NASA’s Hubble Space Telescope has observed an inbound comet
unlike any we’ve seen before, that already is expelling gas
and dust at an enormous distance from the Sun. This makes the
comet a record-breaker, since usually active comets aren’t
discovered until they’re closer to the Sun and warmer. When
trying to learn about our early solar system and the exact
conditions of the materials that went on to form our Sun and
planets, scientists try to find the most primitive objects they
can – objects that haven’t been disturbed by geologic activity,
strong radiation, or outside forces at any point in the past
4.6 billion years. These primitive objects include
asteroids and comets. Comets are more icy than asteroids, and
when they get close to the Sun, the ices sublimate - go from
solid to gas - and release dust that forms the comet’s iconic
coma and tail. We call this an active comet. Once a comet has
orbited close to the Sun multiple times, it’s no longer
as primitive as it once was. But every once in a while, we catch
a comet coming into the inner solar system for the first time,
kicked out of its home in the outer solar system. These
first-time inner-solar-system-visitors give
us a chance to observe a more pristine leftover from the early
days of planet formation. A comet named K2 was discovered in
May 2017 by the Pan-STARRS telescope in Hawaii. Hubble then
pointed its camera at the icy visitor in late June, revealing
this image of its 80,000-mile-wide dust cloud
coma. Researchers estimate that the nucleus - the actual solid
body inside - is less than 12 miles wide, and that the comet
came from trillions of miles away from the Oort Cloud at the
far periphery of our solar system. Astronomers don’t
usually discover active inbound comets until they’re well within
the orbit of Jupiter. But comet K2, at the time of this Hubble
image, was out past the orbit of Saturn. After discovery,
researchers went back through archival images and found that
K2’s coma was actually visible back in 2013, when the comet was
way out between the orbits of Uranus and Neptune. So why is
comet K2 active so early? Usually astronomers see comets
that are activated by the sublimation of water ice, which
requires relatively warm temperatures. Researchers think
that K2 must be so primitive that it still has frozen
volatile gases like nitrogen, carbon dioxide, and carbon
monoxide, which sublimate at much cooler temperatures and are
sublimating right now on K2. The lead researcher on this study
thinks that K2 is the most primitive comet we’ve ever seen.
However, compositionally, K2 probably isn’t a particularly
unusual comet; it’s just been difficult to discover comets
that far away. Even though its coma is almost as large as
Jupiter, K2 is still about 40 times fainter than Pluto, and
discovering a moving object that faint requires improved
technology. Survey programs like Pan-STARRS should allow us to
discover more and more of these faint, distant comets like K2. Since
we are able to see comet K2 so early, we’ll have another five
years to study the comet before it reaches its closest approach
to the Sun, just beyond the orbit of Mars. During that time,
scientists will be able to study this visitor from the remote
past with ground-based telescopes, Hubble, and the
soon-to-be-launched James Webb Space Telescope. Once K2 swings
by the Sun, it will begin its outward journey, and K2’s
trajectory will actually have it leave our solar system forever.
Humanity will never see this particular comet again.
www.nasa.gov/hubble @NASAHubble