Free Astronomy Magazine May-June 2019

43 MAY-JUNE 2019 SPACE CHRONICLES neighbour are incredibly close, at only 12,000 km apart. Ordinarily, if two satellites of such different sizes coexisted in such close proximity, either the larger would have kicked the smaller out of orbit or the smaller would crash into the larger one. Instead, it appears that billions of years ago a comet collision chipped off a chunk of Proteus. Im- ages from the Voyager 2 probe from 1989 show a large impact crater on Proteus, almost large enough to have shattered the moon. “In 1989, we thought the crater was the end of the story,” said Showalter. “With Hubble, now we know that a lit- tle piece of Proteus got left behind and we see it today as Hippocamp.” T his artist’s impression shows the outermost planet of the Solar System, Neptune, and its small moon Hippocamp. Hippocamp was discov- ered in images taken with the NASA/ESA Hubble Space Tele- scope. Whilst the im- ages taken with Hubble allowed as- tronomers to dis- cover the moon and also to measure its diameter, about 34 kilometres, these images do not allow us to see surface structures. [ESA/Hub- ble, NASA, L. Calçada]