Orcus (2004 DW)
Orcus (2004 DW) |
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Greg Bacon, STScI, NASA --
larger illustration and
discovery images
(Like Quaoar, Orcus is one of the largest known EKOs
-- more).
Breaking News
According to astronomer Mike Brown's blog entry for December 31, 2007, Orcus has a tiny moon. Orcus may be a good example of a large planetary body with a lot of ice that did not have a giant collision like Quaoar. Orcus (which is about the same size as Quaoar or around half the size of Pluto) has a moon like Quaoar. Observations with the Hubble Space Telescope in late 2006 have been used to track the orbit of Orcus' satellite and to determine that Orcus is much icier than anticipated. While this moon could have been formed in a giant collision, the satellite is somewhat larger than the recently discovered moons of other large planetary bodies beyond Neptune's orbit, which look like collisional remnants. Hence, it is more probable that Orcus never experienced a giant impact and that its moon is simply another planetary body captured from Solar space. Brown hopes to better determine whether Orcus' moon was created from a collision or captured from space by looking more closely at the composition of its surface to see whether it is very different in surface composition from collisionally formed satellites, by analyzing the spectrum of sunlight after it has reflected off the surface of the satellite with the Keck telescope at the end of March 2008 (more).
Orcus
On February 17, 2004, a team of astronomers
(Chad Trujillo,
Mike Brown, and
David
Rabinowitz, who include discoverers of Quaoar) announced the
discovery of an Edgeworth-Kuiper Object (EKO) that is similar to
Quaoar in size, which was given the provisional designation of
2004 DW.
Based on its current distance of around 48 AUs from Sol,
brightness, and presumed albedo or ability to reflect light of
nine percent, 2004 DW was initially estimated to have a larger
diameter than Quaoar at around 990 miles
(1,600 kilometers), but an
August
2006 table posted by Mike Brown (a member of the discovery
team) now lists a diameter of around 680 miles (1,100 kilometers),
or just under half the size of Pluto. On August 24, 2006, the
International Astronomical Union
(IAU) voted to establish a new category of Solar System objects
called "dwarf planets." Orcus (2004 DW)
is large enough to have a spherical shape and so is a candidate
for IAU designation as a dwarf planet.
2004 DW has an orbit similar to Pluto's. Hence, it has been classified
as "Plutino" -- an EKO in an orbit that goes around the Sun
twice for every three times that Neptune orbits around the
Sun. As a result, the IAU approved the permanent name "Orcus"
for this object, which comes a Roman god of the dead and is
another name for the Greek deity of the dead Hades.
Based on preliminary orbital characteristics determined
using images of the object traced back to a First Palomar
Sky Survey photograph of November 23, 1954 and to a
November 8, 1951 photograph by Australia's Siding Spring
Observatory, Orcus' orbital distance from the Sol varies
between 30.9 and 48.1 AUs with an orbital inclination of
about 20.6°, which is larger even than the orbital
inclination of Pluto. It takes 248 years to complete its
orbit. Having reached its farthest point from the Sun in
1989, Orcus is now moving inbound towards Sol, reaching
perihelion in 2113
(more
details, an orbital illustration, and an animated image
are available from astronomer
Chad Trujillo).
Martin Kornmesser, IAU
Larger illustration.
Orcus is a candidate
for dwarf planet
status like many
other EKOs, smaller
asteroids, and Sedna,
a member of the inner
Oort Cloud
(more).
Chad Trujillo,
Mike Brown,
and
David
Rabinowitz
Larger illustration.
Like Pluto (black ellipse),
Orcus (red ellipse) has a
2:3 resonance orbit with
Neptune
(more).
Other Information
Astronomer Chad Trujillo's discovery team maintains an Orcus (2004 DW) FAQ page.
More information and images of Pluto, Charon, and the Edgeworth-Kuiper Belt are available at NASA's Pluto-Kuiper Belt Mission and Planetary Photojournal. Fact sheets on Pluto and the Centaur object Chiron are also available from NASA's National Space Science Data Center.
David C. Hewitt has collected more images and information on Edgeworth-Kuiper objects at his Kuiper Belt Page.
David Seal (a mission planner and engineer at NASA's Jet Propulsion Laboratory at CalTech) has a web site that generates simulated images of the Sun, planets, and major moons from different perspectives and at different times of the year. Try his Solar System Simulator.
For more information about the Solar System, go to William A. Arnett's website on "The Nine Planets", or to Calvin J. Hamilton's web pages on "Kuiper Belt Objects" and "Pluto."
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