The Little Spacecraft that Could ¿ and Did

Decades after their original objectives were completed, Pioneer, Voyager and IMP-8 keep on trucking
Pioneer 10

A Deep Space Network antenna in Madrid picks up a feeble and unexpected radio signal--a cry from a lost spacecraft. It is April 2001. Pioneer 10, missing, feared dead for eight months, was phoning home to let scientists know that it was still alive and kicking. Intrepid Pioneer 10 went on to celebrate a milestone 30th birthday in 2002, and today 10 belongs to an elite group of spacecraft that just refuse to die.

More than a quarter of a century after their launches, Pioneer 10, Voyager 1, Voyager 2 and the IMP-8 spacecraft are still going strong. During their primary missions in the Seventies and Eighties, they gave us a unique insight into the atmospheres of Jupiter, Saturn, Uranus and Neptune, and the solar-wind conditions around Earth. And today, thanks to some nifty long-distance repair work by dedicated space scientists on the ground, the probes continue to radio-signal postcards home to tell us about the path to interstellar space.

Pioneer 10 was launched on March 2, 1972, on an expedition to Jupiter. To mark its 30th birthday, scientists sent a special transmission to the retired spacecraft. After 22 hours, having crossed 7.4 billion miles, Pioneer 10¿s reply--equal in power to a nightlight--was heard loud and clear. But without careful remote control maneuvering, Pioneer¿s latest messages home could well have been lost in space.

Maneuvers in the Dark

After two and a half decades in outer space, Pioneer 10¿s antenna had drifted off-point. In 1997, NASA researchers realized they needed to re-point the antenna to Earth to keep in contact with the craft. But the targeting process carried the risk of muting Pioneer 10 permanently. To garner enough power for the maneuver, Pioneer 10¿s transmitter had to be shut down. "We were concerned that turning the transmitter¿s traveling wave tube off in the deep cold of space, and then back on again, would cause a thermal shock that might shatter the helix [a component that¿s critical for transmission] in the tube," explains Larry Lasher, Pioneer project manager at NASA Ames Research Center. But "the hearty spacecraft successfully executed the [maneuvering] procedure in the blind for 90 minutes," he says.

Not bad for a craft originally slated only for a 21-month mission to Jupiter. To reach the giant planet, Pioneer 10 first had to cross the (then) seemingly impenetrable asteroid belt. By the 1960s, when Pioneer's mission was being planned, the orbits of more than 3,000 large asteroids had been determined and thus could be avoided, but tiny particles in the belt were still thought to pose a big threat. Scientists had no way of estimating the number of grain-size particles that might severely damage a craft after multiple collisions. "Only by going there could the danger be properly assessed, " says Ed B. Massey, manager of the Voyager and Ulysses projects, describing the debt later missions owe to Pioneer. "When Pioneer successfully traversed the asteroid belt, it had demonstrated that a concentration of small debris sufficient to harm the spacecraft did not exist."

After three decades that saw Pioneer 10 blaze a trail for all future spacecraft through the asteroid belt, take the first close-up images of Jupiter and its satellites and then head out of the solar system, the spacecraft was officially decommissioned in 1997. But, as Lasher notes, "in retirement Pioneer 10 still served the public as a valuable space resource at no extra cost to the taxpayers." It has been used to train spacecraft controllers in tracking station protocol and continues to be tracked as a part of study in communication technology.

Edge of Darkness

Pioneer 10 also continues to investigate a puzzle posed by its own early findings: Where exactly is the edge of the sun¿s influence? One of the greatest surprises raised by the original Pioneer mission was, according to Lasher, the persistence of the solar wind, the stream of charged particles that radiates from the sun. Prior to Pioneer¿, the effect of the solar wind was thought to extend out to Jupiter; but even now, with Pioneer at a distance of 7.5 billion miles, it has yet to escape the long arm of the sun¿s reach.

The elusive edge is known as the heliopause boundary, where the pressure of the solar wind becomes equal to that of the interstellar wind of cosmic rays. It¿s thought to be immediately preceded by the termination shock, where the solar wind is abruptly slowed from super- to sub-sonic flow. Pioneer¿s Geiger tube telescope should be able to locate the boundary by measuring an increase in cosmic-ray counts.

But, after a history filled with dramatic firsts, Pioneer 10 will have to settle for being a runner-up in this particular marathon. The first to encounter interstellar space will most likely be the more recently launched Voyager 1 spacecraft, itself an accomplished long-distance runner.

Voyage to Jupiter and Beyond

Also celebrating a milestone birthday this year, turning 25 on September 5, 2002, Voyager 1 overtook Pioneer in February 1998, to become the farthest man-made artifact. Like its pioneering cousin, Voyager 1 defied the odds, surviving for more than six times its original four-year mission length.

Voyager 1 and Voyager 2 set out in 1977, to Jupiter and beyond. The timing took advantage of a rare arrangement of the planets, occurring once every 175 years, which allowed a bargain tour of Jupiter, Saturn, Uranus and Neptune. To save weight on fuel, the twin spacecraft both employed the "slingshot" method, using the gravity of each planet passed to bend the flight path and increase velocity. Following in the footsteps of Pioneer 10, Voyager 1 traveled to Jupiter and Saturn before embarking on its interstellar mission, while Voyager 2 took the full four-planet tour. The two Voyagers found the first signs of volcanic activity outside Earth, on Jupiter¿s moon Io, and discovered that the atmosphere of Saturn consists almost entirely of hydrogen and helium.

The Voyagers¿ longevity can be attributed in part to last-minute design changes made following feedback from the Pioneer 10 and 11 missions. (The mission of Pioneer 11, which also flew by Jupiter, ended after its last transmission was received on September 30, 1995.) "The Pioneers discovered that Jupiter¿s radiation belt was much more intense than anticipated, and the Voyagers, while more sophisticated than the Pioneers, were also more vulnerable," Massey explains. "Optical glasses and electronic components that would withstand the intense radiation had to be found, and instruments and other space subsystems had to be modified."

Credit must also be given to the original design team¿s foresight when preparing the spacecraft for potential mission extensions. Each craft houses on-board backup systems that can be activated from the ground if needed. In April 2002, the team had to call on one of those reserve systems when Voyager 1¿s position-sensing capability was in jeopardy, setting a new record for most-distant spacecraft maintenance in the process. "We were switching to a system that had not been used for 20 years, and, while we felt confident, we were not positive that it would work," Massey says. During a temporary changeover, on-board computers became confused about the spacecraft¿s location, mistaking the sun for Earth, almost leading to loss of communications with the ground. The team had to instruct Voyager 1 to try to keep itself steady with gyroscopes during the final switch because it couldn¿t rely on its computer sun-position sensors. "There was only about a 15-minute interval to analyze the data after the switch and decide whether to continue with the permanent switch," Massey says. But the team had faith that the transition had been successful, and today Voyager 1¿s backup is functioning as smoothly as the original did 25 years ago.

Now both Voyagers are also heading out to the heliopause. With more sophisticated instrumentation than Pioneer, they may be used to study interstellar fields, particles and waves, in regions unaffected by the solar wind. To make sense of the data received, however, it¿s important for scientists to have a clear understanding of how galactic cosmic rays are modulated by the solar wind. And who better to look to for advice than IMP-8, a spacecraft with almost 30 years of experience collecting data on long-scale solar processes?

On the Home Front

IMP-8, the last in a series of Interplanetary Monitoring Platforms (IMPs), was launched into orbit on October 26, 1973, to examine the solar wind. Previous IMP lifetimes ranged up to six years, usually ending when the crafts succumbed to atmospheric drag. Sent into a higher orbit than its siblings--nearly two thirds of the distance to the moon--IMP-8 was pencil-marked for a five- to 10-year working lifetime. Instead, the small drum-shaped craft, 135.6 centimeters across and 157.4 centimeters high, clocked 28 years of active service before being retired, temporarily, in 2001.

Scientists at the NASA Goddard Space Flight Center had fitted IMP-8 with solar panels to allow it to continue its mission after its battery died. However, other setbacks on its way, such as the eventual obsolescence of its transmission frequencies, were harder to anticipate and resolve.

IMP-8 had no on-board tape recorder; instead it beamed its data to the Earth at 6,000 bits per second. This constant stream of data was vital in understanding long-scale solar processes; more than a thousand scientific papers cite IMP¿s results. However, the lack of a tape recorder made it crucial to have enough coverage on the ground to capture the spacecraft¿s signals, which were being broadcast at increasingly outdated VHF frequencies. Joseph H. King, the IMP-8 project scientist since 1974, found himself approaching everybody he thought might be able to support VHF capture "Once in the early 1980s an ESA [European Space Agency] scientist visited Goddard and I asked him to help. As a result the ESA station at Redu, Belgium, captured IMP data for about 16 years," King says. "As another example, I asked a GSFC [Goddard Space Flight Center] engineer while we were cooling down from a Goddard 'fun run' in about 1994. He was involved in the Antarctic with NASA/NSF [National Science Foundation] ozone programs. He then built an IMP-specific antenna at McMurdo Sound, which was operational for two to three years and was then cloned at Canberra, Australia." Ironically, with the ad-hoc stations at Redu and Canberra, in addition to the "regular" NASA station at Wallops Island, Va., IMP had greater coverage toward the end of its original mission than it had during the preceding decade.

NASA officially stopped listening to IMP-8 on its 28th anniversary in 2001, after the failure of its magnetometer, which was needed for studying magnetic fields. But the plucky little IMP continued on its 12.5-day Earth orbit transmitting regardless, while scientists lobbied NASA to continue using IMP-8 as a cosmic-ray monitor. As a reward for its perseverance, IMP-8 was called out for an encore performance three months later to provide support for the current Voyager mission. Scientists can use IMP¿s near-Earth data on solar plasma and energetic particles, in conjunction with that of the Voyagers, to evaluate changes in solar-wind conditions as particles are carried out to the distances of the Voyagers.

NASA intends to operate IMP-8 in this adjunct role until 2005. Meanwhile, Voyagers 1 and 2 have longer-term plans; their radioactive power sources should keep them chugging along until at least 2020. And Pioneer 10? It¿s on course to reach the Taurus constellation in about two million years. Pioneer 10,the Voyager twins and IMP-8 show you just can¿t keep a good spacecraft down.

Zeeya Merali is based in Providence, RI.
"The Quest for the Limits of the Heliosphere," by J.R. Jokipii and Frank B. McDonald Scientific American, April 1995), is available for purchase at the Scientific American Archive.
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