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A massive parabolic dish is seen at the Goldstone Deep Space Network complex in the Mojave Desert near Barstow, California.

SCOTT NELSON/AFP

Scientists are gearing up to observe a mountain-size space rock as it hurtles past our planet on Monday.

The 500-metre-wide asteroid designated 2004 BL86 is larger than most of the celestial interlopers that periodically whiz by the globe. However, the encounter poses no threat to life on Earth. At its closest approach, about 11:20 a.m. ET on Monday, the asteroid will be about 1.2 million kilometres away, or just over three times farther from us than the moon.

No asteroid as large as this is expected to pass this close to Earth before 2027. To take advantage of the chance event, scientists with NASA's Jet Propulsion Laboratory will use giant radio dishes in Goldstone, Calif., and Arecibo, Puerto Rico, to bounce radar signals off the speeding asteroid's rocky surface.

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"The exciting research opportunity is that the asteroid comes close enough for resolution of the surface," said Alan Hildebrand, a planetary scientist at the University of Calgary.

The radar observations should help pin down the asteroid's size, shape and rotation with better precision and could potentially reveal interesting details such as the size and number of boulders on its surface, Dr. Hildebrand said.

As NASA and other agencies work toward a new generation of asteroid missions with the goal of returning samples to Earth, observations of close encounters such as this will help put the detailed information expected from those missions in broader context.

"And the more we learn, the better the missions can plan," said Dr. Hildebrand, who is a Canadian principal investigator on OSIRIS-REx, a NASA-led mission to a similar size asteroid, scheduled for launch in September of 2016.

Asteroid 2004 BL86 was spotted more than a decade ago by a telescope in New Mexico that is part of an automated search to find potentially hazardous objects that are adrift in our part of the solar system.

About 12,000 asteroids have now been identified with orbits that bring them near Earth, though only a fraction of those, about 1,000, are a kilometre or larger in size – large enough for a collision with Earth to have global environmental consequences.

Such a collision has never occurred during human history; others are known to have affected Earth over the course of geologic time. The extinction of the dinosaurs 66 million years ago has been linked to the impact of an asteroid that was at least 10 kilometres in diameter.

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Even an asteroid as large as 2004 BL86 could cause regional damage over thousands of square kilometres or raise devastating tsunamis if it struck the ocean. It is this rare but real danger that has, in recent years, motivated further study of asteroids that pass near Earth.

Although this asteroid will be too faint to observe with the unaided eye, it should be within the range of most backyard telescopes for those who know where and when to look.

The asteroid will be found travelling through the constellation Cancer on Monday evening. Detailed finder charts can be found online courtesy of Sky & Telescope magazine.

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