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A double asteroid – in the manner of a large ship followed by a lifeboat – passed the Earth in late May 2019, providing researchers with an excellent opportunity to study it.
The 1999 KW4 asteroid includes a primary car about 1.3 km in diameter, with a much smaller rock trailing in its wake at high speed, about 2.6 km behind.
The binary is very well known and does not pose a danger to the Earth, but its orbit brings it close enough – at a distance of at least 5.2 million kilometers – to serve as a substitute for any future impact risk.
To this end, astronomers at the Sothern European Observatory in Chile have deployed an extremely sensitive kit called a spectro-polarimetric instrument for high-contrast exoplanet research (SPHERE), above the very large observatory telescope (VLT), to get the sharpest images ever taken.
The SPHERE instrument was originally designed to observe exoplanets, but Diego Parraguez's researchers decided to test it on 1999 KW4 to see if it could be useful in case of an asteroid threat – and to learn a bit more about the binary itself.
"The double asteroid was firing more than 70,000 kilometers at the time near Earth, making observation with the VLT difficult," he says.
And even though the asteroid will never come close enough to graze Earth, it looks a lot like another binary system, known as Didymos, which has been considered a potential threat of impact in many centuries .
Didymos also has a miniature partner, fondly nicknamed Didymoon. The system will be the subject of NASA's next global defense exercise, called the Double Asteroid Redirection Test Mission (DART), where a craft will be fired at a smaller object to alter its orbit.
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