When a massive asteroid is hurtling toward Earth, the solution seems straightforward; smash a spacecraft into it and knock it off course. That’s exactly what NASA successfully did with the DART mission in 2022, they proved this concept works and dramatically altered the orbit of the asteroid Dimorphos. But new research reveals the chilling possibility that an asteroid hit in the wrong spot, and you might just be postponing the impact!

This high-resolution view of Dimorphos was created by combining the final 10 full-frame images obtained by DART’s Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO)before impact (Credit : NASA/John Hopkins)

Scientists at the University of Illinois have discovered that poorly aimed asteroid deflection attempts could accidentally steer space rocks through dangerous regions in space known as "gravitational keyholes” that would alas, still mean they hit Earth, just years or decades later!

A gravitational keyhole is a small region of space where a planet's gravity can modify a passing asteroid's orbit such that it returns on a collision course with that planet at a later date. Think of it like a rather funky pinball machine where hitting the wrong bumper sends the ball ricocheting back toward the flippers.

"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" - Rahil Makadia from NASA

To solve this problem, Makadia's team has developed "probability maps" that identify the safest spots to strike each asteroid. Each point on the surface of an asteroid has a different probability of sending the asteroid through a gravitational keyhole after deflection by a kinetic impactor.

Creating these maps requires detailed knowledge of the asteroid's characteristics such as its shape, surface features, rotation, and mass. Ideally this would be done with a space mission to rendezvous with the asteroid, producing high resolution images and data. However, if an asteroid is discovered late with little time before impact, scientists can create preliminary, lower quality maps using ground based telescope observations alone.

The researchers have already created probability maps for well studied asteroids like Bennu, complete with crosshairs marking the optimal impact zones. These maps account for the inevitable uncertainties in any space mission since even the most precisely aimed spacecraft might miss its target by several meters.

The asteroid Bennu, captured here by NASA’s OSIRIS-REx spacecraft, has been mapped and probability maps created to identify the most appropriate and lowest risk impact point (Credit : NASA/Goddard/University of Arizona)

While DART's target, Dimorphos, was chosen precisely because the Didymos system is too massive to be deflected onto a collision course with Earth, future asteroid threats won't be so forgiving. Real planetary defence missions will require a high level of precision planning.

The European Space Agency's Hera mission, set to reach the DART impact site in December 2026, will provide valuable data to refine these techniques. We have been really quite lucky so far, nothing significant has been found to be heading straight for us but as we continue exploring the sky, eventually, one day we will spot an asteroid with Earth’s name on it! Thanks to Makadia and his team, when we do need to play planetary defence for real, we'll know exactly where to aim…..hopefully!