Science probes that just won't quit
Old space probes don't necessarily die, or even fade away, when they have fulfilled their missions. Some of them show a surprising endurance that can lead to valuable new roles in space. That's why the twin probes Pioneer 10 and 11, which opened the exploration of the outer planets, continue to hold the interest of solar system scientists. The more spectacular close-ups of Jupiter and Saturn which Voyagers 1 and 2 -- the latter's stuck camera platform notwithstanding -- have given do indeed eclipse the earlier views sent by the two Pioneers. But when Pioneer 10 reached its "silver astronomical unit" mark -- 25 times farther from the sun than is Earth -- on July 26, the event symbolized the opening of a new field of exploration, not the passing of a retired spacecraft.Skip to next paragraph
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At a distance of 3,739,947,300 kilometers from the sun (on that date), Pioneer 10 is well into unknown space as far as direct sampling is concerned. This is the farthest a man-made object has yet gone from Earth -- especially one with which Earth-bound scientists still can communicate and from which they receive data. Pioneer 11, at 10.5 astronomical units, is the next most distant artifact, with Voyager 1 (10.2 a.u.) and Voyager 2 (9.4 a.u.) being the third and fourth.
These probes are sampling the outer parts of the sun's sphere of influence. It is a region of space still dominated by the "solar wind" of particles, laced with magnetic fields, that moves outward from the sun. Pioneer 10, especially, is showing that region to be far more complex than scientists had suspected.
Meanwhile, back at Earth, the launching a few weeks ago of a pair of Dynamics Explorer satellites to study the near-Earth environment has given a new operational role to two other craft now long past their designed lifetimes. One of these, called Helios and launched in 1974, is orbiting the sun. The other, ISEE-3 launched in 1978, is one of the International Sun-Earth Explorer satellites, whose original mission was study of the interaction between the solar wind and Earth's magnetic field.
The US National Aeronautics and Space Administration (NASA) sent up the Dynamics Explorers to learn more about the impact of the solar wind and solar radiation at the boundary between Earth's magnetic shield and interplanetary space and on the upper atmosphere. This can affect auroral displays, radio transmission, and perhaps even the weather. Helios and ISSE-3, which is 1.6 million km from Earth in the direction of the Sun, can now give scientists early warning of solar disturbances whose effects the new Dynamics Explorer satellites are to detect.
Yet another long-playing spacecraft should enable planetary scientists to make analogous studies of Venus. The Pioneer-Venus orbiter, which reached that planet in December 1978 along with four atmospheric probes, is still working well. It has surveyed the planet by radar and at ultraviolet and infrared wavelengths. Now, as its orbit slowly changes, the high point above the planet will rise, allowing the spacecraft to sample the space environment at various distances near Venus and measure effects of the solar wind. The orbit will begin to decay in about three years, eventually causing the spacecraft to enter Venus's atmosphere and burn up. Meanwhile, it will return a bonus of data.