When NASA's 30-year Space Shuttle Program ends on Thursday as Atlantis touches down for the last time, space-watchers will be looking toward our next step into space.
We've already "done" the moon, but Mars still beckons like some interplanetary Brigadoon ; visible through the eyes of clever little rovers and orbiters, but just beyond the reach of human footsteps.
Despite several decades of research and development, a long-duration voyage to Mars is still on the drawing board. Putting aside the enormous financial costs of an interplanetary mission, there are still major engineering and physiological hurdles to overcome.
Radiation
The combined effects of background cosmic rays from extragalactic sources and extreme radiation events from the sun make space travel too hazardous for an estimated six months there and six months return.
"The estimate now is you would exceed acceptable levels of fatal cancer," said Francis Cucinotta, chief scientist for NASA's space radiation program at the Johnson Space Center in Houston. "That's just cancer. We also worry about effects of radiation on the heart and the central nervous system."
Cucinotta says these estimates do take into account protective shielding around a crew vehicle, probably some form of polyethylene plastic. Lead shields actually create secondary radiation when struck by cosmic rays, while water, perhaps the best form of protection, would have to be several meters thick to get enough protection. ("Houston calling Water Balloon 1, do you copy?")
Lead and water, in any case, are very heavy for the quantities that would be required, making them an expensive shielding to launch.
Solution: Pick astronauts that have never smoked, never been around smokers, and have a built-in genetic resistance to radiation damage. "We didn't know about this (ability) five or 10 years ago, we should have an answer in another 10 or 15 years," Cucinotta said.
Genetic protection plus a special shielded shelter may do the trick.
Bodies Without Gravity
Let us count the ways that the human body falls apart without gravity:
1) Bone loss of 1 percent per month.
2) Fainting spells (women more than men) after re-entering a gravitational field.
3) Cognitive problems including Alzheimer's-like symptoms.
4) Weakness and lack of cardiovascular fitness.
5) Muscle atrophy.
All of these medical conditions would make it tough for the crew to build a shelter when they land on the Red Planet, for example.
"What happens if they land on Mars and try to lift an object that's fairly or reasonably heavy, they could herniate their discs," said Alan Hargens, an orthopedic surgeon at the University of California-San Diego who studies the effects of gravity on astronauts. "One of the main issues is that when they arrive at Mars, there's nobody there to take care of them. If they have some issue due to deconditioning in that six-month period, they'll definitely have a problem."
Solution: Lots of time on a treadmill while in microgravity conditions and some kind of artificial gravity may help, but not eliminate the risk.
The crew needs either a small unit inside the ship or a vehicle design that rotates around a central pivot point (think "2001: A Space Odyssey"). Hargens said a rotating arm of 1-kilometer diameter will produce the equivalent of the gravity felt on the Earth at sea-level.
Smaller centrifuges have produced nausea among astronauts, according to Hargens, and take up a lot of space inside a cramped vehicle.
Another possibility is a special compression suit that forces body fluids into the legs, and helps maintain fitness.
Cabin Fever
Put six or seven people in a confined space for 18 months, send them to a place nobody's been before, with no way to escape, is likely to produce stress, tension and perhaps even severe psychiatric problems, according to NASA’s 2009 Human Research Program report.
Based on studies in Antarctica and other isolated environments underwater, the report cited the risk of "increased human performance errors due to sleep loss, fatigue, work overload and circadian desynchronization; and, increased errors due to poor team cohesion and performance, inadequate selection/team composition, inadequate training, and poor psychosocial adaptation."
Cosmonaut squabbles aboard the Russian Mir space station brought one mission home ahead of time, while NASA has also reported crew disputes among its astronauts.
Solution: Pick the astronauts very carefully ahead of time. NASA is also looking at special voice and facial movement monitors to diagnose early signs of stress before they turn into a big fight. An on-board counselor (a la Star Trek’s Deanna Troi) may help as well, but who counsels the counselor?
Food
Only vegetarians will be allowed on a Mars trip, since meat can't be preserved in space.
Food provides a natural morale boost for the crew, and proper nutrition wards off oxidative damage to the astronauts' bodies.
However, studies show that radiation can damage the vitamins in food supplies, and the loss of even one vitamin in the food chain could cause serious health effects over a long trip. Little is known about the long-term effects of radiation on food supplies, since International Space Station (ISS) crews have been partially sheltered by Earth's magnetosphere.
It's expected that the crew will have to grow its own food in some kind of greenhouse, something researchers already do at the South Pole during winter-over.
Hardware/Propulsion
At closest approach, Mars is about 35 million miles from Earth, but that figure increases six-fold depending on the alignment of the two planets' orbits. Some experts say the answer to all the medical, radiation, food and psychological issues is to get there faster.
Former astronaut Franklin Chang-Diaz is developing an electric-nuclear plasma-powered rocket (called the Variable Specific Impulse Magnetoplasma Rocket, or "VASIMR") to cut the trip to 39 days. President Obama mentioned the new propulsion system during a recent trip to Cape Canaveral.
But the Mars Society's founder, Robert Zubrin, is highly critical of this plan, saying we already have the rocket technology needed to mount a Mars expedition. Zubrin also went on the record to call VASIMR a "hoax."
"The insistence that we need a faster propulsion system just allows politicians to postpone a Mars mission," said Zubrin, author of the recently re-released book "The Case for Mars." Zubrin proposes a three-stage, 18-month round-trip Mars expedition that will send a crew habitat ahead of time, as well as devices to produce fuel for the return trip.
"The technical challenges are considerably less today than (planning on) sending men to the moon in 1961 and we did it eight years later," Zubrin said. "It's going to require hard work, but it's not beyond our technology; it's just a question of the focus and the will. We could do it by the end of the decade."