Can we safely deflect a killer asteroid without making it worse? Only if we avoid the gravitational ‘keyhole,’ scientists say

Smacking a planet-threatening asteroid comes with big responsibility.

If we slam an impactor into an asteroid in exactly the wrong spot, the space rock may pass through a “gravitational keyhole” that actually brings it to Earth — which is exactly what planetary defense scientists and mission planners don’t want to happen.

The keyhole is an area in space where the gravity of a planet, like Jupiter, could change the asteroid’s altered trajectory once more — potentially putting it on a collision course with our planet. The findings were presented at the Europlanet Science Congress division for planetary sciences meeting in Helsinki on Wednesday (Sept. 9), and submitted to the peer-reviewed Icarus journal.

“Even if we intentionally push an asteroid away from Earth with a space mission, we must make sure it doesn’t drift into one of these keyholes afterwards. Otherwise, we’d be facing the same impact threat again down the line,” lead author Rahil Makadia, a NASA-funded graduate fellow at the University of Illinois Urbana-Champagne, said in a statement.

To avoid this scenario, Makadia’s team plotted the best spots on an asteroid’s surface to avoid the keyhole. Their technique borrows from results obtained during a 2022 test mission for asteroid collision, called Double Asteroid Redirection Test (DART). (Makadia was a member of the DART science investigation team, his website states.)

DART deliberately tested how to impact an asteroid at a location presenting no threat to Earth. The NASA-funded mission slammed a kinetic impactor into an asteroid moonlet, Dimorphos, which circles the asteroid Didymos. The spectacular results on Sept. 26, 2022 not only proved asteroid deflection was possible, but showed a haunting dust trail visible in telescopes stationed all over the solar system.

Each asteroid is different, Makadia’s team emphasized, so finding the ideal spot for a successful deflection involves looking at the space rock’s shape, rotation, mass and surface topology (its distribution of hills and craters).

Ideally, just like with DART, scientists would use a spacecraft to check out all this stuff first. But if time is of the essence, Makadia stated it would be “possible using ground-based observations alone.”

The team then created “probability maps” showing where the asteroid is most likely to veer after an impact, depending on where it is hit. (The asteroid Bennu, visited by the OSIRIS-REx mission in 2020, was used for some of the keyhole mapping simulations, although the study is not examining Bennu in particular.)

Looking at all of these trajectories therefore allows scientists to determine where the safest zone would be for asteroid deflection.

“With these probability maps, we can push asteroids away while preventing them from returning on an impact trajectory, protecting the Earth in the long run,” said Makadia.