A First in Planetary Defense History
NASA has confirmed a remarkable outcome from its Double Asteroid Redirection Test mission: the DART spacecraft's impact on the asteroid Dimorphos in 2022 not only changed the small asteroid's orbit around its companion Didymos, as intended, but also measurably altered the entire binary asteroid system's orbit around the sun. The finding represents the first time humanity has verifiably changed the solar orbit of a celestial body.
The DART mission was designed as a proof of concept for planetary defense — demonstrating that a kinetic impactor could deflect an asteroid that might one day threaten Earth. The primary success metric was changing Dimorphos's orbit around Didymos, which DART accomplished dramatically, shortening its orbital period by approximately 33 minutes against an expected change of roughly 10 minutes.
The Unexpected Solar Orbit Change
While the change in Dimorphos's local orbit was the mission's primary objective, continued tracking of the Didymos-Dimorphos system has revealed that the entire binary system's orbit around the sun was also affected. The change is small in absolute terms but measurable with modern tracking techniques, and it represents a genuinely unprecedented achievement in human spaceflight history.
The heliocentric orbit change was caused by the momentum transferred during impact. When DART struck Dimorphos at approximately 6.1 kilometers per second, it transferred momentum not just to Dimorphos relative to Didymos but to the entire system relative to the sun. The impact also ejected a significant amount of debris, and the recoil from this ejecta amplified the momentum change beyond what the spacecraft's mass alone would have produced.
Measuring the Change
Detecting the solar orbit change required precise tracking observations conducted over an extended period after the impact. Ground-based telescopes and radar systems monitored the position and velocity of the Didymos system with extreme precision, building up enough data to distinguish the impact-induced orbit change from natural gravitational perturbations and measurement uncertainty.
The measurement campaign involved observatories around the world, coordinating observations to build a comprehensive picture of the system's post-impact trajectory. The fact that the change was detectable speaks both to the magnitude of the DART impact and to the remarkable precision of modern astronomical measurement techniques.
Why This Matters for Planetary Defense
The confirmation that DART altered a solar orbit has significant implications for planetary defense planning. If a potentially hazardous asteroid were discovered on a collision course with Earth, the goal would be to change its heliocentric orbit enough to convert a direct hit into a near-miss. DART's results demonstrate that kinetic impactor technology can produce exactly this type of orbit modification.
The amplification effect from impact ejecta is particularly important. The debris cloud ejected by the DART impact multiplied the momentum transfer by a factor of several times compared to the spacecraft's mass alone. This means that a kinetic impactor mission targeting a real threat could achieve greater deflection than simple mass-and-velocity calculations would predict, effectively making the technology more capable than its raw specifications suggest.
The ESA Hera Follow-Up
The European Space Agency's Hera mission, launched in 2024, is currently en route to the Didymos system to conduct a detailed post-impact investigation. Hera will arrive later in 2026 and conduct close-up observations of both asteroids, measuring the crater left by DART, studying the structural properties of Dimorphos, and making precise measurements of the orbital changes.
Hera's observations will provide crucial data for refining planetary defense models. Understanding exactly how the energy and momentum from DART's impact were transferred through Dimorphos's structure and into the debris cloud will improve predictions for future deflection scenarios against asteroids of different compositions and structures.
Historical Significance
Changing the orbit of a celestial body around the sun — even slightly — is an achievement without precedent in the history of space exploration. Humans have landed on the Moon, driven rovers on Mars, and sent probes to the outer solar system, but actively altering the trajectory of a natural body around our star represents a qualitatively different type of interaction with the cosmos.
The achievement also carries symbolic weight for humanity's relationship with existential risk. Asteroid impacts have caused mass extinctions in Earth's past. The demonstration that human technology can prevent such events transforms an extinction-level threat from an inevitable certainty on geological timescales into a manageable risk — if the investment in detection and deflection capabilities continues.
Next Steps
Building on DART's success, NASA and partner agencies are developing plans for more advanced planetary defense missions. These include concepts for larger kinetic impactors, gravity tractor spacecraft that could slowly alter asteroid orbits through gravitational interaction, and nuclear deflection options for larger or faster-approaching threats. DART's success provides the empirical foundation on which all future planetary defense planning will build.
This article is based on reporting by New Scientist. Read the original article.
