The asteroid, which is officially designated 2010 WC9, will make its closest approach at 6:05 p.m. EDT (2205 GMT) while traveling at a speed of 28,655 mph (46,116 km/h), according to the Minor Planet Center.
Astronomers estimate that the asteroid measures 125 to 390 feet (38 to 119 meters) in diameter. That means it's about as big as New York City's Statue of Liberty, though it could be even longer than a football field. [Images: Potentially Dangerous Asteroids]
While this isn't exceptionally large for a near-Earth asteroid, it is rare for asteroids this big to venture so close to Earth. According to EarthSky.org, this will be "one of the closest approaches ever observed of an asteroid of this size."
Asteroid 2010 WC9 was first spotted by the Catalina Sky Survey in 2010, but astronomers lost track of it once it became too faint to see. The "lost" asteroid was rediscovered on May 8, and astronomers have been tracking its approach ever since.
The asteroid isn't visible to the naked eye, but it can be spotted through some telescopes. Astronomers with The Virtual Telescope Project in Italy and Tenagra Observatories in Arizona captured a view of the asteroid today at 2:46 a.m. EDT (0646 GMT). At the time, the asteroid was about 454,000 miles (730,000 km) from Earth.
The Slooh community observatory has also been tracking the asteroid, and yesterday (May 14), the observatory showed a live webcast of the asteroid as seen from its telescopes at the Institute of Astrophysics of the Canary Islands. (You can learn more about how Slooh.com tracks near-Earth asteroids here.)