NASA finds crashing spacecraft into asteroids is a viable defence strategy

Recoil from DART impact changed Dimorphos's orbit more than expected

Plumes of dust and rocks kicked up from the surface of asteroid Dimorphos after NASA's DART spacecraft smashed into it altered the space rock's orbit more than the kinetic impact alone, according to research published on Wednesday.

Boffins successfully changed the position of an astronomical body in space for the first time in history during the Double Asteroid Redirection Test mission last October. A 610kg (1,345lb) spacecraft was directed to crash headfirst into an binary asteroid system to see whether it was possible to knock it off its original orbit.

DART was a huge success. The impact from the collision with Dimorphos shortened its orbit by 33 minutes – it now takes less than 11 and a half hours to circle around its parent asteroid, Didymos. The change is significantly greater than had been expected.

Researchers analyzing data from the experiment calculated that material ejected from Dimorphos's surface during the collision changed its momentum by a greater amount than DART's impact.

The spacecraft alone would have altered Dimorphos's orbit by seven minutes, but the recoil and ejecta created by the impact shortened it even further, according to a paper published in Nature. A second study estimated that the loss of material from the asteroid's surface contributed to the change in momentum by a factor between 2.2 and 4.9.

"We can't stop hurricanes or earthquakes yet, but we ultimately learned that we can prevent an asteroid impact with sufficient time, warning and resources," declared Derek Richardson, a professor of astronomy at the University of Maryland (UMD) and a DART investigation working group lead, in a statement. "With sufficient time, a relatively small change in an asteroid's orbit would cause it to miss the Earth, preventing large-scale destruction from occurring on our planet."

Astronomers would have to spot the object in advance – "several years at a minimum, but preferably decades" – to give space agencies sufficient time to plan and execute a kinetic impact mission. 

Although several efforts are dedicated to mapping near-Earth objects, potential threats aren't always easy to spot before they're close. For instance, a truck-sized asteroid made one of the closest approaches ever recorded in January 2023 and was only discovered a week before it passed a couple of thousand miles from Earth. 

"The closer the object is to impacting the Earth, the harder it is to deflect it," Richardson told The Register. "In practical terms, that means we would need a bigger spacecraft (or more than one) moving faster and launched sooner compared to a hazard further away. So, the more warning we have, the better. We can't put an exact limit on this because it also depends on the size of the hazardous object: the bigger it is, the bigger the kick we would need to apply, etc."

Still, DART's success "builds optimism about humanity's capacity to protect the Earth from an asteroid threat," researchers concluded in a third paper also published in Nature. A fourth study found that the ejected material formed a tail trailing Dimorphos like a comet, turning it into a so-called "active" asteroid. 

There's still plenty we don't know about how the collision changed Dimorphos and its parent asteroid, Didymos. DART was equipped with only a camera and was destroyed upon impact. The European Space Agency is launching its Hera probe in a follow-up mission to analyze the binary asteroid system in more detail in 2024.

"The [number one] question the DART team wants answered is the exact mass of Dimorphos,' Richardson explained. "Knowing that, coupled with the observed change in its trajectory, will definitively tell us how effective the impact was. But we would also like to know lots of other things: did we make a crater so big that the overall shape of the moon changed significantly? Did we change the moon's rotation significantly?

"Is there any debris left over orbiting in the system? Did a substantial amount of debris hit Didymos and affect its surface in a measurable way? What is the internal structure of Dimorphos (is it a rubble pile all the way through, or just on the surface)? What can we learn about how asteroid moons are formed in the first place, and how they evolve over long timescales?"

Scientists are working to enhance planetary defence techniques, which could involve building new types of technology designed to deflect an object in space. Tony Farnham, principal research scientist at the UMD, said there are plans to look for potential targets on which to test future Asteroid-bumping machines.

He told us "There is an effort to explore Apophis during the 2029 passage, with plans to redirect the OSIRIS-Rex spacecraft to rendezvous with the asteroid. O-Rex will characterize its physical properties and study what happens to the Apophis during the Earth flyby." ®