The DART mission slightly altered an asteroid’s orbit around the Sun
In September 2022, NASA’s Double Asteroid Redirection Test (DART) mission marked a major milestone in planetary defense. Its objective was to test, for the first time, humanity’s ability to alter the trajectory of an asteroid through a controlled spacecraft impact.
The target was Dimorphos, a small asteroid roughly 160 meters in diameter orbiting the larger asteroid Didymos (777 m). The system posed no threat to Earth but provided an ideal natural laboratory for testing an asteroid deflection strategy.
On 26 September 2022, the DART spacecraft deliberately crashed into Dimorphos at a speed of 22,500 km/h. The outcome exceeded expectations: the orbital period of Dimorphos around Didymos was shortened by 33 minutes, far surpassing the predicted 73-second change. This strong effect was largely due to the massive ejection of debris produced by the impact, which acted like an additional thrust, amplifying the momentum transfer.
A study published in March 2024 in Science Advances has now revealed an unexpected consequence: the collision also slightly altered the heliocentric orbit of the Didymos system. This represents the first documented case in which human activity has measurably modified the solar orbit of an asteroid.
The discovery relies on extremely precise observations, particularly through stellar occultation measurements, a technique that records the temporary dimming of a star as an asteroid passes in front of it. By analyzing several observation campaigns, researchers detected a small deceleration of about 35.5 km/h in the system’s motion.
Scientists from the Observatoire de la Côte d’Azur (OCA) played a key role in these high-precision measurements, contributing to the dynamical analysis of the system and the interpretation of the occultation data. Their work highlights the OCA’s expertise in asteroid orbital dynamics and celestial mechanics, disciplines that are central to planetary defense.
The change in orbit remains extremely small, and orbital calculations confirm that no future collision scenario with Earth is possible.
The observations also shed new light on the internal structure of the two asteroids. Dimorphos appears to be a loosely bound rubble pile, with a density only slightly higher than that of water, whereas Didymos is significantly denser and more compact.
Understanding these structural differences is essential for designing future asteroid deflection strategies. Depending on their internal composition, some asteroids may fragment upon impact, while others could require multiple kinetic impactors or more energetic methods to achieve the desired deflection.
The DART experiment therefore marks the beginning of a new era in experimental planetary defense. Its results will soon be complemented by the European Space Agency’s Hera mission, which is scheduled to reach the Didymos system to investigate the impact site and the debris produced by the collision in unprecedented detail.