Cosmic time capsule
Hale-Bopp offers rare glimpse of universe's origin
(CNN) -- Comets like Hale-Bopp appear to come from nowhere:
dirty snowballs with tails millions of miles long, blowing
gases and interplanetary dust.
These travelers were born on the frigid outskirts of our
solar system at the same time as the Big Bang, 10 to 20 billion
years ago.
And they've hardly changed since the time of their formation,
says Hal Weaver, an astronomer at Johns Hopkins University.
"So we think when we look at comets today we're essentially
looking backwards in time and seeing the physical and
chemical conditions that existed at the time of the formation
of the solar system," Weaver says.
Scientists think many comets that were formed on the edges of
the universe are still there, in a giant, spherical mass
called the Oort cloud.
Every now and then a comet is jogged loose, then falls into
our solar system and into an orbit around the sun.
At that point, the comet forms a tail. The sun's heat
releases gases in the comet, creating a cloud around its
core. Particles hurled out by the sun, called solar winds,
distort the cloud to create the comet's tail.
Comet Hale-Bopp is now on a 2,000- to 3,000-year trek around
our sun.
Another great comet, Halley's, comes around every 76 years.
Halley's put on an impressive show in 1910; its last visit in
1986 was less spectacular. But spacecraft flew through the
comet, sampled its gases and dust, and proved that it was,
indeed, a dirty snowball. (1.5M/46 sec. QuickTime movie) 
In 1973, there was the comet the ran out of gas, Kahoutek.
"It didn't have as much gas as they thought it would," says
Kelly Beatty, senior editor of Sky & Telescope Magazine. "So
it didn't have the activity and throw off all the dust to
make it bright and obvious in the sky."
Another great comet, Hyakutake, made a quick appearance last
spring, its tail a faint blue.
Comet Hale-Bopp will pass about 120 million miles away,
there's no chance of it hitting the Earth. But because it's
so big and bright, and was first spotted a year and half ago,
scientists have a rare opportunity to take a long look at
this churning time capsule.
For the first time, scientists are observing how gases are
burning off the core, and with infrared and radio telescopes,
they hope to find molecules never before seen in a comet.
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