Daddy Longlegs in the Sky

Some time this week, the newest NASA satellite is scheduled to perform a complex series of operations in orbit. If all goes well, Radio Astronomy Explorer-A will unreel a collection of booms and antenna until it turns into a veritable space spider, with two pairs of appendages reaching 1,500 ft. from tip to tip--a distance greater than the height of the Empire State Building (which is 1,472 ft.). With those great legs foraging for information, RAE-A will act as a flying radio telescope capable of monitoring signals that even the largest earth-bound installations cannot detect.

Releasing the Yo-Yos. Lofted from the U.S.A.F.'s Western Test Range at Lampoc, Calif, on July 4, the daddy longlegs in the sky is a masterpiece of technological ingenuity. It is guided from the Goddard Space Flight Center at Greenbelt, Md. NASA scientists there had to perform a series of intricate maneuvers before they could call for the unreeling of the satellite's four main antennas. First they had to nudge the 417-Ib. satellite into a circular, near-polar orbit about 3,640 miles above the earth with precisely timed bursts of a small rocket called an apogee-kick motor. Tho operation evened out the varying gravitational tugs of the original elliptical orbit, which would have bent and distorted the antennas. Next, RAE-A's masters had to stop its 92-r.p.m. spin, which would have wrapped the antennas uselessly around the exterior of the satellite. Two 3/4-lb. "Yoyo" weights were released at the end of 27-ft. wires, reducing the satellite's rate of spin like a whirling skater who slows himself down by extending his arms. Their task accomplished, the Yo-yos were cut loose to drift in space. RAE-A's remaining rotation was stopped when three electromagnetic coils were energized and the spacecraft lined up with the earth's magnetic field. To eliminate oscillations caused by gravitational pull on the tended antennas, a 630-ft. boom also be deployed, much like a tightrope-walker's balancing pole.

Mapping the Milky Way. This week the satellite will receive a radio command from earth and begin to unfurl its giant antenna system. Carried as flat ribbons of silver-plated copper alloy coiled on spools within the satellite, the tubular legs will be formed as the outer edges of the unwinding ribbons curl towards each other and meet. Tiny tabs along the edges of the ribbon will hook together as the tubeb forms, adding rigidity. At first, the antenna legs will be extended only 358 ft. from the craft, to test their stability. Then they will grow to their full length--750 ft. apiece. Already oriented towards the earth by magnetism, RAE-A's lower V antenna will also be pulled earthward by gravity, which will further stabilize the spidery satellite.

After the onboard television cameras have checked out the legs, the upper V will begin monitoring long-wave radio signals from the universe; the underside V will listen for radio emissions from the inner Van Allen radiation belt and from the earth itself. In addition, a dipole antenna in two 60-ft. sections will be deployed to monitor short bursts of radio energy on the same frequencies from various points within the solar system. Such longwave signals are reflected by the earth's ionosphere and are lost to ground-based radio astronomers. But the information that is gathered and rebroadcast to earth by RAE-A should provide U.S. scientists with the first low-frequency radio map of earth's home galaxy, the Milky Way.

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