Dec. 1, 2015

Excerpt from sciencenews.com
Pluto and Charon might have been the stars of the New Horizons mission, but the dwarf planet’s four smaller moons have some surprises to share as well.
With images of Kerberos transmitted from the spacecraft on October 20, the Pluto family portrait is complete. The tiny moons Nix, Hydra, Kerberos and Styx are no longer pinpricks of light but textured, misshapen balls of ice that look quite different from both Pluto and Charon.

“It’s really cool that the Pluto system has all these different things,” says project scientist Hal Weaver of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. With data from the four moons, “we can put together a more complete picture of how the system formed and evolved over time.”
Three of the moons — and possibly all four — appear to have been assembled from two smaller bodies squished together. Images of Kerberos in particular are reminiscent of early pictures of comet 67P/Churyumov–Gerasimenko taken by the Rosetta spacecraft. The two lobes of 67P were probably once separate comets that gently collided and stuck (SN: 10/31/15, p. 17). Kerberos and its siblings might have a similar history.

A long-ago collision could have split proto-Pluto into Pluto and Charon, with the smaller satellites assembling out of the debris. Theorists, however, have trouble figuring out how to make that scenario work. “It’s a little bit mysterious how the four moons got there,” says Mark Showalter, a planetary scientist at the SETI Institute in Mountain View, Calif., who discovered Kerberos and Styx several years after New Horizons launched.
All four moons are tiny. Hydra — the largest — measures just 55 kilometers along its long axis; teeny Kerberos spans only 12 kilometers. Most of the sizes and even the elongated shapes were deduced ahead of New Horizons’ arrival by watching reflections change on their surfaces with the Hubble Space Telescope. “We were pretty much dead on,” says Showalter.

Image: Excerpt from sciencenews.com