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Emily LakdawallaFebruary 18, 2015

New Horizons spots Nix and Hydra circling Pluto and Charon

A series of images just sent to Earth from New Horizons clearly shows Pluto's moons Nix and Hydra orbiting the Pluto-Charon binary. These were not easy observations to make; the moons are tiny and dark, reflecting few photons to New Horizons' LORRI camera. New Horizons is still at a distance where it operates more like an astronomy mission than a geology mission, using long exposures, stacking images, and binning data to detect the faint signals of the little moons from among the background noise. But the moons are unmistakably there:

New Horizons sights Nix and Hydra (animation)

NASA / JHUAPL / SwRI

New Horizons sights Nix and Hydra (animation)
The moons Nix and Hydra are visible in a series of images taken by the New Horizons spacecraft from January 27 to February 8 at distances ranging from 201 million to 186 million kilometers. Each frame is a combination of five long-exposure images, taken in a mode that allows the LORRI camera to capture better signal at the expense of resolution. At left, Nix and Hydra are just visible against the glare of Pluto and its large moon Charon, and the dense field of background stars. The bright and dark streak extending to the right of Pluto is an artifact of the camera electronics. The right-hand images have been processed to remove most of Pluto and Charon’s glare, and most of the background stars.

Nix orbits Pluto and Charon once in 25 days; Hydra, once in 38 days. To make these animations, LORRI was commanded to stare at Pluto for 10 seconds at a time, gathering 5 of these observations every 2 days. At its native resolution, LORRI's camera produces images 1024 pixels square, but the faint signal from Nix and Hydra would not be easy to detect in such images. To improve the signal-to-noise ratio, the original images were binned as they were read out of the chip: 16 pixels were read out as one, producing images only 256 pixels on a side. Binning reduces the resolution of the images, but reduces noise; and binning has the added benefit of reducing the data volume required for these observations. Even at full resolution, Nix and Hydra were just point light sources anyway.

New Horizons will continue to gather these kinds of observations to watch the motions of Nix and Hydra every two days through March 6, seeing a complete orbit of Hydra. Then it will pause for a long period of data downlink. It will pick up the observations again on April 5 and gather data through May 14, again long enough for a complete orbit of Hydra. Although New Horizons cannot yet match Hubble's resolution on the Pluto system, New Horizons can now do what Hubble can't: perform continuous observations. By measuring the positions of Nix and Hydra with respect to background stars in these images, scientists will be able to predict the future paths of the moons with much greater precision, and navigators will be able to target New Horizons with less uncertainty.

Nix and Hydra are much smaller than Pluto and Charon. They are only roughly 110 and 80 kilometers across, repsectively. We don't know what they look like yet, but I'm betting that they are potato-shaped. We'll see when we get closer! New Horizons will obtain much better photos of Nix than of Hydra -- there are many things in the Pluto system demanding New Horizons' attention during the few hours around closest approach, and the New Horizons team elected to characterize a single one of Pluto's smaller moons more completely rather than get less thorough observations of all of them. The best photo of Nix will have a resolution of about 300 meters per pixel, so it will appear about 360 pixels across, while poor Hydra will be seen well-lit at a best resolution of 1.1 kilometers per pixel, only 74 pixels across. Still, that will be enough to resolve their shape, and see if they are globes or potatoes!

Relative sizes of objects in the Pluto system represented by objects from the Saturn system

NASA / JPL / SSI / chart by Emily Lakdawalla

Relative sizes of objects in the Pluto system represented by objects from the Saturn system
The Pluto system contains two large worlds (Pluto and Charon) as well as at least four small moons (Nix, Hydra, Kerberos, and Styx). We will not have high-resolution images of any of them until the New Horizons flyby in 2015. In the meantime, this graphic uses objects photographed by Cassini in the Saturn system to represent the relative sizes of the objects in the Pluto system.

Read more: trans-neptunian objects, New Horizons, pretty pictures, Pluto, Charon, dwarf planets beyond Neptune, Pluto's small moons, animation

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
Read more articles by Emily Lakdawalla

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