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Double problem: the binary asteroid photographed as it approaches.

At the end of May 2019, a double asteroid, rather like a large ship followed by a lifeboat, sped through Earth, giving researchers an excellent opportunity to study it.

The 1999 KW4 asteroid comprises a primary bolide, approximately 1.3 kilometers in diameter, with a much smaller rock that crawls in its wake of high speed, about 2.6 kilometers behind.

The binary is well known and does not constitute a danger to Earth, but its orbit brings it close enough, at a minimum distance of approximately 5.2 million kilometers, to serve as a proxy for any danger of future impact.

To this end, the astronomers of the Sothern European Observatory in Chile deployed a highly sensitive kit, known as the high-contrast spectro-polarimetric exoplanet research instrument (SPHERE), at the top of the Observatory's Very Large Telescope (VLT), to ensure the sharpest images ever taken.

The SPHERE instrument was originally designed to observe exoplanets, but researchers led by Diego Parraguez decided to test it on the 1999 KW4 to see if it could be useful in any asteroid threat emergency, and also to learn a little more about the binary itself.

"The double asteroid was at full speed across the Earth at over 70,000 kilometers per hour, which makes its observation with the VLT a challenge," he says.

And although the asteroid will never get close enough to touch the Earth, it has a great resemblance to another binary system, known as Didymos, which has been evaluated as a possible threat of impact several centuries later.

Didymos also has a companion in miniature, affectionately nicknamed Didymoon. The system will be the target of an upcoming NASA planetary defense exercise, called the Double Asteroid Redirection Test Mission (DART), in which a ship will fire at the smaller object in an attempt to alter its orbit.

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