“on the timescales involved in this event, we’re really catching it in the moment of happening”

When the comet-like asteroid P/2010 A2 was identified as a suspected asteroid-asteroid collision in January this year it was the first such collision caught “in the act”.  Images Credit: NASA, ESA, and D. Jewitt (UCLA).

As the BBC notes, subsequent observations by the Hubble Space Telescope and the Rosetta probe suggest that the collision probably occurred in early 2009.

Here’s a sequence of Hubble observations from January to May 2010, with scale.

As the associated text explains

The asteroid debris, dubbed P/2010 A2, appears to be shrinking in each successive image because Earth’s faster orbit is carrying the planet away from the object. Between January and May, Earth rotated more than 100 million miles away from the debris field. The object was 102 million miles from Earth when Hubble first observed it in January 2010.

P/2010 A2 was found cruising around the asteroid belt, a reservoir of millions of rocky bodies between the orbits of Mars and Jupiter.

The images were taken in visible light and artificially colored blue.

And from the Hubble site

The Hubble images, taken from January to May 2010 with Wide Field Camera 3, reveal a point-like object about 400 feet (120 meters) wide, with a long, flowing dust tail behind a never-before-seen X pattern. The observations also show that the object retained its X shape even as the debris field slowly expanded. Particle sizes in the tail are estimated to vary from about 1/25th of an inch (a millimeter) to an inch (2.5 centimeters) in diameter. P/2010 A2 was 102 million miles from Earth when Hubble first observed it in January 2010.

The 400-foot-wide object in the Hubble image is the remnant of a slightly larger precursor body. Astronomers think a smaller rock, perhaps 10 to 15 feet (3 to 5 meters) wide, slammed into the larger one. The pair probably collided at high speed, about 11,000 miles (18,000 kilometers) an hour, which smashed and vaporized the small asteroid and stripped material from the larger one. Jewitt estimates that the violent encounter was as powerful as the detonation of a small atomic bomb.

Radiation pressure from the Sun then swept the debris behind the remnant asteroid, forming a comet-like tail. The tail contains enough dust to make a ball 65 feet (20 meters) wide, most of it blown out of the bigger body by the impact-caused explosion.

The two asteroids were probably no strangers to collisions. They were themselves most likely relics from impacts between larger asteroids that occurred tens or hundreds of millions of years ago. This collisional grinding from large sizes down to small is thought to be one of the main processes by which asteroids are destroyed.

And a short video of those observations [Credit: NASA, ESA, and G. Bacon (STScI).  Science Credit: NASA, ESA, and D. Jewitt (UCLA)].