Uncovering the Secrets of PlutoSpecial Feature
By NASA’s Amazing Space reporters June 2015
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IMAGE: NASA, ESA, and M. Showalter (SETI Institute)
A P U B L I C A T I O N O F N A S A ’ S “A M A Z I N G S P A C E ” E D U C A T I O N P R O G R A M
WITNESSSTARTHE
National Aeronautics and Space Administration
L ONG BEFORE THE ICY dwarf planet Pluto was discovered on the outskirts
of our solar system, amateur astronomer Percival Lowell began his search for an object he called “Planet X.” It was 1905, and, at the time, our solar system had eight known planets.
Astronomers had found Neptune, the
eighth planet, in 1846 after noticing
that Uranus, the planet’s next-door
neighbor, had a slightly irregular
motion around its orbit. Researchers
calculated that the gravity of Neptune
was tugging on Uranus and creating
its inconsistent motion.
Lowell, along with some astronomers,
extended this idea, and predicted
there was at least one other planet
orbiting far away in our solar system.
The hunt is on
Objects far away in our solar system
are small pinpoints of light and are
This Hubble Space Telescope image shows five moons orbiting the distant, icy dwarf planet Pluto. Pluto and its bright inner moon, Charon, were observed with a short exposure, while the fainter, outer moons required a longer exposure time.
Pluto
Hydra
Nix
StyxKerberos
Charon
hard to see with the best telescopes.
Lowell, however, was ready to give
the search a try. He conducted a
photographic search for Planet X
using a telescope at his observatory
in Arizona. Lowell hunted for the
planet for 11 years before giving up
in 1916. It would later be revealed
that Pluto showed up in two of his
images. The object was overlooked
because it was much dimmer than
anyone expected.
More than 10 years later, a young
amateur astronomer named Clyde
Tombaugh took up the search
at Lowell’s Arizona observatory.
Beginning in 1929, he spent 10
months photographing the sky,
looking for any slight movement
of objects over that time. While
comparing two images taken in
January 1930, Tombaugh found an
object that changed position in the
expected manner. The discovery
of a new planet was officially
announced on March 13, 1930.
Interestingly, Pluto’s size and orbit
did not match the expectations for
Planet X. In fact, Lowell’s predictions
had been incorrect, and it is
Tombaugh’s careful observing that
really deserves credit for
the discovery.
Pluto has a family
Astronomers would have to wait
another 48 years before finding the
first of Pluto’s five known moons.
Charon, the largest and innermost
moon, was discovered in 1978
with ground-based telescopes. The
sharp vision of the Hubble Space
Telescope spotted the other four,
smaller, outer moons. Nix and Hydra
were found in 2005; Kerberos in
2011; and Styx in 2012.
Charon, Pluto’s largest moon, is
746 miles (1,200 kilometers) across.
Nix and Hydra range from 20 to
70 miles (32 to 113 kilometers) in
diameter. Kerberos has an estimated
diameter of 8 to 21 miles (13 to 34
kilometers), and Styx is estimated to be 6 to 15 miles (10 to 24
kilometers) across.
Borrowing names from Greek mythology
The names for Pluto and its
family of moons come from Greek
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mythology. Discovered in 1930,
Pluto was named for the Greek
god of the underworld, who was
perhaps better known as Hades. The
moon, Charon, discovered in 1978,
was named for the ferryman who
carried souls to the underworld.
Nix, one of two moons discovered
in 2006 with Hubble, is named for
Nyx, the goddess of the night and
Charon’s mother. Hydra, the other
moon spied in 2006, is named for
the many-headed, serpent-like
beast of ancient times. Kerberos,
discovered in 2011, was named for
the many-headed dog that guarded
the entrance to the underworld.
Styx, spotted in 2012, is the name
of the goddess who ruled over the
underworld river.
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Pluto is demoted
During the flurry of Hubble
observations of Pluto, the faraway
object lost its planetary status. The
International Astronomical Union
(IAU) voted in 2006 to demote Pluto
from planet to dwarf planet. The
reason for the change is because
Pluto is now known to be part of
a vast collection of objects called
the Kuiper Belt. The Kuiper Belt
is a region beyond Neptune that
contains small, icy objects much
like the asteroid belt contains small,
rocky objects. Pluto is the second-
largest of five currently recognized
dwarf planets. Eris, the largest dwarf
planet, is only slightly bigger
than Pluto.
The discovery of Charon
IMAGE: USNO
In 1978, the U.S. Naval Observatory was making measurements to refine Pluto’s orbit. Astronomer James Christy noticed that a number of the Pluto images appeared elongated. These Pluto images show a normal, roughly circular image on the right, and an elongated one on the left. Upon further study, Christy concluded that the only explanation was a previously unknown moon, now called Charon, with an orbital period of just over six days.
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Pluto’s dismissal from the planetary
ranks is not unusual. During the
1820s, astronomers counted 11
planets. Years later, several of them
were dropped from the planetary
list, including Ceres, now considered
a dwarf planet. Ceres is the largest
object in the asteroid belt.
The dwarf planet is full of surprises
Despite Pluto’s demotion,
astronomers are still interested in
the dwarf planet and its moons.
In fact, recent Hubble observations
of the frigid system have revealed
another surprising discovery. Two
of the moons, Nix and Hydra, are
rotating chaotically as they orbit
Pluto. If you lived on Nix or Hydra,
you would have a hard time setting
your alarm clock. You would not
know when the Sun would rise or
even in which direction it would
rise. You might even have a hard
time figuring out when to celebrate
your birthday.
In preparation for the New Horizons
flyby, Hubble has been making
detailed observations of Pluto and
its neighborhood. Launched in 2006,
the New Horizons spacecraft has
been traveling for nine years to the
far-flung dwarf planet. The satellite
will arrive at the Pluto system on
July 14, 2015, and will take the first
detailed close-up views of the dwarf
planet and its moons. Following that
encounter, the spacecraft may study
other objects in the Kuiper Belt.
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The dwarf planet, Ceres
IMAGE: NASA, ESA, and J. Parker (Southwest Research Inst.)
The dwarf planet Ceres is the largest object in the asteroid belt, and has a diameter about 40 percent that of Pluto. Ceres has a rocky core with an icy outer layer and a dusty surface. This Hubble image is low resolution compared to the high-resolution ones coming from the Dawn mission, which began to orbit Ceres in early 2015. Dawn will map its features and measure its characteristics for about a year.
The New Horizons mission encounters Pluto in July 2015
IMAGE: JHUAPL/SwRI
As depicted in this artist’s drawing, New Horizons will be the first spacecraft to visit the Pluto system. It will be able to study the dwarf planet and its moons in greater detail than ever before. Following that encounter, the spacecraft may be re-directed in order to study another object in the Kuiper Belt.
SEE MORE Hubble images and read more
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