Human Missions to Mars

Human flights to Mars will likely be the next major milestone in humankind's expansion into the solar system. Solving the complex problems of Mars' origin and history, such as whether life ever existed there, is likely to require direct scientific exploration by humans. However, sending humans to Mars will not be easy.

Mission Planning

Much of the mission planning for human exploration missions deals with finding appropriate trajectories for the trips out to Mars and back to Earth. Earth revolves around the Sun about twice as fast as Mars does. A spacecraft launched from Earth must "lead" Mars, aiming at the place where that planet will be in 5 to 9 months. Opportunities to do this only occur at 26-month intervals. By the time a spacecraft arrives at Mars, Earth has moved and it is necessary to wait for a similar leading trajectory opportunity from Mars to Earth. Trajectory options exist for long travel times (9 months) and short stay times (30 to 60 days) on Mars or somewhat shorter travel times (5 to 8 months) and long stay times (500 days). A total round trip requires 21 to 36 months. It is possible to shorten the transit time by increasing the velocity with which the spacecraft leaves Earth. However, the round trip times will remain about the same because of the need to wait for the correct planetary alignment. Chemical rockets using hydrogen and oxygen as well as nuclear rockets have been studied. Nuclear fission rockets can provide higher velocities for transit but have not been developed. Even higher-energy propulsion systems, such as nuclear fusion rockets, are being studied but will not be available for a long time.

Because of these orbital and propulsion considerations, trip times for human missions will be much longer than any previous missions. In addition, the infrequent mission opportunities will not permit resupply or rescue missions once a spacecraft has been launched from Earth. Human health and safety therefore will be a major consideration on these missions. For example, methods will have to be found to prevent the loss of calcium, deconditioning of the heart, and other detrimental effects of weightlessness that occur in spaceflight. The mechanical systems required for life support and surface activities will also have to be far more reliable than those developed thus far.

Chemical propulsion, which is used in the space shuttle, requires large quantities of propellant. For a spacecraft that is launched from low Earth orbit (LEO) to Mars, three times as much propellant is required. Five times a spacecraft's mass in propellant is required for a rocket launched from the surface of Mars into space. Therefore, approximately 15 kilograms (33 pounds) of mass must be launched from LEO to get 1 kilogram (2.2 pounds) of mass back to Earth. Because of this unfavorable relationship, designers have looked for ways to reduce the mass of spacecraft and other materials that must be launched from Earth. Reducing crew size is one possibility; however, considering the range of skills that will be necessary, crew sizes of five to eight are probably minimal. Inflatable habitation systems provide more crew space for the same amount of mass of the hard modules used in the International Space Station. Aerobraking, or using the atmosphere to slow spacecraft down when landing on Mars or Earth, is one way of reducing the amount of propellant that is needed in space. Manufacturing propellant from the atmosphere of Mars also could reduce the mass of propellant that must be hauled to Mars. That is the fundamental premise of the Mars Direct mission proposed by Robert Zubrin and has been incorporated into some of NASA's Design Reference Missions.

"Split" mission options are designed to launch a habitat, a power and propellant production system, and a return vehicle twenty-six months before sending humans from Earth. Humans would not be launched until all systems were tested and found to be working well. This strategy allows greater support capability on Mars, although the equipment must be able to work unattended for the twenty-six months during which it awaits the crew.

Exploring Mars

On the surface of Mars, astronauts would conduct several types of activities. Astronauts riding on long-range motorized vehicles, some of which might be able to traverse hundreds of kilometers from an outpost site, would conduct field studies of Martian geology, search for evidence of past or current life, collect rocks, and place geophysical instruments. Automated vehicles operated by astronauts from their Martian control center could explore and collect samples at even greater distances. Astronauts would use an analytical laboratory to study samples. Data would be sent back to Earth, and information from the initial investigations would be used to plan later investigations. The search for a usable source of water would have a high priority. Within the habitat astronauts would conduct plant growth and medical experiments aimed at determining the possibility of establishing permanent settlements on Mars. They would also select and package samples that would be returned to Earth for more detailed analysis. While they were accomplishing their scientific mission, the astronauts would carry out the operations and maintenance required to keep the systems and themselves fit and productive.

The search for existing life on Mars and for usable resources will focus on looking for liquid water beneath the surface. Drilling for water and analyzing its organic and inorganic constituents will be a major task for the human crews. The need to prevent terrestrial organisms from invading Martian water deposits and to protect astronauts from exposure to Martian organisms will be one of the most difficult technical challenges of a human exploration mission.

Many of the questions surrounding the design of the first human missions to Mars can be addressed by using automated missions that precede humans. These missions should include reconnaissance surveying activities (images and surface properties) and the return of samples that can be used to determine whether surface materials might be detrimental to astronauts' health.

see also Apollo (volume 3); Human Factors (volume 3); Humans versus Robots (volume 3); International Space Station (volumes 1 and 3); Life Support (volume 3); Living in Space (volume 3); Long-Duration Spaceflight (volume 3); Lunar Rovers (volume 3); Microgravity (volume 2); Mir (volume 3); Nasa (volume 3); Nuclear Propulsion (volume 4); Weather, Space (volume 2); Why Human Exploration? (volume 3).

Michael B. Duke

Bibliography

Budden, Nancy Ann. Mars Field Geology, Biology, and Paleontology Workshop: Summary and Recommendations. LPI Contribution Number 968. Houston: Lunar and Planetary Institute, 1998. Also available at <http://www.lpi.usra.edu/publications/reports/CB-968/CB-968.intro.html>.

Hoffman, Steven J., and David L. Kaplan, eds. Human Exploration of Mars: The Reference Mission of the NASA Mars Exploration Study Team. Houston: NASA Johnson Space Center, 1997. <http://spaceflight.nasa.gov/mars/reference/hem/hem1.html>.

Lupisella, Mark. "Humans and Martians." Earth Space Review 9, no. 1 (2000:50-60).

Nicogossian, Arnauld E., Carolyn L. Huntoon, and Sam L. Pool, eds. Space Physiology and Medicine. Philadelphia: Lea & Febiger, 1993.

Zubrin, Robert and Richard Wagner. The Case for Mars: The Plan to Settle the Red Planet and Why We Must. New York: The Free Press, 1996.