RELEASE NO: 98-179

IMP-8 SPACECRAFT CELEBRATES 25 YEARS ON DUTY

The Interplanetary Monitoring Platform (IMP-8) spacecraft continues to make valuable observations a quarter century after its launch on Oct. 25, 1973. IMP-8, built and operated at NASA's Goddard Space Flight Center in Greenbelt, Md., provides important space physics data as part of NASA's Sun-Earth Connections research program.

"IMP-8 is the Cal Ripken of spacecraft. Space is a very harsh environment to work in, with radiation and temperature extremes. Nevertheless, Goddard engineers made IMP-8 so well that it is still active and contributing importantly today," said Dr. Joseph King, project scientist for IMP-8 at Goddard.

Over the past 25 years, more than one thousand 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 endorsed by independent experts chosen by the editor of a scientific journal before they are accepted for publication.

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 miles per second and is known as the 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. IMP-8 data, observations from the 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.

Very high speed subatomic particles, called galactic cosmic rays, bombard Earth from space.

"The same fleet of interplanetary spacecraft that worked with IMP-8 to study the solar wind also used IMP-8 as the anchor point near Earth to determine how solar wind variability and the large scale structure of the interplanetary magnetic field affect the motion and number of cosmic rays able to penetrate to the inner solar system," said King.

Additionally, some of the cosmic rays are radioactive, and can be used as atomic "clocks" to determine when they were created. This is accomplished by comparing their abundance to the amount of their isotopes or to products of their radioactive decay found in cosmic rays.

"IMP-8 analysis of beryllium isotopes in cosmic ray abundances at Earth, especially the radioactive 10Be isotope, revealed the mean lifetime of cosmic rays in the galaxy to be ten to twenty million years, thus constraining models of cosmic ray origin and propagation," said King.

"Like a detective interviewing eyewitnesses, we can assemble a portrait of our galaxy from the clues cosmic rays provide. We know that some cosmic rays collide with thinly distributed material between stars as they travel our galaxy. This can shatter the atomic nuclei of both particles, creating cosmic rays with a different elemental composition. IMP-8 and earlier measurements of the relative composition of cosmic ray elements tells us the amount of material traversed by cosmic rays. This knowledge, when combined with the age determination, requires that cosmic rays spend most of their lives in regions only one third as dense as the galactic plane. This points to the galaxy having a very extended and dilute "halo" of material," said King.

Closer to home, plasma "clouds" in the solar wind occasionally slam into the region of space occupied by the Earth's magnetic field, called the magnetosphere. Processes associated with the physics of plasma trapped in the magnetosphere cause geomagnetic storms and substorms. Intense auroral (northern lights) displays, magnetic field fluctuations, and occasional power system disruptions are associated with these storms. IMP-8 observations, in coordination with a fleet of spacecraft from the International Solar Terrestrial Physics program, enhanced understanding of how the solar wind causes these events.

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. However, IMP has had no choice but continue to use the now largely obsolete VHF telemetry frequencies. (IMP is not Space Shuttle-accessible, as is the Hubble Space Telescope, whereby onboard technologies can be swapped out by new ones.) 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," said King.

The person-sized IMP-8 spacecraft is in a nearly circular orbit about the Earth, at a distance a little more than half way 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, seven 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 IMP's intended for (and achieving) geocentric orbit and two "anchored IMP's" intended for lunar orbit. These ten spacecraft were launched by NASA in the ten year period 1963-1973. 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.

NOTE TO EDITORS: An image of IMP-8 is available on the internet at:  http://www.gsfc.nasa.gov/ftp/newsmedia/IMP8