An asteroid known as "2004 BL86" is being compared in size to an ocean liner or a mountain, but the massive planetoid is expected to pass Earth by Monday.

According to Reuters, NASA projected the half-kilometer wide asteroid to make its approach Monday night, though it poses no threat. 2004 BL86 will be some 1.2 million kilometers from Earth, which is about three times farther than the moon.

"While it poses no threat to Earth for the foreseeable future, it's a relatively close approach by a relatively large asteroid, so it provides us a unique opportunity to observe and learn more," Don Yeomans, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, said in a statement.

Jonathan Powell, secretary of the Abergavenny and District Astronomical Society, told BBC News the asteroid's approach is far enough not to worry and close enough to take notes.

"We get an asteroid almost daily passing earth but this one is exciting because it is exceptionally large and because of the timing - we should get a good view of it in the night sky above Wales," he said. "Most asteroids are so small they cannot be seen, even with amateur telescopes.

"But you this one is so large and so close - just powerful binoculars and telescopes will be able to spot it in Welsh skies from around 8 p.m. onwards."

NASA estimated the next time Earth would see another asteroid like this one will be in 2027 when 1999 AN10 makes its approach.

Slooh.com is offering a live stream of 2004 BL86's flyby complete with commentary from their own astronomers and some from NASA.

"Realistically speaking it's only a third of a mile, and the moon by comparison, is 2,160 miles wide," Bob Berman, a Slooh astronomer, told Space.com "So you figure this is one 6,000th the diameter of the moon. Plus it's three times further away. So if you put those together it'll appear 18,000 times smaller than the moon. So that means it's a dot through any telescope.

"For people who want to look in the right direction you just have to find the brightest 'star' in the sky... and that's Jupiter."

The asteroid will fly between the stars Betelgeuse and Sirius.