Astronomers have been keenly watching the asteroid Didymos since the historic DART mission successfully crashed a spacecraft into it last month. And now, Hubble has detected something unexpected – the asteroid has sprouted two tails.

NASA’s DART mission was designed as a test run for a planetary defense system. A spacecraft was deliberately smashed into a small asteroid called Dimorphos, to see if the impact was enough to change its orbit around a larger rock called Didymos. If this worked, it could give us a way to defend ourselves if we were to ever spot an asteroid on a collision course with Earth.

Sure enough, follow-up observations showed that the test was a success, with its original 12-hour orbit being shortened by 32 minutes. But that wasn’t the only outcome – two days after impact, the SOAR Telescope in Chile imaged the asteroid and found that it had grown a tail that stretched more than 10,000 km (6,000 miles).

Hubble continued to observe the asteroid, snapping 18 images of Didymos in the weeks after the impact, and spotting a few interesting developments. The first was that the ejected material has expanded and faded over time – nothing too surprising there. But the unexpected discovery was that sometime between October 2 and October 8, the asteroid had grown a second tail.

In the above image, the upper tail is the new one, while the DART spacecraft approached from the top left of frame. The Sun is off to the lefthand side of the image.

It may be unexpected in this case, but the find isn’t entirely unprecedented – comets and other active asteroids have been known to grow extra tails in the past. Exactly how this second tail formed remains uncertain, but the data will be analyzed further to find an answer, with several scenarios thought possible.

The Didymos system will continue to be observed for any other changes.