How Nasa’s Dart mission could save Earth from rogue asteroids

Nasa‘s Double Asteroid Redirection Test, or Dart mission, witnessed a spacecraft slam into an asteroid on Monday evening in a bid to prove it’s possible to change the space rock’s trajectory in space.

It’s a first of its kind proof of concept test of a technology that could one day save Earth from a civilization-ending asteroid strike, or even a minor infrastructure damaging one. The key will be ensuring Dart succeeds in striking its target, an asteroid called Dimorphos some 68 million miles from Earth, and then carefully measuring the results.

“Dart is demonstrating what we call the kinetic impact technique for changing the speed of the asteroid in space and therefore changing its orbit,” Nasa planetary defense officer Lindley Johnson told reporters at a Thursday press conference about the mission. “This demonstration is extremely important to our future here on the earth, and life on earth.”

At 7.14pm EDT on Monday, the Dart spacecraft will slam into Dimorphos at around 14,000 miles per hour. Nasa and other space agencies will be watching in the aftermath using ground and space based telescopes in order to see if the impact changes the speed at which Dimorphos orbits its larger asteroid companion, Didymos.

Neither Dimorphos or Didymos pose a threat to Earth, so the test provides a safe means to the principle of kinetic impact as a means to alter an asteroid's course.

“This is the perfect natural laboratory,” Nasa’s Dart program scientist, Tom Statler, told reporters on Thursday. “We're doing this test when we don't need to on an asteroid that isn't a danger, just in case we ever do need to and we discover an asteroid that is a danger.”

Scientists will consider the Dart impact successful if it changes Dimorphism’s orbit around Didymos anywhere from 73 seconds to 10 minutes.

That may seem modest, University of North Dakota assistant professor of Space Studies Sherry Fieber-Beyer told The Independent in an interview, but if a hazardous asteroid is detected far enough in advance, “a velocity change as small as a few centimetres per second implemented several years ahead of time could provide the change in the arrival time to convert a potential impact into a clear miss.”

The Earth travels along its orbit around the Sun at 29.79 kilometres per second, which means it crosses its own diameter of about 12,724 kilometres in just more than seven minutes.

“So if a [near Earth object] initially on a trajectory to impact dead centre on the face of the Earth, arrives as little as five minutes early or late a potential disaster would be changed into a near miss,” Dr Fieber-Beyer said.

While Dart will help Nasa better understand how a kinetic impact mission would work, that doesn’t mean an actual mission would look exactly like Dart does.

“Dart is specifically sized to have the desired effect on Dimorphos,” Dr Johnson said. If faced with a real asteroid threat, “It would depend on the size of the asteroid how much we would need to hit it. In the case of the kinetic impactor, it probably need to be larger than Dart, and we also might hit it with several kinetic impactors.”

There are lots of mission options, but many depend on just what type of asteroid you’re dealing with and how it will react to an impact, according to Dr Statler. The reality is that while Nasa has tracked asteroids and observed them and modeled them, “we do not know exactly how these how these asteroids will behave, because we don't have samples of real asteroids on the earth,” he said “As a scientist, I fully hope to be surprised by the results of the experiment. Although as a planetary defender, I don't want to be too surprised.”

Dart will help “ground truth” scientists’ understanding of asteroids and the materials they are made of. It could turn out that kinetic impact strategies are not appropriate for some of them, according to Dr Fieber-Beyer, particularly metallic iron-nickel asteroids.

“A kinetic impactor will definitely work on a rocky body [asteroid], or a large comet,” she said. “If you have an iron-nickel asteroid, it's not going to happen.”

Nasa’s Psyche mission will be the first to visit what scientists think is just such a solid metal bodied asteroid in 2026, providing the first direct assessment of such an asteroid. There may be alternatives to kinetic impactors for deflecting such a body, if it ever threatened Earth.

“Some of the other things that have been studied are what we call a gravity tractor,” Dr Johnson said, “which is just taking a spacecraft keeping with the asteroid and using nature's tug rope, gravity; the mutual attraction between the spacecraft and the asteroid will slowly tug that asteroid out of its impacting trajectory into a more benign one.”

Another alternative would be to use ion beam deflection, essentially strapping a thrust like that which powered Dart to Dimorphos to an asteroid to change its course. Thrusters use electrical energy to accelerate charged propellant particles out of a nozzle, generating weak, but very efficient thrust that can build up to a large velocity change over time.

“Of course, a technique like that takes longer to implement,” Dr Johnson said. “We would have to have more warning time to be able to implement it.”

And that’s the main mission of Nasa’s Planetary Defense Coordination Office anyway, he added. While Dart marks the first test of a method to deflect an asteroid, the key thing is making sure we know where all the potentially threatening asteroids are, long before they could pose a threat to Earth.

“ Hollywood and movies, they have to make it exciting. You know, we find the asteroid only 18 days before it's going to impact and everybody runs around as if they're on fire,” Dr Johnson said. “That’s not the way to do planetary defence.”

There are just more than 2,250 known potentially hazardous near Earth asteroids, and Nasa hopes its upcoming Near Earth Object Surveyor mission, a space telescope currently in its preliminary design review phase, will help scientists locate any asteroids that could be hiding in the glare of the Sun.

“Neo Surveyor will be able to find the population of asteroids 140 meters and larger within about a 10 year period,” Dr Johnson said. “Our strategy is to find that population out there with a mission like Neo Surveyor and know where all the potential hazards are, and then have sufficient time to then build the right campaign of missions to go out there.”