All realistic architectures of crewed missions to Mars now incorporate the basic principles of the Mars Direct design. There are two main options for the round-trip flight plan. One of these is the conjunction mission, where the Earth and Mars are on opposite sides of the Sun at the time of launch. The other is the opposition mission, where the two planets are on the same side of the Sun. In the conjunction mission, the outward leg of the journey would take about 180 days. And there would be about 180 days on the return journey, after a stay on the Martian surface of about 550 days.
An opposition mission would also involve an outward journey of 180 days. Returning from Mars, however, would have to be done by sending the spacecraft into the inner part of the Solar system, to use the gravity of Venus to give it a boost to get back to Earth. This leg of the journey would take about 430 days. This mission would allow a stay of only about 30 days on the surface of Mars, which would not be enough time to do a lot of exploration.
Radiation Risk on Mars Direct Mission
Solar flares are storms of protons that are emitted periodically by the Sun. They are not frequent (occurring approximately once per year) but can be very intense. An unprotected astronaut would receive a lethal dose of radiation in a matter of minutes. Fortunately the energy of these protons is comparatively low, so they can be stopped by modest amounts of shielding.
Cosmic rays do not come in floods, but arrive in small amounts, from interstellar sources. The amount of cosmic rays that arrive in the Solar system varies, because in times of high sunspot activity on the Sun its magnetic field expands and provides a shield that provides a significant amount of protection. Although the amounts of cosmic radiation are small, they have very high energies and it is not practical to provide shielding sufficient to stop it. For most of the Mars mission, the crew will be located on the surface. Cosmic rays from below would be blocked by the planet itself, and the use of sandbags for roof shielding would reduce the amount of radiation from above.
The basic unit used (in the USA) to measure radiation doses is the rem. A reasonable estimate of the amount of radiation that an astronaut would receive over the duration of the mission would be about 50 rem. This amount would increase the lifetime risk of a 40-year old developing cancer from about 20% to about 21%. Exposure to radiation, then, does not add enormously to the risks of the Mars mission.
Exposure to Zero Gravity on the Mars Mission
Long-term exposure to the effects of zero gravity has been well-studied over many years, with the cosmonauts in the Mir space station, and more recently with the crews operating the International Space Station currently in Earth orbit. One effect muscular atrophy, in which the muscles are not being used and lose mass and tone.Muscular atrophy can be somewhat ameliorated by a vigorous exercise regime.
Other effects include cardiovascular changes and demineralisation of the skeleton. Recovery from all of these conditions takes place once the astronaut has returned to normal gravity, but in the case of mineral loss, recovery can be prolonged.
Normally crew members become basically functional again after a 48-hour period. However all of these effects that would be experienced during the journey to and from Mars can be avoided by the simple measure of providing artificial gravity, by spinning the spacecraft.
The way it can be done is to stretch a tether between the (empty) final stage that boosted the crewed habtat into space, and the habitat occupied by the crew members. Using an appropriate combination of spin rate with the length of the tether the desired gravitational force can be obtained. It would make most sense to set the value that matches the gravitational force at the surface of Mars, i.e., about one-third of that on Earth. When the astronauts land on Mars, they will therefore be already acclimatised to Mars gravity. Spinning the craft similarly on the return journey to Earth will mean that the recovery period to get used to 1g again would be shorter than if the crew had spent six months in zero gravity.
Human Factors on the Mars Mission
Many people have raised doubts about the feasibility of a crew functioning successfully for the three years or so of a mission to Mars. One of the major concerns that has been raised is that the crew will be in rather cramped conditions for the duration of the mission. Another is that they will be cut of from direct contact with people on Earth, as a result of the fact that radio signals will take about 20 minutes to travel each way. 'Conversations' will have to be by email only. or possibly by short, prerecorded video clips. Two-way, realtime conversations will be impossible. History, however, is replete with explorers enduring much more onerous and dangerous conditions.
Earlier this year (2009) a crew of six people completed a stay of 105 days in an isolation facility in Moscow, Russia. This is part of the Mars500 study that next year (2010) will see another crew of six members sealed into a unit for 510 days,to simulate a full mission to Mars. During this time the crew will carry out duties similar to those they would do on a real mission. There will be simulated emergencies, as well. The purpose of the study is to investigate the psychological and medical aspects of a long-duration space mission.
The 105-day study has produced encouraging results, boding well for the longer study next year.
All in all, there are no aspects of the proposed Mars mission that are complete showstoppers. No space mission is free of risk, but the risks are manageable.
Reference: The Case for Mars by Robert Zubrin
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