In a research paper called "The population of natural Earth satellites", astronomers say that Earth has a second moon at any given time. While these moons are small, the scientific implications of this discovery are phenomenal.
Think about it: instead of having to send crews to asteroids, now we know that they come to us—they orbit Earth and we can intercept them to learn more about the origins of our solar system. All with a small price tag.
What has been discovered?
Cornell University's Mikael Granvik, Jeremie Vaubaillon and Robert Jedicke have calculated the population of "irregular natural satellites that are temporarily captured" by Earth.
They say these secret moons come and go quietly and without notice, but at least once we detected one. In fact, their calculations were confirmed by this observation: a mysterious titanium white object that was discovered rotating around Earth by the Catalina Sky Survey in Arizona, back in 2006.
Our results are consistent with the single known natural [temporarily-captured orbiter] 2006 RH120, a few-meter diameter object that was captured for about a year starting in June 2006.
That object was actually a small asteroid captured by Earth's gravitational field. It rotated as a second moon until June 2007, when it left our home planet's orbit. This study demonstrates that, even while they are not easily detected, these little moons come and go often, staying around for about ten months to about three spins around Earth and then wave goodbye.
Why is this a cool discovery?
Now that we know they come and go, and there's always one around, astronomers can work in detecting them. Once we detect one, we can send a few astronauts to analyze it instead of getting a full crew to a distant one. While they wouldn't be able to land on it, they could certainly study it up close.
The data about the formation of the solar system that we can obtain from these asteroids—without having to spend a lot of money—could be amazing.