Mars has played an important role in the mythological and astrological traditions of human cultures since time immemorial. But it was with the invention of the telescope that scientists began to appreciate Mars for what it was: a planet similar to Earth, and located almost right next door.
By the 19th century, the resolution of telescopes improved to the point where astronomers were able to discern features of the surface.
In 1877, Italian astronomer Giovanni Schiaparelli was able to create the first detailed map of Mars and noted the existence of strange features he called "canali" (canals).
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This gave rise to the myth of a Martian civilization, which would endure until well into the 20th century. But thanks to the many robotic missions sent to the Red Planet since the 1960s, scientists have learned that Mars is actually a very cold, dry, and inhospitable place. They also learned that it was not always this way, and once had a thicker atmosphere and oceans on its surface.
The recent surge in exploration, as well as the discoveries that Mars was once habitable, has led to renewed interest in sending humans to Mars, and not just to explore. There are even plans to send people there to create a permanent presence there, which may or may not involve ecologically engineering the planet to make it more Earth-like; that is, terraforming it.
Maybe it's time to dust off all the proposals that have been made over the years and see if they are still of value!
Even before robotic missions began to study Mars up-close - whether it was from space, orbit, or on the ground - scientists have contemplated what it would take to send a crewed mission to the Red Planet.
The Mars Project (1952):
In 1952, German-American rocket scientist Werhner von Braun releasedThe Mars Project, the world's first technical treatise for a proposed crewed mission to Mars. The inspiration for the treatise came largely from the large Antarctic expeditions that were being mounted at the time - particularly by the US Navy, named Operation Highjump (1946-47).
The plan called for a fleet of 10 spacecraft (7 passenger ships and 3 cargo ships) which could be assembled in Earth's orbit using reusable space shuttles. The flotilla would travel with a crew of 70 and would launch in 1965 (by his reckoning) and take three years to get to Mars and back.
Once in orbit around Mars, the crew would use telescopes to find a suitable site for their base camp near the equator.
A landing party would then use a series of winged craft, which would be mounted on the outside of the hull, and glide down to one of the Martian poles and use hull-mounted skis to land on the ice.
Using crawlers, the crew would then travel overland for 6,500 km (over 4000 mi) to their identified base camp site and begin construction of a landing strip.
The rest of the ground crew would then descend using wheeled gliders to the landing strip, leaving a skeleton crew to run the ships.
After spending 443 days on the surface conducting science operations, the crew would use the gliders as ascent craft and return back to the flotilla.
Because of the detailed nature, calculations, and planning, The Mars Project remains one of the most influential books on planning human missions to Mars.
All told, Von Braun calculated the size and weight of each ship, how much fuel each of them would require for the round trip and even computed the length of rocket burns necessary to perform the required maneuvers.
In a 2001 report compiled by NASA's Johnson Space Center, author Annie Platoff described Von Braun as "[w]ithout a doubt, the most influential figure in the history of human Mars mission planning."
Between the 1950s and 1970s, many concepts were floated by NASA for sending astronauts to Mars. As the next logical step beyond crewed missions to the Moon (the Apollo Program), it was only natural to assess what a mission like that would entail, and whether or not it would be technically feasible.
From 1957 to 1962, two proposals were made that could have made crewed missions to Mars possible. The first was Project Orion, which proposed a nuclear pulse propulsion (NPP) spacecraft that would have nuclear warheads propel itself to other planets (and even other star systems) within a relatively short space of time.
Such a mission, which would have been able to carry much heavier payloads, would have made missions to Mars feasible.
However, the Limited Test Ban Treaty of 1963 forbade the use of nuclear warheads in space and the project was abandoned.
Project EMPIRE and Nuclear Spacecraft:
In 1962, NASA's Marshall Spaceflight Center launched "Project EMPIRE" (Early Manned Planetary-Interplanetary Roundtrip Expeditions), which called on industry partners to make proposals for possible Mars missions.
These studies were the first to make use of actual NASA spaceflights and indicated that such a mission could be done using eight Saturn V boosters, or an upgraded rocket.
These studies laid the groundwork for further theoretical work on the subject. In the 1970s, following the success of the Apollo Program, Von Braun advocated for a crewed mission to Mars by the 1980s. The missions would rely on a Saturn V rockets with a nuclear-powered upper stage.
This stage of the rocket would take a crew and landing/return craft all the way to Mars using Nuclear Engine for Rocket Vehicle Application (NERVA) reactors. This proposal was considered by President Nixon but passed over in favor of the Space Shuttle Program.
Mars Direct (1990):
In 1990, aerospace engineers Robert Zubrin and David Baker wrote a research paper titled "Mars Direct", in which they made proposals for a cost-effective plan to send a mission to Mars using current technology.
In 1996, Zubrin released a condensed version of the study for the public, titled The Case for Mars: The Plan to Settle the Red Planet and Why We Must.
In the book, Zubrin paints a picture of a series of regular Martian missions that ultimately coalesce into colonization efforts. This would begin with astronauts leaving habitation modules on the surface for future crews to use.
The construction of large subsurface habitats would follow, where people would have natural radiation shielding.
Over time, hard plastic geodesic domes (that are radiation-resistant and abrasion-resistant) would be deployed to the surface to create larger modules and indoor crops.
Local industries would also begin to use indigenous resources to manufacture plastics, ceramics, and glass. These industries and commercial opportunities will draw settlers, workers, and investment to Mars.
For the first few generations, Zubrin indicated that the colony will still be largely dependent on Earth for supplies. But eventually, a Martian settlement would be able to become profitable because of its large caches of precious metals and the fact that concentrations of deuterium are five times higher on Mars than Earth, which can be converted into hydrogen and liquid oxygen fuel.
Between 1956 and 1962, a series of studies were conducted in which Soviet rocket pioneer Mikhail Tikhonravov recommended taking the necessary steps for making a crewed Mars expedition.
This included the creation of a Martian Piloted Complex (MPK) and using the then-proposed N1 rocket - a heavy-launch vehicle designed to send Soviet cosmonauts to the Moon.
During the 1960s, proposals were also made for a Heavy Interplanetary Spacecraft (TMK in Russian) that would be used to send crews to Mars and Venus without landing.
Between the MPK and TMK, missions were designed that envisioned a three-year or a 21-month roundtrip to Mars. Neither project succeeded because the N-1 rocket was never flown successfully.
By the turn of the century, NASA and other space agencies began to seriously contemplate taking the "next great leap".
While this would involve conducting renewed missions to the lunar surface, the Moon was seen as a stepping stone at this point. To quote famed Apollo astronaut Buzz Aldrin, the second man to walk on the Moon and a major proponent for Mars exploration:
"NASA’s Apollo program adopted a get-there-in-a-hurry, straightforward space race strategy that left the former Soviet Union in the lunar dust. Doing so meant don’t waste time developing reusability. Let’s close that chapter in the space exploration history books... In my view, U.S. resources are better spent on moving toward establishing a human presence on Mars."
NASA's Journey to Mars (2010 - 2030s):
NASA’s proposed crewed mission to Mars began in earnest with the passage of the NASA Authorization Act of 2010 and the U.S. National Space Policy that was issued that same year. Among other things, the Act directed NASA to take all the necessary steps:
"In developing technologies and capabilities... the Administrator may make investments in space technologies such as advanced propulsion, propellant depots, in situ resource utilization, and robotic payloads or capabilities that enable human missions beyond low-Earth orbit ultimately leading to Mars."
The crewed missions were originally envisioned to take place during the 2030s using the Space Launch System (SLS) and the Orion Multi-Purpose Crew Vehicle (MPCV). The plan called for three phases and a total of 32 SLS launches between 2018 and the 2030s to send all the necessary components to cis-lunar space and near-Mars space before sending a crewed mission to the surface.
Phase I - “Earth Reliant”: This phase includes the restoration of domestic launch capability to the US with the completion of the SLS and Orion. It also involved the continued utilization of the International Space Station until 2024 to test deep space technologies and study the effects that long-duration space travel (and the increased exposure to solar and cosmic radiation that results) have on the human body.
Phase II - "Proving Ground": Once the SLS and the Orion spacecraft are up and ready to go, NASA will begin mounting a series of missions to cis-lunar space to test the systems and develop the necessary expertise. The first, dubbed Exploration Mission-1 (EM-1), is scheduled to take place in June of 2020.
This uncrewed mission will see the Orion capsule being launched by the SLS for the first time and sending it on a journey around the Moon. Exploration Mission-2 (EM-2), scheduled for June of 2022, will be the first crewed mission of the Orion, and will similarly involve the spacecraft flying around the Moon.
By 2024, Exploration Mission-3 will involve a crewed Orion flying to the Moon to deliver the first of several pieces of the Lunar Orbital Platform-Gateway (LOP-G) – the next big piece of the overall mission architecture. Formerly known as the Deepspace Gateway, the LOP-G is a NASA-led international project to create a solar-powered habitation module in orbit of the Moon.
The station is scheduled to be complete by the mid-2020s and is intrinsic to NASA's plan to conduct renewed lunar exploration, as well as long-duration missions to Mars and other locations. These missions will be conducted once the Deep Space Transport (DST) is incorporated into the station.
This spacecraft - the Mars Transit Vehicle (MTV) - will consist of two elements: an Orion capsule and a propelled habitation module. Basically, after a crew is launched from Earth aboard an Orion spacecraft, they will rendezvous with the LOP-G and reintegrate the capsule to the DST to travel to Mars.
The DST will then be used to transport the necessary components to Mars so the final piece of the mission architecture can be built: the Mars Base Camp and Lander, both of which are being developed by Lockheed Martin.
Phase III - "Earth Independent": In this final phase of the "Journey", astronauts will assemble another habitat in orbit around Mars. Known as the Mars Base Camp (MBC), this habitat will be similar to the LOP-G, consisting of a series of integrated modules and powered by solar arrays.
The station will have all the necessary amenities for a four-person crew and will include a laboratory module for conducting key science operations on the Martian surface. The crews will also rely on the reusable Martian Lander to make trips to and from the surface.
Once complete, this infrastructure will allow for repeated missions to Mars, which will go beyond NASA and include international and commercial partners. The plan was well summarized by Buzz Aldrin:
"I envision a comprehensive plan that would lead to permanent human settlement on Mars in the next 25 years. To get under way, the International Space Station can serve as a test bed for long-duration life support and for technologies that can safely, reliably and routinely transport crews to the distant shores of Mars. I’ve championed the creation of spacecraft to be placed on continuous loops between Mars and Earth, thereby putting in place a pathway to sustainability that forever links the two planets."
European Space Agency:
The ESA also has long-term plans for Mars, though they have yet to build a crewed spacecraft. However, between 2007 and 2011, the ESA cooperated with Roscosmos to conduct the Mars500 Study, a series of cooperative isolation experiments meant to simulate a long-duration mission to Mars.
In addition, the ESA has indicated in the past that it hopes to send astronauts to Mars by the mid-2030s. This would take place after crewed lunar missions are conducted and the ESA completes a number of robotic missions to the Martian surface. The proposed Ariane 5 heavy rocket would be the likely candidate for the launch vehicle.
Chinese National Space Administration:
China’s future efforts are focused primarily on the Moon (the Chang’e program), which it hopes to accelerate in the coming decades. This three-phase lunar exploration program, which has sent several orbiters, landers, and rovers to the surface, will culminate with a sample return mission.
Once this is done, the CNSA hopes to send Chinese astronauts ("taikonauts") to the lunar surface and possibly collaborate with agencies like the ESA to build a lunar base.
So far, all plans for a mission to Mars have been somewhat ambiguous, with robotic missions expected to take place between 2020 and the early 2030s followed by crewed missions between 2040 and 2060.
Similarly, the Russians have stated that they hope to conduct crewed lunar missions in the coming decades and put off missions to Mars until mid-century.
In 2011, first deputy Nikolay Panichkin of the Central Research Institute of Machine Building - an institute of Roscosmos – indicated that crewed lunar missions were expected by the 2030s, with an expedition to Mars happening in 2040 or 2045.
In April 2013, the head of Roskosmos, Vladimir Popovkin, said that the agency's latest conceptual designs envisioned sending a 450-tonne expeditionary complex to Mars.
This plan would involve the creation of a small fleet of partly-reusable super-heavy launch vehicles that would deliver the components to orbit.
Russia has also been pursuing nuclear-electric propulsion concepts since 2009, which would enable long-duration missions to Mars and beyond. These would have the payload capacity to transport heavy components, like the habitat modules envisioned by Popovkin.
Aside from federal space agencies, a number of aerospace companies have also drafted plans for colonizing the Red Planet. This is in keeping with the age of New Space, where private industry is assuming a greater role than ever before and space exploration itself is becoming commercialized.
In 2012, a group of Dutch entrepreneurs revealed plans for a crowdfunded campaign to establish a human Mars base, beginning in 2023.
Known as MarsOne, the plan called for a series of one-way missions to establish a permanent and expanding colony on Mars, which would be financed with the help of media participation.
Base powered by 3,000 square meters of solar panels and the SpaceX Falcon 9 Heavy rocket would be used to launch the hardware. The first crew of 4 astronauts was scheduled to land in 2025, followed by an addition 4 crew members every two years.
Unfortunately, insolvency issues caused MarsOne to declare bankruptcy in 2019.
Elon Musk has been upfront about his long-term plans to establish a colony on Mars for many years. And while the development of reusable rockets and the Dragon crew capsule were stepping stones to this goal, it is recent developments with the Starship and Super-Heavy completely reusable launch system that SpaceX's Mars missions have truly begun to take shape.
Currently, SpaceX is hoping to commence cargo missions using theStarship and Super Heavy as early as 2022, followed by the first crewed flight in 2024.
According to recent statements by Musk, he hopes to create a permanent outpost (Mars Base Alpha) by 2028.
Virgin CEO Richard Branson has long been known for his efforts to create a viable aerospace industry. But looking ahead, he has also expressed interest in creating a tourist venture that would take customers to Mars. As he explained in a 2013 interview with CBS This Morning:
"In my lifetime, I'm determined to be a part of starting a population on Mars. I think its absolutely realistic. It will happen. I think over the next 20 years, we will take literally hundreds of thousands of people to space and that will give us the financial resources to do even bigger things."
Making a Life on Mars
The challenges posed by the distance between Mars and Earth and the natural hazards of the planet have led to some creative suggestions.
In particular are ideas for Martian habitats that will offer protection against the environment and the radiation, and which can be built using local resources - a process known as in-situ resource utilization (ISRU). In the end, any habitat on Mars will have to meet a long list of requirements.
Air, Heat, and Shielding:
For starters, all will need to be airtight and sealed to keep an atmosphere of pressurized air (22% oxygen and 78% nitrogen) from leaking out. This atmosphere will be recycled, which will necessitate carbon scrubbers to make sure that excess CO² is removed.
Every habitat will also need an airlock to ensure the internal atmosphere is contained, and pressure suits to allow the inhabitants to reconnoiter outside.
The habitats will also need to be nice and cozy since they will be built on a planet where the average surface temperature makes a cold night in Antartica look balmy by comparison (-63 °C; -82 °F). This will mean plenty of internal heating, which could be provided using solar heating units, thick insulating walls, and warm clothing.
Protection against radiation is also a must and will require either lead-lining, depleted uranium, or some other shielding material on the exterior of the habitat. Otherwise, settlements will have to be built beneath the surface, taking advantage of the natural shielding the regolith provides.
Time outside will also have to be limited, and Martian settlers will have to monitor their radiation levels routinely (and most likely take anti-radiation meds).
Aside from relying on technology and construction methods to see to our needs on Mars, the geography could also be used as a defensive measure.
As scientists have noted many times, there are multiple locations on Mars that would make for good base sites because they provide natural shielding, will be easier to pressurize, have access to water, or are naturally warmer.
For example, like Earth and the Moon, Mars has a number of stable lava tubes that are the result of past volcanic activity.
In the region of Arsia Mons, which is near the raised region known as the Tharsis Bulge, multiple "skylights" have been observed that are indicative of underground lava tubes.
These tubes are considered by many to be an ideal location for building a base. Not only would the skylights allow for access to the surface, but the tunes themselves are far enough beneath the surface to provide protection against radiation and seasonal temperatures. They would also be easy to pressurize since the walls are rock-solid.
Other advantages of building settlements in the equatorial region include higher average temperatures. During summer and midday, temperatures reach a high of 35°C (95°F), which is as hot as it gets on Mars.
It is also here that temperature variations are the least extreme. Recent studies have also indicated that there may be ample supplies of water ice beneath the surface in and around the equator.
The idea is to create bases in the polar regions and lower latitudes, where permafrost and the polar ice caps make for abundant supplies of water. Yet another idea is to build settlements in the massive canyon system known as Valles Marineris, where the air pressure is 25% higher on average than the rest of the planet.
NASA has launched numerous Incentive challenges in the past few years. They are focused on getting public input on the creation of Mars housing. These include (and were hosted by) the Journey to Mars Challenge (NASA), the Makerbot Mars Base Challenge (NASA and Makerbot) and the 3D Printed Habitat Challenge (NASA and America Makes).
Together, these competitions sought proposals that would leverage recent technological advances, like additive manufacturing (3D printing) and ISRU methods to create habitats made from regolith, ice, and preformed modules that would provide protection while also allowing for comfort, work, research, and recreation.
Advocacy and Training
Hawaii Space Exploration Analog and Simulation (aka. Hi-SEAS):
Funded by the NASA Human Research Program, this program consists of a habitat on the slopes of the Mauna Loa volcano in Hawaii.
This habitat acts as a human spaceflight analog for Mars, where crews reside for up to a year and perform research missions designed to simulate a crewed mission to Mars.
Located at an elevation of 2500 meters (8,200 feet) above sea level, the analog site is in a dry, rocky environment that is very cold and subject to very little precipitation (a lot like Mars). Crews live in a sealed dome-like habitat and wear spacesuits when traveling outdoors to explore.
To complete the illusion, the crews use composting toilets that turn their feces into a potential source of fertilizer for the next crew. Communications are conducted through NASA-issued email addresses – with an artificial delay to simulate the time lag from Mars.
In 1998, Dr. Robert Zubrin and colleagues founded the Mars Society, a non-profit advocacy group that works to educate the public, the media, and government on the benefits of Mars exploration. Like Hi-SEAS, they also conduct research and training programs to simulate the challenges of mounting a crewed mission to Mars.
These missions involve crews of six or seven people that train together at the Mars Desert Research Station (MDRS) in southern Utah. Once their training is complete, the crew is sent to the Flashline Mars Arctic Research Station (FMARS), located on Devon Island in northern Canada.
Here too, the crews engage in activities designed to simulate conditions on another planet. For the duration, they will live and work in a Mars Analog Research Station (MARS) – a prototype habitat that the Mars Society plans to eventually land on Mars someday.
Making Mars Green (Terraforming)
If humanity intends to make Mars a permanent outpost of our civilization, then there is a good chance the inhabitants will try to make the planet more amenable to a human presence. This would involve large-scale ecological engineering, otherwise known as terraforming, to make Mars more "Earth-like".
Since we know that Mars once had a denser atmosphere and was warm enough to maintain rivers, lakes, and oceans, terraforming Mars would be tantamount to restoring its past self.
But in the process, we would also destroy the perfectly-preserved Martian landscape and upset the natural balance of the planet. If there is any life there today, it would surely be affected as well.
But the ethics of terraforming aside, the only remaining question is, "Can it be done?" For decades, scientists have been attempting to answer that very question and come up with possible methods for doing it.
What they concluded was, if we want to terraform Mars, we need to do three things:
- Warm up the planet
- Thicken the atmosphere
- Make the atmosphere breathable
Luckily for us, these three objectives are complementary. It's just that getting the ball rolling on any one of them would require a titanic effort on our part, not to mention a massive commitment in time and resources.
So how do we get started?
Most scientists agree that the best bet for warming the planet is to trigger a greenhouse effect. An early proposal came from American aerospace engineer and futurist Dandridge M. Cole in 1964. In his study, "Islands in Space: The Challenge of the Planetoids, the Pioneering Work", Cole recommended importing ammonia ices from the outer Solar System and then impacting them on the surface.
In addition to being a powerful greenhouse gas, ammonia (NH³) is mostly nitrogen by weight. Therefore, it could provide the necessary buffer gas which, when combined with oxygen gas, would create a breathable atmosphere for humans.
Another method has to do with albedo reduction, where the surface of Mars would be coated with dark materials in order to increase the amount of sunlight it absorbs. One of the greatest proponents for this was famed astronomer, author and science communicator, Carl Sagan.
In 1973, Sagan published an article titled “Planetary Engineering on Mars“, where he proposed two scenarios for darkening the surface of Mars: transporting low albedo material and/or planting dark plants on the polar ice caps to ensure they absorbed more heat, melted, and converted the planet to more “Earth-like conditions”.
In 1976, NASA produced their own study titled "On the Habitability of Mars: An Approach to Planetary Ecosynthesis", in which they concluded that photosynthetic organisms, the melting of the polar ice caps, and the introduction of greenhouse gases could all be used to create a warmer, oxygen and ozone-rich atmosphere.
In 1982, Christopher McKay, a planetologist with NASA's Ames Research Center, wrote a paper titled “Terraforming Mars”, where he recommended building a self-regulating Martian biosphere, which included both the required methods for doing so and the ethics of it.
In 1984, famed environmentalist James Lovelock (who also proposed the Gaia Hypothesis) and Michael Allaby, wrote the novel The Greening of Mars. This was a fictional account of a future Mars where the planet had been converted into an Earth-like planet thanks to the importing of chlorofluorocarbons (CFCs) to trigger global warming.
In 1993, Dr. Robert M. Zubrin and Christopher McKay co-wrote "Technological Requirements for Terraforming Mars", where they proposed using orbital mirrors to warm the poles and sublimate the frozen carbon dioxide there, thus contributing to global warming. They also argued how asteroids could be redirected to impact on the surface, kicking up dust and warming the atmosphere.
In 2001, a team of scientists from the Division of Geological and Planetary Sciences at Caltech produced a study titled “Keeping Mars warm with new super greenhouse gases". Here, they recommended using gases like fluorine compounds to heat the planet, which would also act as a long-term climate stabilizer.
Importing methane and other hydrocarbons from the outer Solar System – e.g. from Saturn’s moon Titan – has also been suggested. There is also the possibility of mining it locally, thanks to the Curiosity rover’s discovery of a "tenfold spike" of methane that pointed to a subterranean source.
In 2014, the NASA Institute for Advanced Concepts (NAIC) program and Techshot Inc. began work on a concept called the "Mars Ecopoiesis Test Bed". This entailed the creation of sealed biodomes build on the surface of Mars where colonies of oxygen-producing cyanobacteria and algae would grow.
If this proves successful, NASA and Techshot plan to build several large biodomes on Mars to produce and harvest oxygen for future human missions to Mars.
While technically not ecological engineering, Eugene Boland (chief scientist of Techshot Inc.) has stated that it is a step in that direction:
“Ecopoiesis is the concept of initiating life in a new place; more precisely, the creation of an ecosystem capable of supporting life. It is the concept of initiating “terraforming” using physical, chemical and biological means including the introduction of ecosystem-building pioneer organisms… This will be the first major leap from laboratory studies into the implementation of experimental (as opposed to analytical) planetary in situ research of greatest interest to planetary biology, ecopoiesis and terraforming.”
In 2015, Elon Musk proposed using thermonuclear weapons as a speedier way to melt the polar ice caps to release CO² and water vapor into the atmosphere. This would have the effect of thickening the Martian atmosphere, creating liquid water on the surface, and triggering a greenhouse effect. The downside to this “fast” plan is the fallout; though most of the radiation would probably escape into space.
During the Planetary Science Vision 2050 Workshop in February 2017, NASA scientist Jim Green proposed a concept of placing an artificial magnetic shield at the Sun-Mars L1 Lagrange Point. This shield would prevent stripping of Mars' tenuous atmosphere by solar wind, which would allow the planet to replenish its atmosphere.
According to their calculations, this would lead to an average temperature increase of about 4 °C (~7 °F), which would be enough to melt the carbon dioxide ice in the northern polar ice cap. This would trigger a greenhouse effect, warming the atmosphere further and causing the water ice in the polar caps to melt.
Clearly, there are no shortages of ideas when it comes to making Mars more suitable for human habitation. And if and when we establish a human presence on Mars, we will need to figure out if we intend to put any of them into action. This will raise all kinds of questions; not the least of, which are ethical.
But assuming we can alter the Martian environment with a clean conscience, there's still the logistical challenges and the incredible amount of time and energy involved.
In the end, only time will tell if humanity chooses to make Mars a "backup location" for humanity, or leave it alone.
- Mars Society - Mars Direct
- National Space Society - Islands in Space
- NYT - The Call of Mars, by Buzz Aldrin (2013)
- Icarus - Planetary Engineering on Mars (by Carl Sagan)
- RussianSpaceWeb - Russian human space flight in the 2010s
- PNAS - Keeping Mars warm with new super greenhouse gases
- NASA - On the habitability of Mars: An approach to planetary ecosynthesis
- NASA - Eyes on the Red Planet: Human Mars Mission Planning, 1952-1970 (2001)
- Robert Zubrin and Christopher McKay - Technical Requirements for Terraforming Mars