Nearly a year has passed since NASA deliberately crashed a $300 million spacecraft into an asteroid. In a first-of-its-kind feat, the agency’s Double Asteroid Redirection Test (DART) craft slammed into the space rock Dimorphos at 14,000 miles per hour on September 26, 2022. Dimorphos orbits another asteroid called Didymos, and while neither object posed a threat to Earth, the feat showed the world that NASA could nudge an asteroid and alter its trajectory—an action that might come in handy if a planetary defense scenario were to materialize in the future.
The tiny-but-mighty DART craft shortened Dimorphos’ orbital period, or the time it takes to circle Didymos, by about 33 minutes. It looked like a tidy conclusion to a tale of success, but according to new research, the story didn’t end there. From September to early November last year, a high school teacher and his students used the observatory at their California school to study the asteroid, and they found its orbit had decreased by another full minute compared to NASA’s numbers, reports New Scientist’s Jonathan O’Callaghan.
The finding brought the asteroid’s orbital period to 34 minutes shorter than it was pre-impact. “That was inconsistent at an uncomfortable level,” Jonathan Swift, a math and science teacher at the Thacher School who took part in the research, tells New Scientist. “We tried our best to find the crack in what we had done, but we couldn’t find anything.”
Dimorphos’ unexpected behavior is not a cause for alarm—the asteroid still poses no threat to Earth—but it could teach scientists more about asteroid deflection. Prior to DART, no other mission had ever conducted a planetary defense test, so Dimorphos is the only real-world example to learn from.
The mission always had uncertainties. “If an asteroid were constructed as a monolithic rock, similar to the rocks we find on the ground, the deflection and calculations would be straightforward,” Peter Veres, an astronomer at the Center for Astrophysics, Harvard & Smithsonian, who was not involved in the study, tells Smithsonian magazine in an email. “Instead, we already knew from ground-based observations and several space missions that asteroids are often rubble piles—loosely aggregated spheres of large and small boulders, dust and sometimes ice, with numerous empty spaces.” Hitting one of these asteroids, Veres says, is almost more like striking a sponge than a rock, and it’s harder to predict the impact’s results.
Swift’s team shared their surprising new research at a meeting of the American Astronomical Society in June and posted a preprint of their paper on the online server Arxiv in August. According to Veres, the team relied on a widely used method and gathered enough data points to conclude that Dimorphos’ orbit is degrading.
As for why this may be happening, scientists only have guesses so far. One idea is that Dimorphos was once tidally locked to Didymos—meaning that, like Earth’s moon, the same side of the satellite always faced its host. But now, after the DART mission, this alignment might have been broken, causing Dimorphos to tumble around as the system’s tidal forces change its orbit. If this is the case, Veres says, Dimorphos will likely settle back into a tidally locked state over time.
Perhaps more likely, however, is the second idea, as Harrison Agrusa, a DART team member at the Côte d’Azur Observatory in France who was not involved in the new research, tells New Scientist. Material disturbed from Dimorphos during impact might have orbited the asteroid for a while before falling back to its surface, resulting in more collisions that reduced its orbital period even further.
If the new findings are true, this wouldn’t be the first unintended consequence of NASA’s DART mission. Over the year since the ill-fated spacecraft met its doom at Dimorphos, researchers have been learning increasingly more about the crash’s effects.
A series of surprises
Following the DART collision, the Hubble Space Telescope made 18 observations of Dimorphos and Didymos. But one of the images revealed an “unexpected development”: The impacted asteroid had not one, but two tails of debris coming off of it, per the European Space Agency.
The first part of the tail appeared shortly after DART smashed into Dimorphos, and photographers on Earth captured an image of it: a trail of debris that stretched for some 6,000 miles. Hubble revealed a second, more northern tail formed sometime between October 2 and 8, 2022, roughly one to two weeks after the impact. Astronomers aren’t exactly sure why this double tail was created, though the trait is common in active asteroids and comets.
Then, this year, researchers studying Dimorphos got another shock: The asteroid was surrounded by a “boulder swarm” of rock fragments that got broken off during the NASA spacecraft’s collision, according to a paper published in The Astrophysical Journal Letters in July. The cloud of newly made space debris consists of 37 boulders that range in size from 3 feet to 22 feet across.
While some amount of ejected material from the crash was expected, last year’s collision was not meant to blow Dimorphos to smithereens. In the event of a real asteroid threat, blasting pieces from the rock might actually increase the danger to Earth, sending a “cluster of threatening boulders heading in our direction,” per NASA. But in the case of Dimorphos, none of these newly created rocks can damage our home planet—they’re all too far away. And right now, they’re moving at about half a mile per hour, or the walking speed of a giant tortoise.
Scientists have hypothesized these rocks either broke off from Dimorphos at the impact site after the crash or were shaken from the asteroid due to seismic waves caused by the spacecraft’s impact. These boulders are “another mystery that could be related to the unexpected behavior” of Dimorphos revealed in the new study, Veres says.
The threat of near-Earth asteroids
Dimorphos, at roughly 6.8 million miles away, is just one of several space objects that astronomers have their eyes on. According to NASA’s latest count, published August 31, surveys have identified more than 32,000 near-Earth asteroids, or those that come within 30 million miles of our planet’s orbit. The agency estimates some 14,000 near-Earth asteroids larger than 140 meters might still be undiscovered, as well as 50 massive ones that measure more than one kilometer across.
Still, the likelihood of an impact is low—over the next century, no known asteroid poses a significant threat to Earth, NASA wrote in June. The object known to be the highest risk to our planet is an asteroid called 2009 FD—and even that rock has only a 1 in 714, or less than 2 percent, chance of hitting Earth in the year 2185.
But asteroids are more than just flying potential hazards—they could also hold natural resources such as water, precious metals and organic material that might one day fuel space exploration or enable manufacturing for missions. NASA’s OSIRIS-REx spacecraft is slated to return to Earth later this month, toting a piece of the asteroid Bennu that it collected and is carrying back from more than 46 million miles away. The sample could reveal information on the dawn of our solar system, as well as the properties of asteroids that could potentially collide with our planet.
Space agencies will also continue to study Dimorphos in the coming years, watching the consequences of the DART mission unfold. The European Space Agency plans to launch its Hera spacecraft in October 2024, which will rendezvous with Dimorphos in December 2026 and potentially get a much clearer idea of what happened after the crash.
Meanwhile, the DART team has been making its own observations of the asteroid over the last year, and it will release its findings within the next few weeks. These, like the new data from California, are expected to show that Dimorphos’ orbital period has continued to drop—but only by about 15 seconds, New Scientist reports.
“The Didymos-Dimorphos system has not been forgotten, and we will learn a lot of new outcomes and theories in upcoming weeks,” Veres says. “All the unexpected findings from the DART mission will help us comprehend how such impacts function and prepare for an effective kinetic impact mission should one ever be necessary to divert an object that could potentially be on a direct collision course with Earth.”