The recent discovery of possible fossilized life forms within a Martian meteorite has rekindled interest in human exploration of the planet. While the global scientific community must carefully study this physical evidence as well as await the results of 4 space probes to Mars in the coming years, others are looking ahead to the next logical step of sending humans to the planet. Such a feat, while a tremendous scientific undertaking, is nearly within the grasp of current technology.

The travel distance involved and the harsh Martian environment itself impose great challenges to human physiology as well as to spacecraft design. Mission planners face challenges from the long-duration microgravity environment, complex life-support systems, interplanetary radiation, and the psychological stressors involved in such an endeavor.

• While the human body readily adapts to weightlessness, significant deconditioning of the musculoskeletal and cardiovascular systems occur that could preclude safe landing and operations under the 1/3 G environment of Mars. Further, the long-term effects of microgravity are poorly studied, although a Russian cosmonaut colleague has spent 438 consecutive days on-orbit and has returned to normal daily activities here on Earth. This re-adaptation was due to his intensive exercise regime, including both resistive and aerobic training every day of his flight. Space travel longer than this might require "artificial gravity," but these systems might be prohibitively complex and expensive. Studies from the soon-to-be-built International Space Station will address the problem of decondtioning in-depth, and help define improved countermeasures.

• Interplanetary space places astronauts at significant risk of irradiation from solar particle events (SPEs) as well as low-level galactic cosmic rays. Moreover, even extravehicular activity (EVA) on the planet's surface is not without risk, since Mars has essentially no magnetic field to shield the spacewalking astronaut. SPE forecasting from Earth, as well as "space weather" instrumentation onboard the spacecraft will be essential during the expedition. Enroute to Mars, EVA should be minimized, and the spacecraft will need to be designed for maximum shielding from radiation.

• Crew selection and compatibility is a key consideration for travel to Mars. Not only must the crew have a wide skill base (scientific, engineering, medical) to enable self-reliance, but also they will need extensive compatibility screening preflight and considerable psychological support inflight. Notably, a busy training and science schedule in-transit and frequent communications with family and friends back on Earth will be a formula for success. Results from long duration space crews, submariners, Antarctic scientists and Biospherians support a team approach with a well-defined command structure.

Until recently, planned Mars trajectories have involved hundreds of days in interplanetary space and have relied on conventional rocket technologies. A developmental tunable exhaust plasma rocket, utilizing nuclear electric propulsion, reduces the transit time to the order of 90 days each way. Additionally, system redundancy and the power capabilities of such a vehicle would allow a powered abort capability for return to Earth.

• A "split-sprint" mission is favored, the first spacecraft being a payload tugboat (instrumentation, supplies, fuel, etc.) that would establish the Martian outpost and potentially generate supplies needed for the return home. Once the outpost is fully established robotically, a human-operated speedboat will transfer the crew from low-earth orbit to Mars. Depending on phasing, Martian stays could range from 30 days to 2 years.

• Assembly of the craft including the nuclear reactors will occur in low earth orbit for safety. The vehicle will be comprised of at least 3 redundant plasma rocket engines and 3 redundant nuclear reactors due to their criticality as well as the propulsion and power needed for such a mission. Other critical life-support systems, including environmental control and rendezvous systems must be designed to be highly reliable, modular, and fault-tolerant.

• Shielding from SPEs and other sources of radiation will be provided by liquid hydrogen fuel tanks surrounding the crew modules, as well as an induced magnetic field from the plasma rockets themselves.

• Due to the short transit times enabled with the tunable exhaust plasma rocket, complex countermeasure systems like artificial gravity will not be required. Daily exercise en route to preserve musculoskeletal and cardiovascular health will be necessary, as well as to preserve neuromuscular coordination once the crew arrives on Mars.

In the not-too-distant future, the global scientific community may require human investigation of the planet Mars. Lessons learned from Skylab, the Space Shuttle Program, Mir, and the planned International Space Station, as well as technological advancements in rocketry, will take us there.

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