Asteroid 2011 MD, a chunk of rock estimated to be 20 to 65 feet (5 to 20 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'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.
Nor is it a piece of space junk from a 1962 launch, as was suggested early on. Additional observations have made it possible to calculate 2011 MD's orbit past and future quite accurately. Bill Gray, a well-known expert on orbital dynamics, has run the orbit backward in time, and is now quite sure that this asteroid could not have been close enough to Earth any time during the space age to have started off as a rocket booster. So it seems to be a genuine chunk of rock after all.
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. According to asteroid specialists at NASA's Jet Propulsion Laboratory, an asteroid of this size should pass this close to Earth every six years on average.
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.
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 either the Minor Planet Center or 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.