An asteroid wiped out the dinosaurs; now the earthlings are fighting back. The sight of lizard fossils in most science museums is a powerful reminder that asteroids can threaten Earth as they orbit our sun, occasionally coming perilously close to our planet, or, 66 million years ago, too close. Now scientists have tested a method that could save our planet from future doomsdays. In the last hour, NASA’s Double Asteroid Redirection Test (DART) spacecraft crashed into a small asteroid called Dimorphos.
As the full name of DART implies, this impact was not an accident. It’s meant to change the space rock’s trajectory by a small but noticeable amount, a change that observers will confirm and carefully track from afar with a host of ground-based and space-based telescopes. In the future, if a dangerous asteroid is found on a collision course with Earth, we could use this same technique to deflect it and avert disaster. “We’re not going to blow up the Death Star,” says Andy Rivkin, DART research team leader at Johns Hopkins University’s Applied Physics Laboratory (APL), which is leading the mission. “We are using the momentum of the spacecraft to change the orbit of the asteroid.”
DART launched in November 2021 on a collision course with Dimorphos, a small 160-meter-sized asteroid that orbits another asteroid, Didymos, which is nearly five times its size. For nearly a year, the vending-machine-sized, around 600-kilogram (1,300-pound) spacecraft kept up with the asteroids, snapping ever-clearer images as it got closer. That was until today, at 7:15 pm ET, when APL mission control engineers stopped receiving signals from the spacecraft, confirming its self-destructive impact on Dimorphos, about 11 million kilometers from Earth.
“We are embarking on a new era for humanity,” said Lori Glaze, director of NASA’s planetary science division, in post-impact comments during the space agency’s live broadcast of the event. “An era where we have the potential ability to protect ourselves from something like a dangerous asteroid impact.”
Traveling at about 14,000 miles per hour, the spacecraft slammed into the asteroid with an energy of about three metric tons of TNT and exploded in a superheated shower of metal and debris from the asteroid. A small Italian spacecraft called LICIACube (Light Italian Cubesat for Imaging of Asteroids) following a three-minute delay took images of the impact that will be released in the coming days. However, the real mission has just begun. Now scientists will observe Dimorphos with everything from ground-based telescopes to deep space observatories, and see exactly how much effect DART’s dramatic impact had on its target. “We’re showing for the first time that if humanity needed to alter the course of an asteroid, we would be able to do it,” says Harrison Agrusa of the University of Maryland, a member of the DART team.
The DART mission was originally conceived about two decades ago, when US and European scientists began discussing a joint mission that could practice an asteroid kinetic deflection technique. Originally called AIDA (Asteroid Impact and Deflection Assessment), the mission would involve NASA’s DART spacecraft and Europe’s AIM (Asteroid Impact Mission) spacecraft, which would orbit the target and observe the impact. Unfortunately, European officials canceled AIM in 2016 due to lack of funds. In 2019, however, the mission was reborn as the Hera spacecraft (named after the Greek goddess of marriage). But that restart in development meant a delayed launch: Hera won’t take off until 2024 and won’t arrive at Didymos until 2026, too late to witness the impact of DART but still in time to study its lasting effects.
Scientists wanted DART to target an asteroid binary, where one asteroid orbits another, because such celestial configurations allow easier measurements of small impact-induced orbital changes. “The drift is almost instantaneous,” says Patrick Michel of the French National Center for Scientific Research, a former AIM principal scientist and now Hera principal investigator. In 2013 scientists selected the Didymos system as a target. First found in 1996, that larger asteroid earned its name (Greek for “twin”) after the discovery of a small orbiting companion in 2003, later named Dimorphos, or “having two forms.”
Dimorphos completes an orbit of Didymos every 11.92 hours. Asteroids share a similar orbit with Earth, but they do not pose a threat, as they never come closer than a few million kilometers from our planet. But its orbit angle means that Dimorphos regularly “eclipses” in front of Didymos, allowing its orbital period to be precisely measured. After the impact, a variety of telescopes, including the James Webb Space Telescope and Hubble, and even spacecraft like NASA’s Lucy probe, which is currently en route to visit asteroids near Jupiter, will track this eclipse, which It will allow scientists to calculate exactly how much Dimorphos’s orbit has changed.
DART hit the asteroid almost head-on, meaning it lowered Dimorphos’s orbit. However, the asteroid is so small that mission scientists did not know its exact shape or composition, whether Dimorphos was a rigid, solid object or rather a looser “rubble pile” of rocks and boulders have accumulated smoothly. During the final moments of its approach, DART transmitted images of Dimorphos’ debris-covered surface, indicating that the asteroid was far from rock-solid. If it had been, the change in its orbit could have been just over a minute, since DART would have transferred only a relatively small amount of momentum to the asteroid. “We need at least 73 seconds of orbit change” for the mission to be heralded as a success, says Rivkin. Instead, the ramshackle appearance of Dimorphos suggests the force of the material flung outward (perhaps as much as a few tens of millions of kilograms). it could cause a much larger change in momentum, shortening the asteroid’s orbit by 10 minutes or more. Such an event could completely reshape Dimorphos or even cause him to collapse. upside down. “The fainter the asteroid, the larger the crater,” says Sabina Raducan of the University of Bern in Switzerland, a member of the DART team. “Of course, we want there to be a lot of diversion and ejection because that’s more interesting.”
Observations from the telescopes and LICIACube should reveal roughly how much the orbit changed and how much ejecta was released, and the DART team is set to announce the mission’s preliminary results this December at a meeting of the American Geophysical Union in Chicago. But no one will know for sure how successful the mission was until Hera arrives in 2026. Observations from that spacecraft will accurately measure Dimorphos’ mass and get a more exact idea of how much its orbit around Didymos has changed, perhaps 10 times. better than I would. otherwise it would be possible only from more remote observations. “We’ll understand how big the push was and better understand what Dimorphos is made of,” says APL’s Angela Stickle, a member of the DART team.
That could be crucial information if something like DART is ever called upon to save Earth in the future. “This is one of the most important things we are doing right now,” says Detlef Koschny, deputy director of ESA’s Planetary Defense Office. “We have been talking for many years about the need to show that we can deflect an asteroid.” While no dinosaur-killing asteroids several kilometers in size are known to be on an impact course with our planet, smaller asteroids like Dimorphos are less limited, with only an estimated small percentage of their total population currently known. “We still don’t know enough to feel safe,” says Koschny. The impact of a Dimorphos-sized space rock could instantly destroy a city and cause widespread damage across an entire country, meaning there’s good reason to keep an eye out for such asteroids.
Future telescopes, such as the Vera C. Rubin Observatory, which will become operational in Chile at the end of this decade, will better track these asteroids. If we ever find one on a collision course with Earth, the results of the DART mission may well dictate what action to take. “It’s going to validate a tool that we could use,” says Rivkin. To deflect a dangerous asteroid, perhaps a larger version of DART or even a series of DART-sized spacecraft could be used to crash into the offending space rock, one after another, gradually deflecting its target. “It depends on how much warning time we have,” says Rivkin. Such a dangerous event is unlikely to plague humanity any time soon. But perhaps, in the distant future, our distant descendants will have this little spaceship to thank. “If we can deflect Dimorphos, chances are we can deflect any other near-Earth asteroids,” says Agrusa.