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IMP 8 (Explorer 50), the last satellite of the IMP series, was a drum-shaped spacecraft, 135.6 cm across and 157.4 cm high, instrumented for interplanetary and magnetotail studies of cosmic rays, energetic solar particles, plasma, and electric and magnetic fields. Its initial orbit was more elliptical than intended, with apogee and perigee distances of about 45 and 25 earth radii. Its eccentricity decreased after launch. Its orbital inclination varied between 0 deg and about 55 deg with a periodicity of several years. The spacecraft spin axis was normal to the ecliptic plane, and the spin rate was 23 rpm. The data telemetry rate was 1600 bps. The spacecraft was in the solar wind for 7 to 8 days of every 12.5 day orbit. Telemetry coverage was 90% in the early years, but only 60-70% through most of the 1980's and early 1990's. Coverage returned to the 90% range in the mid to late 1990's.

The objectives of the extended IMP-8 operations were to provide solar wind parameters as input for magnetospheric studies and as a 1-AU baseline for deep space studies, and to continue solar cycle variation studies with a single set of well-calibrated and understood instruments.

In October, 2001, IMP 8 was terminated as an independent mission. Telemetry acquisition resumed after about three months at Canberra only (30-50% coverage), as an adjunct to the Voyager and Ulysses missions.

The last useful science data from IMP 8 was acquired on 07 October 2006.

Alternate Names

  • 06893
  • Explorer 50
  • IMP 8
  • PL-723A
  • imp8

Facts in Brief

Launch Date: 1973-10-26
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Mass: 371 kg
Nominal Power: 150 W

Funding Agency

  • NASA-Office of Space Science Applications (United States)


  • Space Physics

Additional Information

Questions and comments about this spacecraft can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Mr. Paul J. PashbyProject ManagerNASA Goddard Space Flight Center
Dr. Robert L. CarovillanoProgram ScientistNASA Headquarters
Dr. James B. WillettProgram ManagerNASA Headquarters
Dr. Robert E. McGuireProject ScientistNASA Goddard Space Flight

Other IMP 8 Data/Information at NSSDCA

Retrieve/browse selected IMP 8 datasets (FTPBrowser)
Retrieve selected IMP 8 datasets (FTP)

Daily Resolution Geocentric Position Information
Solar Wind and Magnetosphere Residence Times

IMP-8 Bibliography 1996-2000

Related Data/Information at NSSDCA

Information on other IMP spacecraft
Information on other Explorer spacecraft

Other Sources of IMP 8 Data/Information

Retrieve/browse most recent magnetic field data (CDAWeb)
Retrieve/browse OMNI (hourly solar wind field/plasma) data (OMNIWeb)
Heliocentric Positions of IMP/Earth System
Bow Shock Crossings

IMP 8 Project page

PI Sites

Magnetometer (NASA GSFC)
Magnetic field and plasma (UCLA IGPP)

Solar Plasma Electrostatic Analyzer (LANL)
Faraday cup (MIT)

Electrostatic Energy-Charge Analyzer (EECA) (U. Maryland)
CPME (U. Kansas)
CRNC (U. Chicago)

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