Although we have tabs on most of the big, kilometer-sized ones that could wipe out humanity if they hit Earth, most of the smaller ones go undetected.
A 500-foot (150 m) asteroid like Dimorphos wouldn’t wipe out civilization, but it could cause mass casualties and regional devastation. However, these smaller space rocks are harder to find: we think we have only spotted around 40 percent of them so far.
The DART mission
Suppose we did spy an asteroid of this scale on a collision course with Earth. Could we nudge it in a different direction, steering it away from disaster?
Hitting an asteroid with enough force to change its orbit is theoretically possible, but can it actually be done? That’s what the DART mission set out to determine.
Specifically, 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 was in orbit around its larger cousin, Didymos, in a loop that took just under 12 hours to complete.
The impact from the DART spacecraft was designed to slightly change this orbit, slowing it down just a little so that the loop would shrink, shaving an estimated seven minutes off its round trip.
A self-steering spacecraft
For DART to show the kinetic impactor technique is a possible tool for planetary defense, it needed to demonstrate two things:
- That its 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 to Dimorphos’ orbit as a result of the impact, “DART has successfully done both”.
The DART spacecraft steered itself into the path of Dimorphos with a new system called Small-body Manoeuvring Autonomous Real Time Navigation (SMART Nav), which used the onboard camera to get into a position for maximum impact.
More advanced versions of this system could enable future missions to choose their own landing sites on distant asteroids where we can’t image the rubble-pile 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 make 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 have described in detail how the ejected material was kicked up by the impact and streamed out into a 930-mile (1,500 km) tail of debris that could be seen for almost a month.