Twinkle, Twinkle Little Star

Millions of people have watched Echo, the U.S. balloon satellite, as it crosses the sky. And most of them have noticed that it twinkles like a star and also brightens and dims slowly in a way that no star does. Why does it perform in this odd fashion? Last week the explanation came from Dr. Leonard Jaffe of the National Aeronautics and Space Administration. Echo's quick twinkling, said Jaffe, is caused by the same atmospheric irregularities that make stars wink. Some of its slower dimming may be due to thin patches of clouds, invisible at night--but most of it is Echo's own doing.

When Echo first took to space on Aug. 12, it was as round and polished as a giant ball bearing, its aluminized Mylar film kept tightly inflated by 20 Ibs. of vaporized anthraquinone, a normally solid organic chemical. When its 100-ft. sphere moved on its orbit 1,000 miles away from the surface of the earth, it covered about one-tenth the angle of the planet Venus at 40 million miles, but it did not show as a disk even in a powerful telescope. The sun reflecting on its spherical surface showed as a mathematical point, as stars do.

But space is a tough neighborhood for frail balloons. Microscopic meteorites punctured Echo's skin, allowing the gas inside to seep out. Sunlight exerted a slight but persistent pressure. Gradually Echo lost its regular shape; flat places and wrinkles appeared on its shiny surface. "She's prune-faced already," says Richard Slater of G. T. Schjeldahl, Northfield, Minn., the company that made the balloon. When Echo turns deliberately about once in eight to ten minutes, flat places sometimes act as mirrors, making the sun's reflection momentarily brighter. Wrinkled places dim the reflection. The radio waves that are bounced off Echo show the same variations.

For two weeks after launching, Echo stayed entirely outside the shadow of the Earth, but on Aug. 24 it dipped into darkness for two minutes while passing over the U.S. West Coast. Each day its stay in the shadow will increase, until in late December the balloon satellite will be in darkness for 35 minutes of its 118-minute orbit. When it goes into the shadow, it shrinks a bit, but Dr. Jaffe does not know how much.

Echo's orbit has changed very little, but no one can say for sure how long it will last. All its gas pressure is probably gone by now. The only reason it keeps its shape is that the forces that tend to shrink or distort it are extremely small. Slater estimates that meteorites nibble away about 1 1/4 sq. in. of its skin per day. Eventually the sphere may collapse, pushed to a pancake by air drag and pressure of sunlight, or drawn together by the Mylar's "memory" of the way it was folded in the launching rocket. But a flattish or crumpled shape may continue to serve for years as a good radio reflector, which is the basic job that Echo was sent up to perform.

Echo is visible over much of Russia, and is the most conspicuous space vehicle launched so far. But the Soviet press and radio have made no mention of it. Unless a Soviet citizen follows foreign broadcasts, he does not know what to make of the bright star that creeps repeatedly across the night sky.

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