An
asteroid cruising through the solar system six years ago seemed just another
silent ship sailing in the eternal darkness, until it flared up with the
startling brightness of a comet's halo.
Just
like that, the space rock known as NEO 2001 OG108 was re-classified as C/2001
OG108 in 2002, from asteroid to comet. Scientists now suspect that 5 to 10
percent of other Near Earth Objects (NEOs) may also be comets lurking
in disguise as asteroids.
"That
was the first real evidence we have of objects that look like asteroids but are
comets in the NEO population," said Paul Abell, a planetary scientist with
the Planetary Science Institute who is located at NASA's Johnson Space Center
in Houston. Now he's heading a NASA-funded study to sort out which are which.
The
astronomy files
Telling
apart comets and asteroids matters more than just to sticklers. Knowing the
composition of NEOs is crucial to preventing possible collisions with Earth,
especially when a collection of comet pieces bursting in the atmosphere can
have far deadlier consequences than an asteroid. Finding out about the
materials in comets and asteroids also provides hints about the early evolution
of the solar system.
Abell's
research may even uncover future targets for spacecraft to investigate, similar
to the Deep Impact and Stardust
missions. He is working with Faith Vilas, director of the Multiple Mirror
Telescope Observatory at Mount Hopkins, Ariz., to try and identify suspect
objects for sure as comets or asteroids.
The
most mysterious objects don't give up their secrets easily. Far-off comet
bodies resemble dirty snowballs that lack the halo or "coma" they get
once they approach the warmth of the sun. As a result, such objects can appear
"blacker than coal" to telescopes because they reflect just 3 percent
of light that hits them, Abell told SPACE.com.
Fingering
the culprits
The
former NEO 2001 OG108 got as close to us as the orbit of Mars before acquiring
its coma, which kept astronomers guessing up until then. Another object that
continues to arouse controversy is 3200 Phaethon, a suspected asteroid that
some observers believe to be the husk of a burned-out comet.
The
uncertainty goes to the heart of comet evolution. Scientists argue about
whether some comets simply lose their surrounding cloud of dust and gas, or
form hardened shells to contain the loose icy material that makes up comets.
So
far, Abell has set his sights on three comet culprits in the crowd of space
rocks. NASA's Infrared Telescope Facility at Mauna Kea in Hawaii allows his
group to see the chemical composition of different objects, and even find the
unique fingerprints of comets depending on their origins.
Stranger
and stranger
For
instance, some comets come from the Kuiper Belt, a disk-shaped icy cloud past
the orbit of Neptune. Other passing comets such as Halley's
Comet start from much further away in the Oort
Cloud, which lies far beyond Pluto's orbit at 1,000 times the distance from
the sun to the Kuiper Belt.
"There
are some indications that there may be spectral differences between things that
come from the Oort Cloud and things that come from the Kuiper belt," Abell
noted.
The
survey has a long way to go after analyzing just three objects, Abell said, but
that's how science works. You get a result, and then you can start asking
better questions.
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