By now you have likely seen the images of NASA’s DART spacecraft smashing into the asteroid, Dimorphos, a small asteroid in orbit around a larger asteroid, Didymos. Here is the streaming video from DART itself, showing right up to the moment of impact. Many telescopes were focused on this event, including (for the first time) both the Webb and Hubble space-based telescopes. Why did NASA deliberately crash a spacecraft?

DART stands for “double asteroid redirect test” – it is designed to test a system for redirecting the path of an asteroid by smashing stuff into it. It’s a double asteroid because Dimorphos and Didymos are a double asteroid system. DART included a DRACO camera – Didymos Reconnaissance and Asteroid Camera for Optical navigation, used to support the SMART-nav system – Small-body Maneuvering Autonomous Real Time Navigation (SMART Nav). NASA loves its acronyms. Part of the DART test was to see if these systems worked, if the craft could successfully navigate itself precisely enough to score a direct hit on a tiny asteroid. It did.

But also NASA wanted to see the results, and this is where Webb and Hubble come in, as well as many ground-based telescopes. They wanted to image the composition and speed of the ejecta. Specifically – would DART kick off a cloud of fine powder, or would larger chunks of rock break off the asteroid? When redirecting an asteroid the last thing we would want to do is create a debris field of large chunks of rock. Which brings us to the overall goal of the mission, to protect the Earth from an asteroid impact.

Asteroid impacts are inevitable, with large impact increasingly rare, but they do occasionally happen. Just ask the dinosaurs, except you can’t because they were all killed by an asteroid impact. Statistically it will likely be a long time before a biosphere-killing asteroid hits the Earth again, but it will likely happen eventually. There can also be smaller, but still catastrophic, asteroids headed our way, ones large enough to take out a major city, cause a devastating tsunami, or even threaten a continent. We don’t have to wait helplessly while a killer asteroid head our way. We can push it into a different trajectory, so that it safely misses us.

The first step in preventing major asteroid impacts is to detect incoming asteroids. There are several projects to do this. NASA has its NEO (near-Earth objects) observation program. The ESA has a planetary defense office. The UN also has an international asteroid warning network. The goal is to catalogue all the objects large enough to threaten the Earth, and map their orbits precisely enough to predict if they will intersect the Earth at any point in the future. Sometimes we don’t learn about an NEO until just after they passed by the Earth. Obviously that’s not ideal. Also, we would need the longest warning time frame possible. The longer we have to redirect an asteroid, the easier it will be. There are several proposed methods for doing so.

First the movie idea of blowing up an asteroid is not a good idea. It would be difficult to impossible to pulverize a larger asteroid into dust. More likely the result would be many large chunks, which could potentially do even more damage than a single impact. Inertia being what it is, all the chunks of the asteroid would still be heading for Earth. The blast is unlikely to be powerful enough to significantly alter that path of the individual chunks. It is far better to keep the asteroid intact as one solid piece (if that’s what it is – some asteroids are already piles of rubble) and to redirect the whole thing away from Earth.

DART is a test of one such method, probably the most promising method. One of the advantages of this approach is its simplicity – you simply hit the asteroid like a billiard ball and push it into a different orbit. This requires a fairly direct hit in the asteroid’s center of gravity, but DART proves that current autonomous navigational systems can get the job done. Momentum is proportional to mass and velocity, and the asteroids we would be concerned about can be massive. So likely any significant redirection would require many impacts of heavy spacecraft going at high velocity. And again, the earlier we can do this, the smaller the redirection necessary to get the object to miss Earth. If it gets too close, it becomes too late for this strategy.

Another method is to attach rocket engines to the asteroid and move it like a spaceship. The challenge here is that asteroids are almost always rotating with respect to their orbital trajectory, so the rockets would not always be facing in the correct orientation. They would have to fire intermittently, timing it precisely as the asteroid spun around. But if the asteroid were spinning too fast, this could prove impossible. Perhaps first rockets could slow the spin of the asteroid, but this adds more complexity and time. Similarly you could paint one side of the asteroid white and let the sun push it away, or use lasers to give it a push. But again – spinning is a problem. Or we could place powerful atomic bombs on the surface of the asteroid and use their detonation not to blow it up but as a push to the object.

A more promising method is to park a heavy ship near the asteroid and pull the object with its gravity. This requires that the ship is constantly pushed away from the asteroid, either with rockets or perhaps solar sails, so that it doesn’t just get pulled into the asteroid. This is a tricky maneuver, but it could be a more precise method that the impact method, perhaps used just for some final tweaks to the orbit.

After reviewing all the possibilities, the impact method seems the most plausible and technologically feasible. The real trick is the precise navigation, and it seems that hurdle has been passed. We seem, therefore, to be on the cusp of an effective Earth defense system. We need a sufficient early detection system, and there are multiple efforts developing this. And we need a fleet of impact rockets to push it out of the way. These either need to be in a warehouse read to be fueled and launched, or at least the designs and infrastructure are ready and they can be rapidly assembled. We would hopefully have years of warning, so taking a few months to get the mission ready is likely not critical. We would want our response to be as rapid as possible, however. We still have a way to go, but a successful DART mission is a great milestone toward a planetary defense system.