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NEWS! from the
NAVAL OBSERVATORY

Public Affairs Office
U.S. Naval Observatory
Washington, D.C.

Press Contact:
Geoff Chester, grc{at}usno.navy.mil

For Immediate Release: June 20, 2003

25th Anniversary of the Discovery of Pluto's moon CHARON

On 22 June 1978, an astronomer at the U.S. Naval Observatory in
Washington, D.C. was making routine measurements of photographic
plates taken with the 1.55-meter (61-inch) Kaj Strand Astrometric
Reflector at the USNO Flagstaff Station in Arizona. The purpose of
these images was to refine the orbit of the far-flung planet Pluto to
help compute a better ephemeris for this distant object. 

Astronomer James W. Christy had noticed that a number of the images
of Pluto appeared elongated, but images of background stars on the
same plate did not. Other plates showed the planet as a tiny, round
dot. Christy examined a number of Pluto images from the USNO
archives, and he noticed the elongations again. Furthermore, the
elongations appeared to change position with respect to the stars
over time. After eliminating the possibility that the elongations
were produced by plate defects and background stars, the only
plausible explanation was that they were caused by a previously
unknown moon orbiting Pluto at a distance of about 19,600 kilometers
(12,100 miles) with a period of just over six days.

On 7 July 1978, the discovery was formally announced to the
astronomical community and the world by the IAU Central Bureau for
Astronomical Telegrams via IAU Circular 3241. The discovery received
the provisional designation "1978 P 1"; Christy proposed the name
"Charon", after the mythological ferryman who carried souls across
the river Acheron, one of the five mythical rivers that surrounded
Pluto's underworld. 

Over the course of the next several years, another USNO astronomer,
the late Robert S. Harrington, calculated that Pluto and its
newly-found moon would undergo a series of mutual eclipses and
occultations, beginning in early 1985. On 17 February 1985 the first
successful observation of one of these transits was made at with the
0.9-meter (36-inch) reflector at the University of Texas McDonald
Observatory, within 40 minutes of Harrington's predicted time. The
IAU Circular announcing these confirming observations was issued on
22 February 1985. With this confirmation, the new moon was officially
named Charon. 

Pluto was discovered at Lowell Observatory in 1930 by the late Clyde
W. Tombaugh, an amateur astronomer from Kansas who was hired by the
Observatory specifically to photograph the sky with a special camera
and search for the planet predicted by the Observatory's founder,
Percival Lowell. 

Lowell had deduced the existence of a "Planet X" by studying small
anomalies in the orbits of Uranus and Neptune. As it turned out,
Pluto's discovery was almost entirely serendipitous; Pluto's tiny
mass was far too small to account for the anomalies, which were
resolved when Voyager 2 determined more precise masses for Uranus and
Neptune. 

The discovery of Charon has led to a much better understanding of
just how tiny Pluto is. Its diameter is about 2274 km (1413 miles),
and its mass is 0.25% of the mass of the Earth. Charon has a diameter
of about 1172 kilometers (728 miles) and a mass of about 22% that of
Pluto. The two worlds circle their common center of mass with a
period of 6.387 days and are locked in a "super-synchronous"
rotation: observers on Pluto's surface would always see Charon in the
same part of the sky relative to their local horizon. 

Normally Pluto is considered the most distant world in the solar
system, but during the period from January 1979 until February 1999
it was actually closer to the Sun than Neptune. It has the most
eccentric and inclinced orbit of any of the major planets. This orbit
won't bring Pluto back to its discovery position until the year 2178!

Pluto is in a 2:3 resonance with Neptune; that is, it orbits the Sun
twice in the time it takes Neptune to orbit three times. It is the
largest of a family of objects called "Plutinos", objects about the
size of Charon and smaller, which also have a 2:3 resonance with
Neptune. Plutinos are one of several classes of minor planets that
make up the Kuiper Belt, a region just beyond the orbit of Pluto
thought to be a source of periodic comets. A current plot of these
objects shows their relationship to Pluto and other transneptunian
objects in the outer solar system.

The U. S. Naval Observatory continues to monitor the positions of
Pluto and Charon, as well as those of other bodies in the solar
system as a part of its continuing Mission. Ephemerides for Pluto and
Charon may be found in our annual Astronomical Almanacs and Computer
Almanacs.

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