The threat of an asteroid hurtling towards Earth may be the stuff sci-fi thrillers are made of, but it’s also a real-life concern. In 2013, for instance, a roughly 20-meter-wide meteor exploded over Russia, injuring about 1,500 people with its shock wave.
“The hazard of asteroids crossing Earth’s orbit and being a potential for collision and impact with the Earth is a real thing,” says NASA planetary defense officer Lindley Johnson. He leads the Planetary Defense Coordination Office, which finds, tracks and characterizes potentially hazardous asteroids and comets.
But what if we could nudge an asteroid out of our way by shooting a missile at it, changing the asteroid’s orbit just enough to avoid a collision with Earth? With its new Double Asteroid Redirection Test (DART) mission, NASA is hoping to do just that: The refrigerator-sized missile is now in a preliminary design phase. If built, DART would take aim at an asteroid called Didymos B, which will (non-threateningly) approach Earth in 2022, and again in 2024.
“The DART mission is a technology demonstration,” DART co-investigator Andy Cheng says. “It will demonstrate the method of deflecting an asteroid by running a spacecraft into it and changing its orbit.”
Cheng explains that after leaving Earth’s orbit, the spacecraft would use a long-range telescope to home in on Didymos. The craft would ultimately ram into the asteroid at a speed of roughly six kilometers per second, “which is about nine times the speed of the bullet from an AK-47.”
Didymos was chosen as a target because it’s actually a binary asteroid system — the 160-meter Didymos B orbits the 780-meter Didymos A. Striking Didymos B with DART would change its orbit around the larger body, which Cheng says could help scientists measure the effect of the deflection.
“We can show that we deflected the asteroid, we can show the change, that the orbit has changed, and we can measure the amount of the change,” he says. “And we can do that from Earth with ground-based observations.”
Another reason for choosing Didymos B? It’s a relatively safe testing ground. “Because it is orbiting another one, we are not going to be changing the orbit of the system around the sun to any appreciable degree,” Cheng says. “We’re not going to knock it into a dangerous orbit.”
While the Didymos asteroid system itself may not threaten Earth — in fact, no currently known asteroids pose a significant risk to us in the coming century — scientists are discovering more near-Earth asteroids all the time.
“The population of all sizes of asteroids that might come near Earth is probably in the millions,” Johnson says. Of those, he estimates that about 93 percent of asteroids 1 km wide and larger have already been found. But medium-sized asteroids — the kind that DART could potentially deflect — are abundant, and based on current estimates, we have many more yet to find.
“Our current objective is to find those down to 140 meters [wide], and we think the population of those is somewhere around 25,000,” Johnson says. So far, his program has only found about 7,700 — less than a third of the estimated total. INASA’s catalog includes more than 16,000 near-Earth asteroids of all sizes.
Detecting, tracking and characterizing near-Earth asteroids costs $35 to $40 million a year, Johnson says. “The total amount of our program — including the preliminary development for DART and some other capabilities — this fiscal year 2017 was $60 million.” According to the NASA press release, however, DART is not mentioned as a specific line item in the FY 2018 federal budget.
For Cheng, DART is a modest investment, compared to the scale of disaster it could potentially avert, “even from one of the smaller asteroids, a few-hundred-meter asteroid impact.”
“If we’re talking about something like that over a metropolitan area, you’re talking many billions of dollars of damage and potentially millions of casualties,” he says. “So we are really talking about a major disaster that’s being averted here.”