Journey to Jupiter

In the 13 years since Russia launched Sputnik 1, man has steadily pushed back the frontiers of space. Astronauts have walked on the moon, the Soviet spacecraft Venera 7 has soft-landed on Venus, and three U.S. Mariner spacecraft have swept past Mars, transmitting detailed pictures back to earth. Now scientists are preparing for an even more far-reaching journey. Last week NASA discussed its plans to launch the first unmanned planetary probe to the outer part of the solar system--a 550-lb. spacecraft that will fly past Jupiter.

To reach the solar system's largest planet, a flight that could take two years or more. Pioneer F will have to survive a hazard never before encountered by a spacecraft: it will have to pass through the asteroid belt, which consists of some 50,000 asteroids that circle the sun between the orbits of Mars and Jupiter. If Pioneer runs the rocky gauntlet successfully, the way will be cleared for further explorations of the outer planets by unmanned spacecraft making Grand Tours* later in the decade, as well as future flights by man himself. A serious accident, on the other hand, might well cause space scientists to reconsider their plans.

Mysterious Spot. As it passes within 100,000 miles of Jupiter, Pioneer F will conduct a total of 13 experiments and radio the results back to mission controllers at NASA's Ames Research Center in Mountain View, Calif. A complex array of detectors, which poke out of the cone-shaped spacecraft like antennae on a monstrous insect, will measure, among other things, magnetic fields, ultraviolet and infrared radiation, cosmic rays, meteoroid density and the intensity of the solar wind (charged atomic particles streaming from the sun).

These readings may help to explain some of Jupiter's more puzzling features. Except for the earth, it is the only planet believed to have a magnetic field. It is also producing a great quantity of heat, the origin of which is still a mystery. In addition, it has twelve satellites, three of which are larger than the earth's moon. By analyzing the radio signals that Pioneer emits just before it ducks behind one of the larger moons, possibly lo (pronounced eye-oh), scientists may be able to tell whether the satellite has an atmosphere.

Most intriguing of all, light measurements by Pioneer's imaging photo-polarimeter will enable computers on earth to construct about ten pictures of the planet that will show features as small as 250 miles across. Although the resolution is not much greater than that achieved by terrestrial telescopes, the pictures will be shot from glare-free angles completely unobtainable on earth.

The pictures and measurements should give scientists an entirely new perspective on Jupiter. They may help, for example, to explain the origin of the planet's Great Red Spot: the huge (8,000-by 30,000-mile) changing blemish on the planet is almost as much a mystery now as it was when Robert Hooke discovered it more than three centuries ago. Pioneer should also provide important new knowledge about Jupiter's atmosphere, now thought to be composed of turbulent clouds of hydrogen, helium, methane and ammonia. It may closely resemble the earth's own early atmosphere, and could contain the chemical seeds for the beginnings of life. Cornell Astronomer Carl Sagan, for one, speculates that Jovian life could already exist--in the form of ballasted gas bags that absorb organic matter as they float through atmosphere, just as certain whales swallow tons of plankton as they swim through the earthly seas.

Even after it has passed Jupiter and headed into the outer reaches of the solar system, Pioneer's scientific usefulness should continue. The spacecraft's nuclear-powered instruments have a life expectancy of six years and may still be in good working order when it passes the orbit of Uranus, the seventh planet from the sun. It may even be able to detect the unknown limits of the heliosphere, the region in space influenced by the sun's gases and magnetic field, and chart the fringes of interstellar space. Later, having finally ceased transmitting its data, it may become the first spacecraft to fly beyond Pluto's orbit and leave the solar system. If it achieves those awesome goals, Pioneer will indeed have been well named.

-In the late 1970s, the five outer planets will be so aligned that a single spacecraft can pass close to Jupiter, Saturn and Pluto--or Jupiter, Neptune and Uranus--using the gravitational pull of each planet to hurl itself on toward the next. That favorable configuration of the outer planets will not occur again for another 179 years.

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