NASA has raised a few eyebrows (and even the ire of some lawmakers) with its recently announced, far-flung plan to capture an asteroid and tow it into orbit around the moon, where it can be explored and sampled relatively easily from Earth. The fact that NASA aims to complete this asteroid snatch-and-grab mission by 2025 makes it seem all the more audacious. Yet, it turns out such a mission would be possible today, with current technology, for no less than 12 different asteroids. Scientists at the University of Strathclyde in the UK have published a new paper surveying the nearly 10,000 near-earth objects, or all comets and asteroids that come within 120 million miles of Earth, looking for those that can be redirected into Earth's orbit by today's spacecraft.

As it turns out, each of the 12 identified asteroids, or "easily retrievable objects," as the scientists deem them, could be brought back to Earth's orbit or nearby within a relatively short period of between 3 and 7.5 years. In order to find asteroids that would be good candidates for pickup, the scientists presumed that a 6-ton (US) spacecraft would be available, and that it would be able to carry enough liquid fuel to have 9-tons by the time it reached any one of the asteroids. "With a spacecraft of those characteristics, the total asteroid mass that could be transferred with the trajectories described in this paper is close to 400 tons," and between 2 to 5 meters in diameter, the authors write.

"There could be a justified concern regarding the possibility of an uncontrolled re-entry."

The main limiting factor the scientists found was the energy it would take to transfer the asteroids from their current orbits into orbits between the Earth and the Sun, or the Earth and the moon. As long as an asteroid could be moved at below a speed of 500 meters-per-second, the transfer should be doable, according to the researchers. These 12 asteroids fit the bill, so whenever NASA decides to select its target, it would be well advised to consider these candidates. However, researchers also note that whenever NASA or anyone else attempts this mission, they need to be very careful: "Regarding the safety of such a project, there could be a justified concern regarding the possibility of an uncontrolled re-entry of a temporary captured asteroid into Earth atmosphere.