Sky & Telescope March 1991 [antikvár]

I. 'IV -.t-íFOCAL POINTForgotten PioneersIT JUST doesn't seem fair. When mostpeople think of the exploration of the outer solar system, they think of the Voyager deep-space probes. They've forgotten that the successes of Voyagers 1 and 2 were due in no small part to a pair of earlier space probes: Pioneers 10 and 11. These were the first spacecraft to travel beyond the asteroid belt out to Jupiter, Saturn, and beyond.If it's not fair, it's not unexpected. Pioneers 10 and 11 were designed as relatively inexpensive trailblazers. Each used an arsenal of 11 scientific instruments and sensors to find a route that the more capable and considerably more expensive Voyager spacecraft could safely follow. It wasn't a gentle journey; the Pioneers were continually exploited for the Voyagers' sake.For example, Pioneers 10 and 11 were hurled through the asteroid belt at 20 kilometers per second. Scientists had feared that the belt might contain micro-meteoroids and other debris that could destroy a spacecraft traveling so fast. Such objects would act like bullets fired from a high-power rifle, but moving 15 times faster. Fortunately our twin Pioneers suffered just a few tiny impacts, proving the hard way that transit through the asteroid belt is relatively safe.The next assignment was Jupiter. Scientists sent the craft sprinting through the giant planet's magnetosphere to learn whether fast-moving swarms of charged particles could damage delicate computer components on the spacecraft. Pioneer 10 made the first pass, traveling within 130,000 km of the planet's cloud tops. Although the craft wasn't damaged, the charged-particle fields temporarily interfered with the on-board computer system, preventing some instruments from studying the Jovian moon lo. Pioneer II passed three times closer to Jupiter, also with no serious damage. Once again the expendable Pioneers cleared the way for the more "valuable" Voyagers.With Jupiter behind it, Pioneer 10 ended its sole planetary encounter. Pioneer 11, however, still had much work to do. The close flyby of Jupiter gave it the velocity and trajectory change necessary to reach Saturn on the other side of the solar system. This largely unproven technique would prove crucial to the success of the multiple-planet Voyager missions.Meanwhile, NASA space scientists engaged in fiery debates over the plight of Pioneer II. Many wanted the craft to pass inside Saturn's rings. Although this would be dangerous, the potential scientific gain seemed enormous. Others argued for a less risky trajectory outside the rings, on a path similar to those planned for the Voyagers. The final decision: Pioneer-Saturn (as it was christened for this portion of the mission) would steer clear of the rings. Its role as the pathfinder for Voyager was secured. While at Saturn the craft discovered the thin and "braided" F ring and narrowly missed colliding with an unexpected new satellite (later named Epi-metheus).More than deep-space vanguards. Pioneers 10 and 11 also managed to gather a rich fount of information about the solar system and to give scientists their first direct observations of the asteroid belt, the Jupiter and Saturn systems, and interplanetary space beyond. Even as the craft head out of the solar system (Pioneer 10 toward the constellation Taurus and its twin toward Ophiuchus) they continue to do science. They dutifully hunt for the boundary of the heliosphere and search for gravitational signs of a possible tenth planet or "dark star." Sadly, the historic probes' fission-driven generators are gradually losing power, and Pioneer 11 may not survive past 1995 (see page 244).Yet despite their scientific and path-finding accomplishments, the Pioneers remain overshadowed by the more spectacular Voyager missions. Perhaps this is theway space exploration must progress. Each new program is heralded as a grand breakthrough, reaching new heights and achieving the previously unachievable. In time, however, each program is superseded by another more glamorous, more far-reaching effort.But just as the achievements of Pioneers 10 and 11 were lost in the shadows of Voyagers 1 and 2, the Voyager accomplishments will be dimmed by the upcoming Galileo mission to Jupiter and Cassini mission to Saturn. These new probes won't just fly by their targets but will orbit them for several years, gathering more detailed data and producing more enchanting pictures than was possible on past flights.Although the cycle of fame and obscurity is expected and even welcomed, Pioneers 10 and 11 can still claim some mark of distinction with their many firsts. Voyagers 1 and 2 made a bigger scientific splash, but they are just two of what NASA hopes will be a long succession of spacecraft to explore and study the outer solar system. To Pioneers 10 and 11 fell the task of leading the way, a distinction that future exploration, no matter how spectacular, cannot diminish.ROBERT G. NICHOLSThe author, a former contractor for NASA's Johnson Space Center, writes frequently about science and technology.Focal Point invites contributions from readers who wish to comment on contemporary issues in astronomy and space science.236 Sky & Telescope, March, 1991