By looking through the catalog of known asteroids, aerospace engineers have identified 12 candidates that we could reach out and capture using existing rocket technology.

Long overlooked as mere rocky chunks leftover from the formation of the solar system, asteroids have recently gotten a lot more scrutiny. A couple years ago, researchers outlined a seemingly daring plan to lasso and bring an asteroid back to Earth. NASA doesn't seem to think this is too crazy, and is moving forward with plans to capture, tow, and place a small asteroid somewhere near our planet. There are also two different private space companies, Planetary Resources and Deep Space Industries, with plans to seek out and mine precious metals and water from near-Earth asteroids. And finally, the widely filmed Chelyabinsk meteor, which exploded over Russia in February, has focused international attention on the fact that we may one day want to deflect a potentially catastrophic Earth-asteroid crash.

The question is how best to go about moving one of these flying space rocks. After all, most are too far away or traveling too fast to be of any use. Changing their orbits would require massive amounts of fuel or a gravity tug that would take decades or centuries to work. So three engineers from the University of Strathclyde in Scotland took a look at the current catalog of known asteroids — which recently surpassed 10,000 objects — and identified a new category: Easily Retrievable Objects or EROs.

These EROs are all fairly small asteroids, ranging in size from approximately 2 meters to 60 meters in diameter. They already come (cosmically) close to our planet, and it would take a relatively small push to put them in orbits around Lagrange points near Earth. Lagrange points are positions around a million kilometers from our planet where the gravitational influence of Earth and the sun are minimal, and an object can circle without expending too much energy. There are several fancy orbits that these EROs could be placed in, including the bean-shaped Lyapunov orbits pictured at top or the more circular halo orbits seen at left.

Moving one of these EROs would be a "logical stepping stone towards more ambitious scenarios of asteroid exploration and exploitation, and possibly the easiest feasible attempt for humans to modify the Solar System environment outside of Earth," the authors write in a paper in the August issue of Celestial Mechanics and Dynamical Astronomy.

Their number one most easily moved space rock is named 2006 RH120. A single rocket burn in 2021 would be enough to place this roughly 4-meter-wide asteroid into orbit around a Lagrange point by 2026. NASA could then launch people to study this object (which would barely be bigger than the astronauts themselves) and learn about its history.

Little is known about the composition of the 12 EROs in this study. Because of their small size, astronomers have speculated that they might be fragments ejected from the moon after an impact or could possibly even be spent rocket stages from a Space Age mission. Perhaps a dedicated telescope, such as the one Planetary Resources wants to launch next year, could perform reconnaissance to learn what sorts of objects these EROs are. If they turned out to be asteroids and any of them contained a useful amount of precious metals or water, they could be exploited for mining.

The authors acknowledge there are uncertainties regarding the orbits of many of these EROs and the possibility that some miscalculation could send one slamming into Earth. But with active control systems, the asteroids could be herded fairly easily. Even if one did go rogue, most of the objects are small and anything under 5 meters in diameter would burn up in the atmosphere before causing any damage.

Images: D. García Yárnoz, J. P. Sanchez, C. R. McInnes, "Easily retrievable objects among the NEO population," Celestial Mechanics and Dynamical Astronomy, DOI: 10.1007/s10569-013-9495-6