Lunar Probe

Both the Russians (with buttoned lips) and the U.S. (with considerable public discussion) are preparing to probe the moon. Lieut. General Samuel E. Anderson declared last week (and was sternly shushed by the Pentagon) that the Air Force will fly three lunar probes this year, in August, September and October. A lunar probe means neither a landing nor a circumlunar trip with a manned spaceship, both enormously difficult, but an impact on the moon or passage around it by an unmanned Sputnik-like vehicle. A one-way trip ending in impact is probably the easiest, but many scientists oppose it as a childish stunt that may prove serious scientific vandalism.

There is no point in shooting at the moon unless the shooter can tell if he makes a hit. so all sorts of methods have been proposed to signal back to earth that the impact has occurred. An obvious way, advocated by Professor Fred Singer of the University of Maryland, would be to explode a nuclear charge on the lunar surface. It would make a visible flash, and although its crater would probably be too small to be seen with the biggest telescopes, it might toss up a vast amount of fine lunar dust. If the explosion took place on a dark part of the moon near the edge of the lighted area, some of the dust would be thrown into sunlight, making a conspicuous bright patch that could be photographed.

No Litter Bugs. But the moon would never be the same again. Since it has no atmosphere to limit the motion of small particles, the radioactive residue from the explosion would be carried all over the lunar surface. When earth's scientists finally land on the moon, they would not be able to distinguish between its natural radioactivity, perhaps including material formed by cosmic rays hitting the airless surface, and the nuclear litter scattered by earth's vandals.

Chemical explosions (e.g., magnesium flash powder) would not do as much damage, but they would contaminate the moon in their own way. So would powdered dyes or carbon black splashed on the moon's surface to make a visible mark. Even a probe that lands gently on the moon and tells about its feat by radio (no easy trick) might carry earthside germs whose desiccated corpses would confuse later-coming biologists. Many scientists have urged that any vehicle intended to hit the moon should be sterilized inside and out before it leaves the earth.

Most of the earth's space-conscious scientists would be much happier if the first lunar probes merely pass around the moon, examining it with instruments or cameras, and bring or radio their information back to earth. This delicate problem in celestial mechanics has been worked on for more than a century in finer and finer detail. Many factors must be considered, including the speed of the probe, the motion of the moon around the earth, and the overlapping gravitational fields of the earth, moon and sun (see diagram).

Aim Ahead. The rocket for the first probe will be aimed about 40DEG ahead of the moon, like a hunter leading a duck. Its initial speed of 23,827 m.p.h. will bring it to the moon's vicinity in a little more than three days. If aimed correctly, it will cross the moon's orbit slightly ahead of the moon, moving comparatively slowly. In this region the moon's gravitational field is dominant. It will pull the probe around the moon and sling it back toward earth in a lopsided figure eight.

At first the returning vehicle will move slowly, but eventually the pull of the earth will accelerate it to its departure speed. The pull of the sun will also make itself felt, its effect depending on the position of the moon in its orbit. In about 6 1/2 days (with a little bit of luck), the probe will return to earth and enter the thin film of its atmosphere. The whole operation calls for precision at every point. Even a small error of speed, aiming or timing could make a probe crash against the earth, or revolve around it in wild, unpredictable swings, or even escape from its gravitation and fall into the sun. Despite many confident predictions from military and other optimists, a successful first probe of the moon is not likely.

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