Images of Pluto’s tiny moon tiny Kerberos taken by NASA’s New Horizons spacecraft – and just sent back to Earth this week – complete the family portrait of Pluto’s moons.
Kerberos appears to be smaller than scientists expected and has a highly reflective surface, counter to predictions prior to the Pluto flyby in July. “Once again, the Pluto system has surprised us,” says New Horizons Project Scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
The new data, downlinked from the New Horizons spacecraft on 20 October show that Kerberos appears to have a double-lobed shape, with the larger lobe approximately eight kilometres (five miles) across and the smaller lobe approximately five kilometres (three miles) 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’s surface is similar to that of Pluto’s other small moons (approximately 50 per cent) 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 neighbouring moons. That influence was surprisingly strong, considering how faint Kerberos was. They theorised 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 by these new images show that that idea was incorrect, for reasons that are not yet understood.
“Our predictions were nearly spot-on for the other small moons, but not for Kerberos,” says New Horizons co-investigator Mark Showalter, of the SETI Institute in Mountain View, California. The new results are expected to lead to a better understanding of Pluto’s fascinating satellite system.