National Aeronautics and Space Administration
Cool, Curious Comets
What brings comets to the inner solar
system?
Sometimes the gravitational pull of a passing star
stirs up comets in the Oort Cloud. Some might
Sometimes the gravitational pull of a planet can
headlong toward the Sun.
What are comets? The Sun’s gravitational pull really takes over,
Comets are part of our solar system family. Like shaping the comet’s path into a very lop-sided
Grandma’s mystery meat pie, they are made of orbit. The comet is pulled faster and faster toward
old leftovers after the Sun, the planets, and the the Sun, it swings around close to the back-side,
moons were formed. then it heads out again to more or less where it
came from. Some comets dive right into the Sun,
Can we see them? never to be seen again. When the comet is in the
About once a year a comet comes around that inner solar system, either coming or going, that’s
we may be able to see with our unaided eyes. It when we may spy it in our skies.
might look like a fuzzy cotton-ball—maybe faint,
maybe bright—usually with one or two long tails.
Where do comets come from?
Most comets come from the Kuiper Belt, a region
beyond the orbit of Neptune. Comets from this sun
neighborhood usually take 200 years or less to
make one orbit around the Sun. These comets
are called short-period comets.
Orbit of comet
Two-hundred years sounds like a long trip, but
that’s nothing compared to the trips made by
comets from their other hangout, the Oort Cloud.
The Oort Cloud is a What do comets look like up close?
far-far-distant cloud The nucleus, or solid part, of a comet is usually
of comets that sur- less than about 6 miles across, but may be as big
rounds the solar sys- as about 25 miles across. Recent space missions
Earth
Sun
Planetary tem. Scientists think have given us some close-up views, so we don’t
region
there could be about have to guess what they look like anymore.
Inner
Oort cloud
a trillion comets orbit- The Deep Space 1 mission flew close to comet
Oort cloud ing way out there. Borrelly in 2001. It found rugged terrain, smooth
One trip around the rolling plains, deep fractures and very, very dark
Sun could take one of material. A few years later, space mission Deep
Not to scale these comets 30 mil- Impact flew very close to Comet Tempel 1. That
lion years! That’s why comet also appeared very black on the outside,
they are called long- covered with something like the burned grease
period comets. on a barbecue grill.
www.nasa.gov
What’s inside? directly away from the Sun. And a third tail, the
Comets seem to contain a lot of ice, some rocks sodium tail, we usually don’t see. But what the Sun
and dust, and some gas. Deep Impact crashed a gives, the Sun can take away. In 2007, the STEREO
“smart impactor” into Comet Tempel 1 and studied spacecraft recorded Comet Encke’s entire ion tail
the debris that spewed out. It found that the sur- being ripped right off when the Sun got especially
face of the comet is very fragile and weak. Inside it stormy.
is spongy, with lots of holes. It has ice beneath its
surface. It contains material from outer, middle, and Has a comet ever crashed into Earth?
inner parts of the solar system. Other comets may Yes, indeed. In Earth’s babyhood, comets often
be different. The Stardust-NExT mission is on its hit it. Planetary cruelty? No. Actually, scientists
way to Tempel 1 to find out more. think comets may have contributed water for our
oceans or even molecules from which life eventually
OW! What's the evolved. Some believe it may have been a comet
big idea?! Hey man,
you rear-ended collision that did in the dinosaurs.
ME!
Uh oh...
What makes comets look fuzzy?
When they are at home in the Oort Cloud or Kuiper
Belt minding their own business, comets are just
dull, dark chunks of ice, dust, and rock. In this state,
they may not be much different from asteroids. How many comets have been discovered?
But as comets get closer to the Sun and begin to
warm up, some of their materials start to boil off. Humans have discovered thousands of comets.
This material forms a cloud around the nucleus. Comets used to be named mostly after the people
The cloud is called the coma and may be hundreds who discovered them. Many comets are now
of thousands of miles across. One mission, called named for the observatory or spacecraft used to
Stardust, gathered samples of the coma of Comet discover them.
Wild 2 and returned them to Earth for study. How will scientists learn more?
Why do comets have tails? Several space missions have already been sent to
What do comets and lizards have in common? If chase comets and study them up close. Rosetta is
either of them loses its tail, it can grow another a mission of the European Space Agency. It is now
one! Comet tails appear as the comet approaches on its way to Comet Churyumov-Gerasimenko to
the Sun and can grow to study the comet nucleus. Also, the EPOXI mission
be millions of miles long. will fly by and study Comet Hartley 2 in 2010.
Tiny charged particles
are constantly blasted out
Ion tail More comet fun:
from the Sun. This solar Comet game, Tails of Wonder: spaceplace.nasa.
wind pushes the small gov/en/kids/stardust
dust particles in the coma Dust tail Comet Wordfinds: spaceplace.nasa.gov/en/kids/
into a long curved path. cnsr_wordfind2.shtml
rbit
This tail is known as the Come
t‛s O
What’s inside a comet: spaceplace.nasa.gov/en/
dust tail. Another tail, the kids/deepimpact
ion tail, is made of electri- Ask Dr. Marc: What powers a comet? spaceplace.
cally charged molecules Solar wind
and sunlight
nasa.gov/en/kids/phonedrmarc/2004_january.
of gas. the ion tail points shtml
EB-2009-12-031-JPL
