Illustration of Interplanetary Monitoring Platform-8. Photo: NASA

The Interplanetary Monitoring Platform (IMP 8) spacecraft has retired after 28 years on duty being buffeted by the solar wind and zapped by cosmic rays.

Launched on October 25, 1973, IMP 8 was built and operated at NASA's Goddard Space Flight Center in Greenbelt, Md., and provided important space physics data as part of NASA's Sun- Earth Connection research program. Last commands were sent to the spacecraft on Oct. 28.

"We will miss IMP 8 because it reliably provided unique data for so long," said Dr. Joseph King, project scientist for IMP at Goddard. "However, due to the failure of the IMP 8 magnetometer during the year 2000, a senior review panel of Sun-Earth Connection scientists advised NASA's Office of Space Science management that continuing IMP operations may inappropriately divert funding from more science-effective missions."

IMP 8 has deepened understanding of the space environment near Earth in many ways. Observations from IMP 8 provided insight into plasma physics, the Earth's magnetic field, the structure of the solar wind, and the nature of cosmic rays.

Electrically charged gas, called plasma, blows outward from the Sun at typical speeds of 250 to 300 miles per second and is also known as solar wind. IMP 8 helped detail the complex structure of the solar wind.

Magnetic fields embedded in the solar wind plasma get twisted into a spiral pattern due to the Sun's rotation. Explosive events on the Sun hurl clouds of plasma that plow into slower-moving streams in the solar wind, warping magnetic fields carried by both. Observations from IMP 8, Pioneer and Voyager spacecraft in the outer reaches of the solar system, and from the Ulysses spacecraft orbiting over the poles of the Sun, helped paint this elaborate picture.

Consistent coverage for such a long time recently enabled IMP 8 to uncover a curious long-term pattern in the solar wind, which in turn led to new insights on the magnetic dynamo churning within the Sun. Although one would expect that, over time, the solar wind should blow at the same average speed from any place on the Sun, IMP 8 discovered that this is not so. The average solar wind speed varies from place to place on the Sun, from as much as 285 miles per second around 70 degrees longitude to as little as 265 miles per second around 135 degrees longitude. The average strength of the magnetic field carried by the solar wind depends on solar location as well.

This pattern served as a clue for researchers analyzing the solar surface and interior with other observatories on the ground and in space. They discovered that helical, twisting motions of plasma flows and magnetic fields deep inside the Sun contribute to the generation of the solar magnetic field.

"This unexpected pattern persisted for at least 28 years despite unceasing change on the Sun, including the complete reversal of the Sun's global magnetic field direction every 11 years, so we knew it must be telling us something important," said King.

IMP 8's longevity presented operational challenges for Goddard. "It has been satisfying to exploit new technologies to expedite, and make less costly, IMP data flow," King said. "IMP 8 used the now mainly obsolete VHF telemetry frequencies. The communication network that originally captured IMP 8 data, known as the Spaceflight Tracking and Data Network, was largely disestablished many years ago. One of the key challenges to the IMP project over the past 15 years has been to define and evolve an ad hoc IMP 8 VHF telemetry-capture network."

Over the past 28 years, more than 1,000 scientific papers have been published in the refereed scientific literature in which IMP 8 data were the sole data used or were important adjuncts to data from other missions. Refereed papers are only accepted for publication after being endorsed by independent experts chosen by scientific journal editors.

IMP 8 is in a nearly circular orbit about the Earth, at a distance a little more than halfway to the moon. In this orbit, IMP is in the solar wind about seven days per orbit and is within the Earth's magnetosphere/magnetosheath system about five days per orbit. Currently, six of the original 12 instruments on board IMP 8 are operational.

IMP 8 was the last of the series of IMP spacecraft, which included eight IMPs intended for (and achieving) geocentric orbit and two "anchored IMPs" intended for lunar orbit. These 10 spacecraft were launched by NASA from 1963 to 1973. The IMP spacecraft series was a subset of the highly successful and productive Explorer spacecraft series. IMP 1 was Explorer 18 and IMP 8 was Explorer 50.