Dolores Beasley
Headquarters, Washington                      Oct. 30, 2001
(Phone: 202/358-1753)

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-5017)

RELEASE: 01-213

PLUCKY IMP COMPLETES 28-YEAR OBSERVING MARATHON

     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.
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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.

An image of IMP 8 is available at:  
ftp://nssdc.gsfc.nasa.gov/pub/imgsite/imp_8.jpg
and
http://nssdc.gsfc.nasa.gov/space/imp-8.html
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