Kerberos Nix Hydra

If you lived on one of Pluto’s moons, you might have a hard time determining when or from which direction the sun will rise each day. Comprehensive analysis of data from NASA’s Hubble Telescope shows that two of Pluto’s moons, Nix and Hydra, wobble unpredictably.

The moons wobble because they’re in a gravitational field that shifts due to the double planet system of Pluto and Charon, which twirl around each other.

Pluto and Charon are called a double planet system because they share a common center of gravity located in the space between the two. Their variable gravitational field sends the smaller moons tumbling erratically. The football-like, rather than the spherical, shape of the moons enhance the radical tumbling. Scientists believe that Pluto’s other two moons, Kerberos and Styx, share the same characteristic.

Hubble data also reveals that the moon Kerberos is as dark as a charcoal briquette and the other frozen moons are as bright as sand. It was believed that dust blasted off the moons by meteorite impacts would coat all the moons giving their surfaces a homogenous appearance. So the condition of Kerberos is very surprising.

NASA’s New Horizons spacecraft, which will flyby the Pluto system in July, may help settle the question of the asphalt-black moon, as well as the other oddities uncovered by Hubble. These new discoveries are being used to plan science observations for the New Horizons’ flyby.

The turmoil within the Pluto-Charon system offers insights into how planetary bodies orbiting a double star might behave. For example, NASA’s kepler space observatory has found several planetary systems orbiting double stars.

“We are learning chaos may be a common trait of binary systems,” noted Doug Hamilton of the University of Maryland at College Park. “It might even have consequences for life on planets if found in such systems.”

Clues to the Pluto commotion were first revealed when astronomers measured variations in the light reflected off Nix and Hydra. Using images of Pluto taken by Hubble from 2005 to 2012 scientists compared changes in the moons’ brightness to models of spinning bodies in complex gravitational fields.

A collision between a dwarf planet and a similar-sized body early in the history of Earth’s solar system is how scientists believe Pluto’s moons were formed. The collision threw out material that consolidated into the family of moons observed around Pluto today. Its binary companion, Charon, is almost half the size of Pluto and was discovered in 1978. Hubble discovered Nix and Hydra in 2005, Kerberos in 2011, and Styx in 2012. These little moons, measuring just tens of miles in diameter, were found during a Hubble search for objects that could be hazards to the New Horizons spacecraft as it passed the dwarf planet in July.

Researchers say a combination of Hubble data monitoring and New Horizon’s brief close-up look in July, as well as future observations with NASA’s James Webb Space Telescope, will help settle many mysteries of the Pluto system. No ground-based telescopes have yet been able to detect the smallest moons.

The Hubble Space Telescope is a project of International cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope.


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