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NASA’s Hubble Space Telescope
has observed an inbound comet

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unlike any we’ve seen before,
that already is expelling gas

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and dust at an enormous distance
from the Sun. This makes the

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comet a record-breaker, since
usually active comets aren’t

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discovered until they’re closer
to the Sun and warmer. When

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trying to learn about our early
solar system and the exact

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conditions of the materials that
went on to form our Sun and

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planets, scientists try to find
the most primitive objects they

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can – objects that haven’t been
disturbed by geologic activity,

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strong radiation, or outside
forces at any point in the past

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4.6 billion years. These
primitive objects include

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asteroids and comets. Comets are
more icy than asteroids, and

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when they get close to the Sun,
the ices sublimate - go from

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solid to gas - and release dust
that forms the comet’s iconic

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coma and tail. We call this an
active comet. Once a comet has

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orbited close to the Sun
multiple times, it’s no longer

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as primitive as it once was. But
every once in a while, we catch

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a comet coming into the inner
solar system for the first time,

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kicked out of its home in the
outer solar system. These

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first-time
inner-solar-system-visitors give

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us a chance to observe a more
pristine leftover from the early

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days of planet formation. A
comet named K2 was discovered in

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May 2017 by the Pan-STARRS
telescope in Hawaii. Hubble then

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pointed its camera at the icy
visitor in late June, revealing

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this image of its
80.000-mile-wide dust cloud

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coma. Researchers estimate that
the nucleus - the actual solid

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body inside - is less than 12
miles wide, and that the comet

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came from trillions of miles
away from the Oort Cloud at the

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far periphery of our solar
system. Astronomers don’t

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usually discover active
inbound comets until they’re
well within

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the orbit of Jupiter. But comet
K2, at the time of this Hubble

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image, was out past the orbit of
Saturn. After discovery,

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researchers went back through
archival images and found that

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K2’s coma was actually visible
back in 2013, when the comet was

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way out between the orbits of
Uranus and Neptune. So why is

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comet K2 active so early?
Usually astronomers see comets

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that are activated by the
sublimation of water ice, which

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requires relatively warm
temperatures. Researchers think

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that K2 must be so primitive
that it still has frozen

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volatile gases like nitrogen,
carbon dioxide, and carbon

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monoxide, which sublimate at
much cooler temperatures and are

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sublimating right now on K2. The
lead researcher on this study

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thinks that K2 is the most
primitive comet we’ve ever seen.

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However, compositionally, K2
probably isn’t a particularly

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unusual comet; it’s just been
difficult to discover comets

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that far away. Even though its
coma is almost as large as

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Jupiter, K2 is still about 40
times fainter than Pluto, and

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discovering a moving object that
faint requires improved

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technology. Survey programs like
Pan-STARRS should allow us to

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discover more and more of
these faint, distant comets like
K2. Since

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we are able to see comet K2 so
early, we’ll have another five

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years to study the comet before
it reaches its closest approach

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to the Sun, just beyond the
orbit of Mars. During that time,

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scientists will be able to study
this visitor from the remote

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past with ground-based
telescopes, Hubble, and the

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soon-to-be-launched James Webb
Space Telescope. Once K2 swings

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by the Sun, it will begin its
outward journey, and K2’s

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trajectory will actually have it
leave our solar system forever.

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Humanity will never see this
particular comet again.

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www.nasa.gov/hubble
@NASAHubble