By Marty McDowell/NASA
The International Sun-Earth Explorer 3 (ISEE-3) spacecraft was one of
three spacecraft (ISEE 1, 2 & 3) whose purpose was to study the solar
wind and the solar-Earth relationship at the boundaries of the Earth's
magnetosphere. In 1981, it was proposed that ISEE-3 be rerouted towards a
comet. To that point, the mission was not intended to study comets. After
a series maneuvers and flybys of the moon, ISEE-3 encountered Comet
Giacobini-Zinner in 1985 and provide distance observations of Comet
Halley in 1986.
Launched on August 12, 1978, ISEE-3's first mission was to use
instruments for making measurements of plasmas, energetic particles,
waves, and fields to measure the solar wind. On June 10, 1982 the first of
maneuvers necessary to move the spacecraft in position to meet the comet
began which moved the spacecraft out of its halo orbit where it had been
for nearly four years. Fifteen complicated maneuvers that would make the
average teenage Atari Asteroids champion envious were required (this
included five lunar flybys!). The fifth and final lunar flyby on December
22, 1983, passed a mere 119 kilometers above the Moon's surface not far
from the Apollo 11 landing site. The craft and mission was renamed ICE
(International Cometary Explorer) to reflect the new mission goals.
By June 5, 1985, the spacecraft had been maneuvered to within 27,000
kilometers behind Comet Giacobini-Zinner. This allowed the craft's fields
and particles instruments to sample the comet's tail. For the first time
ever, measurements of a comet from the comet were made as the first ions
were detect on September 11 (which is also the date of its closest
approach - a mere 7,862 kilometers). This craft did not include a camera
among its instruments, so the first
close-up pictures of a comet would not appear for another year. The
spacecraft found a region of interacting cometary and solar wind ions, and
encountered a comet plasma tail about 25,000 km wide. Water and carbon
monoxide ions were also identified, confirming what astronomers had long
confirmed. A comet is a "dirty snowball" - dust and ice.
It Takes a Licking and Keeps on Ticking
The venerable spacecraft did not have any protection against the
high-velocity dust and was expected to suffer significant damage. The
damage was minimal as the rate of one dust impacts per second was
significantly below estimates.
The Only U.S. Observations of Halley
In 1986, ICE made distant observations of Comet Halley on the sunward
side of the comet. Because of the cancellation of projects due to the Challenger
tragedy, these observations were the only made by the U.S. from space
of the comet. At its closest approach, it drifted by at a distance of 31M
kilometers from the comet on March 28, 1986. Contrast this with the 9,900
kilometer distance of the Soviet Vega 1 craft, and you get a picture of
how out of the running the Americans were in the pursuit of Comet Halley.
Lassie, Come Home
In the year 2014, the ICE spacecraft will be close enough to Earth that
it might be captured. This would allow scientists and engineers to analyze
its exterior for dust impacts. Who knows? You might even be able to bid on
it on eBay. (Actually, NASA has already committed to donate the spacecraft
to the Smithsonian Institute for display should it be recovered.)
Share Your Memories!
What do you remember about ISEE-3/ICE? Have you any compelling stories to share? Share your stories with the world! (We print the best stories right here!)
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The ISEE-3/ICE spacecraft.
Courtesy of NASA
Launched: August 12, 1978
Destination: Comet Giacobini-Zinner
Arrival: September 1, 1985
Mission: Enter tail of comet and make observations.