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SYFY WIRE Bad Astronomy

New Horizons Has Now Seen All Five Known Moons of Pluto

By Phil Plait

Talk about timing. Just Monday I posted about New Horizons possibly seeing new moons of Pluto when it zips past the tiny world in July, and now a new picture has been released from the space probe showing the four smaller known moons!

Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

In the animation the moons are color-coded; the orbit of Styx is green, Nix yellow, Kerberos orange, and Hydra red. Nix and Hydra were discovered in Hubble images in 2005, Kerberos in 2011, and Styx in 2012. Nix and Hydra were spotted by New Horizons in February 2015, but now all five known moons can be seen by the probe.

In the animation Pluto and Charon are in the center but are so bright their light was subtracted of the images to reduce their glare. Funny—from Earth, Pluto is so faint that you need a decent telescope to see it at all, and Charon so close to it and faint it was only first seen in 1978. But now New Horizons is able to see them so well their brightness needs to be reduced to see fainter objects!

Here’s the original animation showing the original images, then with Pluto and Charon subtracted, and then with the orbits annotated.

Photo by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

These images were taken between April 25 and May 1, when New Horizons was still roughly 100 million kilometers from Pluto. The moons are all too small to be resolved just yet; they’re just dots in the images. For a sense of scale, Nix orbits Pluto about 65,000 km out, Hydra about 50,000.

On July 14, New Horizons will pass a mere 12,000 km from Pluto, right through the bull’s-eye of moon orbits. What will it see? More moons? An atmosphere around Pluto, possibly fed by plumes of outgassing material? Things we haven’t even guessed?

Possibly for the first two. Definitely for the latter. New Horizons is now on the frontier of what’s known about the Pluto system. Things will only get cooler from here.

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