Space Rocks that become Temporary Satellites of Planet Earth

The Moon may not, as most people have always assumed, be the only natural object orbiting the Earth at any one time. Since 2006, astronomers have returned to a question that their profession had largely abandoned decades ago: Are there more moons of Earth, awaiting discovery? Although there probably are no other permanent moons of Earth, astronomers now have evidence that there are a few rocks orbiting the Earth at any one time. They call these “temporary satellites.”

Astronomers have been searching in vain for a second “moon” orbiting the Earth – one much smaller than the actual Moon, of course – since the 19th century. Although their efforts never succeeded, they did manage to establish that there was a respectably large population of near-Earth asteroids with orbits quite close to that of Earth, like Cruithne.

Most near-Earth asteroids are large enough, and far enough away, that over the short term they have reasonably stable solar orbits of their own. Smaller objects, however, occasionally come close enough to Earth that they get briefly trapped in the planet’s gravity well. This is similar to the process by which the gas giant planets are believed to have “captured” some of their moons, except that the tiny rocks entering Earth’s gravity well do not form stable orbits. Instead, after a few unsteady orbits around our planet, they get tugged away again and fall back into new orbits around the Sun. For this reason, they have been dubbed “temporary satellites.”

The search for temporary satellites really first got off the ground in 2006. That year, the Arizona-based Catalina Sky Survey noticed a small asteroid orbiting the Earth, which they named 2006 RH120. RH120 was tiny – just a few metres wide – and at first they thought they had stumbled across an old rocket stage deposited in orbit by NASA’s Apollo missions. Closer inspection ruled this out. But then, a year later and after making four full complete orbits, RH120 was pulled away from Earth and back into a new orbit around the Sun. Astronomers don’t expect to see it up-close again for years.

Excited by the discovery, though, astronomers set to work. A joint team from the University of Helsinki, the University of Hawaii and the Paris Observatory, using a computer simulation of the asteroid population in the inner solar system, calculated that at any given time there must be at least one asteroid more than three feet across in a similar temporary orbit around the Earth, plus many more, maybe even hundreds, of even smaller rocks. “Minimoons,” as they dubbed the temporary satellites, could be very valuable subjects of scientific research. At least during their brief and chaotic periods in near-Earth space, it would be relatively inexpensive to send up a space probe to take samples of a small temporary satellite and bring them back to Earth.

So far, scientists have yet to find any more temporary satellites of planet Earth similar to 2006 RH120. Thanks to that discovery and to the recent modelling studies, however, astronomers have a good idea what to look for and can begin to search for them using highly sensitive telescopes. The search will not be easy: The objects they will be looking for are only a few feet across, in a sphere of space hundreds of thousands of miles across.