heat engine generated a planetary magnetic field that shielded the surface from bursts of solar particles, and the atmosphere filtered the solar illumination.

In the 21st century, creatures of the biosphere aspire to emerge from the terrestrial chrysalis to explore, and even populate, the greater cosmos. Their engineers must build containers in which life support is possible, and space medicine professionals must develop countermeasures for the effects of the space environment on the passengers that cannot be mitigated by engineering (Tribble, 1995; Clowdsley, 2003). Let us review the challenges.

Vacuum

In outer space all human spacecraft must be pressure vessels. In the 21st century, even airliners must be pressure vessels. Naively, one might expect that the appropriate pressure in a spacecraft or a space habitat should be one Earth atmosphere, sometimes referred to as “1 bar.” (The metric measure for 1 bar is approximately 100 kiloPascals.) However, such pressure is inappropriate for a spacesuit because the suit would become so rigid that an astronaut could not perform any tasks.

On the International Space Station (ISS), the spacesuit used for extravehicular activity (EVA) has a pure oxygen atmosphere at 290 millibars. Before the crew can go EVA, they must prebreathe pure oxygen to purge nitrogen from their blood and avoid decompression sickness. They spend the night in an airlock at reduced pressure to reduce the prebreathe time to minutes instead of hours. Thus, the decision to use a 1-bar, sea-level atmosphere on the ISS requires time-consuming preparation for EVA.

The principal research activities by the ISS crew take place inside the pressurized volume. The EVA is a special activity, necessary for maintaining the structure or configuration of the station. On a planetary surface, just the opposite may be true. The EVA is the principal research environment while time in the habitat is needed for rest, nourishment, planning, etc. If the surface habitat is a permanent central facility, the real exploration may be accomplished in long-duration surface excursions in pressurized vehicles. In the field, a quick transition from vehicle to EVA suit and back becomes more desirable. In this case, a 1-bar atmosphere is a hindrance to the mission.

The “correct” habitat pressure is frequently a subject of debate in mission design studies.

Gravity

Chemical rockets fire for relatively short periods of time, providing an impulse designed to place a payload into an orbit or to transfer a payload from one orbit to another. We are most familiar with orbits about the Earth, but an interplanetary trajectory is an orbit about the sun. In orbit, the payload is actually falling and experiences no gravitational acceleration. Engineers have a great deal of experience designing mechanical, electrical, and fluidic systems that operate successfully in the weightless environment. However, many basic functions in a human body need gravity to operate normally and begin to change when gravity is absent. Adaptation to weightlessness is not bad per se, but the changes can be debilitating when the crew member returns to a gravitational environment (i.e., back home on Earth).

Temperature

Temperature is a thermodynamic property of an object that will continuously change until the object reaches thermal equilibrium with its environment. In deep space, far from any star, objects become very cold. In our solar system, a small black object exposed to sunlight continuously will attain an equilibrium temperature that is characteristic of its distance from the sun. At the orbit of the Earth (1 astronomical unit) the blackbody equilibrium temperature is approximately 394 Kelvin [K] or 121 ˚C. At the distance of Mars, the equilibrium temperature falls to approximately 321 K or 48 ˚C. The actual temperature on the surface of the Earth or on Mars is different from (and generally much less than) the solar equilibrium temperature for a variety of reasons. In fact, the coldest temperature measured anywhere in the solar system has been detected in a permanently shadowed crater at the south pole of the Moon (Aye et al., 2014).

Any large body without a massive atmosphere and with one side continuously facing the sun can become quite hot (depending on the distance from the sun), while the side facing away from the sun can become very cold. On the lunar equator at local noon (when the sun is directly overhead), the temperature is close to blackbody equilibrium; but the same spot on the lunar surface just before dawn will reach cryogenic temperatures. Mars has a tenuous atmosphere but, like airless bodies, exhibits surface temperatures that are strongly dependent on the length of exposure to sunlight (i.e., on the length of the day determined by the rotation rate).

The temperature of a spacecraft in orbit around a large body such as Mars or the Moon, can be influenced significantly by heat input from the surface below. Engineers mitigate the effects of extreme thermal environments with special surface coatings, clever heat conduction paths in the spacecraft structure, or operational strategies such as rotating the spacecraft to keep heating distributed more evenly. Thermal design is a major consideration for spacecraft and for surface habitats: poor thermal design can be the downfall of a mission.

Solar Particle Events

Everyone knows that the sun shines: that is, emits electromagnetic radiation. Far fewer people are aware of the solar wind, a continuous flow of ions embedded in magnetic fields streaming outward from the solar surface. Physicists call this miasma of ions and fields a plasma and treat it as a special kind of fluid that follows its own rules obtained from Maxwell’s equations. The solar wind flows more or less continuously throughout the entire solar system and far beyond. Eventually the flow stalls as it weakens and encounters the magnetic fields of the galactic medium between the stars. In the 21st century, the Voyager spacecraft are traversing this boundary of the heliosphere. The heliosphere can be envisioned as a gigantic bubble around our sun, holding back “barbarian” magnetic fields from the Milky Way galaxy (National Research Council, 2008).

The positive ions in the solar wind plasma are overwhelmingly protons, although ionized atoms of other elements are also present. The surface of the Moon is directly exposed to the solar wind on the dayside, and lunar surface grains eventually become saturated with solar wind atoms. (When lunar enthusiasts advocate mining the Moon for helium-3 (3He), they are targeting the surface grains rich in solar wind gases.)

Spacecraft exposed to the interplanetary environment acquire a net electric charge. In fact, the surface of the Moon is charged positively on the dayside and negatively on the night side. The charge itself is not a serious problem until some kind of non-uniform charge distribution accumulates with voltage differentials that can generate sudden electrical discharges. Satellite designers have learned through experience to deal with space charging of their spacecraft. In LEO, enough remnant atmosphere remains to provide a leakage path for excess charge. However, in the hard vacuum at the lunar surface, differential charging could be problematic for human activities. Insufficient data exists to determine whether concerns are warranted.

Violent events occur on the surface of the sun. Occasionally, massive clouds of high-speed ions spew outward in what is called a solar particle event (SPE). A SPE is dangerous for an inadequately shielded human in its path and can be devastating to electronics in satellites in its path—depending on the intensity of the storm. The frequency of such eruptions varies on an 11-year cycle, alternating between an active sun and a quiet sun. An SPE is highly directional.

Each SPE has its own energy spectrum, particle density, and duration. Spacecraft designers look at characteristics of the most dangerous SPE historical events, combine their most severe elements into a hypothetical worst case, and design their craft accordingly. The geomagnetic field protects terrestrial life from the direct effects of solar storms, but human beings traveling away from the Earth must take shelter upon encountering an SPE. In an era of human exploration into the solar system, a warning system is mandatory. Unfortunately, once the eruption is detected, the time to react can be as little as 20 minutes in worst-case situations. Solar physicists are working on observation strategies and models to predict dangerous events before they occur.

A historically dangerous SPE occurred between the return to Earth of Apollo 16 and the launch of Apollo 17. Had a similar event occurred during one of those missions, the crew would have been lost.

Galactic Cosmic Radiation

The solar system is permeated by a second kind of radiation, coming not from the sun but from the Milky Way galaxy. The flux of particles is not as high as what comes from the sun, but the galactic cosmic radiation (GCR) has a much greater percentage of high atomic weight ions traveling at high energies. Each such heavy ion is much more damaging to biological systems than a solar proton. Exposure of a human being to GCR is not immediately life threatening, but prolonged exposure steadily increases lifetime cancer risk. In addition, shielding against the highly penetrating GCR particles is difficult (National Research Council, 2008; Wilson, Miller, Conradi, & Cucinotta, 1997).

When a high-energy, high-mass ionized atom from interstellar space strikes a material surface (e.g., spacecraft structure, lunar rock), it creates microscale nuclear havoc, generating a cloud of neutrons through collisions with atoms in the structure. Some neutrons, in turn, interact with other atoms, creating unstable radioactive isotopes in some cases. Other neutrons lose energy through scattering reactions, eventually becoming low-energy particles called thermal neutrons. Unfortunately, thermal neutrons are more dangerous to humans than the original cosmic particle.

The instinct of the spacecraft designer is to consider structure as part of the shielding, but in this case the structure can be the problem. Thermal neutrons are best moderated by hydrogen, leading designers to consider storing water and other materials associated with the life-support system around living areas and between the crew and the outer spacecraft structure.

The boundary of the heliosphere is defined as that region where the diminishing energy in the solar wind plasma is finally stalled by the resistance to change of the galactic magnetic fields. The volume of heliosphere expands during the portion of the solar cycle when the sun is active. Conversely, it shrinks when the sun is quiet. At the boundary of the heliosphere are shocks: a bow shock on the outer side where the interstellar medium begins to interact with the sun and a termination shock inside the heliosphere where the solar wind slows down to subsonic velocities. Between these two shocks is the heliosheath. This is where huge “magnetic bubbles” aid in deflecting the galactic cosmic particles. When the deflection occurs farther from the sun, fewer particles make it into the vicinity of the planets. Ironically when the sun itself is most dangerous, it is also diminishing the galactic cosmic radiation. Conversely, the GCR flux increases when the sun is quiescent.

Meteoroids

The solar system also contains many solid objects. By far the most numerous in this population are micron- to millimeter-scale particles that can be debris shed by comets, detritus from collisions, or even interlopers from other stellar systems traversing our domain at high velocities. Many spacecraft sent to distant destinations carry dust detectors that register impacts from these micrometeoroids. The strikes are infrequent and stochastic; but some occur in clusters, implying an encounter with a cloud of particles. Encounters are more frequent in certain parts of the solar system such as the asteroid belt. If one avoids certain special environments such as the rings of Saturn, or the proximity to a comet nucleus, the probability of serious damage from an impact is small. Nevertheless, risk assessments and appropriate impact shielding must be part of a spacecraft design.

Orbital Debris

Almost all plans for human expeditions begin with some activity in LEO. The time in LEO can be as short as a few orbits while trajectory planes align, or it can be quite extensive (e.g., if an interplanetary spacecraft were to be assembled in LEO). Whatever the plan, a risk analysis must be performed to assess the danger from collision with man-made orbital debris in LEO.

One scenario developed within the NASA Constellation Program required a spacecraft to loiter for two days either in low lunar orbit or in low Earth orbit. Initially, the LEO option seemed best until analysis showed that an extended loiter in LEO required more shielding mass on the spacecraft, thereby reducing the usable payload.

Currently, all spacecraft launched into LEO are evaluated for collision risk. The entire International Space Station is sometimes moved to avoid the possibility of a “conjunction” with a large piece of orbital junk.

Planetary Surface Environments

The destinations for human expeditions attainable by around the mid-21st century, given the current projections of technology (and political will) are less numerous than assumed by the general public. The limiting capabilities are the lift capacity of future launch vehicles, the size and complexity of spacecraft capable of supporting long-duration space voyages, and the ability of human beings to remain healthy and productive during mission times that take more than a year. Many papers have been written about advanced systems that, for example, could significantly reduce interplanetary trip times; but no credible initiatives currently exist to make the long-term investments necessary to develop and to produce these wondrous systems.

Assessments of technology consistently show that expeditions to the surface of Mars will take place no earlier than the fourth decade of the 21st century. Mission scenarios based on chemical propulsion systems show that enormous mass must be lifted from the Earth to assemble the interplanetary transit vehicle. In addition, the mission times are longer than space medicine specialists are willing to permit.

Because Mars may have independently developed early life forms, human surface activities will be allowed only in “safe zones” where absence of martian biology has been conclusively demonstrated. No robotic missions to Mars have even been conceived to find where such zones may exist. Should credible and affordable human mission architectures come to fruition, the strengths of scientific arguments for planetary protection will be tested.

Asteroids, Phobos, and Deimos

Small bodies have minuscule gravitational fields, which can be regarded as totally negligible for spacecraft operations. Consequently, a spacecraft cannot land in the usual sense but rather must rendezvous similar to a docking with the International Space Station (ISS). During a docking maneuver, the ISS is considered a “cooperative” target because it maintains a stable and predictable attitude during the maneuver, and it provides a mechanism for capturing the incoming spacecraft. An asteroid, on the other hand, is indifferent to the visiting spacecraft and may even be rotating too rapidly or too chaotically for docking. The surface may not be solid and may not provide any structure for grappling. An astronaut who is placed “on” the surface of the body may inadvertently achieve escape velocity after some simple attempt at moving about. As one can imagine, human exploration of a small body is nothing like operation on a planetary surface with significant gravity.

In a highly simplified characterization, the solid bodies of the solar system are classified as “rocky” or “icy” or some combination of the two. If a spacecraft can fly close enough to a body to have its trajectory altered by its gravitational field, and if the spacecraft can transmit a picture, the derived mass and volume can be used to calculate an approximate density. Solid, coherent rocky bodies have densities approaching 3,000 kg/m2; pure ice gives densities approaching 1,000 kg/m2. An intermediate number is interpreted as some mixture of the two, as long as that conclusion is consistent with geological intuition or remote sensing results. The measured density will decrease if the body contains significant void space throughout (i.e., is highly porous). Scientists were startled to discover that some asteroids are so underdense that they must be agglomerates of particles, loosely bound by mutual gravity. Such bodies have been dubbed “rubble piles” and lie outside our geological experience.

Of the two moons of Mars, Phobos is somewhat denser than Deimos, the latter falling midway between the “rocky” and “icy” end points. From early Earth-based reflectance spectroscopy, the martian moons were thought to be similar to the carbonaceous chondrite meteorites, containing significant quantities of hydrated minerals. This interpretation led advocates of Mars exploration to characterize the moons as sources of deep-space water for propulsion or life support. More precise spectral measurements from spacecraft in martian orbit are now connect the moons to the “D” class asteroids farther out in the solar system, whose composition is unknown. From theoretical arguments, such asteroids ought to contain significant volatiles. Nevertheless, the low densities of the martian moons are interpreted as arising from porosity rather than a 50% ice composition.

Phobos is ~20 km in diameter and has a highly cratered surface, similar to other asteroidal bodies. On the other hand, Deimos, with a diameter ~10 km, exhibits unusually subdued relief on its surface, leading to the speculation that it might be sheathed in a thick layer of fine material, accreted from some orbital disintegration. Fine dust on asteroids might seem counterintuitive, given the minute gravitational attraction. However, what appear to be pools of fine material in depressions were imaged on asteroid Itokawa by the Hayabusa spacecraft. The accumulations must involve electrostatic forces, an important data point for planning human operations at the asteroid surface. Spacecraft and suited astronauts will be electrically charged in deep space, and electrostatic interactions with nearby fine particles can be a problem.

The selection of small bodies for human exploration is confounded by a lack of knowledge of their orbital parameters and even greater ignorance of their physical properties. Mars and its moons have well-known ephemerides; mission scenarios can be studied for launch opportunities far into the future, constrained by the 26-month periodicity of the launch window from Earth. While hundreds of near-Earth objects (NEO) have been detected and some orbital parameters approximated, the database is sufficient only for describing which ones might be candidates for future missions. Recent surveys have identified tens of candidates in terms of launch capability, but only a handful currently appear to be large enough to be viable targets. (Politicians would not be happy if a mission costing many tens of billions of dollars arrived at an object smaller than the spacecraft.) Possible launch opportunities to various NEOs are scattered over the next two decades, but almost nothing is known about the objects themselves other than an educated guess as to size. The limited opportunities for future observation do not promise to improve the situation significantly.

Once a candidate NEO is selected for a human expedition, a precursor robotic mission to the object will be necessary to verify what we think we know and to discover if the physical attributes of the target are consistent with investigation and sampling by suited astronauts.

The Moon

The moon is a Near Earth Object whose basic characteristics and location are well understood and to which the in-space transit time is quite modest. However, future human exploration of the Moon would be quite different from human exploration of an asteroid (Schrunk et al., 1999). In the case of the smaller body, the time at the object would be a few days; and the operations would be more spatially extensive (exploratory) than intensive at a single location. For the Moon, future expeditions would almost certainly spend more time on the surface than an Apollo mission and would be designed around rather specific investigations, some of which may involve quite detailed characterization of a specific feature. Expedition objectives may include non-scientific activities such as construction of facilities or demonstration of resource extraction technology. In some scenarios, lunar astronauts will live in a habitat on the surface for months.

The lunar gravitational acceleration is powerful enough for objects to stay on the surface; but according to current medical opinion, it is not enough to counteract the physiological changes that occur in the weightless environment. The moon has no sensible atmosphere and no indigenous magnetic field. The dangers of the space radiation environment apply on the lunar surface with the exception that the body of the planet blocks radiation (and meteoroids) coming from half the sky. Spacesuits are still required for surface operations.

The lunar rotation axis is almost perpendicular to the ecliptic, and the path of the sun on the lunar surface never deviates far from the equator. When the sun is directly overhead (lunar noon near the equator), the temperature of the surface reaches 400 K (123 ˚C). After sunset the surface temperature falls rapidly, reaching almost 90 K (–183 ˚C) before dawn. Near the subsolar point, the combination of heat from the sun overhead and the surrounding surface creates thermal loads on equipment (and astronauts) that are quite challenging to thermal engineers, who can only dissipate the heat load via inefficient radiation to deep space. Consequently, engineers favor locating facilities or habitats at high latitudes where the local horizontal surface never reaches the subsolar temperature. Nevertheless, any surface perpendicular to direct sunlight will heat the same way, whether at the equator or at the poles. With the motion of the sun so constrained in the sky, certain regions associated with craters near the lunar poles can remain in permanent shadow and experience extraordinarily low temperatures.

The surface of the Moon is everywhere covered by a layer of loose material, including very fine particles. Movement on the surface by humans or vehicles disturbs this material and can toss it from place to place on ballistic trajectories. The very fine component, generically called dust, is affected by electrostatic fields and tends to adhere to surfaces, creating potential problems for moving parts in machinery. The combination of impact by the solar wind ions and photoemission of electrons in sunlight charge the surface to varying degrees, depending on time of day and the position of the Moon in the Earth’s magnetic geotail. Dust may be mobilized by charge differentials between adjacent sunlit and shadowed areas. These phenomena are controversial and are not yet documented by systematic observation. The Apollo astronauts were on the surface for only a brief part of the lunar morning and did not observe any dust movement due to electric fields.

Mars

Mars is often described as being the most “Earth-like” planet, leading some to infer that human exploration operations on the surface of Mars will be simpler than similar activities on the Moon. The length of the martian “day” (sometimes called a “sol”) is close to that of Earth, and the diurnal temperature excursions are far less than those on most of the lunar surface. Nevertheless, human explorers will still need spacesuits on the surface and will still need protection from solar and galactic radiation. Much of the operations’ planning and execution will be in the hands of the crew because the communications delay over the Earth-Mars distance (up to 20 minutes one-way travel time) makes real-time monitoring and conversation impossible. When Mars and Earth are on the opposite sides of the sun, communications are completely interrupted unless some kind of interplanetary relay network is in place. This increased isolation from Earth, including the loss of visibility of the home planet in the sky, places additional psychological stress on the crew.

The martian gravitational acceleration is more than twice that of the Moon but still less than half that of the Earth. No data is available as to whether the martian gravity is sufficient to prevent the physiological degradation associated with weightlessness.

The orbit of Mars is more eccentric than that of Earth. When Mars is farthest from the sun (aphelion) the maximum possible surface temperature can be as much as 30 ˚C less than when it is nearest the sun (perihelion). At summer noon at perihelion, the surface temperature can reach 20 ˚C. The pre-dawn surface temperatures at low latitudes are approximately 200K (–75˚C), depending on the grain size and physical properties of the ground. The polar caps can be as cold as 145K (–130 ˚C), the sublimation temperature of solid carbon dioxide.

At perihelion, gigantic dust storms frequently erupt on Mars. Some storms are global and fill the entire atmosphere with fine particles, making it opaque to cameras on orbiting spacecraft. The atmosphere may take weeks to clear but still contains a residuum of suspended particles for a much longer time.

Galactic cosmic rays and solar particle events are still radiation hazards on the martian surface. The thin atmosphere provides some protection from SPE but is not effective in shielding GCR. Calculated doses to surface crews do not exceed mandated limits of exposure, but martian astronauts also spend approximately a year in space traveling to and from Mars. When the transit doses are taken into account, substantial surface shielding is recommended.

The Human System

At the beginning of the Space Age, the aerospace medical community had serious reservations about the ability of human beings to survive and to perform complex tasks in the space environment. Some of their concerns were resolved by research in ground-based facilities, in high-altitude aircraft, and in transient weightless environments in airplanes flying parabolic trajectories. Interestingly, one of the first discouraging data points came from a seven-day test of a subject kept in isolation in “space cabin simulator” in 1956. By the end of the test the subject’s ability to complete tasks had deteriorated significantly, and the subject became increasingly difficult to interact with. In 1958, U.S. Air Force physician Major Charles Berry wrote, “The psychological problems presented by the exposure of man to an isolated, uncomfortable void seem to be more formidable than the physiological problems.”

The orbital flight of Yuri Gagarin in 1961 put to rest the most basic issues of survival and performance. Over the ensuing 50 years, a great deal of data and experience has been accumulated regarding human spaceflight. Currently, the NASA Human Research Program (HRP) defines and studies risks and concomitant research issues associated with long-duration exploration missions as defined by the Integrated Research Plan (IRP) of July 2011: “Risks include physiological effects from radiation and hypogravity environments, as well as unique challenges in medical support, human factors, and behavioral or psychological factors.”

Some countermeasures, such as medication or exercise, have proven effective against many aspects of space adaptation. Certain residual effects are seen in postflight examinations in some crew members but not others, implying that informed crew selection could be an effective tool for deep-space expeditions. Medical confidence in any conclusion or procedure demands many subjects for any particular protocol, and for spaceflight the total number of subjects on orbit for study of any particular medical issue is not large. In addition, the risks of long-duration voyages and/or destination stays in the space environment demand more data and specific experiments necessary to design spacecraft and habitats. These research needs run up against the demands on astronaut time in space for “productive” activities and the natural dislike of astronauts to be medical guinea pigs.

From the perspective of the spacecraft design engineer, the requests from the human factors group add mass, power, volume, and complexity to a problem that is already over-constrained. The Astronaut Office will also have an opinion on the design. Conflicts are passed up to project managers, who almost always come from engineering backgrounds and favor decisions that do not impact cost and schedule.

Senior engineers see a complex project such as a spacecraft as a collection of systems that interact with one another to produce a single functioning product to meet customer requirements. In this context, it is useful to think of the crew as one of the systems, the Human System. High-level program documents identify the requirements and characteristics of the Human System in a way that enables designers to integrate those requirements in the earliest stages of the design process. Not all issues are solved by this approach, but it can streamline communication within large projects.

NASA’s Human Research Program (HRP)

Like any other subsystem in a spacecraft or a launch vehicle, the Human System can suffer malfunctions and failures. A problem with the crew can easily preclude mission success and in extreme cases can result in loss of life. Unlike other spacecraft systems, the design of the Human System antedates initiation of any aerospace program. The space medicine community has, in effect, analyzed the Human System and has performed a failure mode and effects analysis (FMEA). Human spaceflight risks include physiological effects from radiation, hypogravity, and extraterrestrial environments, as well as unique challenges in medical support, human factors, and behavioral health. Within five categories (called “elements”) are identified 34 research thrusts, each designed to evaluate and to mitigate, if possible, a major risk. The five research elements are: (1) behavioral health and performance, (2) exploration medical capability; (3) human health and countermeasures; (4) space human factors and habitability, and (5) space radiation. Details on the biomedical risk matrix and the Integrated Research Program can be found on a NASA website (NASA, 2017a).

Most of the risks can be deemed acceptable or controlled if the mission architecture, including spacecraft design and mission operations, is structured along the guidelines of the HRP. This is not always easy. Control of certain other risks may not directly affect the design of the mission architecture but would benefit from advances in technology. A few risk topics lack sufficient data for an assessment.

The European Space Agency (ESA) published its own assessment of research needed for mitigating risk in future human missions. The study couched the discussion in the context of three notional human exploration mission scenarios: (1) an outpost at the south pole of the Moon; (2) a 1,000-day mission to Mars with a surface stay of 525 days, and (3) a 500-day mission to Mars with a surface stay of 30 days. The risk categories are similar to the ones identified by NASA. However, the ESA report goes on to discuss the technology needs for closed life-support systems, future analogue missions on Earth designed to simulate some aspects of space exploration, and future robotic missions designed to gather more precise information on the environment and potential technologies needed for human planetary surface activities. The findings of the ESA study agree generally with the NASA assessment (Horneck et al., 2003).

Obstacles to Exploration: Space Radiation Exposure

Space radiation comprises high-energy protons, especially from solar particle events (SPE) and energetic heavy ions (HZE’s) in galactic cosmic rays (GCR). Interactions with GCR in shielding materials produces secondary protons, neutrons, and heavy ions in a spacecraft or habitat. Unique damage to biomolecules, cells, and tissues occurs from HZE ions, and no human data exists from which to estimate risk. Models based on laboratory experiments must be applied or developed to estimate cancer and other risks.

The acute somatic effects of ionizing radiation are mass phenomena that deplete cells in a given organ system or tissue. A clinical effect becomes apparent only after a significant number of cells are killed. The threshold for a clinical effect is high, meaning that significant risks for early effects occur only with high radiation fluences. By contrast, the low fluences of GCR result in damage to single cells leading, for example, to genetic alteration or cancer over a period of time.

The Apollo astronauts reported seeing flashes of light during their space journeys. Similar flashes have been reported by astronauts in LEO. These flashes are believed to be caused by the traversal through the retina of a single, charged, highly energetic, high atomic weight (HZE) particle (i.e., a galactic cosmic ray). HZE nuclei can produce a column of heavily damaged cells, or a microlesion, along their path through tissues, thereby raising concern over serious impacts on the central nervous system (CNS). Several experimental studies, which expose rats to heavy ion beams simulating space radiation, show a performance deficit can occur in the animals at doses that are similar to the ones that will occur on a Mars mission. The neurocognitive deficits with the nervous system are similar to those seen in aging. In addition, age-related genetic changes in humans increase the sensitivity of the central nervous system (CNS) to radiation. Acute and late radiation damage to the CNS may lead to changes in motor function and behavior or neurological disorders. The heavy ion component of space radiation presents distinct biophysical challenges to cells and tissues as compared to the physical challenges that are presented by terrestrial forms of radiation.

As uncharged particles, neutrons have high penetrating power and are approximately ten times as effective in causing cancer as equivalent doses of X-rays or electrons. Although the neutron itself does not directly damage cellular components, it is absorbed by a nucleus, creating an isotope that is often unstable. The new element can decay by emitting a gamma ray or a beta particle, both of which are ionizing agents. In addition, the decaying nucleus will undergo recoil and can disrupt chemical bonds. Exposure to neutrons can affect tissue throughout the body.

Occupational radiation exposure from the space environment may result in degenerative tissue diseases (non-cancer or non-CNS) such as cardiac, circulatory, or digestive diseases, as well as cataracts, although the mechanisms and the magnitude of influence of radiation leading to these diseases are not well characterized. Radiation and synergistic effects of radiation cause increased DNA strand and tissue degeneration, which may lead to acute or chronic disease of susceptible organ tissues. Data specific to the spaceflight environment must be compiled to quantify the magnitude of this risk to determine if additional protection strategies are required.

Astronauts who are on missions to the ISS, the Moon, or Mars are exposed to ionizing radiation with effective doses in the range from 50 to 2,000 mSv (milliSievert) as projected for possible mission scenarios. The evidence of cancer risk from ionizing radiation is extensive for radiation doses that are above about 50 mSv. Human epidemiology studies that provide evidence for cancer risks for low-linear energy transfer (LET) radiation such as X-rays or gamma rays at doses from 50 to 2,000 mSv include: the survivors of the atomic-bomb explosions in Hiroshima and Nagasaki; nuclear reactor workers in the United States, Canada, Europe, and Russia; and patients who were treated therapeutically with radiation. New evidence of radiation cancer risks come from populations that were accidentally exposed to radiation (i.e., from the Chernobyl accident and from Russian nuclear weapons production sites). There is also strong evidence for intergender variations due to differences in the natural incidence of cancer as well as additional cancer risks for the breast and the ovaries and a higher risk from radiation for lung cancer in females.

Obstacles to Exploration: Microgravity-Induced Visual Impairment

Some crewmembers on long-duration ISS missions experienced ophthalmic anatomical changes and visual performance decrements of varying degrees that were temporary in some cases and permanent in others. Additionally, future implications for asymptomatic crewmembers that demonstrated anatomical changes via MRI are unknown. It is unknown if exposure to partial gravity will be protective.

Visual acuity changes and visual field defects in spaceflight crews occur at a rate much higher than expected. Observed physical findings in long-duration crewmembers include papilledema, choroidal folds, increased optic nerve sheath diameter, and a posterior flattened globe. Persistent increased post-flight intracranial pressure (ICP) has been inferred in several cases, consistent with a root cause of intracranial hypertension (IHT) possibly secondary to microgravity-induced fluid shifts. The mechanisms that cause IHT in microgravity are not known, and the processes by which eye damage occurs as a result of IHT are not understood. Decreased visual acuity, IHT, and other findings present themselves months and in some cases years after return, indicating that the damage may be permanent. Acuity changes have been noted in short-duration crewmembers, suggesting that the process starts early in spaceflight, although this group has not been closely examined. It is unknown if fractional gravity would mitigate the hazard, but its persistence after the return to Earth suggests not. Likewise, the impact of multiple missions or of cumulative time in space is not yet established. Greater understanding of the mechanisms for eye damage and IHT is necessary to understand and mitigate the hazard and treat the resultant conditions.

Architecture Drivers: Human Health Countermeasures

The most familiar effect of space missions was once called space adaptation syndrome, referring to the physiological changes in the human body in response to the removal of gravity. After attaining orbit, astronauts can experience transient discomfort such as nausea. For some individuals, medication is required to reach a state where productive work can be done. Once past the initial phase, other more serious, long-term changes occur, such as loss of bone mass, muscle atrophy, and cardiovascular deconditioning. Over the past four decades of spaceflight in LEO, apparently effective countermeasures have been developed, particularly the regular performance of resistance exercise. Although much data exists for astronauts spending up to six months in orbit, the confidence in these countermeasures for voyages of a year or more is still lacking.

These protocols require equipment that occupies valuable volume in a spacecraft. Although various clever stowage designs have been implemented, an astronaut needs room to exercise. Such considerations impact the design of the spacecraft and make it difficult to keep the vehicle envelope compact enough to fit under a shroud atop a launch vehicle. In addition, frequent breakdown of equipment such as the treadmill still plagues flight operators.

Nutrition is an essential component of human health. In long-duration spaceflight, a nutrition research strategy will include study of possible nutrient variations that could aid in countermeasures to the deconditioning and physiological change associated with extended weightlessness. Food also has a psychological component, particularly during long voyages where routine and isolation can lead to ennui or irritability. For the ISS, the regular resupply of food can include variety, both in presentation and in nutritional content. On a year-long mission to an asteroid, the food supply is established at launch; and all meal planning must be done in advance.

The spacecraft design engineer is less concerned with the kind of food supplies than with their mass, volume, shape factors, and power requirements for preservation. A food supply in plastic containers forms a mass with a high percentage of hydrogen atoms, making it attractive as shielding against the secondary products of GCR. Generally, however, the designer is less concerned with its structural potential than he is with minimizing the impact on the mass and configuration of the spaceship.

Architecture Drivers: Medical Support

A review of published and non-published (e.g., NASA Longitudinal Study of Astronaut Health database) information on medical conditions that occurred during space missions (including the Russian experience), as well as a review of relevant information from harsh analog environments (e.g., submarine fleet health databases, Antarctic expeditions, mountaineering expeditions) provides evidence that medical conditions of different complexity, severity, and emergency will inevitably occur during long-term exploration missions.

The level of medical care required will depend on the nature of the medical problem and, possibly, on the time until the return to Earth. Resources for medical care include diagnostic equipment, tools, facilities, and medication. Unfortunately, the limited mass, volume, and power for space exploration missions limits the equipment, quantity of consumables, and available protocols for treating medical problems. Consequently, there is a significant probability that certain untreatable medical conditions will arise, particularly in the longer duration expeditions. Plans for care to support both the health and the safety of astronauts and mission success must be made with regard to balancing the most likely conditions with those that pose the most catastrophic outcome. All medical problems have the potential to affect the mission, but significant illnesses or trauma will result in a high probability of mission failure or loss of crew.

At least one crewmember must also have appropriate medical training to utilize whatever resources are available. For larger crews, it makes sense to include a physician. For crews as small as two on long voyages, training in skills necessary to operate the spacecraft and carry out mission objectives compete with time needed for medical training.

A stark reality is that eventually a crewmember will become incapacitated or will die for reasons that have nothing to do with the quality of the engineering design and construction of the space system. Any mission plan for a long-term expedition must allow for such a contingency.

Architecture Drivers: Psychological and Behavioral Factors

Spaceflight, whether of long or short duration, occurs in an extreme environment that has unique stressors. Even with excellent selection methods, behavioral problems among space flight crews remain a threat to mission success. Assessment of factors that are related to behavioral health can help minimize the chances of distress and, thus, reduce the likelihood of behavioral conditions and psychiatric disorders arising among crew members. Similarly, countermeasures that focus on prevention and treatment can mitigate the behavioral conditions and psychiatric disorders that, should they arise, would impact mission success. Among these is a thoughtful spacecraft design that emphasizes habitability as well as systems whose operation is intuitive and whose maintenance is easily accomplished by the crew.

Based on spaceflight and analog evidence, the average incidence rate of an adverse behavioral health event occurring during a space mission is relatively low. While mood and anxiety disturbances have occurred, no behavioral emergencies have been reported to date in space flight. Anecdotal and empirical evidence indicates that the likelihood of a behavioral condition or psychiatric disorder occurring increases with the length of a mission. Further, while behavioral conditions or psychiatric disorders may not immediately and directly threaten mission success, such conditions can, and do, adversely impact individual and crew health, welfare, and performance, thus indirectly affecting mission success.

Behavioral issues are inevitable among groups of people, no matter how well selected and trained. Spaceflight demands can heighten these issues. Earth analog studies show an incidence rate of behavioral problems ranging from 3%–13% per person per year. Transposing these figures to six-to-seven-person crews on a three-year mission, there is a significant likelihood of behavioral problems and psychiatric disorders. Impacts of behavioral issues are minimized if they are identified and addressed early.

Many factors predict or otherwise play a role in the occurrence of a behavioral condition or psychiatric disorder. These include: sleep and circadian disruption, personality, negative emotions, physiological changes that occur when adapting to microgravity, lack of autonomy, daily personal irritants, physical conditions of life in space, workload, fatigue, monotony, cultural and organizational factors, family and interpersonal issues, and environmental factors. Positive or salutary aspects of space flight also contribute to behavioral health outcomes. In a long-duration mission where communication delays are significant, crew training in dealing with interpersonal issues becomes important.

Long mission durations and remoteness of planetary expeditions imply that a crew will have a great deal of autonomy in decision making during the mission. Once a Mars expedition leaves Earth orbit, the administrative leverage of Mission Control vanishes. Missions to LEO in both the Soviet and the American program have had instances of tension between the crew on orbit and ground controllers. Training and crew selection must take autonomy into account. The mission objectives of the crew must align with the objectives of the sponsoring agencies.

Architecture Drivers: Human Factors

Human factors are significant (and sometimes dominant) components in the design of consumer products. The interface between the driver and the automobile has evolved over a century, spurred by competition and technology. Military aircraft are complex human systems where functionality (and safety) is paramount. Although competition does not occur at the level seen in the consumer market, the perspective of the operator (i.e., pilot) has been an important driver in the evolution of the cockpit.

Spacecraft design begins with the concept of the crew as cargo and then is modified to allow the crew control over some functions. The Space Shuttle commander landed the vehicle, but it was capable of landing on its own. Russian design philosophy minimizes crew operation except in the case of emergencies. In the United States, astronauts have had more influence in having some control over the spacecraft.

As spaceships are designed to spend longer periods time in space, habitability increases in importance as more productivity in space is expected of the crew. The habitability of the Gemini capsule cannot compare to that of the Space Shuttle, but the expected productivity of the Gemini crew pales in comparison to that of the Space Shuttle.

The International Space Station (ISS) takes design in the direction of a habitat rather than a flight vehicle. The ISS must house a crew while operating reliably for decades. As a facility constructed in orbit, the ISS has a large volume for implementing systems related to human factors. The productivity of the facility is closely tied to the psychological well-being of the crew, who value such niceties as personal space and showers. The volume also accommodates crew health countermeasures such as exercise equipment.

A number of scenarios for lunar surface expeditions were studied within the Constellation Program under the White House initiative called the Vision for Space Exploration (VSE). In a sortie mission, astronauts stay on the surface up to two weeks, living in the lunar lander that brought them there. A second class of expedition studies assumed establishment of a lunar base, including a habitat delivered separately from the lunar lander. In the former class, the lander serves both transportation and habitation; and the habitability of the design is compromised. Limitations on habitability were deemed acceptable due to the brevity of the mission.

The ISS experience has provided valuable data on the amount of living volume and its design, including privacy issues, for deep-space habitats, either for long journeys to Mars or on planetary surfaces. As mentioned earlier, the limitations imposed by launch vehicles restrict somewhat the mass and volume brought from the Earth. Inflatable habitats currently in testing address the volume issue to a large degree. However, the partial gravity of the Moon or Mars introduces different design issues by introducing a defined up-down direction as well as new considerations regarding how crew will perambulate. Video of the Apollo astronauts suggests that walking on the Moon is more like bounding than strolling, implying that living volumes may need higher ceilings and more maneuvering space than on Earth.

Logistics must be rethought. For expeditions to Mars, resupply is not possible; and the design reference mission has all necessary supplies prepositioned on the martian surface before the crew arrives. The interplanetary transits are months in length, and all supplies must be onboard before launch. For a lunar outpost, a logistics train similar to ISS is conceivable, but the cost is much greater. Supply ships docked at ISS are emptied and then de-orbited filled with garbage. That disposal method does not work so well on the Moon.

Settlement of the Solar System

Historic Human Migrations

The archaeological and genealogical history of hominids reveals migrations across the continents of the Earth over the span of 100,000 years. Emerging from Africa, humans spread across the Eurasian land mass and ultimately into the Western Hemisphere. More recently the Polynesian migration populated islands in the Pacific Ocean. In historical times, waves of conquest have resulted in the displacement of one ethnic group by another. The opening and settlement of the American West, largely by peoples of European descent, is a relatively modern example (Finney & Jones, 1986).

Every migration event took place under conditions of breathable air and an acceleration of gravity with a value of 9.6 meters per second per second. Potable water and sources of food were available throughout the journey. Often the destination was chosen because food and water were more easily obtained than at the point of origin. Human waste could be disposed of in the “environment” without polluting the immediate environs of family dwellings. Transportation during the journey was the responsibility of the individual or the family unit and may be by foot in many cases. In the case of Polynesia, the journey took place in boats. Beginning in about the 15th century, European intercontinental migrants could charter sailing ships, whether as groups or as individuals.

Should people choose to depart planet Earth for permanent relocation to other places in the Solar System, the conditions of the journey and at the destination will not be so benign.

Mission Architectures for Emigration Off Earth

In February 2018, SpaceX successfully completed a test flight of its new rocket, the Falcon Heavy. Its performance rivals that of the SLS Block 1, projected to have its first operational flight about five years later. The launch cost of the Falcon Heavy should be more than a factor of five lower than that of the SLS. This new development fuels the hope that launch costs for human transportation could be reduced to the level of affordability by individuals or groups, creating an opportunity analogous to the transatlantic sailing ships of the 17th century. Such launch vehicles could carry passengers to a waiting interplanetary ship somewhere in cislunar space, possibly in Earth orbit.

Alternatively, the settlers might be headed for a space colony, located at one of the stable Lagrange points in the Earth’s orbit. According to the Colonies in Space text on the website of the National Space Society, the O’Neill cylinders are the most ambitious of several concepts. Two such cylinders would constitute a colony. Each of them would be over 6 km in diameter and over 30 km in length. The total land area would be approximately 130,000 hectares with a population of several million. The cylinders would provide artificial gravity by spinning and would be largely self-sufficient through agriculture and closed cycle regenerative life support systems. However, colonies at the Lagrange points would lie outside the protective shield of Earth’s magnetic field. At the time of conceptualization of the space colony paradigm, the radiation hazards presented by solar particle events were not fully appreciated. Design concepts dealt with architectural problems such as structural integrity, power generation, and the production of an idealized Earth-like environment having comfortable and attractive living conditions. Issues presented by the space environment have been addressed only in principle.

Obviously, the design, construction, and operation of such enormous structures would be a project of epic proportions. Given that no agency or enterprise with appropriate expertise and resources is pursuing technology development of space colonies, the probability of this path to space settlement appears to be low.

NASA has produced studies of operations on a planetary surface, most particularly the Moon and on Mars to a lesser extent. The substantive elements of those studies focused on the transportation systems to and from the planet and mobility on the surface, with less attention to the architecture and design of the surface habitation. Crew quarters were usually assumed to be derivatives of modules for the LEO space station. Drawings often show such modules covered with lunar regolith for radiation protection. A sizable professional literature exists on lunar base concepts, most not funded by NASA. In particular, schools of architecture or civil engineering frequently choose a lunar base design for a semester project, with NASA providing the environmental constraints and other technical information. The design solutions conceived by the design/build young professionals never used cylindrical space station modules placed on the lunar surface. Their concepts arose from an emphasis on the living and working conditions from the crew and on standard terrestrial construction techniques with use of local materials where possible.

Aerospace engineers, by contrast, are trained to avoid use of designs, materials, or techniques that have not already been proven in the spaceflight environment. The NASA processes for designing, testing, and verifying new approaches are lengthy and expensive. NASA employs a rating system called Technology Readiness Level (TRL) to determine when a new technology is sufficiently reliable to use in a spaceflight project. New ideas that are untried but do conform to the known laws of physics are given a TRL of 1, while systems that are flight proven are rated at TRL 9. The construction technologies so familiar to the design/build professionals have a low to mid-range TRL in the lexicon of NASA, and no funded project would be made dependent on them without lengthy prior development for use in space (NASA, 2017b).

In-Situ Resource Utilization on Planetary Surfaces

Until now, a spaceship had to be launched with all equipment for operations, consumables for life support, and supplies for maintenance. A Space Shuttle mission is an example. The ISS in Earth orbit is resupplied periodically with unmanned logistics spacecraft. The logistics train for a station on a planetary surface is expensive and complex, but the planet may have materials that can be utilized to reduce the mass for supplies imported from Earth. The concept of In-situ Resource Utilization (ISRU) on the Moon began to be discussed in the professional literature in the mid-1980s. Since then, NASA advanced planners have begun to incorporate ISRU into design reference mission architectures for the Moon and Mars (Sullivan & McKay, 1991).

Obviously, materials found on a planetary surface will need some sort of processing to a product that can be incorporated into operation and upkeep. The mass of equipment to process local raw materials will increase the total launch cost. Therefore, the processed material must be something that replaces launched supplies, and the total production from local materials over the lifetime of the equipment or the facility must more than offset the otherwise imported mass (McKay et al., 1992a).

In the mid-1980s when new mission architectures were being studied for lunar surface bases, the assumption was that a LEO space station would be an element of the transportation system and that the transport ships would use cryogenic oxygen and hydrogen for propellants. The hydrogen-oxygen combination has the highest specific impulse among potential propellants. Fuel for the deployment to the lunar surface and for the return to the space station from the lunar surface would be launched from Earth for each round trip. As engineers constructed manifests for steady-state logistics trains between the Earth and the Moon for lunar base sustainability, they found that most of the mass flowing between the two bodies was not equipment (payloads) but rather liquid oxygen (fuel). If oxygen could be provided from lunar materials to fuel only the shuttle craft that traveled between lunar orbit and the lunar surface, the total launch mass from Earth could be reduced by almost half. That calculation alone provided a strong incentive to investigate possible ISRU for producing oxygen (McKay et al., 1992b).

The formation of the Moon involved very high temperatures, resulting in the loss of elements with low volatilization temperatures, including those elements associated with gases and liquids on Earth. The moon is a dry and desiccated planetary body. Nevertheless, the bulk of the Moon, like the Earth, consists of silicate compounds (“rocks”), whose chemical structure contains approximately 50% oxygen. The chemical bonds in silicates are generally quite strong; extraction of oxygen from rocks requires energy to melt them or strong chemical reagents. Ilmenite, an iron titanium oxide mineral, can be found in the mare areas of the Moon; and the bonds between iron and oxygen are relatively weak. Certain young, small lunar craters are surrounded by visually dark mantles and are thought to be volcanic pyroclastic deposits rather than impact features. The dark mantle material incorporates more volatile elements than usual. Laboratory experiments on these two types of lunar material suggest they could be “ores” for oxygen production, even though the yield is small, and the process would still require high temperatures. Exploiting these deposits would require some kind of strip mining operation and a modest processing plant.

Other sources for volatiles may lie near the lunar poles (Feldman et al., 2000). Unlike the Earth, the subsolar point (where the sun is directly overhead) never strays far from the lunar equator. The “arctic circle” for the Moon lies at latitude 88.8 degrees, rather than the approximate latitude 67 degrees on the Earth. In 1967, a trio of scientists published a paper speculating that parts of the interiors of craters near the lunar poles would not be illuminated (or heated) by the sun for billions of years. The temperatures in these locations would be very low. Impacts on the Moon by comets or volatile-rich meteoroids would create a small, temporary enhancement of the highly tenuous lunar atmosphere. As the local pressure enhancement dissipated, some volatile atoms could find their way into the polar cold traps and remain there to the present day.

One measurement of a reflected radio signal from the Clementine spacecraft orbiting the Moon in 1994 was interpreted by the scientific team as being consistent with reflection from a body of ice at the lunar south pole. Although the scientific publication of the result described the presence of ice as one interpretation of the signal, some scientists took the result as proof of vast quantities of water ice in craters of the lunar south pole.

Beginning in 1998, a neutron spectrometer aboard the orbiting Lunar Prospector spacecraft gathered data for 19 months counting neutrons emerging from the lunar surface. The returned data had very low spatial resolution but verified an enhancement of hydrogen concentration in the lunar surface in the lunar polar regions and was consistent with the possibility of large deposits of ice in permanently shadowed regions (PSR) without being able to conclusively prove it. LEND, a Russian neutron spectrometer with enhanced spatial resolution aboard the Lunar Reconnaissance Orbiter, validated the hydrogen enhancement at the poles but specifically ruled out water deposits in some PSRs. It is unclear why a lunar polar PSR should not have a volatile deposit when others nearby do have them.

LCROSS, a two-part spacecraft launched to the Moon with the Lunar Reconnaissance Orbiter was deliberately crashed into a crater thought by the LEND team to have volatile deposits. A spectrometer aboard the trailing part of the spacecraft measured gases ejected by the impact of the leading half of the spacecraft. The spectra showed a number of volatile compounds, thereby validating the premise that such deposits do exist at the lunar south pole.

The spate of measurements related to lunar polar volatiles has spurred an optimism among lunar base advocates that self-sustaining human presence on the Moon is possible. The major space agencies all have plans for robotic missions to the lunar south pole to carry out measurements intended to elucidate the nature of the volatiles and their abundance.

The NASA Design Reference Mission 5.0 for human expeditions to Mars invokes ISRU as a fundamental element of the mission architecture. A major challenge for a human Mars mission is the large amount of mass that must be launched into Earth orbit to construct and outfit the Mars Transit Vehicle. A second concern was the lack of an abort strategy. In other words, if some emergency occurred on the way to Mars, no possibility existed for the crew to return to Earth.

In the DRM, the expedition is broken into two launches to Mars, at successive launch windows, 26 months apart. No crew is involved in the first launch where the objective is to land the Mars Ascent Vehicle (MAV) and an ISRU plant. The MAV is to be used in the future by the crew at the end of their surface stay. The MAV will carry them to orbit around Mars to rendezvous with the interplanetary Earth return vehicle. The ISRU plant will make methane and oxygen from the atmosphere of Mars, which is almost 100% carbon dioxide. The plant has two years to fill the gas tank and can be monitored from Earth. Although methane and oxygen are a perfectly suitable fuel combination for a rocket engine, no such engines were available until NASA invested in their development. When flight controllers are confident that the MAV is fueled and ready, they will authorize the launch of the crew on the second mission. In this scenario, Mars becomes a safe haven in case of emergency during the journey.

Terraforming

Artist renderings of human habitation on a planetary surface commonly show an assemblage of structures imported from Earth. The structures contain the life-support systems and the equipment required for various kinds of surface activities. In other words, technology is employed to create a habitable volume for the crew to exist comfortably in an otherwise uninhabitable environment.

Alternatively, the planet itself could be altered to provide a breathable atmosphere, an appropriate atmospheric surface pressure, a favorable temperature range, and easily accessible water. Nothing can be done about gravity. The hypothetical process for transforming a planet to Earth-like conditions is called terraforming (Oberg, 1981).

Billions of years ago the Earth’s atmosphere underwent an “oxygenation event” during which the composition of the atmosphere was altered, raising the concentration of free oxygen from negligible amounts to the percentage seen today. The transformative agent producing oxygen is believed to photosynthesis by cyanobacteria, taking place over roughly two billion years.

Planetary scientists are finding evidence on the surface of Mars that the red planet was “wet and warm” 3.5 billion years ago. Many believe that climate change on Mars caused much of the early atmosphere to freeze out into the polar caps and into the subsurface. The MAVEN spacecraft has confirmed that the lack of a global magnetic field allows the solar wind to directly impinge on the upper atmosphere, causing loss of elements with lowest atomic weights. It is unknown how much of the original atmosphere has been preserved and how much has been lost. Optimists advocating planetary engineering have produced schemes that, over time, could restore the martian climate sufficiently to support human colonization by melting the polar caps and the martian version of permafrost or by guiding thousands of comets to impact Mars with just the right impact parameters to allow retention of the cometary volatiles in the martian atmosphere.

Venus is the twin of Earth in size and composition with a similar surface gravity. It was an early subject of speculation for being able to support Earth-like conditions. Unfortunately, Venus was found to have a carbon dioxide atmosphere with a surface pressure 100 times that of Earth and surface temperatures hot enough to melt lead. Optimists proposing human exploration there point out that at a certain altitude the atmospheric pressure is equal to Earth’s. The appropriate altitude is above the layer of sulfuric acid clouds so that aerostats floating at that height could house human explorers.

Beyond the orbit of Mars, the prospects for terraforming dwindle. The energy density of sunlight per unit area at Mars is 50% that of Earth. At the distance of Jupiter, it is only 4%. Titan the largest satellite of Saturn has an atmospheric density 1.5 times that of Earth, but the temperature is too cold. The atmosphere contains hydrocarbons, which could be used for human life support. Europa, one of the Galilean satellites of Jupiter, is thought to have an underground ocean; however, the surface is bathed in radiation from the trapped particles in the jovian magnetic field.

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