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How long the voyagers stay on Mars will depend in part on the homeward-bound route. To await the proper alignment of Mars and the earth for an economical Hohmann-ellipse return, the crew would have to remain on Mars for more than a year -- increasing the mission length to what now seems an unbearably long 1,100 days. But with the expenditure of more fuel, the explorers could blast off earlier, head toward Venus and loop around it, using the planet's gravity to whip their craft toward earth at a higher speed. That would cut the mission time to 600 to 700 days.
But the Venus route would also cause the craft to re-enter the earth's atmosphere at 80,000 m.p.h., in contrast to the returning Apollo's 25,000 m.p.h. "We're not sure we know how to build the appropriate heat shields," says Oberg. Also, at that speed, the astronauts would have a much smaller "window" for re-entering the atmosphere. "Come in too low, and you burn up," says Oberg. "Come in too high, and you overshoot. You miss the earth, and you'll never see it again." Other plans call for an unmanned cargo ship to precede the manned craft to Mars and for even higher velocities that would cut mission times down to a year.
Yet the problems of sending a spacecraft to Mars and bringing it back to earth pale when compared with the challenge of keeping its human cargo safe and in peak physical and mental condition. The medical consequences of long periods of weightlessness are still not fully understood. And radiation, says NASA's Michael Bungo, "is going to be a showstopper." Once beyond the earth's atmosphere and magnetic field, which protects terrestrial life from most lethal radiation, crew members would be vulnerable to cosmic rays. These highly energetic particles travel through space at close to the speed of light and can produce hazardous secondary radiation when they strike atoms in the aluminum walls of a spacecraft. During a single Mars mission, says Frank Sulzman, chief of NASA's space-medicine and biology branch, unprotected astronauts could receive an unacceptably high dose of radiation -- more than is now allowed workers in a lifetime on jobs that expose them to radioactivity.
Even more dangerous are solar flares, which usually blossom on the sun around the peak of the eleven-year solar cycle. During these massive explosions, which astronomers can spot in the form of extra-bright splotches suddenly appearing on the sun, bursts of X rays and charged particles are hurled outward at high velocities. Because protons from a large flare can easily penetrate the walls of a spacecraft and fatally riddle the body of an astronaut in half an hour, planners envision an onboard shelter into which the crew could repair as soon as a solar-flare warning was sounded. One idea is to build the shelter with the heavy-walled oxygen and water tanks that must be brought along anyway. Soviet scientists are experimenting with generating strong electrically charged fields around the spacecraft. These would have an effect similar to that of the earth's magnetic field, deflecting the speeding particles around the ship.