I stopped by the Jet Propulsion Laboratory today to see NASA Administrator Charles Bolden talk about the proposed Asteroid Retrieval Mission. The Planetary Society is ambivalent about the mission; though the concept of moving an asteroid and exploring it is compelling and could produce fascinating science, it's not clear how NASA plans to pay for it. We don't want this mission to raid existing science programs.
NASA is going around the country trying to sell this mission to its various constituencies. Today, the NASA Administrator was highlighting the advanced solar electric propulsion engine developed at JPL. Similar hardware has already flown in space, most notably on the Dawn mission to Vesta and Ceres.
This special type of engine uses ionized xenon to generate small amounts of force over long periods of time. It's more efficient than chemical propulsion, but it does require extra time to achieve similar amounts change in velocity. Even though this engine is currently in use on a real mission, the requirements of the asteroid retrieval mission call for a much larger, more robust ion engine than previously existed. For example, Dawn had about 450 kg of xenon "fuel" to utilize throughout the course of its lifetime. For asteroid retrieval, NASA needs about 10,000 kg. This is due to the much larger size of the spacecraft plus the heavy asteroid itself.
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