Sentries in Orbit

The rocket pads at Cape Canaveral have been comparatively quiet for months. Only an occasional missile roars aloft, and to jaded Florida bird watchers, the Atlas-Agena that lifted off last week was far from novel. But this time the familiar workhorse carried a brand-new payload: its nose was fitted with two icosahedrons (two-sided solid figures) about 4 ft. in diameter. And the angular cargo was destined to play a large part in policing the cold war.

After separating from the Agena second stage, the two odd objects headed for orbits 60,000 miles above the earth. There, well above the Van Allen radiation belt, they will act as the outermost sentries of the U.S. nuclear alarm system. If any nation explodes a nuclear test in space, the orbiting icosahedrons should promptly report a violation of the test ban treaty.

Developed by Los Alamos Scientific Laboratory and Space Technology Laboratories, the space sentries are parts of a many-sided program to detect clandestine nuclear tests. One tempting possibility for a nation that has signed a test ban but intends to cheat is to shoot a test device deep into space and observe the results by means of instruments carried on a nearby spacecraft. Since a nuclear explosion in a vacuum gives little visible light, it might well go unnoticed by observers on earth.

But such explosions are far from invisible to eyes designed to see them. Most of their energy goes into X rays that travel unhindered through space and are stopped by the earth's atmosphere. A sensitive X-ray detector above the atmosphere can spot them 200 million miles away, and the satellite sentries launched last week carry twelve cylindrical X-ray detectors poking out in all directions. Inside the satellites' skins are instruments that will watch for the neutrons and gamma rays that also come from explosions in vacuum.

The first two sentry satellites were rigged to orbit on opposite sides of the earth, thus reducing the probability that the earth will ever be between both of them and a space test. Next year the U.S. intends to put eight more sentries in orbit. While they watch for treaty violations, they will make themselves peacefully useful by reporting bursts of X rays coming from the sun.

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