Dubbed 2012 LZ1, the near-Earth asteroid was discovered in images snapped on June 10 and 11 by comet and asteroid hunter Robert McNaught at the Siding Spring Observatory in Australia.
(Read about a green comet spotted by McNaught in June 2010.)
So far astronomers have cataloged almost 9,000 near-Earth objects—asteroids and comets that come within about 120 million miles (195 million kilometers) of our home world.
Of these, just over 1,300 are classified as potentially hazardous, because they come within 4.6 million miles (7.4 million kilometers) of Earth and are more than 500 feet (150 meters) wide—large enough to pose a threat.
At its closest approach, the newfound space rock will swing by our planet at a distance of roughly 3.3 million miles (5.4 million kilometers), or about 14 times as far away as the moon.
What's more, 2012 LZ1 is unexpectedly large: The asteroid is about 1,650 feet (500 meters) wide.
"This one is interesting, because we've found about half of the [near-Earth] objects that are this large already," said Tim Spahr, director of the International Astronomical Union's Minor Planet Center in Cambridge, Massachusetts.
"So it's getting rarer to get snuck up on like this."
How to Spot Asteroid 2012 LZ1
The asteroid will be too faint to see with the naked eye, but sky-watchers with the right equipment could potentially make it out, said Patrick Paolucci, president of Slooh, an online sky-watching service.
"It's not going to be like some fireball streaking through the sky," Paolucci said.
"You're going to need decent equipment to see it. You couldn't track it with a hundred-dollar retail telescope from Walmart. But you could probably do it if you spent thousands of dollars [on equipment] and [you] know how."
Meanwhile, Slooh will be tracking the asteroid with its telescope on the Spanish Canary Islands and will broadcast the flyby over the Web, starting at 8 p.m. ET.
"We love it when stuff like this happens, because it's fun to do and the public appreciates it," Paolucci said.
No Chance for Collision
Despite qualifying as a near-Earth asteroid, 2012 LZ1 is too far away to threaten our planet or the moon, the Minor Planet Center's Spahr said.
During tonight's close approach, the asteroid's trajectory will get modified slightly by Earth's gravity, but the effect is predicted to be minimal, he added.
"It's really just a small tug on the object," Spahr said.
The next approach for 2012 LZ1 will be in 2016, but it's expected to clear Earth with plenty of room to spare.
Then and now, Spahr said, "there is no possibility of a collision for this object."