Nobody understands exactly how binary asteroid systems formed, or even how stable the current binary systems are, that is, how they might evolve, with the two components either separating completely or collapsing onto one another to form a contact binary. The theoreticians really have their work cut out for them now. Nolan said that the near-Earth binary systems might have formed during certain kinds of collisions. Or, if they came from loosely bound, unconsolidated piles of rubble instead of solid rocks, binary asteroids might have formed during close passages by Earth when gravity pulls them apart. The first binary asteroid was found in August 1993 when NASA's Galileo spacecraft took pictures of asteroid Ida and revealed its tiny moon Dactyl.
Current statistics suggest that at least several percent of the near-Earth asteroids are binaries.
Ostro said that the existence of binary asteroids on potentially hazardous orbits means that we have to start figuring out how to maneuver spacecraft close to such objects. The three frames show several-hour time exposures of radar echoes from binary asteroid 1999 KW4. The distance from Earth increases toward the bottom, and speed from Earth increases toward the left. Gaps in the trail are due to breaks in the data-taking. The primary appears much wider than the secondary because it is a few times bigger and is rotating much faster.