Originally Posted by
Dreamwoven
Notes for Society for Popular Astronomy:
Another visitor from the Oort Cloud:
FARTHEST ACTIVE INBOUND COMET YET SEEN
NASA
The Hubble space telescope has photographed the most distant active
inbound comet ever seen, currently beyond the orbit of Saturn.
Slightly warmed by the remote Sun, it has already begun to develop an
80,000-mile-wide coma, enveloping a tiny, solid nucleus of frozen gas
and dust. The observations represent the earliest signs of activity
ever seen from a comet entering the Solar System's planetary zone for
the first time. The comet, called C/2017 K2 (PANSTARRS) or 'K2', has
been travelling for millions of years from its home in the frigid
outer reaches of the Solar System, where the temperature is about
minus 262 degrees Centigrade. The comet's orbit indicates that it
came from the Oort Cloud, a spherical region almost a light-year in
diameter and thought to contain hundreds of billions of comets.
Comets are the icy leftovers from the formation of the Solar System
4.6 billion years ago and therefore pristine in icy composition.
The Hubble observations of K2's coma suggest that sunlight is heating
frozen volatile gases -- such as oxygen, nitrogen, carbon dioxide, and
carbon monoxide -- that coat the comet's frigid surface. Those icy
volatiles lift off from the comet and release dust, forming the coma.
Past studies of the composition of comets near the Sun have revealed
the same mixture of volatile ices. The volatiles are spread all
through K2, and in the beginning, billions of years ago, they were
probably all through every comet presently in the Oort Cloud. But the
volatiles on the surface are the ones that absorb the heat from the
Sun, so, in a sense, the comet is shedding its outer skin. Most
comets are discovered much closer to the Sun, near Jupiter's orbit, so
by the time we see them, the surface volatiles have already been baked
off. That is why astronomers think that K2 is the most primitive
comet ever seen.
K2 was discovered in 2017 May by the Panoramic Survey Telescope and
Rapid Response System (Pan-STARRS) in Hawaii, a survey project of
NASA's Near-Earth Object Observations Program. Hubble revealed the
extent of the coma and also helped to estimate the size of the nucleus
-- less than 12 miles across -- though the tenuous coma is 10 Earth
diameters across. That vast coma must have formed when the comet was
even further away from the Sun. Digging through archival images,
astronomers uncovered views of K2 and its fuzzy coma taken in 2013 by
the Canada-France-Hawaii Telescope (CFHT) in Hawaii. But the object
was then so faint that no one noticed it. It is likely that the comet
has been continuously active for at least four years. In the CFHT
data, K2 had a coma already, when it was at 2 billion miles from the
Sun, between the orbits of Uranus and Neptune. As it approaches the
Sun, it is getting warmer and warmer, and the activity is ramping up.
But curiously, the Hubble images do not show any tail flowing from K2.
The absence of such a feature indicates that the particles lifting off
the comet are too large for radiation pressure from the Sun to sweep
them back into a tail. Astronomers will have plenty of time to
conduct detailed studies of K2. For the next five years, the comet
will continue its journey into the inner Solar System before it
reaches its closest approach to the Sun in 2022 just beyond Mars'
orbit. The James Webb space telescope, an infrared observatory
scheduled to be launched in 2018, could measure the heat from the
nucleus, which would give astronomers a more accurate estimate of its
size.
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