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.”

“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.”