Astronomers didn't notice the oncoming asteroid until February 28, when it showed up as a faint dot in pictures taken at the Siding Spring Observatory in Australia.
At that point the asteroid was already a mere 2.4 million kilometers from Earth, and closing in fast.
Astronomers now know that the asteroid is moving within the inner solar system and that the space rock completes an orbit around the sun every 1.56 years.
This means the asteroid could swing close by Earth again someday—though that doesn't seem to be any cause for alarm, if Monday's flyby is any indication.
(Highlighted Image of the Asteroid)