Aviation Week is carrying the news that NASA's FY2014 budget will include a $100 million line item to start planning a robotic mission to snatch an asteroid and relocate it to near the Moon, where it could be studied up-close by NASA—and possibly even visited by astronauts (hat-tip to the Houston Chronicle's SciGuy blog for the news).

The idea is based on a report by the Keck Institute for Space Studies, which outlines an entire robotic mission to locate and retrieve an NEA—a Near Earth Asteroid—of about 500,000kg in mass and a diameter of about 7 meters. Such an asteroid would be a C-type or carbonaceous asteroid, and would have the consistency of "a dried mudball." The asteroid would be hauled back via a robotic probe and positioned in an orbit above the far side of the Moon at the second Earth-Moon Lagrange Point, where the vagaries of gravity and inertia would keep the asteroid in a roughly consistent location. Once positioned there, the asteroid would—at least in theory—be within the range of a manned visit.

To go out and grab the asteroid in the first place, the report recommends a probe weighing about 18,000 kg, which could be lofted into space using an existing launch vehicle (such as an Atlas V). Shifting 500,000 kg of mass with conventional rockets would require a tremendous amount of propellant to be carried along with the probe so, rather than chemical rockets, the probe would be equipped with a "~40-kW solar electric propulsion system with a specific impulse of 3,000 s."

That's an ion thruster, as the technology is more commonly known. This type of engine works not by burning propellant in a thrust chamber, but rather by using electricity generated by solar panels to accelerate charged particles away from the spacecraft. The thrust from such an engine is relatively low, but its specific impulse—a measurement of the engine's efficiency at extracting force from a given amount of fuel—is extremely high. By way of comparison to the 3,000 s figure above, the RS-25 engines used on the Space Shuttle burn liquid hydrogen and have a specific impulse of 453 seconds in a vacuum.

The probe could use various gravity assist methods to reach its candidate asteroid, depending on where the target happens to be. Most of the potential rendezvous methods include a slingshot around the Moon to give the probe a big velocity boost out of the Earth-Moon system. Once the probe has reached its asteroid target, it would enfold the asteroid in a large bag and stabilize any tumble or rotation it happens to have using xenon-powered maneuvering thrusters. Once stable, it would begin a long ion-powered trek back to the Moon. Once the probe has returned to the Moon's vicinity with its cargo, it would initiate a braking maneuver (again using the Moon's gravity to assist) and stabilize itself in high lunar orbit at the EML2 point.

Because of the low available thrust and the high amount of mass involved, the entire retrieval mission could take up to ten years, depending on the candidate asteroid's location and orbit. Once the asteroid has been installed at EML2, the report says that it would remain stable with only a tiny amount of help: "We estimate that the lunar orbit could be maintained with station-keeping on the order of 10 m/s ∆V per year." The probe would remain attached to the asteroid in order to impart this momentum, though it would not have the propellant reserves to do this for very long and would need to have its xenon supply topped off for long-term station-keeping.

So, once it's there, what do we do with it? The report outlines several science possibilities, but NASA and the current administration are focused on sending astronauts out to visit. Manned exploration of a 7-meter asteroid might seem a bit of a stretch, but the report explains thusly:

Placing a 500-t asteroid in high lunar orbit would provide a unique, meaningful, and affordable destination for astronaut crews in the next decade. This disruptive capability would have a positive impact on a wide range of the nation’s human space exploration interests. It would provide a high-value target in cislunar space that would require a human presence to take full advantage of this new resource. It would offer an affordable path to providing operational experience with astronauts working around and with a NEA that could feed forward to much longer duration human missions to larger NEAs in deep space.

There's another reason, too: rendezvous with a NEA is one of the potential objectives outlined in the Augustine report, a document commissioned by the government that attempts to give some direction to America's manned space program. The vast majority of the report's recommendations and conclusions appear to have been largely ignored; however, NEA rendezvous is mentioned as a potential way to keep the manned space program performing some meaningful work while looking for a bigger, more permanent goal:

There is a third possible path for human exploration beyond low-Earth orbit, which the Committee calls the Flexible Path. On this path, humans would visit sites never visited before and extend our knowledge of how to operate in space—while traveling greater and greater distances from Earth. Successive missions would visit lunar orbit; the Lagrange points (special points in space that are important sites for scientific observations and the future space transportation infrastructure); and near-Earth objects (asteroids and spent comets that cross the Earth’s path); and orbit around Mars.... The Flexible Path represents a different type of exploration strategy. We would learn how to live and work in space, to visit small bodies, and to work with robotic probes on the planetary surface. It would provide the public and other stakeholders with a series of interesting "firsts" to keep them engaged and supportive.

A captured asteroid would provide NASA with another destination, besides the International Space Station, that astronauts could visit using the in-development Orion spacecraft. It also will provide a way to demonstrate the heavy-lift capabilities also-in-development Space Launch System. However, adding a manned component to the mission introduces a lot of dependencies—SLS's actual development and commissioning is still not entirely a sure thing. (Detractors often refer to the rocket as the "Senate Launch System" because of the large number of design and assembly contracts being run by the aerospace companies all across the country.)

The mission as outlined comes across as more of a technology demonstration and practice run than anything else, although there is obviously the opportunity for real science in the capturing and up-close observation of an asteroid. Aviation Week cites a number of experts who discuss objectives beyond just capture and rendezvous; the mission itself could serve as a test for technology that could eventually be scaled up to avert a collision with an inbound asteroid, for example. The presence of a carbonaceous asteroid in cis-lunar space also gives private companies the opportunity to test mining techniques to extract resources from the floating rock. Success could lead to additional robotic capture-and-return missions or even remote-controlled mining missions that return not whole asteroids but rather mined ore ready to be processed.

From a cost perspective, the $100 million line expected to appear in NASA's FY2014 budget is just the start, though: the total cost of the mission would be about $2.6 billion dollars (and that excludes any costs associated with later manned visits). Much of that $2.6B is tied up in the research and planning and manufacturing, and repeat missions would be less—perhaps $1B each.

The mission could launch as early as 2017, depending on the asteroid chosen. The combined Orion/SLS system is expected to make its first manned flight in 2021.