Look out for the keyhole: How to find the safest spots to deflect a hazardous asteroid

(Press-News.org) Selecting the right spot to smash a spacecraft into the surface of a hazardous asteroid to deflect it must be done with great care, according to new research presented at the EPSC-DPS2025 Joint Meeting this week in Helsinki. Slamming into its surface indiscriminately runs the risk of knocking the asteroid through a 'gravitational keyhole' that sends it back around to hit Earth at a later date.

"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," said Rahil Makadia, a NASA Space Technology Graduate Research Opportunity Fellow at the University of Illinois at Urbana-Champaign, who is presenting the findings at the EPSC-DPS2025 meeting.

NASA's DART, the Double Asteroid Redirection Test mission, struck the small asteroid Dimorphos, which is in orbit around the larger asteroid Didymos, in September 2022. DART was a 'kinetic impactor' – effectively a projectile that slammed into the asteroid with enough energy to nudge it into a new orbit, thereby proving that it is possible to deflect an asteroid that could be on a collision course with Earth.

A European Space Agency mission called Hera will follow-up on the DART impact when it reaches Didymos and Dimorphos in December 2026.

Where DART struck on Dimorphos was of relatively little concern, since the Didymos system is too massive to be deflected onto a collision course with Earth. However, for another hazardous asteroid orbiting the Sun, even a small variation in its orbit could send it through a gravitational keyhole.

The keyhole effect revolves around 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. In this way, a gravitational keyhole unlocks more dangerous orbits.

Should a kinetic impactor mission similar to DART nudge a hazardous asteroid so that it passes through a gravitational keyhole, then it only postpones the danger.

"If an asteroid passed through one of these keyholes, its motion through the Solar System would steer it onto a path that causes it to hit Earth in the future," said Makadia.

The trick, therefore, is to find the best spot on the surface of an asteroid to impact with a spacecraft so that the chances of pushing it through the keyhole are minimised.

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. Makadia's team has therefore developed a technique for computing probability maps of an asteroid's surface. Their method uses the results from DART as a guide, although each asteroid, with its own characteristics, will be subtly different.

The asteroid's shape, surface topology (hills, craters etc), rotation and mass all must be determined first. Ideally this would be done with a space mission to rendezvous with the asteroid, producing high-resolution images and data. However, this might not be possible for all threatening asteroids, particularly if the time between discovery and impact on Earth is short.

“Fortunately, this entire analysis, at least at a preliminary level, is possible using ground-based observations alone, although a rendezvous mission is preferred,” said Makadia.

By computing the subsequent trajectory of the asteroid following a kinetic impact, and seeing which trajectories would be the most dangerous, scientists can calculate where the safest location to strike on the asteroid's surface will be.

"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.

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