Easier Moons

Plans for voyaging to the moon are a dime a dozen, but according to Astronomy Professor Jan Schilt of Columbia University, they are all aimed at the wrong moon. Last week he explained why man's first round trip to an extraterrestrial body may be to one of the moons of Mars.

The earth's moon is handy, only 238,857 miles away, but its considerable size (2,160 miles diameter) makes it a trap in space. Its gravitational pull is one-sixth as strong as the earth's, which means that unless a spaceship is braked in some way, it will hit the moon's surface at 5,000 m.p.h. Since the moon has no appreciable atmosphere that can be used for braking, the ship will have to cushion its fall by burning precious fuel in its rocket engine. To take off from the moon will cost fuel too, about one-sixth as much as was needed to escape from the earth. So an earth-to-moon spaceship will have to carry a very large payload of fuel if its crew hopes to get home again.

A voyage to the neighborhood of Mars, about 35 million miles away, will take only slightly more fuel than a near approach to the moon. In each case most of the fuel is expended while breaking away from the strong, close-in gravitational field of the earth. A landing on Mars and a take-off from the Martian surface would be extremely costly in fuel, but Dr. Schilt points out that landing on one of the small moons of Mars would cost practically nothing. The outer moon, Deimos, is about five miles in diameter, and has hardly any gravitation. The spaceship could drift toward it and, without expending fuel, come aboard as gently as thistledown. Then the crew would get a free ride around Mars, circling the planet every 30 hours and studying its surface from the fairly convenient distance of 12,500 miles. For a closer look they could shuttle to the inner moon, Phobos, which circles Mars only 3,700 miles away.

When it came time to return to earth, a 10-lb. push would separate a spaceship from its natural merry-go-round. Free of the little moon, it would have satellite velocity, 3,000 m.p.h. in the case of Deimos, so only a moderate additional push would free it from Martian gravitation and start it on the long voyage home.

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