Researchers Work To Determine Why Comet Sometimes Explodes
When comet Holmes unexpectedly erupted in 2007, professional and
amateur astronomers around the world turned their telescopes toward
the spectacular event. Their quest was to find out why the comet
had suddenly exploded.
Observations taken of the comet after the explosion by NASA's
Spitzer Space Telescope deepen the mystery, showing oddly behaving
streamers in the shell of dust surrounding the nucleus of the
comet. The data also offer a rare look at the material liberated
from within the nucleus, and confirm previous findings from NASA's
Stardust and Deep Impact missions.
"The data we got from Spitzer do not look like anything we
typically see when looking at comets," said Bill Reach of NASA's
Spitzer Science Center at the California Institute of Technology,
Pasadena, CA. Reach is lead investigator of the Spitzer
observations. "The comet Holmes explosion gave us a rare glimpse at
the inside of a comet nucleus." The findings were presented at the
40th meeting of the Division of Planetary Sciences in Ithaca,
N.Y.
Every six years, comet 17P/Holmes speeds away from Jupiter and
heads inward toward the sun, traveling the same route typically
without incident. However, twice in the last 116 years -- in
November 1892 and October 2007 -- comet Holmes exploded as it
approached the asteroid belt, and brightened a millionfold
overnight.
In an attempt to understand these odd occurrences, astronomers
pointed NASA's Spitzer Space Telescope at the comet in November
2007 and March 2008. By using Spitzer's infrared spectrograph
instrument, Reach was able to gain valuable insights into the
composition of Holmes' solid interior. Like a prism spreading
visible-light into a rainbow, the spectrograph breaks up infrared
light from the comet into its component parts, revealing the
fingerprints of various chemicals.
In November of 2007, Reach noticed a lot of fine silicate dust,
or crystallized grains smaller than sand, like crushed gems. He
noted that this particular observation revealed materials similar
to those seen around other comets where grains have been treated
violently, including NASA's Deep Impact mission, which smashed a
projectile into comet Tempel 1; NASA's Stardust mission, which
swept particles from comet Wild 2 into a collector at 13,000 miles
per hour (21,000 kilometers per hour), and the outburst of comet
Hale-Bopp in 1995.
"Comet dust is very sensitive, meaning that the grains are very
easily destroyed, said Reach. "We think the fine silicates are
produced in these violent events by the destruction of larger
particles originating inside the comet nucleus."
When Spitzer observed the same portion of the comet again in
March 2008, the fine-grained silicate dust was gone and only larger
particles were present. "The March observation tells us that there
is a very small window for studying composition of comet dust after
a violent event like comet Holmes' outburst," said Reach.
Comet Holmes not only has unusual dusty components, it also does
not look like a typical comet. According to Jeremie Vaubaillon, a
colleague of Reach's at Caltech, pictures snapped from the ground
shortly after the outburst revealed streamers in the shell of dust
surrounding the comet. Scientists suspect they were produced after
the explosion by fragments escaping the comet's nucleus.
In November 2007, the streamers pointed away from the sun, which
seemed natural because scientists believed that radiation from the
sun was pushing these fragments straight back. However, when
Spitzer imaged the same streamers in March 2008, they were
surprised to find them still pointing in the same direction as five
months before, even though the comet had moved and sunlight was
arriving from a different location. "We have never seen anything
like this in a comet before. The extended shape still needs to be
fully understood," said Vaubaillon.
He notes that the shell surrounding the comet also acts
peculiarly. The shape of the shell did not change as expected from
November 2007 to March 2008. Vaubaillon said this is because the
dust grains seen in March 2008 are relatively large, approximately
one millimeter in size, and thus harder to move.
"If the shell was comprised of smaller dust grains, it would
have changed as the orientation of the sun changes with time," said
Vaubaillon. "This Spitzer image is very unique. No other telescope
has seen comet Holmes in this much detail, five months after the
explosion."
"Like people, all comets are a little different. We've been
studying comets for hundreds of years -- 116 years in the case of
comet Holmes -- but still do not really understand them," said
Reach. "However, with the Spitzer observations and data from other
telescopes, we are getting closer."