NASA’s DART Mission Caused Unexpected Changes in Asteroid Dimorphos’ Orbit After Collision
TL;DR
Nearly a year after NASA’s DART spacecraft collided with Dimorphos, scientists are discovering unexpected consequences. The asteroid’s orbit around Didymos shortened by 34 minutes, a minute more than NASA originally calculated. This shift might be due to chaotic rotational forces or debris collisions from the impact. New findings, including a second debris tail and a surrounding boulder swarm, have added layers of complexity. These surprises offer crucial insights for future asteroid deflection efforts in planetary defense scenarios. The European Space Agency’s upcoming Hera mission will further investigate in 2026.
It’s been almost a year since NASA intentionally crashed a $300 million spacecraft into an asteroid. On September 26, 2022, the agency’s Double Asteroid Redirection Test (DART) spacecraft hit the asteroid Dimorphos at a speed of 14,000 miles per hour, marking a groundbreaking achievement. Dimorphos orbits a larger asteroid called Didymos, and although neither asteroid posed a danger to Earth, the mission demonstrated NASA’s ability to shift an asteroid’s trajectory—something that could prove crucial in a future planetary defense scenario.
The small yet powerful DART spacecraft shortened Dimorphos’ orbital period, or the time it takes to orbit Didymos, by around 33 minutes. This seemed like a neat conclusion, but recent research suggests there’s more to the story. From September to early November last year, a high school teacher and his students at a California school observatory studied the asteroid and discovered that its orbit had shortened by an additional minute beyond NASA’s figures, as reported by New Scientist‘s Jonathan O’Callaghan.
This new data brought Dimorphos’ orbital period to 34 minutes shorter than it was before the impact. “That was inconsistent at an uncomfortable level,” said Jonathan Swift, a math and science teacher at the Thacher School who was part of the research, in an interview with New Scientist. “We did our best to find any errors, but couldn’t find anything.”
While the asteroid’s unexpected behavior doesn’t pose any danger to Earth, it provides an opportunity for scientists to learn more about asteroid deflection. No mission prior to DART had conducted a planetary defense test, making Dimorphos the only real-world example to analyze.
Uncertainties were always expected with this mission. “If an asteroid were made of a single solid rock, like the ones we see on Earth, the deflection and calculations would be straightforward,” explains Peter Veres, an astronomer at the Center for Astrophysics, Harvard & Smithsonian, who wasn’t part of the study, in an email to Smithsonian magazine. “However, based on ground observations and previous space missions, we know asteroids are often made of rubble—collections of large and small rocks, dust, and sometimes ice, with many empty spaces.” Hitting one of these rubble-pile asteroids is more like striking a sponge than a rock, making the outcome harder to predict.
Swift’s team shared their surprising findings at the American Astronomical Society meeting in June and posted a preprint of their study on the Arxiv server in August. According to Veres, the team employed a well-established method and gathered enough data points to conclude that Dimorphos’ orbit is indeed deteriorating.
As for why this is happening, scientists can only speculate for now. One theory is that Dimorphos was once tidally locked to Didymos—like how Earth’s moon always shows the same face. After DART, this alignment may have been disrupted, causing Dimorphos to rotate chaotically as tidal forces in the system alter its orbit. If this is true, Veres suggests Dimorphos could eventually return to a tidally locked state.
However, a more likely explanation, as suggested by Harrison Agrusa, a DART team member at France’s Côte d’Azur Observatory, is that debris dislodged during the DART impact may have orbited Dimorphos for a while before falling back, causing more collisions that further shortened its orbit.
If these new results hold, it wouldn’t be the first unintended consequence of NASA’s DART mission. Over the past year, researchers have learned a lot about the impact’s effects.
A series of surprises
After the DART collision, the Hubble Space Telescope captured 18 images of Dimorphos and Didymos. But one picture showed an unexpected outcome: The impacted asteroid had two debris tails, not one, as reported by the European Space Agency.
The first tail appeared soon after DART struck Dimorphos, with images from Earth showing a debris trail stretching about 6,000 miles. Hubble revealed a second tail between October 2 and October 8, 2022, one to two weeks after the impact. Astronomers aren’t exactly sure why two tails formed, but such behavior is common in active asteroids and comets.
This year brought another surprise for researchers studying Dimorphos: the asteroid was surrounded by a “boulder swarm,” as described in a paper published in The Astrophysical Journal Letters. The cloud of space debris consists of 37 boulders ranging from 3 to 22 feet wide, which were likely shaken loose by the spacecraft’s collision.
While some debris was expected after the impact, the mission wasn’t designed to obliterate Dimorphos. In a real-life asteroid threat, shattering the rock could increase the danger to Earth by sending a swarm of boulders our way, according to NASA. However, in Dimorphos’ case, the rocks are far too distant to harm our planet, and they’re drifting at about half a mile per hour—the speed of a slow-moving tortoise.
Scientists believe these rocks either broke off at the impact site or were dislodged by seismic waves from the collision. According to Veres, these boulders are “another mystery that could relate to Dimorphos’ strange new behavior.”
The threat of near-Earth asteroids
Dimorphos, roughly 6.8 million miles from Earth, is just one of many objects astronomers are monitoring. As of August 31, NASA’s latest count identified more than 32,000 near-Earth asteroids—those that pass within 30 million miles of Earth’s orbit. NASA estimates around 14,000 asteroids larger than 140 meters remain undiscovered, and 50 are massive, over a kilometer across.
Still, the odds of an impact are slim. NASA stated that no known asteroid poses a significant risk over the next century. The highest risk object is asteroid 2009 FD, which has a less than 0.2 percent chance of hitting Earth in 2185.
Asteroids aren’t just threats; they could also contain valuable resources, such as water, metals, and organic materials, potentially aiding space exploration and manufacturing. NASA’s OSIRIS-REx spacecraft is expected to return to Earth later this month, carrying a sample from the asteroid Bennu. This sample could provide insights into the solar system’s origins and the properties of potentially hazardous asteroids.
Space agencies will continue to study Dimorphos in the years ahead to observe the DART mission’s long-term effects. The European Space Agency plans to launch the Hera spacecraft in October 2024, with a rendezvous set for December 2026, to better understand the aftermath of the crash.
In the meantime, the DART team has been conducting its own observations over the past year, and they’ll be releasing their findings soon. The data, like the California team’s, is expected to show a continued reduction in Dimorphos’ orbit—but only by around 15 seconds, according to New Scientist.
“The Didymos-Dimorphos system is still under close observation, and we’ll be learning a lot more in the weeks ahead,” Veres says. “All these surprising findings from DART will help us understand how such impacts work and better prepare for any future kinetic impact missions, should we need to divert an object on a collision course with Earth.”
