By colliding with an asteroid, researchers can see how they can better protect Earth

Atop a SpaceX Falcon 9 rocket, NASA's Double Asteroid Redirection Test, or DART, the spacecraft will take off to collide with an asteroid during the world's first full planetary defense test mission in November 2021.
Enlarge / Atop a SpaceX Falcon 9 rocket, NASA’s Double Asteroid Redirection Test, or DART, the spacecraft will take off to collide with an asteroid during the world’s first full planetary defense test mission in November 2021.

On a fall evening in 2022, scientists at the Johns Hopkins University Applied Physics Laboratory were completing the final stages of a planetary defense mission. As Andy Rivkin, one of the team leaders, prepared to appear on NASA’s live broadcast of the experiment, a colleague posted a photo of a pair of asteroids: the 800-meter-wide Didymos and, orbiting it, a smaller one. called Dimorphos, about 7 million miles from Earth.

“We could see Didymos and this little speck in the right place where we expected Dimorphos to be,” Rivkin recalled.

After the interview, Rivkin joined a crowd of scientists and guests to watch the mission’s finale on several large screens: As part of an asteroid deflection mission called DART, a spacecraft approached Dimorphos and photographed its rocky surface in increasing detail.

Then, at 7:14 p.m., a spacecraft weighing about 1,300 pounds collided head-on with the asteroid.

Within minutes, members of the mission team in Kenya and South Africa posted images from their telescopes showing a bright plume of debris.

In the days that followed, researchers continued to observe the dust cloud and found that it had morphed into various shapes, including clumps, spirals and two comet-like tails. They also calculated that the impact slowed Dimorphos’ orbit by about a tenth of a centimeter per second, a proof-of-concept that a spacecraft – known as a kinetic impactor – could target and deflect an asteroid far from Earth.

The last five and a half minutes of footage from the DART spacecraft as it approached and then deliberately collided with asteroid Dimorphos. The video is 10 times faster than reality, except for the last six images.

NASA/Johns Hopkins APL/YouTube

Ron Ballouz, a planetary scientist at the lab, noted that what is often seen in movies is a “kind of last-ditch effort, what we like to call a final phase of planetary defense.” But if dangerous objects can be detected years in advance, other techniques such as a kinetic impactor can be used, he added.

If a deflection were necessary, scientists would have to change the speed of a dangerous object, such as an asteroid or comet, so that it does not end up in the same place and time as Earth as they orbit the sun. Rivkin said this translates into a change of at least seven minutes in the arrival time: for example, if an object the size of Dimorphos were predicted to collide with Earth in 67 years, the delay caused by DART would be just enough to allow to the seven-minute mark, he added.

With less lead time, researchers could use a combination of multiple deflections, larger spacecraft or speed increases, depending on the dangerous object. “DART is designed to validate a technique, and specific situations would inevitably require adjustments,” says Rivkin.

Researchers use data from DART and smaller-scale experiments to predict the amount of deflection using computer simulations.

Scientists are also focusing on the type of asteroid that Dimorphos appears to be: a “rubber pile,” as they call it, because these types of objects are thought to be made of clumps of many rocks.

In fact, scientists think that most asteroids the size of Dimorphos and larger are debris clusters. As scientists learn more about debris piles, they will be able to make better predictions about the deflection of asteroids or comets. And in 2026, a new mission will arrive in Didymos and Dimorphos to collect more data to refine the computer models.

In the meantime, researchers are trying to learn as much as possible in the untoward event that an asteroid or comet is discovered to pose a threat to Earth and a faster response is necessary.

Scientists first suspected that many asteroids were debris clusters about fifty years ago. Their models showed that when larger asteroids collided, the collisions could throw off fragments that would then reassemble to form new objects.

It wasn’t until 2005 that scientists saw their first mess, asteroid Itokawa, when a spacecraft visited and photographed it. Then in 2018 they saw another planet called Ryugu, and later that year another, asteroid Bennu. DART’s camera also showed that Didymos and Dimorphos are probably the same species.

“It’s one thing to talk about rubble piles, and another to see what looks like a pile of rocks dumped from a truck up close,” said William Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado.

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