Science & Environment

Near asteroid 2004 BL86 has a moon

2004 BL86 Image copyright NASA-jpl
Image caption The 325m-wide object known as 2004 BL86 and its 70m-diameter moon

Scientists who tracked a large asteroid passing relatively close to the Earth say it has a small satellite.

The 325m-wide object, known as 2004 BL86, swept by our planet at the very safe distance of 1.2 million km on Monday, and gave researchers a good view of its shape using radar imaging.

As well revealing information on the space rock's rotation rate and surface properties, the pictures also identified the 70m-diameter moon.

2004 BL86 is not a threat to Earth.

Even before Monday’s pass, scientists had calculated that it would be the asteroid’s closest encounter for at least the next two centuries. And the newly acquired radar data should enable further, confident predictions to be made long into the future.

The next time such a large asteroid will get anything near as close to Earth is in 2027.

That object is called 1999 AN10 and is just over 1km wide.It could flyby beneath the altitude of geostationary satellites, which sit 36,000km above the Earth. But that is an extreme scenario.

Scientists expect the miss distance to be considerably greater by the time they have refined the details of its orbit with additional observations. Identifying and tracking asteroids is a pressing issue.

Rocky debris is constantly raining down from space, but scientists are most concerned with the large objects because of their destructive potential.

Sky surveys suggest that we have probably found a little over 90% of the true monsters out there - the km and bigger-scale objects that could lead to extinctions if they struck the Earth. And the good news is that none look like they will hit us anytime soon.

But data from Nasa's Wise telescope indicates that the population of objects in the 100-1,000m size range may number close to 20,000, and the vast majority of these have yet to be identified and tracked.

The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) operates a network of infrasound detectors around the globe.

This can sense clandestine A-bomb detonations, but also has the capability to “hear” asteroid strikes.

The network counted 26 major explosions between 2000 and 2013, ranging in energy from one to 600 kilotons. By way of comparison, the bomb that destroyed the Japanese city of Hiroshima was a 15-kiloton device.

Fortunately, most of these space rocks disintegrated high up in the atmosphere and caused few problems on the ground.

The object that broke up above the Russian city of Chelyabinsk in 2013 was about 20m wide.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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