What would we do if we spotted a dangerous asteroid on a collision course with Earth? Could we safely deflect it to prevent 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 fridge into an asteroid the size of an Aussie Rules football pitch .
Early results from this first real-world test of our potential planetary defense systems looked promising. However, only now are the first scientific results being published: five papers Nature have recreated the impact and analyzed how it changed the asteroid’s momentum and trajectory, while two studies are investigating debris thrown up by the impact.
The conclusion: “kinetic impactor technology is a viable technique for the potential defense of Earth if necessary.”
Small asteroids can be dangerous, but difficult to detect
Our solar system is full of debris, left over from the early days of planet formation. Today, about 31,360 asteroids are known to orbit the Earth’s neighborhood.
While we have tabs 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 ten years ago, an 18-meter asteroid exploded in our atmosphere over Chelyabinsk, Russia. The shock wave broke thousands of windows, causing havoc and injuring around 1,500 people.
A 150m asteroid like Dimorphos would not wipe out civilization, but it could cause mass casualties and regional devastation. However, these smaller space rocks are harder to find: we think we’ve only spotted about 40% of them so far.
The DART mission
Suppose we spied an asteroid of this scale on a collision course with Earth. Could we push it in a different direction, away from destruction?
Hitting an asteroid with enough force to change its orbit is theoretically possible, but can it really be done? This is what the DART mission was intended to determine.
Specifically, he tried 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, Gemini, in a loop that took less than 12 hours to complete.
The impact from the DART spacecraft was designed to slightly alter that trajectory, slowing it down a bit so that the loop would shrink, shaving about seven minutes off its journey.
A self-driving spaceship
In order for DART to show that kinetic impactor technology is a potential tool for planetary defense, it had to show two things:
that its navigation system could autonomously maneuver and target an asteroid during a high-speed encounter
that such an impact could alter the asteroid’s orbit.
In the words of Northern Arizona University’s Cristina Thomas and colleagues, who analyzed the changes in Dimorphos’ orbit as a result of the impact, “DART did both successfully.”
The DART spacecraft was steered into the path of Dimorphos with a new system called Small-body Maneuvering Autonomous Real Time Navigation (SMART Nav), which used its onboard camera to arrive at a position for maximum impact.
More advanced versions of this system could allow future missions to choose their own landing sites on distant asteroids where we can’t image the rubble-pile terrain well from Earth. This will save the trouble of a scouting trip first!
Dimorphos itself was such an asteroid before DART. A team led by Johns Hopkins University’s Terik Daly used high-resolution images from the mission to build a detailed shape model. This gives a better estimate of its mass, improving our understanding of how these types of asteroids will react to impacts.
The impact itself produced an incredible plume of material. Jian-Yang Li of the Planetary Science Institute and colleagues detailed how the ejecta was ejected from the impact and poured out in a 1,500-kilometer-long debris tail that was visible for nearly a month.
Streams of material from comets are known and documented. They are mostly dust and ice and appear as harmless meteor showers if they intersect with Earth.
Asteroids are made of rockier, stronger materials, so their streams could pose a greater threat if we encounter them. Documenting a real example of the creation and evolution of debris paths in the wake of an asteroid is very exciting. Detecting and monitoring such asteroid streams is a key goal of planetary defense efforts, such as the Desert Fireball Network we operate out of Curtin University.
A result greater 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 indicates 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 ejecta 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.
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 enough warning to use this kinetic impactor technique. The earlier we intervene in an asteroid’s orbit, the smaller the change we need to make to push it away from hitting Earth. (To see how it all works, you can play with NASA’s NEO Deflection app.)
But should I? This is a question that will have to be answered if we ever need to redirect a dangerous asteroid. In changing the trajectory, we had to be sure not to push it in a direction that would hurt us in the future.
However, we are getting better at spotting asteroids before they reach us. We’ve seen just two in the last few months: 2022WJ1, which affected Canada in November, and Sar2667, which came to France in February.
We can expect to detect many more in the future, with the opening of the Vera Rubin Observatory in Chile later this year.
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, Dimorph Orbital Period Change Due to the DART Kinetic Shock, Nature (2023). DOI: 10.1038/s41586-023-05805-2
Jian-Yang Li et al, Ejecta from the active asteroid Dimorphos produced by DART, Nature (2023). DOI: 10.1038/s41586-023-05811-4 Ariel
Graykowski et al, Light Curves and Colors of the Ejecta from Dimorphos after the DART Impact, Nature (2023). DOI: 10.1038/s41586-023-05852-9
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Reference: New results from NASA’s DART planetary defense mission confirm we could deflect deadly asteroids (2023, March 4) Retrieved March 4, 2023, from https://phys.org/news/2023-03-results- nasa-dart-planetary-defense. html
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