IMP 8 Anniversary

Volume 14, Number 3, September 1998

By Joseph King

NASA's IMP 8 spacecraft was launched 25 years ago from Cape Kennedy, late in the evening of October 25, 1973 (October 26, Universal Time), and continues to provide important in situ cosmic ray, energetic particle, plasma, and magnetic field data.

IMP 8 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 moves about 30 deg in the sky for every 24-hour rotation of the Earth beneath it. (That is, the orbit is near 35 Earth radii [Re] and has a 12+ day period.) In this orbit, IMP is in the solar wind about 7 days/orbit and is within the Earth's magnetosphere/magnetosheath system about 5 days/orbit.

The continuing flow of IMP data is very important in providing

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 also used in those papers. In the early years of the IMP 8 mission, most of those papers involved IMP 8 Principal Investigators and/or their immediate colleagues. However, for approximately the second half of IMP 8's life, the dominant contributor to the IMP 8 bibliographic count has been papers by scientists obtaining their IMP data from NSSDC or directly from IMP PI teams.

Among the most significant findings of the past quarter century enabled solely or in significant measure by IMP 8 have been

  1. Galactic cosmic ray modulation has become much better understood (e.g., the major role of charge-dependent gradient and curvature drifts) by studying the temporal variations of near-Earth fluxes and of radial and latitudinal gradients (determined by comparing distant-heliosphere fluxes from the Voyager and Pioneer missions, and high-heliolatitude fluxes from Ulysses, with the long in-ecliptic, 1 AU baseline IMP 8 data set).
  2. Analysis of beryllium isotopes in cosmic ray fluxes at earth, especially the radioactive 10Be isotope, reveals the mean lifetime of cosmic rays in the galaxy to be ten-twenty million years, thus constraining models of cosmic ray origin and propagation.
  3. Solar wind structures known as magnetic clouds and their embedded magnetic field topologies were much better understood after their successful modelling as magnetic force free configurations.
  4. Antisunward flowing plasma volumes (plasmoids) have been ascertained as features of magnetospheric substorms from IMP data in the Earth's magnetotail used in conjunction with data from spacecraft elsewhere in the magnetosphere.

The IMP 8 spacecraft carried investigations from the following groups into space:

Cosmic rays and energetic particles:


Fields and Waves

The IMP 8 spacecraft was built, and has been operated over its lifetime, at NASA's Goddard Space Flight Center. IMP 8 was the last of the series of IMP spacecraft which included 8 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 second of the AIMP's achieved lunar orbit; the first did not but provided valuable data between 1966 and 1971 from high Earth orbit.) A great many Goddard luminaries were associated with this IMP/AIMP series over the years, Frank McDonald, Norman Ness (pre-launch and launch-phase Project Scientist for IMP 8), and Paul Butler to name just a few of many.

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. In its pre-launch life, IMP 8 was known as IMP J. The basic data flow established for IMP 8 in the early 1970's was that each of many sites of Goddard's worldwide Spacecraft Tracking and Data Network (STDN) would use its VHF telemetry capture antennas/electronics to create the 1.6 kbps convolutionally encoded telemetry onto analog tapes to be mailed to Goddard, where data from these stations would be interspersed, digitized, and decommutated by investigation. Finally, with delays of about 2 months from in-space acquisition, digital data tapes would be mailed to PI teams at their university or other sites for further processing, analysis, and archive preparation.

In recent years, data digitization has been accomplished at IMP's telemetry capture stations. Digital data have been transmitted to Goddard from the stations by electronic network, and decommutated data have been transmitted from Goddard to the PI teams by network.

It has been satisfyting 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 obsoleted VHF telemetry frequencies. (IMP is not shuttle-accessible, as is the Hubble Space Telescope, whereby onboard technologies can be swapped out by new ones.) The GSFC STDN 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.

Anchoring this network over the years has been the GSFC facility at Wallops Island. The other two key stations have been, for about 10 years, an ESA (European Space Agency) station at Redu, Belgium, and, for almost a year now, an IMP-dedicated facility on the grounds of NASA's Deep Space Network facility at Canberra Australia. The latter replaced a station that had been operational at McMurdo Sound, Antarctica, for about three years as the result of NASA-NSF collaborations. Many Goddard people and others have contributed to the definition and evolution of this ad hoc IMP ground network. Deserving special recognition is Mike Comberiate who recognized the McMurdo potential upon informal discussions with this author (and IMP 8 Project Scientist) in the cooling down phase of a Goddard "fun run" some years ago.

The IMP 8 home page on the WWW is maintained by NSSDC at It links to IMP 8 network-accessible data at NSSDC and at PI sites and to much IMP positional and other descriptive data.

It is a coincidence of fate that the IMP 8 spacecraft, which in its later years has come to be valued, perhaps more than any other NASA space science mission, primarily as the source of uniquely important data for use by a broad scientific constituency as correlative to other missions and data sources, has as its Project Scientist since months after launch the person also charged as NSSDC Head with ensuring the archiving and public availability of data from all or most of NASA space science missions.

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Editor:Miranda Beall
Programmer: Erin Gardner
Responsible Official: Dr. Joseph H. King, Code 633
Last Revised: 12 Feb 1999 [DRW]
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