In the Feb. 23 issue of the journal Nature, a
team led by Hal Weaver of Johns Hopkins'
Laboratory describes its discovery of two new moons
around Pluto — a finding that made the ninth planet
the first Kuiper Belt object known to have multiple
In a companion paper, discovery team members led by
Alan Stern of the Southwest Research Institute, Boulder,
Colo., conclude that the two small moons were very likely
born in the same giant impact that gave birth to Charon.
They also argue that large binary Kuiper Belt objects like
Pluto-Charon may also have small moons accompanying them
and that Pluto's small moons may generate debris rings that
orbit the planet.
The Kuiper Belt, which has been known since 1992, is a
band of icy, rocky objects and dwarf planets that orbit the
sun in the outer region of our solar system, beyond the
orbit of Neptune. Pluto is its most prominent member.
Using the Hubble Space Telescope's
Advanced Camera for
Surveys, the team first discovered the moons in two
sets of Pluto observations in May 2005; their discovery was
confirmed in new Hubble images taken Feb. 15 and released
"We used Hubble's exceptional resolution to peer close
to Pluto and pick out two small moons that had eluded
detection for more than 75 years," said Weaver, who also
serves as project scientist for NASA's
New Horizons mission,
which is on track to make the first close-up reconnaissance
of the Pluto system in 2015.
Pluto's previously known moon, Charon, was discovered
in 1978, nearly half a century after Pluto's discovery in
1930. With diameters estimated to lie between 35 and 100
miles, the new moons, provisionally designated S/2005 P1
and S/2005 P2, are roughly one-tenth the size of Charon.
They're also about 600 times fainter than Charon and 4,000
times fainter than Pluto and are hidden in the glare of
nearby Pluto and Charon when viewed by ground-based optical
telescopes. The scientists say this is the reason the moons
evaded detection before Hubble looked for them.
The Weaver team writes in Nature that the
satellites were easy to see in the Hubble pictures. "That
was somewhat surprising because ground-based observers had
been trying for more than a decade to find new satellites
around Pluto," said Max Mutchler from the
Space Telescope Science
Institute in Baltimore, the first person to spot the
moons in the May 2005 images. "But I felt almost certain
even when I first saw them that they were real objects
— not any sort of artifact — and that they were
exhibiting orbital motion around Pluto."
That orbital motion — inferred from the
different locations of the moons in pictures taken May 15
and May 18 — is what convinced scientists that they
were indeed looking at moons and not stray light, cosmic
rays or other Kuiper Belt objects that happened to be
"If we assumed the orbits were circular and in the
same orbit plane as Charon, we could predict the exact
positions of the objects on the second day," said William
Merline, a co-author and discovery team member from
Southwest Research Institute, or SwRI "When the objects on
the second day appeared almost exactly where we predicted,
we were convinced. No two artifacts could follow the rules
of orbital physics that 'real' objects must obey."
"The presence of the new moons in orbits with so many
similarities to Charon's sheds light on the formation and
evolution of the Pluto system, as well as on the process by
which satellites are formed in the Kuiper Belt," said
SwRI's Stern, who is principal investigator of the New
The new moons will be important targets of New
Horizons, which was launched Jan. 19 to provide the first
detailed reconnaissance of Pluto and the Kuiper Belt. The
New Horizons spacecraft will fly within several thousand
miles of Pluto and its moons in July 2015.
Weaver says the APL-built Long Range Reconnaissance
Imager telescopic camera on New Horizons should be able to
probe the new moons and resolve surface features down to
600 yards wide. These observations build on primary mission
science plans to characterize the global geology and
geomorphology of Pluto and Charon, map their surface
compositions and temperatures, and examine Pluto's
atmospheric composition and structure. New Horizons also
will map the two smaller satellites in color and in black
and white, and map their surface compositions and
"We're getting four fascinating targets for the price
of two," Weaver said. "The opportunity to explore the
'bookends' of Kuiper Belt object size distribution, with
Pluto and Charon at one end and P1 and P2 at the other, is
an unexpected treat."
The team is already analyzing the new Hubble images,
which confirm the results published in the Nature paper and
provide the most detailed view yet of this fascinating mini
solar system. Hubble is scheduled to take another set of
Pluto images in early March.
"The more we learn about the orbits and physical
properties of P1 and P2, the better we can fine-tune our
spacecraft investigation and focus on the objectives that
are impossible to achieve from Earth-based observations,"
The Hubble Pluto companion search team also includes
Marc Buie, of Lowell Observatory, Flagstaff, Ariz.; and
John Spencer, Eliot Young, Leslie Young and Andrew Steffl,
all of SwRI. New Horizons is the first mission in NASA's
New Frontiers Program of medium-class spacecraft
exploration projects. Stern leads the mission and science
team as principal investigator. APL manages the mission for
NASA's Science Mission Directorate and is operating the
spacecraft in flight.
Related Web sites
Hubble/Pluto System images
NASA's New Horizons mission
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