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.
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.
Likewise on Mars, the Viking 1 Lander is carrying on long beyond its scheduled mission. Although the Viking Orbiters that relayed Lander signals to Earth have been taken out of service, ground stations can still make contact with Lander 1. The survey craft has a search program that enables it to point its antenna at Earth until 1991. Thus, once a month or more often, NASA's more powerful deep-space tracking stations can communicate with the Lander.
Barring unexpected failures, Lander 1 can carry on like this until the end of the decade. Its nominal mission has long since been completed; but its useful life is far from over.
This kind of reliability -- which is by no means always achieved -- has also helped open up the field of ultraviolet (UV) astronomy during the past decade. UV from outer space is absorbed by Earth's atmosphere. Yet studying stars and other project projects at UV wavelengths provides fresh insights for astrophysicists. Thus in August 1972, when NASA launched a UV observing satellite called Copernicus, astronomers hailed a new era in astronomoy. But as Lyman Spitzer of Princeton University, which managed Copernicus, has said, they thought "we'd be lucky if it lasted a year," which was the designed life of the
Copernicus was the fourth in a series of orbiting astronomical observatories. It predecessors had been a mixed success. The first failed after three days. The second worked well and lasted four years. The third never even made it into orbit. As Spitzer explained: "The most difficult part of an operation like this is getting the satellite up and operating in the first place. If you can get past the first week, then chances are you're all right for a while, especially if you build a little redundancy into the instrument."
Copernicus worked well for eight years. It was still in reasonable order, although its efficiency had degraded, when NASA turned it off early last year. By then, it had provided critical information on the origin of the universe and the evolution of stars and galaxies.
It made the first accurate measurements on the amount of deuterium (heavy hydrogen) in the cosmos, a key bit of data in theories of how the universe evolves. It had studied high-speed gases shot from exploding stars -- gases whose shock effect on interstellar clouds may compress these clouds and start new star formation. And it solved an old astronomical puzzle by discovering what had been an invisible partner in a binary star system in the constellation Lyra. Astronomers had looked for that companion star for 40 years and couldn't find it. They needed UV "eyes" to see it.
Thus it was that a honey of a spacecraft with "a little redundancy" gave a new dimension to astronomy. And when copernicus was eventually retired, a newer , more powerful spacecraft -- the International Ultraviolet Explorer -- had taken over. A few months ago, this craft also exceeded its designed life. It is still going strong. Now astronomers wonder how many bonus years of data it will yield.
Space scientists are not always that fortunate. Many well-designed, carefully made spacecraft fail prematurely or lose some of their capacity early. Yet, when a spacecraft does have endurance, skillful ground control can capitalize on this to extract rich dividends of knowledge. This is why, despite tight budgets and stringent schedules for its deep-space tracking network, NASA is careful to maintain contact with Pioneer 10. When the spacecraft passes Pluto's orbit in June 1989 and moves beyond the solar system, it quite likely will give humanity its first direct report on conditions in interstelar space.