NASA's DART Crashes Into an Asteroid, On Purpose

"Dimorphos is the smallest object that has ever been a mission target, and we are coming in very fast with a need to impact on the first try, without knowing fundamental things like Dimorphos' shape or exact size," explains Andy Rivkin, the DART investigation co-lead from the Johns Hopkins Applied Physics Laboratory, which is leading the project for NASA. "It's about about 3,600 feet (1,100 meters) from the center of Didymos to the center of Dimorphos, and it's probably less than 1,968 feet (600 meters) from the surface of one to the surface of the other."

Worse yet, the spacecraft had to overtake that target at such a high speed that there was very little margin for error — "literally the blink of an eye," says DART program scientist Tom Statler via email. To achieve the necessary precision, the spacecraft had to be guided by SMART Nav, a totally automated navigation system that required no human input. The spacecraft also utilized the Didymos Reconnaissance & Asteroid Camera for OpNav imaging instrument, aka DRACO, to see where it was going.

But it's all good, because that sort of technology may come in handy someday. "If we ever need to carry out a kinetic impact to prevent a natural disaster, we might need to do so quite far from Earth, which would make autonomous control by the spacecraft absolutely essential," Statler says. "That's why we want to demonstrate and validate this technology with DART."

"The asteroid itself is the hardest thing to predict. We know what spectral type of object it is, which means we have a reasonably good idea of the type of material it's made of," explains Cristina A. Thomas, an assistant professor in the department of Astronomy and Planetary Science at Northern Arizona University, who's spent years studying Dimorphos and will continue to monitor it after the impact.

"Didymos is similar to what we call an ordinary chondrite meteorite. It's rocky, but not metallic. That gives us a good place to start with our thinking. We don't know if Dimorphos is a solid object or if it's a rubble pile — lots of smaller things held together by gravity. This will change the impact itself and how much material is ejected from the crater. That material, called ejecta, has its own momentum which gives extra energy to the deflection. This enhancement factor is known as 'beta.'"

"The uncertainty of the value of beta gives us an uncertainty in how much we predict the orbit will change," Thomas continues. "Dimorphos currently has an orbital period around Didymos of approximately 11 hours and 55 minutes. We anticipate changing that orbital period by at least 10 minutes. That might not seem like much, but if we were trying to deflect something away from Earth, the change would not have to be large, especially if we do it far in advance."

DART is an early step in protecting human life from being wiped out by a space rock, but it also changes humanity's relationship to the cosmos. Up to this point, space has been something that we watch from afar and occasionally send brave souls to visit for brief periods. But now, it's going to become something that humans can tinker with, just as we've altered our own planet.

"Perhaps the biggest point is that DART will be humanity's first attempt to deliberately change the orbit of a solar system body," Martin Elvis, an astrophysicist at the Center for Astrophysics Harvard & Smithsonian and author of the 2021 book "Asteroids: How Love, Fear, and Greed Will Determine Our Future in Space," explains via email.

"The amount we will change the orbital speed of Dimorphos, the moon of the near-Earth asteroid Didymos, will only be by less than a snail's pace (literally) — 4.6 feet (1.4 meters)/hour," Elvis says. "Yet it is not zero. The architecture of the solar system will be subtly altered." He says that while this doesn't have any immediate importance, it is symbolic. "There are those who will thrill to this stepping out of humanity. There are others who will say, "'Not again. Must we repeat our environmental mistakes, only now on a far larger scale?'"

Dimorphos might look puny compared to the massive asteroid that wiped out the dinosaurs, whose size has been estimated at about 6 miles (10 kilometers) across. But even a small asteroid is capable of inflicting serious harm if it slams into Earth. Johnson notes that it's three times the size and possibly five times the mass of the asteroid that created the Barringer Crater in eastern Arizona some 50,000 years ago.

"It would impact with an estimated energy of roughly 10 megatons of TNT — larger than any nuclear bomb — and create a crater of a few miles in diameter and a quarter mile (0.4 kilometers) in depth," Johnson notes. "Blast effects might extend for 150 miles (241 kilometers) in all directions from the impact site." The prospect of such a disaster makes it conceivable that some future asteroid defense mission might need to target an object of Dimorphos' size.

If DART works as planned, "it will validate both the kinetic impactor technique for planetary defense purposes and that current technology enables our ability to perform the deflection," Johnson explains. But that doesn't mean that NASA will rush to build a spacecraft that can perform the same feat and have it ready to launch at the first glimpse of an asteroid that's a threat to hit Earth.

"A significant asteroid impact is an extremely rare natural disaster, and what techniques might be used to deflect one detected in advance would be very scenario-dependent, especially on how many years in advance it was discovered," Johnson says. "Decades may pass before the next major impactor is discovered and the planetary defense program of that time in the future may want to use the more advanced technology that will likely be available by then."

On the other hand, "how DART compares to what might get used in an actual emergency will depend in part on how the experiment goes," Rivkin says. That future planet protector "may not be terribly different" from DART's design.