Bearing Witness to an Asteroid’s Assassination

The asteroid belt, hanging out between Mars and Jupiter, is not like the cluttered debris field in “The Empire Strikes Back.” It may contain millions of rocky and metal objects, but the distances separating them are vast, and collisions are rare.

That is what makes P/2016 G1 such an exciting object. Spotted zipping through the asteroid belt in early 2016, this object had a strange orbit and a tail of dust that resembled a comet. Through a careful analysis of telescopic imagery, scientists identified multiple showers of debris shooting up from its surface, the sort that could have only been produced by an impact.

What they had stumbled across was not a comet, but the immediate aftermath of an asteroid’s assassination.

On or around March 6, 2016, an asteroid at least 1,300 feet in diameter was minding its own business when another space rock, weighing around 2.2 pounds and perhaps a foot long or so, slammed into the larger asteroid at roughly 11,000 miles per hour. That’s about five times as fast as a bullet fired from a sniper rifle. The projectile was obliterated upon impact; the target then broke up in stages over the coming months before becoming impossible to see.

Without this collision, these two small objects would have remained forever anonymous. Instead scientists gained a serendipitous insight into the destructibility of asteroids, which could help defend Earth against future asteroid hazards. After all, “the best way to see how hard something is, is to break it,” said Olivier Hainaut, an astronomer at the European Southern Observatory and lead author of the study published earlier this year in Astronomy & Astrophysics.

Astronomers first discovered P/2016 G1 with the Pan-Starrs1 telescope in Hawaii in April 2016. Backtracking through archived images, astronomers realized that it had first been visible the previous month as a centralized collection of rocky clumps: the fractured, rubbly remnants of the asteroid, surrounded by a fine dust cloud, most likely the immediate debris jettisoned by the impact.

Over the ensuing weeks, an expanding ring of debris could also be seen emerging from the object. Computer simulations revealed this to be the beginning of a cone of uplifted rubble, a signature feature of an impact event.

After the initial debris cloud was created, the cratering process lost energy and subsequent streams of debris were more slowly excavated from the asteroid’s new scar. On Earth, this ring of debris would land around the crater. But on a tiny asteroid with little gravity, this debris ring simply flew into space, expanding as it went.

There is no clear date when the asteroid disappeared. Documenting the vanishing of P/2016 G1 was like tracking a drop of milk in your coffee, Dr. Hainaut said: Parts spread out and faded away individually. In any case, as of December 2018, the asteroid could no longer be seen.

While the asteroid may be gone, the collected data could be helpful in the future. With sufficient warning time, an asteroid heading toward Earth would ideally be deflected away by ramming a spacecraft into it at remarkable speeds. But an overzealous impact could break an asteroid into fragments that could still disastrously crash into Earth.

Knowing what types of impacts cause deflections and disruption is key to Earth’s protection from errant asteroids. That makes the demise of P/2016 G1 a vital source of information, said Megan Bruck Syal, a planetary defense researcher at the Lawrence Livermore National Laboratory who was not involved with the study.

This spectacularly documented event may not be such a rarity for much longer. Increasingly comprehensive sky-scanning surveys, including the upcoming Large Synoptic Survey Telescope in Chile, will catch many more of these impacts on camera, giving planetary defense researchers more data to play with in cutting-edge simulations.

“An asteroid cannot misbehave anymore” without us seeing it, Dr. Hainaut said.