On October 22, 2015, NASA released images of Pluto’s tiny moon Kerberos, thereby completing the family portrait of all of Pluto’s moons. All the moons are displayed in the image above. Charon is by far the largest of Pluto’s moons, with a diameter of 751 miles (1,212 kilometers). Nix and Hydra have comparable sizes, approximately 25 miles (40 kilometers) across in their longest dimension above. Kerberos and Styx are much smaller and have comparable sizes, roughly 6-7 miles (10-12 kilometers) across in their longest dimension.
All four small moons have highly elongated shapes, a characteristic thought to be typical of small bodies in the Kuiper Belt.
The New Horizons spacecraft took the pictures as it swept through the Pluto system on July 14.
The most recent images – of the moon Kerberos – downlinked from the New Horizons spacecraft on October 20.
Kerberos appears to be smaller than scientists expected and has a highly-reflective surface, counter to predictions prior to the Pluto flyby in July. New Horizons Project Scientist Hal Weaver said:
Once again, the Pluto system has surprised us.
Data show that Kerberos appears to have a double-lobed shape, with the larger lobe approximately 5 miles (8 kilometers) across and the smaller lobe approximately 3 miles (5 kilometers) across.
Science team members speculate from its unusual shape that Kerberos could have been formed by the merger of two smaller objects. The reflectivity of Kerberos’ surface is similar to that of Pluto’s other small moons (approximately 50 percent) and strongly suggests Kerberos, like the others, is coated with relatively clean water ice.
Before the New Horizons encounter with Pluto, researchers had used Hubble Space Telescope images to “weigh” Kerberos by measuring its gravitational influence on its neighboring moons. That influence was surprisingly strong, considering how faint Kerberos was. They theorized that Kerberos was relatively large and massive, appearing faint only because its surface was covered in dark material. But the small, bright-surfaced Kerberos – now revealed in these new images – shows that the idea was incorrect, for reasons that are not yet understood.
New Horizons co-investigator Mark Showalter, of the SETI Institute said:
Our predictions were nearly spot-on for the other small moons, but not for Kerberos.
Bottom line: Images of Pluto’s tiny moon Kerberos – downlinked from the New Horizons spacecraft on October 20, 2015 – complete the family portrait of Pluto’s moons. The New Horizons spacecraft took the Kerberos pictures on July 14, 2015, a few hours before its closest flyby to Pluto.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.