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2011 and visible in March 2013. Astronomers estimate
its orbit at more than 100,000 years, so if you missed
it, well, sorry. 12 Sometimes comets go missing: They
may get bumped out of orbit, crash into another
object or simply run out of particles to shed, and ice
to boil off, after a last swing around the sun. 13 Comet
85D/Boethin likely suffered such a fate. Discovered in
1975 and spotted again in 1986, it was expected back in
1997 and 2008. It was a no-show both years, however.
In 2017, astronomers gave it a D for “disappeared.”
14 Then there’s the X factor: Comets for which a reli-
able orbit has not been calculated get an X prefix. This
category consists mostly of historical flybys, likely
from the distant Oort Cloud, that have yet to make a
return visit. 15 According to NASA, more than 3,500
comets have been identified so far, but that’s small
potatoes compared with the billions, possibly trillions
that astronomers believe may be out there. 16 While
researchers have a solid understanding of what com-
ets are, they can still surprise us. In April, for the first
time, astronomers reported in Nature Astronomy that
they’d found comet particles embedded in a meteorite
found in Antarctica. 17 Our solar system is not alone
when it comes to dirty snowballs whizzing around.
Astronomers have found several exocomets — the
most recent in March — circling Beta Pictoris, a young
star that’s 63 light-years from our own. 18 Back on
Earth, a technique called the comet assay, created in
the 1980s, has been an invaluable tool for analyzing
DNA and identifying damaged segments. 19 Formally
known as single-cell gel electrophoresis, the comet
assay suspends cells in gel and, using an electric field,
knocks apart their molecules — kind of like a cue ball
hitting racked balls on a pool table. 20 The traveling
molecules and other fragments separate by size and
charge. Intact DNA tends to stay put, more or less, but
the broken bits of damaged double helix travel further
through the gel, creating a pattern that resembles the
tail of a comet. And that’s no tall tale.
D
Gemma Tarlach is senior editor at Discover.
1 Gotta love those long-hairs. We’re talking comets, of
course. The word derives from kometes, ancient Greek
for “a head with long hair.” 2 While the Greeks began
using the term about 2,500 years ago, astronomers
in both China and Mesopotamia logged appearances
of the long-tailed objects at least 500 years earlier.
3 We don’t know when humans first observed comets,
but in 2018, two researchers claimed that some of
France’s famous Lascaux cave art — created about
17,000 years ago — depicted cometary activity. 4 It
wasn’t until the 17th century that early astronomers
determined comets were celestial objects with
extremely elongated elliptical orbits around our sun.
5 Building on the “dirty snowball” model developed in
the middle of the 20th century, we now know a comet’s
nucleus of ice, rock and other components (such as
dust and frozen gas), ranges from less than a mile to
a few dozen miles in diameter. 6 For most of its cold,
lonely journey, a comet is a tail-less, very dark object,
coated in a layer of dust and other particles. The grime
reduces its albedo, or reflectivity, to the equivalent
of charcoal. 7 As a comet nears the sun and heats
up, its ice begins to melt and bits of released grime
form a coma: a dusty cloud around the nucleus. Solar
winds and other emissions create at least two sepa-
rate tails of dust and gas — the “long hairs” that give
comets their name. 8 Astronomers believe all comets
originate in one of two icy expanses of debris left over
from the formation of our solar system. One of them,
the Kuiper Belt, is a doughnut-shaped
cloud of these scraps that
starts beyond Neptune,
about 3 billion miles from
the sun. 9 Periodic com-
ets, which have orbits of
less than 200 years — such
as the famous 1P/Halley —
typically come from the Kuiper Belt. 10 The
more distant cometary homeland, the Oort Cloud,
is about 18 trillion miles from the sun. Non-periodic
comets originate way out there. Those comets,
denoted with a C, have orbits lasting 200 years or
more — sometimes a lot more. 11 Take C 2011/L4, bet-
ter known as Comet PANSTARRS. Named for the
instrument that detected it, it was discovered only in
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DISCOVERMAGAZINE.COMComets
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BY GEMMA TARLACH
Comet 67P/
Churyumov–
Gerasimenko (above)
originated in the
Kuiper Belt, one
of two cometary
homelands in our
solar system (below).
Comets with orbits of
more than 200 years
hail from the more
distant Oort Cloud.
Both debris fields
are left over from the
system’s formation.
A comet spends most
of its time as a dark lump.
Its bright coma and tails appear
only as the nucleus nears the sun.
1P/Halley
SaturnSun
Jupiter
Uranus
Neptune
OORT CLOUD
Nucleus
To sun
Coma
D
us
t tailG
a
s
t
ail
K
U
IP
ER
BE
LTPluto