A Sense of Direction

Man or machine, the navigator of a spacecraft traveling toward a distant planet has a delicate problem: when there is no gravitation, there can be no sense of direction. In three-dimensional space, it is impossible even to tell which way is up unless sights can be taken on a pair of reference points. But how to find those two landmarks? The easily sighted sun may glaze as usual, but the familiar earth quickly fades into a background of dim, average celestial bodies.

One promising answer to the problem has been produced for the Air Force by General Precision Aerospace of Little Falls, N.J.: a device that watches the ever constant stars and uses them to keep a spacecraft from losing its way. The device is deceptively simple in conception, but like most space hardware, it is complex in construction. Essentially, it is a mechanical eye that sweeps the sky and is rigged to notice only the 50 brightest stars. Its main working part is a small mirror that rotates inside a window, scanning narrow strips of black space. When the mirror's field of view crosses one of the 50 stars, a photocell reports the star's position to a computer. When three bright stars have been reported, the computer measures the angles between them and compares them with known angles stored in its memory. No two groups of three stars have exactly the same angular arrangement, but if the computer picks up one group that might possibly be mistaken for another, it observes a fourth star and uses it to remove all uncertainty. When the computer has decided firmly what stars the mirror was looking at, it tells the spacecraft how it is heading.

Such measurements are as important to spacecraft as the celestial fixes by which oldtime navigators found their way across the oceans. If the spacecraft is guided by a mechanical navigator, the findings of the star scanner can be fed directly into the navigator's electronic brain. If the craft has a human crew, the heading can be read from dials or other display devices. One promising version of the new system will use the sun as a kind of North Pole. But as man's machines get farther into space, the scanner might locate a prominent star such as Sirius and use it as the changeless anchor point of the spacecraft's sense of direction.

* The earth may have disappeared in just such a manner for the Russians' Mars Probe 1. Last week the Soviets announced that the vehicle failed to keep its directional antenna pointed accurately toward home when its orientation system failed.

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