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

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

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: