Most members of the asteroid belt are primitive celestial bodies, almost unchanged from the early days of the solar system. But a small handful of objects are surprisingly active, streaming debris in their wake. A new, close-up look at one of these active asteroids has revealed the impact that triggered its activity, providing a rare glimpse of real-time collisions in the asteroid belt.

On 1 April 2016, the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) discovered a comet-like object inside the asteroid belt. A massive survey based out of Hawaii, Pan-STARRS’s primary mission is hunting down objects that might eventually collide with Earth. Along the way, the enormous survey has discovered new astronomical objects and events, including active asteroids.

Tracing how the dust moved as it interacted with the Sun’s gravity and radiation, astronomers established that the new object, P/2016 G1, became activated on 6 March 2016.

“We can safely conclude that the cloud of dust that we observed continued to disperse into nothingness—it’s gone.” “Because of the speed of things in the asteroid belt, that impact was really dramatic,” said Olivier Hainaut, an astronomer at European Southern Observatory who works to understand how cometary dust behaves.

Hainaut led observations of the new active asteroid with the Canada-France-Hawaii Telescope (CFHT) over the 3 months after the object was discovered. These observations allowed researchers to follow the collision and trace how the impact created a crater and shattered the tiny asteroid. The research was published in the journal Astronomy and Astrophysics.

By 2017, P/2016 G1 was completely gone, most likely blown apart by the collision.

“We can safely conclude that the cloud of dust that we observed continued to disperse into nothingness—it’s gone,” Hainaut said. “Most likely, we will never know whether there is one or a few largish chunks left, representing the bulk of the original body, or if it was completely disintegrated into pebbles and dust.”

Too Small to See

Active asteroids come in several flavors. Some act like comets, material streaming from them only when they draw close to the Sun. Others suddenly produce plumes of debris after a collision with another object or after they’ve begun to slowly break apart on their own. But with only about 30 active asteroids known to date, what causes most of the asteroid belt’s activity remains a mystery.

“For practical reasons, it’s hard to get a lot of data on these objects, and we need quite a lot of data to understand what causes them,” said David Jewitt, an astronomer at the University of California, Los Angeles. Jewitt, who studies active asteroids, was not part of the new research.

Dust grains in our solar system suffer primarily from two opposing forces. Gravitational forces tug the grains toward the Sun, whereas solar radiation pushes them away. The forces affect dust grains differently on the basis of their radii.

“If you take a small asteroid and instantly destroy it into dust, then you will see a line forming, all of the dust grains sorted by size,” Hainaut said. The smaller grains drift the fastest, midsized grains less so, and the largest barely move at all. Rewinding the drift of the grains allowed Hainaut and his colleagues to narrow down the start of the disintegration process to within a few days.

Most active asteroids suffer from another problem: They are incredibly small. P/2016 G1 was no more than 400 meters across before its collision put it in the spotlight, and Hainaut suspects it was closer to 200 meters. At this size, it remained hidden from modern telescopes.

Only after something slammed into the tiny asteroid did the billowing cloud of dust make it large and bright enough to be seen. That’s par for the course; “a lot of them are too small to [be seen] until they become active,” said Henry Hsieh, a senior scientist at the Planetary Science Institute who was not part of the current study.

A review of Pan-STARRS images prior to the impact revealed no sign of P/2016 G1. It became visible only on 7 March, the day after the suspected impact.

Although scientists have been simulating asteroid impacts in the laboratory on small scales, the glimpse of an almost-real-time collision can help validate the way they have scaled up the process.

Understanding how impacts work can make a difference in programs that hope to, one day, use collisions or explosions to move asteroids out of the way of a potential impact with Earth. “It’s much better to see the real thing,” Jewitt said.

Understanding how impacts work can make a difference in programs that hope to, one day, use collisions or explosions to move asteroids out of the way of a potential impact with Earth.

“We’re learning that a lot of things can happen with impacts,” Hsieh said.

At the same time, understanding active asteroids can help planetary scientists to better glean how asteroids evolve and change over time, especially when they experience a collision.

“With this story, we can really rebuild the movie of what happened,” Hainaut said.

—Nola Taylor Redd (@nolatredd), Freelance Science Journalist