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July 11, 2012

Hubble Discovers a Fifth Moon Orbiting Pluto

A team of astronomers using NASA’s Hubble Space Telescope is reporting the discovery of another moon orbiting the icy dwarf planet Pluto.

The moon is estimated to be irregular in shape and 6 to 15 miles across. It is in a 58,000-mile-diameter circular orbit around Pluto that is assumed to be co-planar with the other satellites in the system.


This image, taken by NASA's Hubble Space Telescope, shows five moons orbiting the distant, icy dwarf planet Pluto. The green circle marks the newly discovered moon, designated P5, as photographed by Hubble's Wide Field Camera 3 on July 7. The observations will help scientists in their planning for the July 2015 flyby of Pluto by NASA's New Horizons spacecraft. P4 was uncovered in Hubble imagery in 2011. (Credit: NASA; ESA; M. Showalter, SETI Institute)



“The moons form a series of neatly nested orbits, a bit like Russian dolls,” said team lead Mark Showalter of the SETI Institute in Mountain View, Calif.

The discovery increases the number of known moons orbiting Pluto to five.

The Pluto team is intrigued that such a small planet can have such a complex collection of satellites. The new discovery provides additional clues for unraveling how the Pluto system formed and evolved. The favored theory is that all the moons are relics of a collision between Pluto and another large Kuiper belt object billions of years ago.

The new detection will help scientists navigate NASA’s New Horizons spacecraft through the Pluto system in 2015, when it makes an historic and long-awaited high-speed flyby of the distant world.

The team is using Hubble’s powerful vision to scour the Pluto system to uncover potential hazards to the New Horizons spacecraft. Moving past the dwarf planet at a speed of 30,000 miles per hour, New Horizons could be destroyed in a collision with even a BB-shot-size piece of orbital debris.

“The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system,” said Harold Weaver of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

“The inventory of the Pluto system we're taking now with Hubble will help the New Horizons team design a safer trajectory for the spacecraft,” added Alan Stern of the Southwest Research Institute in Boulder, Colo., the mission’s principal investigator.

Pluto’s largest moon, Charon, was discovered in 1978 in observations made at the United States Naval Observatory in Washington, D.C. Hubble observations in 2006 uncovered two additional small moons, Nix and Hydra. In 2011 another moon, P4, was found in Hubble data.

In the years following the New Horizons Pluto flyby, astronomers plan to use the infrared vision of Hubble’s planned successor, NASA’s James Webb Space Telescope, for follow-up observations. The Webb telescope will be able to measure the surface chemistry of Pluto, its moons, and many other bodies that lie in the distant Kuiper Belt along with Pluto.

Other Moons of Pluto

Charon's diameter is about 1,207 kilometres (750 mi), just over half that of Pluto, with a surface area of 4,580,000 square kilometres (1,770,000 sq mi).

Two moons discovered in 2006. If the moons' albedo is similar to Charon's at 35%, then their diameters can be estimated at 46 kilometres for Nix and 61 kilometres for the brighter Hydra.

On July 20, 2011 Mark Showalter of the SETI Institute announced the discovery of a fourth moon of Pluto, provisionally named S/2011 P 1 or P4. It was noticed by NASA's Hubble Space Telescope during a survey searching for rings around the dwarf planet. It has an estimated diameter of 13 to 34 km and is located between the orbits of Nix and Hydra.

New Horizons was launched on January 19, 2006, directly into an Earth-and-solar-escape trajectory with an Earth-relative velocity of about 16.26 km/s (58,536 km/h; 36,373 mph) after its last engine was shut down. Thus, the spacecraft left Earth at the greatest-ever launch speed for a man-made object. It flew by the orbit of Mars on April 7, 2006, the orbit of Jupiter on February 28, 2007, the orbit of Saturn on June 8, 2008; and the orbit of Uranus on March 18, 2011. As of February 2012, its distance to Pluto is less than 10 AU (more than 20 AU from Earth).

New Horizon spacecraft


Moons of the Planets

There are now 177 known natural satellites in the solar system

Jupiter has 66 confirmed moons

Saturn has 62 confirmed moons

Uranus has 27 confirmed moons

Neptune has 13 known moons

Mars has two moons.
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