New results from NASA’s DART planetary defense mission confirm we can deflect deadly asteroids

CTIO / NOIRLab / SOAR / NSF / AURA/ T. Kareta (Lowell Observatory), M. Knight. Credit: US Naval Academy

What would we do if we discovered a dangerous asteroid on a collision course with Earth? Can we divert it safely to prevent the impact?

Last year, NASA’s Double Asteroid Redirection Test (DART) mission tried to find out if a “kinetic impactor” could do the job: smash a 600kg spacecraft the size of a refrigerator into an asteroid the size of an Aussie Rules football pitch.

Early results from this first real test of our potential planetary defense systems looked promising. However, it is only now that the first scientific results are being published: five articles in Nature have recreated the impact, and analyzed how it changed the asteroid’s momentum and trajectory, while two studies examine debris that was knocked off by the impact.

The conclusion: “kinetic impactor technology is a viable technique to potentially defend Earth if necessary”.

Small asteroids can be dangerous, but difficult to spot

Our solar system is full of debris, left over from the early days of planet formation. Today, around 31,360 asteroids are known to be loitering in Earth’s neighborhood.

Although we have plumes on most of the large, kilometer-sized ones that could wipe out humanity if they hit Earth, most of the smaller ones go undetected.

Just over ten years ago, an 18 meter long asteroid exploded in our atmosphere over Chelyabinsk in Russia. The shock wave broke thousands of windows, caused chaos and injured around 1,500 people.

A 150 meter long asteroid like Dimorphos would not wipe out civilization, but it could cause mass damage and regional devastation. However, these smaller space rocks are harder to find: we think we’ve only seen about 40% of them so far.

The DART mission

Suppose we spied an asteroid of this scale on a collision course with Earth. Can we nudge it in a different direction and steer it away from disaster?

Hitting an asteroid with enough force to change its orbit is theoretically possible, but can it actually be done? That is what the DART mission decided.

In particular, it tested the “kinetic impactor” technique, which is a fancy way of saying “hitting the asteroid with a fast-moving object”.

The asteroid Dimorphos was a perfect target. It orbited its larger cousin, Didymos, in a loop that took just under 12 hours to complete.

The impact from the DART spacecraft was designed to alter this trajectory slightly, slowing it just a bit so that the loop would shrink, shaving an estimated seven minutes off the round trip.

An autonomous spacecraft

In order for DART to show that the kinetic impactor technique is a viable tool for planetary defense, it needed to demonstrate two things:

  • that the navigation system could autonomously maneuver and target an asteroid during a high-speed encounter

  • that such an impact could change the asteroid’s orbit.

In the words of Cristina Thomas of Northern Arizona University and colleagues, who analyzed the changes in Dimorphos’ orbit as a result of the impact, “DART has successfully done both”.

The DART spacecraft steered into the path of Dimorphos with a new system called Small-body Maneuvering Autonomous Real Time Navigation (SMART Nav), which used the onboard camera to get into a position for maximum effect.

More advanced versions of this system could enable future missions to choose their own landing sites on distant asteroids where we cannot image the rubble mound terrain well from Earth. This would save the trouble of a scouting trip first!

Dimorphos itself was one such asteroid before DART. A team led by Terik Daly of Johns Hopkins University has used high-resolution images from the mission to create a detailed shape model. This provides a better estimate of the mass, and improves our understanding of how these types of asteroids will react to impacts.

Asteroid statistics and the threats posed by asteroids of various sizes. Credit: NASA’s DART press card

Hazardous debris

The impact itself produced an incredible amount of material. Jian-Yang Li of the Planetary Science Institute and colleagues have detailed how the ejected material was kicked up by the impact and flowed out in a 1,500 km long tail of debris that could be seen for almost a month.

Streams of material from comets are well known and documented. They are mainly dust and ice, and are seen as harmless meteor showers if they cross paths with Earth.

Asteroids are made of rockier, stronger stuff, so their streams may pose a greater danger if we encounter them. To record a real example of the creation and development of debris paths in the wake of an asteroid is very exciting. Identifying and monitoring such asteroid streams is a major goal of planetary defense efforts such as the Desert Fireball Network we run from Curtin University.

A bigger result than expected

So how much did the impact change Dimorphous’ trajectory? Much more than the expected amount. Instead of changing by seven minutes, it had become 33 minutes shorter!

This larger than expected result shows that the change in Dimorphos’ orbit was not just from the impact of the DART spacecraft. Most of the change was due to a recoil effect from all the ejected material flying into space, which Ariel Graykowski of the SETI Institute and colleagues estimated to be between 0.3% and 0.5% of the asteroid’s total mass.

A first success

The success of NASA’s DART mission is the first demonstration of our ability to protect Earth from the threat of dangerous asteroids.

At this stage, we still need quite a bit of warning to use this kinetic impact technique. The earlier we intervene in an asteroid’s orbit, the less change we have to make to push it away from hitting Earth. (To see how it all works, play around with NASA’s NEO Deflection app.)

But should we? This is a question that will need answering if we ever have to redirect a dangerous asteroid. When we change the trajectory, we must be sure that we are not going to push it in a direction that will affect us in the future as well.

However, we are getting better at detecting asteroids before they reach us. We’ve seen two in just the last few months: 2022WJ1, which affected Canada in November, and Sar2667, which came in over France in February.

We can expect to discover much more in the future, with the opening of the Vera Rubin Observatory in Chile at the end of this year.

More information:
R. Terik Daly et al, Successful Kinetic Impact into an Asteroid for Planetary Defense, Nature (2023). DOI: 10.1038/s41586-023-05810-5

Andrew F. Cheng et al, Momentum Transfer from the DART Mission Kinetic Impact on Asteroid Dimorphos, Nature (2023). DOI: 10.1038/s41586-023-05878-z

Cristina A. Thomas et al, Orbital Period Change of Dimorphos Due to the DART Kinetic Impact, Nature (2023). DOI: 10.1038/s41586-023-05805-2

Jian-Yang Li et al, Ejecta from the DART-produced active asteroid Dimorphos, Nature (2023). DOI: 10.1038/s41586-023-05811-4 Ariel

Graykowski et al, Light Curves and Colors of the Ejecta from Dimorphos after DART Impact, Nature (2023). DOI: 10.1038/s41586-023-05852-9

Journal information:

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