First results of planetary defense test published – spacecraft impact shortens asteroid orbit period by 33 minutes
Observations of NASA’s Double Asteroid Redirection Test (DART) impact on the 150-m sized asteroid moonlet called Dimorphos orbiting around a bigger 800-m sized near-Earth asteroid Didymos are reported in five papers published in Nature this week with contribution of University of Helsinki scientists.

Understanding how the impact of the DART spacecraft changed the orbit of the moon sheds light on how this approach might be able to provide a defence system against potential collisions of asteroids with the Earth. 

The DART mission ran a spacecraft into Dimorphos, the moon of near-Earth asteroid (65803) Didymos, on 26 September 2022. The aim of the collision was to change the mutual orbit of the binary asteroid system, testing a potential method of asteroid deflection. Detailed physical characterization of the target asteroids, documentation of the DART impact consequences including large-scale cloud of ejected debris, measurement of Dimorphos 33-minute orbital period change, and a description of how momentum was transferred from the spacecraft to the moon are revealed in five papers published in Nature. University of Helsinki researchers directly contributed to two of these. 

Cristina Thomas and colleagues determine the change in the orbital period of Dimorphos around Didymos after the DART impact. Two independent approaches to measure this change suggest that the orbital period was reduced by around 33 minutes.

Ronald Terik Daly and colleagues including UH’s Docent (Adjunct Professor) Tomas Kohout reconstruct the impact of the DART spacecraft on Dimorphos, which may help with the planning of future missions and could help to predict outcomes with more certainty. They also describe the location and nature of the impact site, noting that it was between two boulders, one of which was grazed by the spacecraft as it made contact with the moon.

Jian-Yang Li and colleagues including UH’s Associate Professor Mikael Granvik and Docent (Adjunct Professor) Tomas Kohout report Hubble Space Telescope observations of the ejecta plume (debris created by the impact), the speed and evolution of which may explain the momentum change caused by the impact.

Andrew Cheng and colleagues note that the orbital velocity of Dimorphos reduced after the impact. They propose that the momentum transfer from the spacecraft to the moon was enhanced by recoil from ejecta streams produced by the impact. Observations of Dimorphos before, during and after the impact — made by a global network of citizen science telescopes (including three on Réunion Island and one in Nairobi that captured the moment of impact) — are reported by Ariel Graykowski and colleagues. They estimate the mass and the energy of the ejected dust, and its evolution over time, which may help us to understand the outcomes of impact missions. 

“Despite its small size, the target asteroid Dimorphos turned out to be a rubble pile – a loose conglomerate of boulders and pebbles. This is why we observed such a massive cloud of ejected material. The impact ejecta also significantly enhanced the spacecraft momentum transfer and amplified the resulting orbital period change.” says Tomas Kohout. 

“A scientifically interesting aspect of the DART-produced ejecta cloud is that it will most likely be used as a very-well-calibrated reference point for telescopic observations of the aftermath of natural collisions occurring in the asteroid belt. The significance of DART is thus greater than what one would perhaps initially assume” adds Mikael Granvik. 

Neither Dimorphos nor Didymos poses any hazard to Earth before or after DART’s controlled collision with Dimorphos.

Johns Hopkins Applied Physics Lab built and operated the DART spacecraft and manages the DART mission for NASA’s Planetary Defense Coordination Office as a project of the agency’s Planetary Missions Program Office. LICIACube is a project of the Italian Space Agency (ASI), carried out by Argotec. For more information about the DART mission, visit or

Links to the articles prepared by the DART Investigation Team: 

Daly et al.: Successful Kinetic Impact into an Asteroid for Planetary Defense

Thomas et al.: Orbital Period Change of Dimorphos Due to the DART Kinetic Impact  

Li et al.:  Ejecta from the DART-produced active asteroid Dimorphos

Cheng et al.: Momentum Transfer from the DART Mission Kinetic Impact on Asteroid Dimorphos 

Italian Space Agency’s LICIACube satellite acquired this close-up image on Sep. 26, 2022 at a distance of approximately 50 km moments after DART’s impact on Dimorphos. Larger Didymos can be seen on top-right and smaller Dimorphos on bottom-left with clearly visible spreading curtain of ejected material. Credit: ASI/NASA