Scientists say an asteroid mission isn't in the cards in the near future, since the space rocks are lesser known and nearly impossible to map. The Obama administration had asked NASA to land astronauts on nearby asteroids by 2025, but the idea has not been well-received in the space community. "There are still no good asteroid targets for such a mission, a necessary prerequisite for determining mission length and details such as the astronauts' exposure to radiation and the consumables required," according to a report released by the U.S. National Research Council.