Double asteroid tracked by VLT in planetary defence test

The left frame shows asteroid 1999 KW4 as imaged by the European Southern Observatory’s Very Large Telescope and the SPHERE planet-hunting instrument. The image on the right is an artist’s impression of the double asteroid. Image: ESO

The European Southern Observatory’s Very Large Telescope participated in test coordinated by the International Asteroid Warning Network, successfully targeting, tracking and imaging a double asteroid as it flew within 5.2 million kilometres (3.2 million miles) of Earth on 25 May.

Using the sensitive SPHERE planet-finding instrument attached to VLT’s Unit 3 telescope, along with state-of-the-art adaptive optics, asteroid 1999 KW4’s two components, separated by just 2.6 kilometres (1.6 miles), were clearly seen despite their rapid track across the sky and somewhat unstable air above the observatory.

The double asteroid was moving through space at some 77,000 kilometres per hour (48,000 mph). While the larger member of the pair has a diameter of about 1.3 kilometres (0.8 mile), the duo’s orbit is well known and there is no risk of impact with Earth.

But by studying the passing asteroids with a variety of instruments, the IAWN campaign aims to learn more about detecting, tracking and studying threatening asteroids to give engineers and scientists time to develop possible countermeasures.

“These data, combined with all those that are obtained on other telescopes through the IAWN campaign, will be essential for evaluating effective deflection strategies in the event that an asteroid was found to be on a collision course with Earth,” said ESO astronomer Olivier Hainaut.

“In the worst possible case, this knowledge is also essential to predict how an asteroid could interact with the atmosphere and Earth’s surface, allowing us to mitigate damage in the event of a collision.”

Imaging asteroid 1999 KW4 was a challenge for the VLT given its distance and rapid movement across the sky. Image: ESO

Observing the fast-moving double asteroid was a major challenge for operators of ESO’s VLT.

“During the observations the atmospheric conditions were a bit unstable,” said Mathias Jones, a VLT astronomer. “In addition, the asteroid was relatively faint and moving very fast in the sky, making these observations particularly challenging, and causing the AO system to crash several times. It was great to see our hard work pay off despite the difficulties.”

1999 KW4 is similar in appearance to Didymos, a double asteroid made up of a 780-metre-wide (2,560-foot-wide) component – Didymos A – and a smaller sibling measuring 160 metres (525 feet) across.

Assuming funding questions are resolved, NASA’s Double Asteroid Redirection Test, or DART, spacecraft will deliberately crash into Didymos B in 2022 in an attempt to slightly change its orbital velocity and in so doing, demonstrate the feasibility of deflecting a threatening asteroid. Observations like those of 1999 KW4 will be carried out to document the mission’s results.