Double trouble: a binary asteroid zooms past

A double asteroid – rather in the manner of large ship followed by a lifeboat – sped past Earth in late May 2019, affording researchers an excellent opportunity to study it.

Asteroid 1999 KW4 comprises a primary bolide, about 1.3 kilometres in diameter, with a much smaller rock trailing in its high-speed wake, about 2.6 kilometres behind.

The binary is very well known and does not constitute a hazard to Earth, but its orbit does bring it close enough – at a minimum distance of roughly 5.2 million kilometres – to serve as a proxy for any future impact hazard. 

To this end, astronomers at the European Sothern Observatory in Chile deployed a highly sensitive bit of kit, known as the Spectro-Polarimetric High-contrast Exoplanet Research instrument (SPHERE), on top of the observatory’s Very Large Telescope (VLT), to secure the sharpest images of it ever taken. 

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

“The double asteroid was hurtling by the Earth at more than 70,000 kilometres per hour, making observing it with the VLT challenging,” he says.

And while the asteroid will never come close enough to graze the Earth, it bears a close resemblance to another binary system, known as Didymos, which has been assessed to be a possible impact threat several centuries hence.

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