A double asteroid – more in the nature of a post-mortem – sped past Earth in late May 2019
Asteroid 1999 KW4 comprises a primary bolide, about 1.3 kilometers in diameter, with a much smaller rock trailing in its high-speed wake, about 2.6 kilometers behind.
5.2 million kilometers – 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 originally designed to observe exoplanets, researchers by Diego Parraguez decided to test it on 1999 KW4 to see if it could be useful in any asteroid threat emergency – and so to learn more about the binary itself.
"The double asteroid was hurtling by the Earth at more than 70,000 kilometers per hour, making it difficult to observe the VLT," he says.
And while the asteroid does not come close enough to graze the Earth, it bears a close resemblance to another binary system, known as Didymos, which has been assessed as having a possible impact threat several centuries hence.
Didymos also has a miniature partner, affectionately nicknamed Didymoon. NASA's planetary defense exercise called the Double Asteroid Redirection Test Mission (DART), in which a craft wants to be fired at the smaller object in an attempt to alter its orbit.