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Today sees the close flyby, at only 12,000 km from Earth, of the little asteroid 2011 MD. Its trajectory will take it over the southern Atlantic ocean and at its closest approach it will be over the Antarctic coast.
After this it will begin to distance itself once more, but with a trajectory modified by the encounter, making it uncertain whether this object will visit us again in the future.
For now, 2011 MD presents no danger.
Also, given its distance, brightness and assumed albedo, the diameter is somewhere in the range 5-20 metres, and would be significantly reduced after a passage through the terrestrial atmosphere (should this ever happen), and so would have only a limited impact. To give some idea of the consequences, Meteor Crater near Flagstaff, Arizona, was made by the impact of a body about 50 metres in diameter, a would have been even bigger before passing through the atmosphere.
Nonetheless, 12,000 km is really very close, considering the diameter of our planet is only 800 km more and that geostationary satellites orbit at a distance of 36,000 km, a radius that the asteroid passes twice.
Of course, given the volume of space involved and size of the bodies in question, a collision is extremely unlikely, but still cannot be ruled out. Researchers at JPL's Near Earth Object Program estimate that an event of this type happens about once every 6 years.
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