"It is coming very close to Earth, but by close we mean within the orbit of the moon," said Telus World of Science staff scientist Trevor Prentice. "It's a rare occurrence. This asteroid is about nine tenths of the way to the moon."

The spinning spherical mass is expected to pass closest to Earth around 6:28 p.m. ET Tuesday evening, as it approaches from the direction of the sun.

It's the nearest an asteroid of this magnitude has come to our little rock in 35 years, but Prentice said it's not quite close enough to see with the naked eye.

"It would be difficult even to spot it with an amateur telescope," said Prentice. "I'd be surprised if we even catch it with our telescopes out at the Telus observatory."

Scientists have been tracking the asteroid, dubbed the 2005 YU55, since 2005.

The 400-metre carbon-based asteroid was picked up by Puerto Rican radio telescopes in April 2010, and scientists took that chance to map a trajectory for the space rock.

"This asteroid is unique because we've seen it before, and we actually know its projected path, which is rare," said Prentice. "Which is how we know there's no chance of it hitting the Earth."

So, no need for mass grocery shopping Armageddon style.
Although, Purdue University professor Jay Melosh has released some chilling calculations for a theoretical collision.

According to Melosh, if the asteroid were to make impact it could create a crater almost seven kilometres across and more than 500 metres deep.

Prentice says Tuesday's fly-by offers astronomers a chance to practice tracking techniques, in the event that a near-Earth asteroid is ever headed straight for us.

"It's an opportunity for astronomers and scientists to study the asteroid, and understand how to deflect it if and when one was incoming," said Prentice.

The solar system is home to millions of asteroids, but only a few are known to cross Earth's path, usually much smaller in stature that 2005 YU55.

"Asteroids actually hit our planet all the time," said Prentice, adding they're usually too small to detect, or burn up on their way into our atmosphere. "But if one this size hit us, it would have catastrophic consequences."

Fortunately for us, Prentice estimates an actual strike by an asteroid of this size might occur every 10,000 years or so.

Another slightly smaller asteroid named Apophis, which measures 300 metres across, will come even closer to us on April 13, 2029.