Free Astronomy Magazine May-June 2019

44 MAY-JUNE 2019 SPACE CHRONICLES Hippocamp is only the most recent re- sult of the turbulent and violent history of Neptune’s satel- lite system. Proteus itself formed billions of years ago after a cataclysmic event in- volving Neptune’s satellites. The planet captured an enor- mous body from the Kuiper belt, now known to be Nep- tune’s largest moon, Triton. The sudden presence of such a massive object in orbit tore apart all the other satellites in orbit at that time. The debris from shattered moons re-coalesced into the second generation of natural satellites that lite. “Based on estimates of comet populations, we know that other moons in the outer Solar System have been hit by comets, smashed apart, and re-ac- creted multiple times,” noted Jack Lissauer of NASA’s Ames Research Cen- ter, California, USA, a coauthor of the new research. “This pair of satellites provides a dramatic illustration that moons are some- times broken apart by comets.” T his com- posite image shows the location of Neptune’s moon Hip- pocamp, for- merly known just as S/2004 N 1, orbiting the giant planet Nep- tune, about 4.8 billion kilo- metres from Earth. [NASA, ESA, and M. Showalter (SETI Insti- tute)] T his diagram shows the or- bital positions of Neptune’s inner moons, which range in size from 17 to 420 kilometres in di- ameter. The outer moon Triton was captured from the Kuiper belt many billions of years ago. [NASA, ESA, and A. Feild (STScI)] we see today. Later bombardment by comets led to the birth of Hip- pocamp, which can therefore be considered a third-generation satel- !