|Asteroid 2011 MD, a chunk of rock estimated to be 25 to 55 feet (8 to 18 m) across, is expected to pass less than 8,000 miles above Earth's surface around 1 p.m. EDT (17:00 UT) on Monday, June 27th. The actual event will be observable only from South Africa and parts of Antarctica, but the approach will be visible across Australia, New Zealand, southern and eastern Asia, and the western Pacific.
The asteroid was spotted on June 22nd by LINEAR, the Lincoln Near-Earth Research project. Its discovery was announced on Thursday morning by the Minor Planet Center.
The asteroid's orbit is uncannily similar to Earth's orbit. But there's no chance that the asteroid will hit Earth on this approach, and almost no risk at its next close approach, in 2022. If the asteroid did strike, it would probably explode in the upper atmosphere — a fine spectacle, but harmless.
This is not the closest known asteroid approach; in fact, a smaller asteroid actually struck Earth in 2008. In addition, three other asteroids have come closer than 0.00012 astronomical units (11,000 miles) from Earth's center, the estimated distance of 2011 MD at its closest.
However, this is probably the biggest known asteroid to have come this close. Note the phrase "known asteroid." No doubt many asteroids much bigger than this one made close approaches without being detected before the near-Earth-object (NEO) surveys ramped up in the 1990s.
The logistics for seeing the moment of closest approach are poor. It takes place in broad daylight and halfway between the southern tip of South America and the northernmost point in Antarctica. The event is visible fairly low in the sky in deep twilight from South Africa.
However, the asteroid should be visible in the hours leading up to the closest approach across Australia, New Zealand, southern and eastern Asia, and the western Pacific. The farther south you are, the better. The farther west you are within this zone, the shorter the period of visibility, but the closer to Earth the asteroid will be when it disappears.
The asteroid peaks brighter than magnitude 11.0 at the places where the closest approach is visible, and it's already about magnitude 12.5 — fairly easy to spot in an 8-inch telescope — by 14:30 UT, 2½ hours before closest approach. At that point it's visible from Southeast Asia, eastern China, and Japan, as well as Australia and points between.
The asteroid will be very hard to observe after its closest approach, since it's departing more or less toward the Sun.
To observe the asteroid you will need a good telescope (the bigger the better), excellent charts and the know-how to use them, and ephemerides from the Jet Propulsion Laboratory. Make sure you enter the proper latitude and longitude. With an object this close, a small difference in the observer's location makes a huge difference where it appears in the sky.