Lead
In 2022, NASA’s Double Asteroid Redirection Test (DART) intentionally collided with the small asteroid Dimorphos to test planetary defense. New research published in Science Advances reports that the collision not only shortened Dimorphos’ orbit around its companion Didymos but also produced a measurable, permanent acceleration of the pair’s orbit around the sun. The system’s heliocentric orbital period—about 770 days—was reduced by roughly 0.15 seconds, and the binary’s mutual dynamics also shifted significantly. Scientists say the result demonstrates that a kinetic impactor can change the long-term trajectory of a solar-orbiting body.
Key Takeaways
- DART struck Dimorphos in 2022 as a planetary defense demonstration; the test released an estimated 16 million kilograms (35.3 million pounds) of ejecta.
- Dimorphos lost about 0.5% of its mass; the expelled material had roughly 30,000 times the spacecraft’s mass, amplifying the momentum transfer.
- The mutual orbit of Dimorphos around Didymos shortened by 33 minutes after impact; the pair’s solar orbital period decreased by ~0.15 seconds (<1 second) permanently.
- Measured change in system orbital speed was about 11.7 microns per second (≈1.7 inches per hour), as reported by the study team.
- Twenty-two stellar occultations between October 2022 and March 2025, combined with ground-based tracking, enabled the high-precision measurements.
- The European Space Agency’s Hera mission, launched 2024, will provide follow-up close-up data later this year to refine these results.
Background
Didymos and Dimorphos form a binary asteroid pair: Didymos, a roughly spinning-top-shaped primary, and Dimorphos, a smaller satellite about 170 meters across, orbit one another while the pair circles the sun every ~770 days. Both objects are believed to be rubble-pile asteroids—aggregates of rock and dust held together by gravity—an architecture that affects how impacts and ejecta modify momentum. NASA chose this non-threatening binary for DART because a measurable change could be produced without creating an Earth hazard.
Planetary defense planning aims to demonstrate practical techniques for altering a hazardous object’s trajectory if one were found on an impact course with Earth. Kinetic impactors—spacecraft that collide with asteroids to impart momentum—are one such method; the DART mission served as the first real-world test of this approach on a solar-orbiting body. Understanding both the direct impulse from the spacecraft and the secondary impulse from ejecta is essential to predict deflection outcomes.
Main Event
On impact in 2022, DART struck Dimorphos at high speed, excavating an enormous plume of debris. Researchers estimate about 16 million kilograms of material were ejected, while the target lost roughly 0.5% of its total mass. Although the spacecraft’s own mass was small by comparison, the momentum carried by the escaping rubble multiplied the effective impulse delivered to the system by about 30,000 times the mass of the spacecraft.
That amplified momentum altered both the mutual orbit (Dimorphos around Didymos) and the pair’s heliocentric motion. Earlier studies documented that Dimorphos’ 12-hour orbit around Didymos shortened by about 33 minutes. The new analysis extends that result to the system’s solar orbit, showing a cumulative decrease of ~0.15 seconds in the ~770-day orbital period—a tiny absolute change but detectable with precision timing.
To detect such a subtle shift in solar orbital period, astronomers combined traditional ground-based astrometry with 22 stellar occultation observations collected between October 2022 and March 2025. In a stellar occultation, an asteroid briefly blocks the light of a background star as seen from particular locations on Earth; timing and chord measurements from many observers yield precise positions, sizes and motion vectors for the occulting body.
Analysis & Implications
The DART result provides the first demonstration that a human-made impact can measurably change the orbit of a solar-orbiting minor body. While a 0.15-second change in a ~770-day period is vanishingly small in the short term, orbital dynamics are cumulative: given enough lead time, even minute velocity changes can grow into large positional offsets at Earth-crossing timescales. That is the core rationale behind kinetic deflection as a viable mitigation technique.
The study also underscores the importance of ejecta momentum enhancement (sometimes called β, the momentum multiplication factor). For rubble-pile targets, loose material can be launched at high velocities after impact, producing a greater net impulse than the projectile alone. The DART observations suggest ejecta dominated the effective deflection in this case, which has implications for modeling future interventions against different asteroid structures.
Operationally, the findings support two-pronged planning: improved early detection (so there is time to act) and refined impact models that account for target composition and geometry. Missions such as NASA’s planned Near-Earth Object Surveyor (NEO Surveyor) and ESA’s Hera follow-up are complementary—Surveyor expands the detection window, Hera will provide ground truth on the post-impact crater, mass distribution and ejecta properties.
Comparison & Data
| Parameter | Pre-impact | Post-impact change |
|---|---|---|
| Heliocentric orbital period | ~770 days | decrease of ~0.15 seconds |
| Mutual orbital period (Dimorphos–Didymos) | ~12 hours | decrease of ~33 minutes |
| Ejecta mass | — | ~16,000,000 kg (35.3 million lb) |
| Mass loss (Dimorphos) | — | ~0.5% of mass |
Putting those numbers in context: the fractional change in the solar orbital period is on the order of 10^-9, extremely small in immediate terms but detectable through coordinated, high-precision observations. The mutual-orbit change is comparatively large because the impact directly altered the binary dynamics; ejecta directed relative to the binary geometry can produce much larger short-term shifts in the satellite’s orbit.
Reactions & Quotes
Team members and outside experts praised the measurement and its implications, while noting the technical difficulty of the observations.
The measured change in the system’s orbital speed was only about 11.7 microns per second, but over long timescales such a tiny shift can determine whether an object intersects Earth or not.
Rahil Makadia, DART planetary defense scientist (study lead)
Context: Makadia emphasized that small, sustained velocity changes matter for long-range deflection scenarios.
Combining occultation data with years of ground-based tracking made this precise calculation possible—volunteer observers around the world were essential.
Steve Chesley, senior research scientist, NASA JPL (study co-lead)
Context: Chesley highlighted the logistical and weather-dependent challenges of occultation campaigns that required global coordination.
Measuring a change this small required exceptional international organization; Hera will return the first close-up images this fall to validate and extend these findings.
Patrick Michel, principal investigator, ESA Hera mission
Unconfirmed
- Long-term orbital evolution: projections of how the 0.15-second change maps to impact probability decades to centuries ahead remain model-dependent and require further simulation and Hera data.
- Ejecta distribution details: precise velocity distribution and directional asymmetries of the expelled material are still being refined pending Hera’s in-situ measurements.
Bottom Line
DART demonstrated, for the first time, that a human-made impact can change the heliocentric orbit of a small body—albeit by a vanishingly small amount in the short term. The dominant role of ejecta in amplifying momentum transfer shows that target structure matters as much as the projectile’s mass and velocity when planning a deflection.
For planetary defense, the practical takeaway is twofold: improve early detection to maximize intervention lead time, and improve predictive models (informed by Hera and other missions) that account for rubble-pile behavior and ejecta dynamics. Together, these elements strengthen the case that kinetic impactors can be a credible tool in the arsenal to protect Earth from hazardous asteroids.
Sources
- CNN — (news report)
- Science Advances — (peer-reviewed journal; study published)
- NASA DART Mission — (official mission/agency)
- NASA Jet Propulsion Laboratory — (official research/agency)
- European Space Agency — Hera — (official follow-up mission)