This article assumes visiting an asteroid in its natural orbit. It does not take into account an asteroid diverted into Earth orbit, as may occur under some proposals.
Billion of asteroids pepper the vast spaces between the planets, debris left over from the formation of the solar system. Near-Earth asteroids--large space rocks that regularly drift near or cross Earth's orbit--number in the thousands. After the moon, they are the closest celestial bodies to our home planet, and are natural targets for future visits by astronauts.
Many view near-Earth asteroids as the natural mid-range step between manned missions to the Moon and manned missions to Mars, a chance to test techniques and technologies for taking the next big step after establishing a lunar base.
Such missions are sure to be fuel intensive. Even if an asteroid is passing close to Earth, chances are that it won’t stay there long as its orbit carries it past. So a spaceship carrying astronauts will have to carry enough fuel to go out up to several million miles and back fairly quickly. This is very different for planning a trip to the Moon, which is only a three-day flight away, or a flight to Mars, where fuel can be manufactured from the environment. This will add a great deal of bulk and expense to the mission.
Depending on the distance, trajectory, and orbital speed of a target asteroid, the crew may have to spend anywhere from a few weeks to a few months in transit. Even though life support recycling has made great leaps and strides in the past few decades, such a mission would still have to carry a great deal of consumables with them, enough to last for the entire round-trip journey. This will add mass to any spacecraft, which will in turn require more fuel for maneuvering.
The current vision for the mission is for NASA to use its Project Constellation system, currently under development. The modular components would likely include an Orion Crew Exploration capsule, a service module, an additional booster, and a specially designed lander. An extended habitation module may or may not be sent along as well depending on the projected length of the mission, either as part of the spaceship "stack" or sent separately, perhaps landed on the target asteroid ahead of the main manned mission.
Stepping foot on the target rock may prove tricky, as the asteroid would have negligible but not zero gravity, making it like neither landing on a large body nor like docking in orbit. The craft would have to be anchored onto the surface afterward.
Once on the surface the crew could carry out a number of experiments, including mineralogical analysis, sample collection, testing mining techniques, tapping frozen ice for life support and fuel, and other projects that could prove valuable for space science and the possible future exploitation of asteroids.
On Manned Asteroid Missions