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(meteorobs) [ASTRO] Going Comet Wild
------- Forwarded Message
From: Ron Baalke <BAALKE@KELVIN.JPL.NASAdot gov>
Date: Tue, 29 Feb 2000 19:29:04 GMT
Subject: [ASTRO] Going Comet Wild
http://spacescience.com/headlines/y2000/ast27feb_2.htm
Going Comet Wild
Space Science News
NASA's STARDUST spacecraft, bound for comet Wild-2, celebrated its first
year in space this month.
February 28, 2000: In February 1999, NASA's STARDUST spacecraft blasted off
from the Kennedy Space Center for a daring encounter with periodic comet
Wild-2. Its ambitious goal is to intercept Wild-2 in 2004, to capture tiny
bits of comet dust and debris, and then return them to Earth for analysis in
2006. On the way, STARDUST will also sample a stream of dust particles from
outside the solar system.
STARDUST is the first comet rendezvous mission since the European Giotto
spacecraft's fly-by of Comet Halley (1986) and Comet 26P/Grigg-Skjellerup
(1992), and the first ever to attempt to return a comet sample to Earth.
It's a long 7-year mission, but one most scientists feel is worth the wait.
"I'm Here! I'm OK!"
After one year in space STARDUST is doing well, say project officials. The
spacecraft has executed several flawless course adjustments, and last week
it deployed its aerogel collector for a first-ever sampling of interstellar
dust particles. Still, STARDUST has given mission controllers their share of
sleepless nights.
"There have been storms to sail through," recounts STARDUST project manager
Dr. Kenneth Atkins in a recent commemorative essay. "The first attempt to
move from gyro-stabilized control to pure star-referencing found a software
bug that caused the spacecraft [to go into safe mode]. When the ship invokes
this routine, it shuts down all unnecessary activities, including
communications with Earth, and turns to the Sun to ensure the lifeblood of
solar energy floods its batteries and electronics with electricity. When it
deems all is well, it sets up a plan to contact us on Earth, tell us what
happened, and let us tell it what to do next. This routine, while carefully
designed to protect the spacecraft, is still an 'anxiety event' for the crew
back on Earth. It's a bit like the feeling when your teenager is late coming
home, and you get no phone call. The anxiety builds fear until the dutiful
signal comes through. 'I'm here!' 'I'm O.K.!'"
STARDUST and its crew have successfully navigated three more safe mode
events, all involving data handling by on-board software.
What's all the fuss about Wild-2?
Scientists are curious about comets because they are thought to be the
oldest, most primitive bodies in the solar system. Comets are made up of the
same stuff as the early Solar Nebula that collapsed to form the sun and
planets. It is now known that comets contain significant amounts of water
ice, dust, and carbon based compounds. They may have been an important
source of water and organic molecules for Earth when many comets collided
with our planet during a period of heavy bombardment over 4 billion years
ago. Modern-day comets are like a time machine. They offer a window into the
past when the Solar System was young and life on Earth was just beginning.
History is filled with famous comets. Halley's comet, Hale-Bopp, Hyakutake
and others have dazzled observers with their brilliant nuclei and dramatic
tails. Recent comets like Hale-Bopp have been viewed by hundreds of millions
of people, and Halley's comet has had a real impact on history, as in 1066
when it was so bright that it terrified millions of Europeans and was widely
credited with the Norman victory at the Battle of Hastings.
Unlike its famous cousins, comet Wild-2 is a relatively dim, new arrival to
the inner solar system. Before September 1974, when it passed within 0.006
AU of Jupiter, Wild-2 circled the Sun in an orbit between Jupiter and
Uranus. That encounter with the giant planet, at only 10 times the distance
which fragmented P/Shoemaker-Levy 9 in 1994, altered Wild-2's orbit so that
its closest approach to the Sun now lies just inside the orbit of Mars.
During its first passage by Earth (1.21 AU) on January 6, 1978, the comet
was discovered by Paul Wild. Since then, the best apparition of Wild-2 was
in March 1997 when it passed within 0.85 AU of our planet, brightening to an
unimpressive 10th magnitude. That's too faint to be seen with the naked eye,
but bright enough for modest amateur telescopes.
So, why visit an obscure, hard-to-see object like Wild-2, when there are so
many more notorious comets to choose from? There are two important reasons:
#1 It's fresh. Before its near miss with Jupiter in 1974 comet Wild-2 was
well-preserved in the frigid outer solar system. With its new orbit, Wild-2
now comes much closer to the Sun. When a comet passes close enough to the
Sun, some of its material is boiled off into interplanetary space. After
about a thousand trips past the Sun, it loses most of its volatile materials
and no longer generates a coma or tail. Since Wild-2 has passed the Sun only
a few times, it still has most of its dust and gases - it is "pristine." By
the time STARDUST encounters the comet, Wild-2 will have made only five
trips around the Sun. By contrast, Comet Halley has passed the Sun more than
100 times.
#2 It's in the right place at the right time. Wild-2 presents a unique
opportunity -- it is in the right place at the right time. Scientists have
found a flight path that allows the spacecraft to fly by the comet at a
relatively low speed, only 13,600 mph. Because of this "low velocity" flyby,
comet dust can be captured by collectors on the spacecraft, rather than
blowing right through the collectors and out the back side! This comet dust
can then be brought back to the Earth to be analyzed.
If comet Wild-2 had passed much closer to Jupiter in 1974, it might
have ended up like comet Shoemaker-Levy 9. Comet SL-9 was often referred to
as the "string of pearls" comet. It is famous for its suggestive appearance
as well as its collision with the planet Jupiter! The comet's original
single nucleus was torn to pieces by Jupiter's strong gravity during a close
encounter with the solar system's largest planet in 1992.
After the flyby is done, STARDUST will return to Earth. In 2006 the craft's
aerogel sample collectors will descend by parachute toward the U.S. Air
Force Test and Training range in Utah, about 100 miles southwest of Salt
Lake City in the desert. By the time the mission is over, comet Wild-2 --
dim, obscure, and little-known -- will take its rightful place in the
pantheon of historic comets.
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