NASA is testing a plan to deflect killer asteroids — by crashing into one

Originally published on October 6, 2019.

On July 25, 2019, we were 73,000 km away from having a very bad day here on planet Earth. An asteroid the size of a football field — big enough to annihilate a major city — passed that close. Near misses by asteroids are a relatively commonplace occurrence, and there's little doubt that one of these days one will have our number.

But researchers at NASA and the European Space Agency (ESA) aren't waiting for an emergency. They're planning a test of a "planetary defence" mission designed to deflect an incoming asteroid.  

The AIDA project, short for Asteroid Impact and Deflection Assessment, is a collaboration between the two space agencies. It involves two different missions. NASA's DART (Double Asteroid Redirection Test) spacecraft will launch in 2021 and the following year will smash into an asteroid to attempt to deflect it in its orbit.

ESA's Hera mission will follow a couple of years later to rendezvous with the same asteroid and make observations about how the deflection worked. 

An asteroid will get a nudge

The coordination lead for the DART mission is Nancy Chabot, a planetary scientist at Johns Hopkins University Applied Physics Laboratory. 

"Our plan is definitely deflection not disruption," Chabot told Quirks & Quarks host Bob McDonald. "So we're not looking to blow the asteroid up. Instead we're going to give it a slight little nudge which would change its future path."

The target is the smaller of the two bodies that make up the Didymos binary asteroid system, which orbits its larger companion once every twelve hours. The smaller of the two asteroids is 160 metres across and the larger is 780 metres, and they orbit about one kilometre apart.

"You can't actually make out the fact that it's two asteroids through a telescope, but what you can make out is that the brightness of that point of light changes roughly on this 12 hour period," said Chabot. "That's because the smaller asteroid passes in front of or behind the larger one. So it's like a little mini clock out there."

Kinetic Impactor Technology

Dart's impact will reset that clock using something NASA is calling "kinetic impactor technology," which is simply a high-speed crash. The spacecraft is about two metres on a side, which is small compared to the size of the asteroid it's meant to move. But it will crash into the smaller asteroid with tremendous speed — about 24,000 km/h.  

This will transfer enough momentum to slightly change the path of the smaller asteroid around the larger asteroid. The 12-hour orbit will get a little tighter.

"We're gonna make that about ten minutes shorter," said Chabot. "So change it by about one per cent."

Asteroid deflection animation. (European Space Agency)

The DART spacecraft itself won't be able to observe the effects of its impact, since it will be completely destroyed by the collision, but it will see everything up to that last moment. DART will be equipped with an on-board camera that will not only help it aim itself at the smaller of the two asteroids, but will also record images up to the final second.

"You can imagine it getting closer and closer and closer until there are no more images streaming back," said Chabot. "We'll be streaming those back to Earth in nearly real time, taking one image per second."

DART will also be equipped with a CubeSat, a miniature satellite with a camera, courtesy of the Italian Space Agency. This will be launched five days prior to the crash and will record images of the impact from a safe distance. As the CubeSat has no engines it won't be able to remain on scene to observe the after-effects of the collision, but will fly past at the same speed that DART strikes with.

To see the results of their mission, those on the DART team will use telescopes here on Earth to observe the asteroid pair and see what they hope will be the successful change that DART will have made to the orbital dance of the two Dydimos asteroids. Further close-up observations of the impact will be made by the ESA's Hera mission when it arrives in 2026.

DART hitting an asteroid (APL)

Saving the planet in good time

The key to the success of asteroid deflection is planning and time, said Chabot, not Hollywood-style heroics.

Their test with DART will allow researchers to understand how much they can move an asteroid with an impact, but the plan is to have a small change in the asteroid's path now, magnify into a big change in its path much later.

"You don't want to just try that out when you need it. It wouldn't be like Armageddon where you would do it at the last minute and save the world," said Chabot. "This is something that you would want to do decades in advance because you'd make this small nudge and then it would add up over time and then you would save the Earth."

Produced and written by Sonya Buyting