If an asteroid were on a collision course with Earth, is it possible to alter its trajectory without worsening the predicament? Indeed, thanks to a new system designed to identify the optimal strike point on incoming asteroids.

Diverting an Earth-bound asteroid is a high-stakes operation, and we have little practical experience with it. In 2023, NASA’s Double Asteroid Redirection Test (DART) demonstrated for the first time that it’s possible to divert asteroids by impacting them with spacecraft.

However, engaging in such missions carries risks. Altering an asteroid’s orbit can push it through a narrow window, known as a gravity keyhole, where the gravitational pull from a larger body like Earth may alter its trajectory enough to eventually cause a collision with our planet.

Now, Rahir Macadia from the University of Illinois at Urbana-Champaign and his team have devised a system to determine the best impact sites for satellites to reduce this risk. By analyzing data from the DART mission along with the asteroid’s shape, mass, and rotation speed, they were able to project how different impact points would alter the asteroid’s trajectory. This leads to the creation of a probability map for the asteroid’s surface, identifying various locations with differing chances of pushing the asteroid through a gravity keyhole, allowing scientists to select those with lower probabilities for a strike.

“It’s feasible to map these keyholes to asteroids, and the only cost involved is computational power before the mission is launched. So, on September 9th, during the Europlanet Science Congress (EPSC) in Helsinki, Finland, we should be able to pinpoint the optimum targeting point on the asteroid’s surface for a kinetic impact.

Macadia and his team have tested this approach on the asteroid Bennu, discovering over 2,000 potential keyhole locations and identifying safe spots for a spacecraft to impact.

While gathering specific data on a single asteroid is optimal with bespoke probes designed for the task, it might not always be practical if the asteroid is located close to Earth at the time of its detection. However, Macadia asserts that a rough analysis should still be achievable using data from terrestrial telescopes.

A practical trial for collecting this data will occur when asteroid Apophis passes close to Earth in 2029. Astronomers have determined there is no risk posed by this 450-meter-long asteroid, but close encounters are considered rare, occurring only once every 7,500 years.

“We’ve observed many asteroids, but never one subjected to the kind of stress and natural vibrations due to Earth’s gravity.” Richard Binzel remarked at the EPSC on September 8th, representing the Massachusetts Institute of Technology.

Nobody knows how NASA’s Osiris Apex spacecraft, which initially visited asteroid Bennu, will fare when redirected to study Apophis in proximity to Earth’s passing European Space Agency’s Ramses spacecraft.

Apart from orbiting asteroids at safe distances to gather crucial details about their composition and shape, astronomers aim to monitor the small kilogram-scale spacecraft on the surface, including long-anticipated seismic activity when it’s near a massive body like Earth, to investigate internal conditions.

According to Binzel, understanding these characteristics is vital for asteroids that may pose future threats to Earth. “If you’re addressing an actual asteroid hazard posed by Apophis or other similar objects, you’ll need to be well-informed about characteristics such as angular momentum and tumbling behavior of the asteroid.”

Paolo Martino, the project manager for the mission, indicated that the trajectory of Apophis is excellently mapped, eliminating the danger of deviation since the Ramses spacecraft is equipped with sensors that enable it to autonomously avoid collisions, and its low mass means any impact would minimally affect Apophis.