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New Horizons spies Pluto's smaller moons

Kulvinder Singh, News reporter
May 14, 2015, 2:45 UTC

Sen—New images taken by the New Horizons probe, NASA's mission to study Pluto and other objects in the Kuiper Belt, show the faintest two of Pluto's five moons, Kerberos and Styx.

Although these two tiny moons have been seen previously by the Hubble Space Telescope, this is the first time they’ve been spotted by New Horizons. This marks a new phase in the mission as it travels ever-closer to its flyby of Pluto, which is 7.5 billion km from Earth. New Horizons itself is currently 55 million km from Pluto.

The probe can now see all of the known bodies in the Pluto system including the largest moon, Charon—1,200 km in diameter—as well as Nix and Hydra.

This means that any objects New Horizons spots from now on would be new discoveries. As mission team member John Spencer of the Southwest Research institute has said in a statement: “If the spacecraft observes any additional moons as we get closer to Pluto, they will be worlds that no one has ever seen before.”

But learning about the known worlds orbiting Pluto (plus Pluto itself) is also important. No spacecraft has ever previously been there. For example, the sizes of Nix and Hydra are not accurately known. They are thought to be between 46 and 168 km in diameter. Kerberos and Styx are smaller still. They are thought to be between 13 and 34 km in size.

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The image from New Horizons showing Pluto's four smaller moons. Image credit: NASA

New Horizons was launched on Jan. 19, 2006 to study Pluto and recently-discovered bodies in the Kuiper Belt—designated ‘dwarf planets’ by the International Astronomical Union.

Pluto is an icy, rocky body with a mean diameter of 2,322 km. Hubble images show it to be a patchy world covered in hydrocarbons such as methane. But better data is needed, as well as sharper images. That means getting closer to have a look. Scientists will want to know if Kerberos, Styx and the other moons have a similar composition to Pluto.

These latest images of the two smallest moons consist of five ten-second exposures that have been added together to produce the final image. At this level of exposure, the glare from Pluto, Charon and background stars overwhelmed the original image. It had to be processed by the New Horizon’s science team to reveal Kerberos and Styx. In some of the exposures they come out very faint and difficult to see, and had to be tracked using orbital calculations. This is probably due to changes in brightness as they rotate. There’s little doubt that as the probe makes its closest approach to Pluto in mid-July, its images of the faint moons will improve enormously.