A NASA spacecraft that hurled itself into an asteroid two weeks ago changed the space rock’s orbit as scientists had hoped it would, the mission team announced today from the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
“This mission shows that NASA is trying to be ready no matter what the universe throws at us," said NASA Administrator Bill Nelson.
Data from a group of Earth-based telescopes showed that the Sept. 26 collision of the DART craft — the mission’s name is Double Asteroid Redirection Test — jostled the asteroid closer to its larger neighbor, shaving 32 minutes off its nearly 12-hour orbit.
The asteroid had no chance of striking Earth, nor does any other known asteroid for at least half a century. The mission tested a technique for redirecting this asteroid as proof of concept in case future Earth folk really need to bat one out of the way.
The basic idea is simple: Hit it with a hammer! But the degree of difficulty was high, in part because NASA was aiming at an asteroid no one had ever seen until about an hour before the collision. It is a moonlet named Dimorphos that is about the size of a football stadium.
Sky watchers operating the world’s highest-powered telescopes detect the moonlet only as a shadow that crosses the larger asteroid it orbits, Didymos, as the two circle the sun together. The pair make up a “double asteroid,” a common arrangement in our solar system.
Here’s how the $330 million DART test worked:
The spacecraft was launched on a Falcon 9 rocket from Vandenberg Space Force Base in California on Nov. 23.
For 10 months, it orbited the sun on a collision course with the pair of asteroids until it met them on Monday, when they were relatively close to Earth.
In the final hours, the craft’s autonomous navigation took over the steering.
With an hour to go, the 1,200-pound spacecraft spotted the moonlet and aimed for it, coasting into it at 14,000 mph.
Just making contact with such a relatively small asteroid was a scientific triumph.
The solid whack shortened the moonlet’s orbit time by about 4 percent, more than enough for telescopes on Earth to detect the change.
Why just bump it instead of blowing it apart, "Armageddon”-style? Because exploding a pile of ancient rock — especially one that may contain metal or giant boulders, as many asteroids do — would be messy and unpredictable, mission coordination lead Nancy Chabot said before last year’s launch. The deflection method assumes we will have time for a bit of finesse: A small nudge now could ensure that an asteroid sails well wide of Earth many years down the road.
“You don’t want, necessarily, to make this more complicated than it has to be, right? You would do this well ahead of time, like decades — 10, 20, 30 years ahead,” she said. “Small changes add up to big changes in that amount of time.”
The asteroids in our neighborhood
Thousands of asteroids are large enough and come close enough to Earth’s orbit that researchers need to keep an eye on them.
No known asteroid large enough to cause damage on the ground has any significant chance of reaching our planet in the next 50 years, according to Paul Chodas, director of NASA’s Center for Near Earth Object Studies. His team catalogues and tracks asteroids and comets whose orbits bring them into Earth’s general neighborhood, defined as within 121 million miles of the sun.
Most of these known asteroids were identified by ground-based optical telescopes, and some were located by an infrared space telescope named NEOWISE that detected their heat signatures from its perch in low Earth orbit.
Almost two-thirds of those are so small that they would burn up in Earth’s atmosphere if they came our way. But, of course, some asteroids are huge and dangerous — just ask any dinosaur.
Chodas said scientists have discovered 95 percent of near-Earth asteroids that are large enough to create global catastrophe, meaning a kilometer (about six-tenths of a mile) or wider. The largest is about four miles across, much smaller than the six-mile behemoth that wiped out the dinosaurs.
The unknown ones are the wild cards.
Asteroids that are just a bit smaller but still large enough do a lot of regional damage are tougher to detect with current technology. Models estimate that we have found just 40 percent of those that are 460 feet wide (140 meters) and larger, such as Didymos and its moonlet. That is well below NASA’s goal of identifying at least 90 percent.
“Some asteroids are sneaky, and they have orbits that make an asteroid very hard to find,” Chodas said.
Some may be in orbits that don’t often bring them close to Earth. Some are made of dark material that doesn’t reflect much light, making it difficult for ground-based telescopes to detect them. Others may lurk on the opposite side of the sun.
The truck-size rock that caused a fireball and shock wave over Russia in 2013 arrived with no warning because it came from the direction of the sun, a huge blind spot for existing telescopes.
Fortunately, more high-powered eyes are on the way.
In 2026, NASA plans to launch a very sensitive infrared telescope called NEO Surveyor, which will have a wide view of the skies from a stable vantage point about a million miles up between the Earth and the sun. Like its predecessor NEOWISE, it will detect heat signatures rather than visible light.
Amy Mainzer, principal investigator on the Surveyor team, said it should be able to spot a 460-foot asteroid from at least 50 million miles away.
Around the same time, a new ground telescope in Chile is expected to become operational with a massive 28-foot mirror that will be able to detect objects that are much fainter and farther away than any current ground telescope.
“The two together will get us to 90 percent very quickly,” Chodas said.
Why NASA picked this asteroid
The moonlet Dimorphos seemed to be an ideal target because of its ordinary composition and extraordinary location close enough — but not too close — to Earth.
It is probably chondrite, Chabot said, a common type of asteroid made of rock and metal rubble left over from when planets were formed 4.5 billion years ago. It is the size of something people would definitely want to redirect if it were headed toward Earth.
About a sixth of all near-Earth asteroids are linked by gravity in pairs or small groups the way Dimorphos is linked to Didymos. That is how we knew the moonlet existed: Ground-based telescopes detected the regular dimming and brightening of Didymos as the moonlet passed in front of it and behind it every 11 hours and 55 minutes.
Now, they detect that the orbit is 32 minutes shorter.
The spacecraft’s head-on collision slowed the moonlet enough that Didymos’s gravity pulled it a bit closer, speeding up its orbit. NASA models had estimated a roughly 10-minute change, but the plume of rock and debris that flew out of the crater on impact appears to have provided a good bit of extra push, Chabot said.
The contact occurred about 7 million miles from Earth, roughly 28 times the distance between the Earth and the moon. That’s close enough for high-speed data transmission and for telescopes on the ground to detect a change in the moonlet’s orbit, but it’s far enough away that the whole endeavor presented a significant technological challenge.
The tech that was tested
The DART spacecraft carried quite a bit of sophisticated equipment, including some that NASA was testing for future missions.
Rolled-up solar panels
Unfurled shortly after launch
Lightweight, flexible solar arrays unrolled to power a next-generation thruster.
Took the first photo of Didymos on July 27
Images were sent to Earth and also informed the craft’s navigation system. In the mission’s last few hours, the camera shot and transmitted an image every second.
Was tested during the mission
DART mostly was maneuvered by 12 hydrazine thrusters, which have guided spacecraft for decades. But it fired up a new and more efficient ion thruster for a two-hour test.
Mini cube satellite was deployed from the DART craft on Sept. 12
A briefcase-size satellite provided by the Italian Space Agency trailed the spacecraft by about three minutes and photographed the crash and its aftermath with cameras named LUKE and LEIA.
Autonomous navigation system
Kicked in about four hours before impact
Once the craft got within about 50,000 miles of the target, object-tracking algorithms that evolved from missile defense analyzed camera images and took over the driving.
What’s next? We’ll see.
In 2024, the European Space Agency will launch a spacecraft named Hera to visit Dimorphos and investigate the crater that was left by DART. What it discovers will help planetary defense experts figure out how the deflection technique can be refined, and perhaps they will gain some insight into what other methods might work as well.
Future techniques might include using gravity to tug asteroids out of orbit, zapping them with lasers, or even moving them with tractor beams, said NASA planetary defense officer Lindley Johnson before DART launched last year.
“This,” he said, “is just a start.”
About this story
Most information and visual reference material for the DART mission and its equipment came from NASA, the Jet Propulsion Laboratory at the California Institute of Technology and the Johns Hopkins Applied Physics Lab. NEOWISE and Surveyor information came from Amy Mainzer of the Lunar and Planetary Laboratory at the University of Arizona. Information on near-Earth asteroids came from CNEOS.