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1995-05-24
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Most, if not all comets originate in the outer solar system. Just beyond the orbits of
Neptune and Pluto, there lies the Kuiper belt of comets and cometary debris. Out much
further, perhaps as far as a light-year from the Sun, there is the immense Oort Cloud of
comets surrounding the entire solar system.
Many of the "great comets" throughout history have come from the Oort Cloud, and may
only visit the inner solar system once every several thousand years. Thus, it has been
impossible to predict their orbits, since we have to wait so long in order to observe them
again.
Other comets become captured in smaller, shorter elliptical orbits, mostly due to the
immense gravitational pull of Jupiter. Some of these "short period" comets never venture
much further than Jupiter's orbit, and have revolutionary periods ranging from 3 to 15
years. Halley's comet has the longest period of these short period comets, and visits the
inner solar system every 76 years.
The comet Giacobini-Zinner is an example of a short period comet that originally came
from deep space. This small body is now in a smaller orbit around the Sun, never
venturing much further than the orbit of Jupiter, and coming in very close to the orbit of
Earth. Comets such as these that visit the inner solar system frequently could possibly
pose a threat of collision with the Earth.
In 1985, NASA and JPL did not have the budget to send a space probe to study Halley's
Comet with the armada of other spacecraft. In fact, budget restrictions laid out by the
Reagan administration made sure that America was the only space-faring nation that did
not participate with a space probe in the international Halley's study project, despite the
fact that it would be 76 years until we got another chance.
However, scientists at the Jet Propulsion Laboratory figured out an ingenious method to
use a small spacecraft already deployed to study another comet, called Giacobini-Zinner.
This probe was called the International Sun-Earth Explorer 3. After a series of extremely
delicate maneuvers to bring it near the comet, it was renamed the International Cometary
Explorer (ICE).
Unfortunately, ICE was not originally designed to study comets, and did not carry any
cameras or imaging equipment. Nonetheless, it became the first space probe to "chase" a
comet, and provided some valuable information for the nations pursuing Halley's comet.