The last Explorer has fallen silent, and the current series of U.S.-made satellites is spinning to its end. Last week Roy W. Johnson, director of the Defense Department's Advanced Research Projects Agency (ARPA), announced plans for the U.S.'s next series. The new "Discoverer" series will include Sputnik-sized reconnaissance satellites revolving in north-south orbits that cross both poles.
The first shot, scheduled for next month, will use a Thor IRBM as its first stage and is expected to put 1,300 lbs. in orbit. The instrument payload, said Johnson, will weigh "several hundred pounds." Later shots will use Atlas ICBMs as boosters and will put as much as five tons in orbit. Some of the satellites will carry live animals, including a "primate," and attempts will be made to bring them back alive.
Orbit Geography. The Discoverers will be launched from Vandenberg Air Force Base in California (see NATIONAL AFFAIRS), Florida's Cape Canaveral has the wrong geography for polar orbits. If a satellite launcher is aimed either north or south from the cape, it must pass over densely populated areas while still in the dangerous early stages of its flight. The nearest land south of Vandenberg is the Pitcairn Island group in the South Pacific, more than 4,000 miles away.
A satellite that crosses the north and south poles has one great advantage: it "sees" the whole earth. The plane of its orbit remains fixed in space, while the earth turns inside it. If the satellite's period of revolution is 90 minutes, it makes 16 north-south passes around the earth in a day, each pass being 22.5 degrees of longitude (about 1,560 miles at the equator) farther to the west than its predecessor. So a polar satellite, theoretically at least, can take pictures of the entire earth every twelve hours, thus act as a kind of "spy in the sky."
The first Discoverer, missilemen suspect, will do no more than report the cloud cover of the earth. Later versions may eventually take pictures with real cameras. If the satellite is recovered intact, the films can be developed on earth. Another possible trick would be to have the pictures developed automatically on board the-satellite and sent to earth by facsimile radio. A good telescopic camera orbiting several hundred miles up might photograph objects as small as Russian military bases.
Rotation Control. If the satellite can be made to rotate once in 90 minutes (the period of its revolution), its camera can point at the earth all the timejust as the moon rotates so as to keep one side always facing the earth. A promising way to control the satellite's rotation is to give it apparatus that observes the earth's horizon and keeps the satellite in steady alignment with it by squirting out stabilizing jets of gas.
To bring the satellite back to earth at a desired place and time, designers expect to employ a retrorocket, which will be fired to reduce its speed at the chosen moment and spot. A parachute will slow it further, and a radio will shout an S O S. Finding the satellite with its undeveloped films or its beat-up "primate" should not be much harder than finding a missile's nose cone.