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to have enough coverage on the ground to capture the spacecraft¿s signals, which were being broadcast at increasingly outdated VHF frequencies.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.