Tip on Re-entry

At first, U.S. and Soviet space scientists were far more concerned with what they sent up than with what came down. Now success has forced them to equalize their interest. The North American Air Defense Command, which is responsible for tracking earth-circling traffic, counts more than 1,300 objects in orbit. These have included not only satellites but last-stage boosters, drifting bolts, and an astronaut's glove and camera. By the immutable laws of gravity, all must one day come plunging down toward earth.

Last week, when remnants of the Soviets' Cosmos 61 shot whirled out of space, NORAD's cameras, radar network and computer banks watched the descending debris until it was finally incinerated in the atmosphere. Other eyes also followed its fiery fall. Using NORAD data dubbed TIP (for Target Impact Point), Herbert E. Roth, a Denver-based jet-training planner for United Air Lines, operates a unique one-man satellite-early-warning system. It alerts commercial airline pilots to the possibility of space debris hurtling across their flight paths.

Growing Hobby. Collision danger, to be sure, is still remote, but Roth figures that precaution is called for. Five years ago, as head of the Smithsonian Astrophysical Observatory's Denver Moonwatch Team, he became interested in the problems of satellite reentry. To help scientists predict the debris' drop more precisely, he organized flight crews into the Voluntary Flight Officer Network and asked them to report all satellite sightings.

Originally a hobby, the network has grown far beyond Roth's expectations. The shower from space has increased, and, with United picking up the tab, Roth has stepped up his activities. He now publishes advance word of the time and place of satellite re-entries in a weekly bulletin that goes out to 118 airlines in the U.S. and abroad.

At best, such predictions are still a rough art. Depending on its size, shape and density, as well as atmospheric conditions, a satellite or other piece of space hardware can plunge down on a steep trajectory, glide relatively slowly through the air, or skip along like a pebble across water. To assure accuracy, Roth gets updated calculations from NORAD two or three hours before an expected re-entry and flashes out a final warning over United's communications setup.

Thin-Slcinned SSTs. Happily, the debris has caused scant damage so far. Like meteors, space junk usually burns up in the atmosphere. Even the few pieces that manage to survive have landed harmlessly enough. No planes have been hit, although pilots have sighted 26 descending satellites.

Yet as planes fly higher, the risk of collisions with space fragments may also rise. In the 1970s, supersonic transports (SSTs) will be soaring at 70,000 ft. --nearly twice the ceiling of present-day passenger jets. In that rarefied atmosphere, space garbage is still more of a menace; the tiniest fragment could puncture the metal skin of an SST. Pentagon, NASA and commercial aviation officials all concede the dimensions of the future problem. But at present, the only formal warning system for commercial aviation is Herb Roth's part-time effort.

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