Rocky Gauntlet in Space

As every sci-fi fan knows, one of the great hazards of space travel between the orbits of Mars and Jupiter is the asteroid belt: a doughnut-shaped stretch of floating debris that could fatally pierce the thin metallic skin of a speeding spacecraft. Now, for the first time, a real ship is beginning to run this rocky gauntlet. Success will increase the possibility of future missions to Jupiter and the other outer planets (Saturn, Uranus, Neptune and Pluto).

Launched from Cape Kennedy last March, the instrument-packed Pioneer 10 is scheduled to make the first flyby of Jupiter late next year. But for the next seven months, NASA scientists will be watching to see whether their spaceship can pass unharmed through the 175 million-mile-wide asteroid belt. The greatest danger may not come from any of the 1,831 charted asteroids that range in diameter from one mile to 480 miles, but from untold numbers of tiny fragments, some of them no bigger than a grain of talcum powder. At typical asteroid speeds (30,000 m.p.h.), such minuscule bullets could easily puncture one of Pioneer's vital parts.

Scientists at NASA's Ames Research Center figure that the odds of such a collision are no worse than one in ten. Says Project Scientist John Wolfe: "Pioneer should get through without any trouble at all." Still, the density of the asteroid belt, now estimated as one particle per 10 million sq. km., is one of the questions that scientists are now trying to answer, and they cannot really be sure. In fact, a surprise of the pre-belt journey is that Pioneer was hit by 56 tiny meteoroids, or about 50% more than expected. Fortunately, these impacts, detected by pressure drops in Pioneer's 234 specially designed external gas cells, have not caused any significant damage. In the asteroid belt, Pioneer will use more sophisticated sensors: an array of four small telescopes that have been aligned to measure the brightness, speed and direction of passing particles, and a sensitive light-measuring photopolarimeter that should glean even more detailed information from the reflected light of nearby asteroids.

Such data may help clear up the mystery of the asteroid belt itself. For a long time astronomers suspected that it was made up of the remnants of a small planet that was blown apart, perhaps in a collision with a comet. Now astronomers are leaning to the idea that the asteroid belt consists of primordial matter that failed to coalesce into a full-fledged planet. If so, that could make an asteroid an even more valuable prize than any moon rock--assuming some far-ranging space traveler could bring it home. It would be a piece of material largely unchanged from the very beginnings of the solar system.

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