After several days of continued observations, scientists continue to
work to determine and to understand the fate of Comet ISON: There's no
doubt that the comet shrank in size considerably as it rounded the sun
and there's no doubt that something made it out on the other side to
shoot back into space. The question remains as to whether the bright
spot seen moving away from the sun was simply debris, or whether a small
nucleus of the original ball of ice was still there. Regardless, it is
likely that it is now only dust.
Comet ISON, which began its journey from the Oort Cloud some 3
million years ago, made its closest approach to the sun on Nov. 28,
2013. The comet was visible in instruments on NASA's Solar Terrestrial
Relations Observatory, or STEREO, and the joint European Space
Agency/NASA Solar and Heliospheric Observatory, or SOHO, via images
called coronagraphs. Coronagraphs block out the sun and a considerable
distance around it, in order to better observe the dim structures in the
sun's atmosphere, the corona. As such, there was a period of several
hours when the comet was obscured in these images, blocked from view
along with the sun. During this period of time, NASA's Solar Dynamics
Observatory could not see the comet, leading many scientists to surmise
that the comet had disintegrated completely. However, something did
reappear in SOHO and STEREO coronagraphs some time later – though it was
significantly less bright.
Whether that spot of light was merely a cloud of dust that once was a
comet, or if it still had a nucleus – a small ball of its original, icy
material – intact, is still unclear. It seems likely that as of Dec. 1,
there was no nucleus left. By monitoring its changes in brightness over
time, scientists can estimate whether there's a nucleus or not, but our
best chance at knowing for sure will be if the Hubble Space Telescope
makes observations later in December 2013.
Regardless of its fate, Comet ISON did not disappoint researchers.
Over the last year, observatories around the world and in space gathered
one of the largest sets of comet observations of all time, which should
provide fodder for study for years to come. The number of space-based,
ground-based, and amateur observations were unprecedented, with twelve
NASA space-based assets observing over the past year.
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