Having already discerned the dwarf planet has distinct surface features, perhaps even a polar ice cap, NASA's New Horizons mission has now spotted not just Pluto's largest moons, but the smallest natural satellites too.
Imaged at a distance of over 55 million miles (88 million kilometers) from the Plutonian system between April 25 and May 1, this animated sequence of five 10 second observations by New Horizons' Long Range Reconnaissance Imager (LORRI) instrument show Pluto's largest moon Charon, plus smaller moons Nix and Hydra, and it has also pinpointed, for the first time, the recently-discovered moons Kerberos and Styx.
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"New Horizons is now on the threshold of discovery," said mission science team member John Spencer, of the Southwest Research Institute in Boulder, Colo., in a NASA news release. "If the spacecraft observes any additional moons as we get closer to Pluto, they will be worlds that no one has seen before."
The diminutive Kerberos and Styx measure only 4 to 13 miles (6 to 21 kilometers) and 6 to 20 miles (10 to 32 kilometers) in diameter and were discovered in 2011 and 2012, respectively, by the New Horizons science team using Hubble Space Telescope data. As a comparison, Charon is 750 miles (1,200 kilometers) wide, Nyx is between 29 and 85 miles (47 and 137 kilometers) wide and Hydra is between 37 and 92 miles (60 and 148 kilometers) wide. These uncertainties in moon diameters will be greatly reduced as New Horizons rapidly approaches its July 14 flyby.
"Detecting these tiny moons from a distance of more than 55 million miles is amazing, and a credit to the team that built our LORRI long-range camera and John Spencer's team of moon and ring hunters," said principal investigator Alan Stern, of the Southwest Research Institute.
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The imaging and processing techniques used by the team will now be used to continue exploration of the neighborhood surrounding Pluto. It has long been assumed, with the discovery of the dwarf planet's growing family of natural satellites, that many more moons exist. There may even be a ring of debris, signifying a possible ancient impact event in the Kuiper belt.