As we count down the days to its historic flyby of Pluto, NASA's New Horizons probe has moved within sight of all members of the dwarf planet's system. This has enabled it to return the first images of its smallest moons Kerberos and Styx, and signals the beginning of the spacecraft's hunt for unknown moons and rings.
Kerberos and Styx are the tiniest and faintest of the five known moons orbiting Pluto. Kerberos, first detected by the Hubble Space telescope in 2011, is thought to be between 6 and 20 mi (9.7 and 32 km) in diameter and circles the planet over 32 days. Styx, discovered in 2012 also by Hubble, orbits Pluto over 20 days and is between four and 13 mi (7 and 21 km) in diameter.
The sightings of these moons rounds out New Horizon's first photo series of Pluto and its moons, well, the ones that we know about so far anyway. In February following a 9.5 year journey, the probe kicked things off by firing up its Long-Range Reconnaissance Imager (LORRI) and sending back its first glimpse of Pluto and its largest moon, Charon. Snaps of smaller moons Nix and Hydra soon followed, as did a colored image of the planet itself, the first ever color photo of Pluto taken by spacecraft on approach.
The images were constructed from five 10-second exposures, which were combined and then processed to tone down the blinding glare of Pluto and Charon, as seen above in the image on the left. This also removed the sea of bright background stars, visualizing smaller moons Kerberos and Styx in orbit, alongside bigger siblings Nix and Hydra.
"New Horizons is now on the threshold of discovery," says mission science team member John Spencer, of the Southwest Research Institute in Boulder, Colorado. "If the spacecraft observes any additional moons as we get closer to Pluto, they will be worlds that no one has seen before."
Further to the possibility of stumbling across new moons, New Horizons will also be keeping an eye out for unknown rings that could obstruct its path. As it comes to pass Pluto at a distance of 7,750 mi (12,500 km) on July 14, the probe will use cameras, spectrometers and other scientific instruments to inspect Pluto's atmosphere and the possiblity that Charon may have an atmosphere of its own. From there it will continue on to study the Kuiper Belt.