Fast-approaching asteroid has its own small moon

Earth will soon get a visit from not one asteroid, but two. The first radar images of asteroid 1998 QE2 have revealed that the incoming space rock has an unexpected companion ? a small moon that may be the key to unlocking the asteroid's secrets.

Astronomers have been tracking the larger asteroid since it was discovered in 1998. Calculations of its orbit around the sun show that it will swing about 5.8 million kilometres from our home planet, or about 15 times the distance between Earth and the moon, at 2059 UTC (2159 British Summer Time, 1659 Eastern Standard Time) on 31 May.

There is no risk of an impact, but it is getting close enough to take good images with telescopes on the ground. Asteroid enthusiasts from NASA to the White House have been gearing up to watch 1998 QE2 make its closest approach.

"We find something neat about every one that comes in like this," says Tim Spahr of the Minor Planet Center at Harvard University. "They're all different, and it's very cool what you can learn from close approaches."

Pile of fluff?

On 29 May, astronomers using the 70-metre Deep Space Network antenna in Goldstone, California, got their first glimpse of 1998 QE2 when it was 6 million kilometres from Earth. To their surprise, the radar images showed a small, bright blip moving around the larger craggy body.

Initial readings suggest that the main asteroid is 2.7-kilometres wide, and its bright companion is roughly 600 metres across.

Asteroid duos and even trios make up 15 per cent of near-Earth objects larger than 200 metres. The discovery of the small moon around 1998 QE2 gives us a rare opportunity to weigh the larger object, and that can tell us what it is made of, says Spahr.

Some asteroids are solid chunks of rock and metal, while others are more like piles of rubble loosely bound by gravity. Measuring the orbit of the moon can reveal the larger asteroid's mass. Combine that with its size, and we'll know QE2's density.

"This is information we would not get otherwise, without sending a spacecraft to orbit the thing," says Spahr.

Revealing the object's composition might be important for astronauts planning to visit similar asteroids in future, whether as part of NASA's scheme to bring an asteroid close to Earth, or as part of planned private mining enterprises.

"If you're an astronaut who wants to go walk around on this thing, you could land in a bank of cottonwood fluff," says Spahr. "It's that when we do go visit something, we know what to expect."

Clarification: Since this article was first published on 31 May 2013, we have quantified the commonness of duos and trios of near-Earth objects.

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