Asteroid was Earth's closest encounter in three decades

This image made from radar data by the Arecibo Radar Telescope in Puerto Rico shows asteroid 2005 YU55. Photo / AP

A 400m asteroid by-passed Earth today in the closest encounter by such a massive space rock in more than three decades.

Asteroid 2005 YU55, which is nearly three times the length of a rugby field, came within 325,000km of Earth - 85,000 km closer than the moon - at about 12.30pm today, travelling at speeds of almost 13 km per second.

The asteroid's trajectory has been tracked by Nasa since its discovery in 2005, and according to their data, we will remain well out of the object's path for at least the next 100 years.

But when the asteroid passes close to Venus in 2029 its path may be altered, meaning predictions past that point are not guaranteed, said astronomy educator David Britten.

Asteroid 2005 YU55 travels an elliptical path, meaning it passes the Earth, Venus and Mars regularly - meaning this was not the first time it has crossed our skies, said astrologist Ron Dantowitz.

"The scary thing: This thing has been past the Earth several times and we never even noticed it."

Although the asteroid missed us this time, Dantowitz believes an asteroid impact is ultimately inevitable. It's happened before, and it will happen again.

"It's literally a matter of time," Datowitz told FoxNews.com. "The bad news, the last one hit about 60 million years ago, give or take."

"It's a reminder that we need to pay attention to the things out in space."

The next approach of a large asteroid will be in 2028, when an asteroid up to 1500 m in diameter will pass within 230,000km of Earth.