The Cassini Equinox Mission strikes again with a fresh portrait released today of Calypso, one of the Saturn system's so-called Trojan moons.
Calypso's smooth surface, and its brightness, stem from the ice grains it sweeps up as it orbits within Saturn's E ring. The ice particles come from another Saturnian moon, Enceladus. Water erupts from fissures in the icy surface near the moon's south pole, feeding fresh ice particles to the E ring.
The moon has earned the "Trojan" tag because of its relationship with a much larger moon of Saturn, Tethys.
Astronomers originally gave the title to one of two groups of asteroids that orbit the sun at Jupiter's distance. The Trojans occupy a kind of gravitational no-man's land known as a Lagrange point. In this case, the point – a region of space, actually – is equidistant from the center of mass of the sun and of Jupiter. Jupiter has two of these types of Lagrangian points – one about 60 degrees in front of it as it orbits the sun, the other 60 degrees behind it.
In Calypso's case, it's orbiting Saturn at Tethys's distance and at a point equally distant from the center of mass of Saturn and of Tethys.
Cassini finished up its initial four-year science agenda in 2008. But with many miles left in the tank, NASA extended the craft's assignment through September 2010 and renamed it the Cassini Equinox Mission, because that extension coincided with Saturn's equinox, which occurs once every 15 years. Next up, a close encounter with Titan on April 5.