NASA has announced the prime candidate for an ambitious unmanned mission that will visit a Near Earth Asteroid and return a sample of it to earth. The winner is 101955 Bennu, a member of the Apollo group of asteroids roughly half a klick in diameter:
NASA Homepage — Bennu could hold clues to the origin of the solar system. OSIRIS-REx will map the asteroid’s global properties, measure non-gravitational forces and provide observations that can be compared with data obtained by telescope observations from Earth. OSIRIS-REx will collect a minimum of 2 ounces (60 grams) of surface material.
“The entire OSIRIS-REx team has worked very hard to get to this point,” said Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona in Tucson. “We have a long way to go before we arrive at Bennu, but I have every confidence when we do, we will have built a supremely capable system to return a sample of this primitive asteroid.”
This thing has been cooked for eons, so probably not much left in the way of volatiles, the kind of stuff you’d find on mint condition comet nuclei. The advantage in going for an Apollo ‘roid over a traditional member of the asteroid belt or a distant comet is it doesn’t take anywhere near the delta V most interplanetary objects demand. That’s a big deal when heading out and matching orbits, it’s a humongous deal when coming back with the sample. Starting out the vehicle has lots of stages, lots of fuel, but much of that is used up punching through our atmosphere, getting off the earth and reaching the object. What’s usually left over is just a dot at the top of that big stack, not much room for fuel left. Low delta V and low surface gravity are two critical considerations in a return mission.