A new study challenges a leading explanation for the continued evolution of Dimorphos’s orbit following NASA’s historic planetary defense test. Researchers simulating the aftermath of the Double Asteroid Redirection Test (DART) mission’s impact found that the previously favored mechanism, known as binary hardening, is an unlikely cause for the ongoing shortening of the asteroid moon’s orbital period. The model suggests that ejected material from the 2022 collision would have had the opposite effect, deepening the mystery of the asteroid system’s behavior.
On Sept. 26, 2022, the DART spacecraft successfully slammed into Dimorphos, the smaller of two asteroids in the Didymos binary system, to test if a kinetic impact could alter an asteroid’s trajectory. The impact was a resounding success, shortening the moonlet’s 11-hour and 55-minute orbit around its larger companion by approximately 33 minutes—far exceeding the mission’s minimum goals. However, observations in the following months detected a small but significant additional decrease of about 30 seconds, a subtle change that lacked a clear explanation. While scientists had theorized that the gravitational scattering of crash debris was responsible, new research indicates a different physical process, likely linked to the asteroid’s dramatic change in shape, is the true cause.
The DART Mission’s Historic Impact
NASA’s DART mission was the first of its kind, designed purely as a technology demonstration for planetary defense. The target was the binary near-Earth asteroid system Didymos, which consists of a primary asteroid about 780 meters in diameter and its small moonlet, Dimorphos, measuring approximately 170 meters across. The system posed no threat to Earth, but its binary nature made it an ideal laboratory; changes to Dimorphos’s orbit could be easily measured by ground-based telescopes by timing how often it passed in front of Didymos.
The 570-kilogram DART spacecraft collided with Dimorphos at roughly 22,530 kilometers per hour. The primary goal was to shorten the orbital period by at least 73 seconds, which would have been considered a success. The actual result was a massive alteration, shortening the orbit by about 33 minutes and 15 seconds. This high degree of efficiency showed that a kinetic impactor could be even more effective than predicted, not just from the push of the spacecraft itself but also from the powerful recoil effect of the vast plume of rock and dust ejected from the asteroid’s surface.
A Lingering Orbital Mystery
While the initial, large orbital shift was celebrated as a triumph, the story was not over. Continued monitoring of the asteroid system in the weeks and months after the collision revealed a secondary, more subtle phenomenon. The orbital period was still shrinking, by an additional 30 seconds or so. This ongoing change puzzled investigators. The initial impact event was explosive and brief, but this suggested a more gradual process was now at work within the Didymos-Dimorphos system.
Understanding this secondary evolution is critical for refining models of asteroid deflection. If a kinetic impact has long-term, evolving consequences, these must be factored into any future mission to protect Earth. The leading hypothesis to explain the continued orbital decay was a process called binary hardening, a concept rooted in the gravitational interactions between the two asteroids and the cloud of debris DART kicked up.
Revisiting the ‘Binary Hardening’ Hypothesis
A Theory of Ejecta
The theory of binary hardening posited that the lingering cloud of ejecta from the impact was the culprit for the continued orbital shortening. In this model, particles in the debris cloud would interact gravitationally with both Dimorphos and Didymos. Over time, some of these particles would be flung out of the system entirely. According to the laws of conservation of angular momentum, as these particles escaped, they would carry orbital energy away with them. This loss of energy would cause Dimorphos to draw slightly closer to Didymos, “hardening,” or shortening, its orbit. For a time, this was considered the most plausible explanation for the extra 30-second shift.
Simulations Reveal a Flaw
A new study from researchers Harrison Agrusa and Camille Chatenet, however, casts serious doubt on this explanation. Their detailed computer simulations modeled the evolution of the ejecta particles within the Didymos system’s specific gravitational environment. The results showed that the system is a “weak scatterer,” meaning it is not very efficient at flinging material away into space. Instead, the model demonstrated that the vast majority of ejecta particles would be gravitationally recaptured by one of the two asteroids over time.
Crucially, the simulation showed that as these particles re-accrete onto the asteroids, they would return angular momentum to the binary system. This process would have the opposite of the desired effect, causing the orbital period to increase, not decrease. The study concludes that the gravitational scattering of ejecta cannot account for the observed evolution of the orbit, forcing scientists to look for an alternative mechanism.
An Asteroid Reshaped
From Spheroid to Ellipsoid
The answer to the mystery may lie not in the ejected debris, but in Dimorphos itself. Separate studies analyzing data from the moments before and after the impact have confirmed that the collision profoundly altered the asteroid’s physical shape. Before the DART mission, Dimorphos was thought to be a relatively symmetrical “oblate spheroid”—akin to a squashed ball.
Post-impact analysis, led by Shantanu Naidu at NASA’s Jet Propulsion Laboratory, painted a different picture. By combining data from DART’s final images, radar observations from the Deep Space Network, and ground-based telescopes, the team determined the asteroid was reshaped into a “triaxial ellipsoid.” In simpler terms, it was stretched from a slightly flattened sphere into a form more like an oblong watermelon. This finding confirms that Dimorphos is likely a loosely aggregated “rubble pile” asteroid, which is more susceptible to deformation than a solid, monolithic rock.
A Chaotically Tumbling Moon
The authors of the study rejecting binary hardening suggest this dramatic reshaping is the key to the lingering orbital changes. The impact didn’t just give Dimorphos a push; it sent it into a chaotic rotation. Another study led by Derek Richardson of the University of Maryland concluded the moonlet was likely knocked out of its stable, tidally locked state and may now be wobbling and tumbling unpredictably.
This ongoing physical adjustment could be the source of the orbital evolution. As the deformed and chaotically rotating Dimorphos continues to settle into a new equilibrium, its changing shape alters the mutual gravitational pull between it and Didymos. This gradual settling process, a slow readjustment of the entire system’s dynamics, is now the leading candidate to explain the additional 30-second shortening of its orbit.
Implications for Planetary Defense
Accurately understanding the multifaceted consequences of the DART impact is essential for the future of planetary defense. The mission proved that a kinetic impactor is a viable tool for deflecting a hazardous asteroid, but it also revealed a rich and complex aftermath that scientists are still working to comprehend. The findings that Dimorphos is a rubble pile, that it was dramatically reshaped, and that its orbit continued to evolve long after the impact are all critical data points.
These unexpected results will help refine the strategies and predictive models for any future deflection mission. They underscore the importance of understanding an asteroid’s composition and internal structure before attempting to move it. The European Space Agency’s Hera mission, scheduled to arrive at the Didymos system in late 2026, will perform a detailed post-impact survey. It will measure Dimorphos’s new mass, shape, and internal properties, providing the definitive ground truth for the DART experiment and helping to solidify humanity’s ability to protect the planet.
