Earth has two moons, a group of scientists argues. One is that waxing and waning nightlight we all know and love. The other is a tiny asteroid, no bigger than a Smart Car, making huge doughnuts around Earth for a while before it zips off into the distance. That’s the scenario posited by the scientists in a paper published December 20 in the planetary science journal Icarus. The researchers say there is a space rock at least 1 meter (3.3 feet) wide orbiting Earth at any given time. They’re not always the same rock, but rather an ever-changing cast of temporary moons. In the scientists’ theoretical model, our planet‘s gravity captures these asteroids as they pass near us on their way around the sun.
According to the researchers, surprisingly little attention has been paid to Earth‘s natural satellites other than the moon, despite the fact that they’re sure to exist. According to Jeremie Vauballion, an astronomer at the Paris Observatory in France, there are lots of asteroids in the solar system, so chances for the Earth to capture one at any time is, in a sense, not surprising. The researchers’ results are consistent with observations of one such “temporarily-captured asteroid” that is believed to have orbited Earth for about a year starting in June 2006. The object, labeled 2006 RH120, was discovered by the Catalina Sky Survey in Arizona, estimated to be between 10 and 20 feet (3 and 6 meters) wide, it appeared to be orbiting Earth from two moon-distances away. Mikael Gravnik, a physicist at the University of Helsinki and lead author of the new paper, says 2006 RH120 was probably discovered because it was slightly larger than most of the other “temporary moons” that come traipsing through our planetary system.Most of the hobo moons are only about 1 meter wide. Objects of this size are too faint to be detected when being at a distance of, say, a few lunar distances from the Earth. When coming closer in during their orbit, they are moving too fast to be detected, because the limited amount of photons is spread over too many pixels. These limitations mean we don’t currently have a way of finding our second moons. But an observatory called the Large Synoptic Survey Telescope (LSST), planned to open in Chile in 2015, could change that.