New Timetables for Planetary Tours

Though unmanned spacecraft have already landed on the moon, photographed Mars and crashed onto Venus, the more distant planets of the solar system are still beyond the practical grasp of man. None of the rockets now used in either the U.S. or Russian space programs are powerful enough to reach them. Even the huge and yet-unproven Saturn 5, which will carry men to the moon, would require an additional stage to send only a tiny payload on one-way trips, and would require six years to reach Saturn, 16 years to Uranus and 30.7 years to Neptune. But the planetary timetable may soon be revised. An ingenious navigational technique and new space engines, says a Jet Propulsion Laboratory scientist, could drastically cut travel time to distant planets as early as the 1970s.

Setting out his thesis in the current issue of Astronautics and Aeronautics, Aeronautical Engineer Homer Stewart suggests that the gravity of other planets represents a still-untapped source of energy for long-range space flights. Jupiter's gravity, for example, would exert a tremendous pull on a passing spacecraft, accelerating it greatly and deflecting its course. Thus Jovian gravity could be used, in effect, to gain both thrust and a mid-course correction without the expenditure of fuel. Space scientists, like expert billiard players, can precisely determine the amount of acceleration and degree of deflection by careful control of both the velocity and course of the spacecraft as it approaches Jupiter.

Proved by Mars. Under Stewart's direction, scientists at JPL's Advanced Studies Office have calculated that a Saturn 5-powered craft launched on Oct. 1, 1978, would gain so much speed as it passed Jupiter that it could reach Saturn in only 2.8 years and Uranus in 5.9 years. A flight launched into a proper trajectory on Nov. 1, 1979, would be picked up by Jupiter's gravity and hurled to Neptune -- like a skater at the end of a crack-the-whip line -- in only 8.1 years. The scientists also discovered that the outer planets would be so fortuitously positioned that a spacecraft launched on Oct. 7, 1978, could actually make a 8.9-year "grand tour," passing close to Jupiter, Saturn, Uranus and Neptune in turn and receiving a gravitational boost from each.

While the use of the interplanetary billiard technique drastically cuts travel time, Stewart says, it does little to reduce the large amounts of fuel and great initial thrust required to send a spacecraft to the distant planets. But another rapidly developing propulsion system, the solar-powered ion engine, may well solve that problem in time for the flights of the 1970s. Using electricity generated by solar panels, these engines produce a stream of ions (charged atomic particles) that provide a minute amount of thrust -- usually measured in hundredths of a pound.

Accelerated by Ions. Unlike chemical rockets, which burn most of their fuel in a few minutes, ion engines can operate continuously for months and even years on an incredibly small amount of fuel. One experimental ion engine recently completed 341 days of steady operation. Thus, after a powerful chemical rocket has boosted a spacecraft beyond the earth's gravitational pull, the ion engine can take over, gradually and steadily accelerating the craft to its planned velocity over the months and years of a long space trip.

By using an ion engine instead of chemical fuel for deep space acceleration, Stewart believes, scientists will be able to launch outer planet probes with rockets as small as the Atlas-Centaur, or send considerably larger payloads aloft with the Saturn 5. Combined with gravity assists from the planets, the ion engines should allow sophisticated unmanned probes to give man a close look at the outer planets, regions outside the solar system -- and even the sun itself.

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