Larissa (astronomy)

Larissa (astronomy), small satellite of the planet Neptune. The moon orbits Neptune at a distance of about 73,600 km (about 45,700 mi), completing an orbit about once every 13 hours. Larissa’s orbit is circular and parallels Neptune’s equator.

Larissa is irregular in shape, measuring about 208 km (about 129 mi) at its widest point and about 178 km (about 111 mi) at its narrowest. It could fit inside a large crater on Earth’s moon. Larissa’s density, chemical composition, and internal structure are unknown.

Like the other small moons of Neptune, Larissa’s surface is as dark as soot. Larissa appears egg-shaped in the best pictures taken of it by the United States spacecraft Voyager 2, but few surface details are visible. Larissa appears to be covered with craters, but no signs of internal geological activity are detectable.

Larissa was first detected in 1981 by a number of astronomers around the world who set up their instruments to watch Neptune pass in front of a distant star. This interruption of starlight by a planet or moon is called an occultation, and astronomers hoped that by studying it they could learn more about Neptune’s atmosphere and rings. The star vanished for several seconds before Neptune covered it, then grew bright again and vanished as Neptune covered it. Astronomers theorized that an unknown small moon of Neptune had blocked light from the star before the planet itself had done so. The first to report the detection was American astronomer David Tholen. Larissa was discovered officially, however, in 1989 by American astronomer Stephen Synnott. He found it in pictures taken by the United States Voyager 2 spacecraft. Larissa is named after a character in Greek mythology associated with the sea god Poseidon.