Like many scientists before him, Caltech Graduate Student Thomas McCord was searching for other answers when he made his unexpected discovery. Curious about the odd behavior of the planet Neptune's two moons, Triton and Nereid, he set out to make a mathematical analysis of their unusual orbits. Last week in the Astronomical Journal, he reported that his two-year, computer-aided investigation had not only accounted for the current state of the Neptunian satellites but had also given him a startling glimpse into the future: Triton, largest of the two moons, is doomed to smash into Neptune in a cataclysmic collision.
Some 2,600 miles in diameter, Triton orbits Neptune at a distance of 220,000 miles, about the same distance as the moon is from the earth. But unlike the earth's moon and most other solarsystem satellites, Triton moves in a retrograde direction: it circles the "wrong" way—clockwise—around Neptune which spins counterclockwise on its axis. Nereid, only about 200 miles in diameter, revolves in the direction of Neptune's spin, but its orbital path is highly irregular, swinging as far as 6,000,000 miles into space and as close as 900,000 miles from Neptune's surface.
Neptune's Tides. McCord's calculations seem to support what other scientists have suspected. Long ago, as it sank from a higher orbit, Triton passed close to Nereid. The smaller moon, buffeted by Triton's more powerful gravity, may have been hurled into the elliptical orbit it now follows.
Carrying his analysis further, McCord discovered that the forces that acted on Triton to make it sink to its present orbit will continue to affect the larger moon at an accelerating rate. Tides raised on Neptune's surface by the pull of Triton's gravity exert a drag on the satellite that causes its orbit to decay. The tidal action on Neptune also creates friction that dissipates energy from the rotating Neptune-Triton system, further depressing Triton's orbit.
In as little as 10 million years—a brief interlude in the solar system's 4½-billion-year history—Triton could begin its final plunge toward Neptune's surface. Only one event could prevent the terrible impact, says McCord: a phenomenon that may have occurred before in the solar system. Neptune's gravity could break Triton into little pieces as it draws near, turning the satellite into a set of Saturnlike rings.