We didn't get much notice, but today Asteroid 2011 MD, estimated to be between 25 and 55 feet wide, will come closer to the Earth than the moon. Wired reports that only parts of South Africa and Antarctica will get a glimpse of the rock. They add:
2011 MD was spotted by LINEAR — the Lincoln Near-Earth Asteroid Research centre — on 22 June. It has an orbit remarkably similar to that of Earth's, but there's no chance that it will strike us this time round. When it next approaches Earth, in 2022, there's a slim chance that it could impact with us, but if it does, it'll probably just explode in the upper atmosphere, which will look impressive but do relatively little damage to the surface.
It's the largest known asteroid to have come so close to the Earth, but that's not as impressive as it sounds, as surveys of near-Earth objects only really began in earnest in the early 90s. As such it's likely, and even probable, that plenty of larger rocks may have passed even closer in the past without us knowing about them.
So, not a threat, but the idea got Mark Thompson at Discovery News thinking. The asteroid will pass close enough to Earth that its trajectory will be affected by the Earth's gravitational pull. Yet, Thompson asks, we got just four-days notice?
The path of 2011 MD. NASA
He points out that in the Hollywood blockbuster Armageddon, they got 12-days notice. So could we destroy a dangerous asteroid with four-days notice? He's not so sure.
The asteroid, by the way, will be closest to Earth at 1 p.m. ET.
Update at 10:54 a.m. ET: We've changed the estimated time at which the asteroid will be closest to us because Sky and Telescope provided an updated time, as Kelly Beatty pointed out in the comments. The asteroid is scheduled to be closest to Earth at 1 p.m. ET.