Deflecting an asteroid requires precise aim, otherwise...

Space

Technological Innovation Website Editorial Team - September 12, 2025

One of the keyhole probability maps of asteroid Bennu . The crosshairs correspond to the location on the surface that minimizes the risk of asteroid impact after deflection. [Image: Rahil Makadia]

Space sight

In September 2022, the DART spacecraft collided with the asteroid Dimorpho , in what was the first demonstration of a planetary defense technology , to see how impacts can help deflect asteroids or comets that enter a collision course with Earth.

Scientific results have shown that the technique is feasible , but now scientists have discovered that special attention will be needed to aim when the impact is for real.

The problem is that hitting an asteroid's surface indiscriminately carries a substantial risk of throwing the asteroid through a "gravitational keyhole," which will send it back to hit Earth later.

When the DART probe struck Didymus, there was little concern, as the Didymus system is too massive to be deflected onto a collision course with Earth. However, for another dangerous asteroid orbiting the Sun, even a small variation in its orbit could send it on a dangerous path.

"Even if we intentionally push an asteroid away from Earth with a space mission, we need to ensure it doesn't fall into one of these keyholes later. Otherwise, we'll face the same impact threat again in the future," said Rahil Makadia of the University of Illinois in the US.

Artist's impression of the DART mission, a kinetic impactor that tested asteroid deflection for the first time. [Image: NASA/Johns Hopkins APL]

Gravitational keyhole

The keyhole effect involves a small region of space where a planet's gravity can modify the orbit of a passing asteroid, causing it to return on a collision course with that planet later. Thus, a gravitational keyhole ultimately creates more dangerous orbits.

If a kinetic impactor mission, similar to DART, nudges an asteroid so that it passes through a gravitational keyhole, it will only postpone the danger.

The solution, therefore, is to find the best point on the asteroid's surface to aim the impact, so that the chances of pushing it through the keyhole are minimized.

It's not as simple a task as aiming at a shooting target because there are no concentric circles marked on the asteroid, much less a bull's-eye, the target's center point. In fact, each point on each asteroid's surface has a different probability of sending it through a gravitational keyhole after a kinetic impact.

To address this challenge, the team developed a technique to calculate probability maps of an asteroid's surface. The method uses DART results as a guide, although each asteroid, with its own characteristics, is different, so we'll need more data, which means more impact tests on other asteroids.

Visit the asteroid first

The team's study showed that it is first necessary to characterize the asteroid's shape, surface topology (hills, craters, etc.), rotation, and mass.

Ideally, this would be accomplished with a space mission to approach the asteroid, producing high-resolution images and data. Unfortunately, this may not be possible for all threatening asteroids, especially if the time between the threat's discovery and its impact on Earth is short.

"Fortunately, all this analysis, at least at a preliminary level, is possible using only ground-based observations, although a rendezvous mission would be preferable," Makadia commented.

Other news about:

More topics