[meteorite-list] Exactly 37 Years after Its Discovery, Pluto's Moon Charon Is Being Revealed

[Ron Baalke]

http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20150622-2

Exactly 37 Years after Its Discovery, Pluto's Moon Charon Is Being Revealed
June 22, 2015

[Image]
Charon discovery image.

[Image]
Charon discoverers James Christy (seated) and Robert Harrington in 1978.

In June 1978, U.S. Naval Observatory astronomer James Christy noticed 
something unusual. He was studying highly magnified photos of Pluto, and 
Pluto wasn't round. A small bump marred one side of blurry Pluto.

That bump turned out to be Pluto's largest moon, Charon, whose discovery 
Christy (working with late colleague Robert Harrington), made on June 
22, 1978. Like Pluto in 1930, Charon was found using photographic plates 
taken in Flagstaff, Arizona.

Thirty-seven years later, Charon is about to be revealed by NASA's New 
Horizons mission. As New Horizons draws closer by nearly a million miles 
a day, every observation of it brings new knowledge about this mysterious 
moon - a world far larger than even the largest asteroid, Ceres.

"Even though Pluto and Charon are partners, they are known to be quite 
different in appearance and composition. As New Horizons reveals them 
in far greater detail than ever before possible, we hope to find out why 
that's so," says Alan Stern, New Horizons principal investigator from 
Southwest Research Institute, Boulder, Colorado.

At 750 miles in diameter, Charon is half Pluto's size. However, it weighs 
only 12 percent as much as Pluto. This suggests that Charon may be half 
ice and half rock; Pluto, by contrast, if about 70% rock by mass. The 
pair form what planetary scientist call the only known binary planet in 
the solar system.

Pluto and Charon are tidally locked, circling their common center of gravity 
once every 6.4 days. As a result, an astronaut on Pluto's surface would 
always see Charon in the same part of the sky, but appearing seven times 
larger than Earth's moon, spanning 3.5 degrees on the sky.

Charon and Earth's moon are believed to share a similarity in that both 
are thought to have been born out of giant impacts early in the solar 
system's history. In the case of Pluto and Charon, it may have been more 
of a grazing impact that left both objects largely intact but may have 
also formed Pluto's retinue of at least four other small moons. Additional 
moons, or even dust rings, may await discovery by New Horizons.

Ground-based observations have shown that while Pluto's surface is covered 
with frozen nitrogen and methane, Charon appears to be primarily covered 
in water ice. Charon could even be dotted by icy volcanoes bubbling a 
slushy mixture of water and ammonia, and it may have an atmosphere, perhaps 
siphoned off Pluto.

"Every day brings us closer to seeing Charon not as a fuzzy point of light, 
but as a fully mapped, and maybe even geologically active, world by this 
July," says New Horizons Project Scientist Hal Weaver, of the Johns Hopkins 
University Applied Physics Laboratory, Laurel, Maryland. "We aim to find 
out."