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More Moons Over Pluto?

Posted on: Thursday, 27 February 2003, 06:00 CST

Johns Hopkins University -- Pluto has only one known satellite - Charon - discovered in 1978 by American astronomer James Christy. At slightly more than half the diameter of Pluto, Charon's 1,200-kilometer diameter makes it the undisputed "relative size" king of solar system satellites. In fact, Charon is so big compared to Pluto that the system's center of mass actually falls between the two bodies, making Pluto-Charon the only true binary planet in the solar system.

Recently, astronomers have discovered that some large Kuiper Belt Objects (KBOs) also have large satellites. This, combined with the fact that previous searches for satellites around Pluto could have missed moons as large as hundreds of kilometers across, suggests to the New Horizons mission team that Pluto might have other, as-yet undiscovered satellites.

"Discovering another moon or moons around Pluto-Charon would be exciting in and of itself, and could tell us a lot about the dynamical evolution of this unique, binary system," says New Horizons Principal Investigator Dr. Alan Stern of the Southwest Research Institute's Space Studies Department, Boulder, Colo. "Whether or not we find new moons of Pluto, we expect to learn more about how and where Kuiper Belt satellites form. And, of course, any discoveries of new moons of Pluto would become targets of interest for the New Horizons flyby of the Pluto-Charon system."

Therefore, the New Horizons project is organizing a search for additional satellites of Pluto. "It's something we plan to complete relatively quickly - that is, this year," says Dr. Harold Weaver, New Horizons deputy project scientist from The Johns Hopkins University Applied Physics Laboratory, Laurel, Md. "We will use both ground-based telescopes and, we hope, the Hubble Space Telescope."

Pluto and Charon, image courtesy of NASA.

"With modern tools we should be able to determine if Pluto has any satellites larger than perhaps 10 or 20 kilometers across," Stern adds, "and do so with only a few days of telescope observing time."

"The prospect that Pluto might have one or more, possibly even many satellites lurking about it is intriguing," Weaver says. "It is also important to know just how many satellites are in the system as part of our detailed mission planning activities."

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