From Tokyo: A spacecraft the size of a small fridge slammed into an asteroid 11 million kilometres away in September 2022. Four years later, scientists have finally confirmed what that collision actually accomplished. It turned out to be less dramatic than the impact itself suggests, yet far more important.
NASA's Double Asteroid Redirection Test successfully impacted asteroid Dimorphos on September 26, 2022, becoming the first mission to demonstrate asteroid deflection. But the new analysis, published in Science Advances and led by Rahil Makadia at the University of Illinois Urbana-Champaign, reveals something that might at first seem underwhelming: the collision slowed the asteroid's orbital speed by roughly 1.7 inches per hour.
That microscopic change is precisely the point.The impact caused an instantaneous slowing in Dimorphos' speed along its orbit of about 2.7 millimeters per second, with the recoil from ejecta playing a major role in amplifying the momentum change directly imparted to the asteroid by the spacecraft. Over time, such adjustments compound. An asteroid deflected by even a fraction of a millimetre per second, given years of accumulation, could be nudged from an Earth-impact trajectory to a safe passage.
The measurement itself required precision that reflects a curious partnership between space science and volunteer observation.The research team used observations conducted by volunteer astronomers who recorded 22 stellar occultations between October 2022 and March 2025, which when combined with years of existing ground-based observations, became key in helping calculate how DART had changed Didymos' orbit. These amateur astronomers travelled to remote regions, often facing poor weather and long odds.The collision moved Dimorphos closer to Didymos, and reduced the duration of the smaller rock's orbit.
What makes this result significant is not the size of the change but what it proves about method.The impact demonstrates that kinetic impactor technology is a viable technique to potentially defend Earth from asteroids. The mission showed thata kinetic impactor approach means smashing a spacecraft into an asteroid, with the key being that the push comes not only from the colliding spacecraft but also from ejecta recoil.
There are limits to this approach.Kinetic impactor technology for asteroid deflection requires having sufficient warning time—at least several years but preferably decades—to prevent an asteroid impact with Earth. Australia, like most nations, remains vulnerable because our catalogue of near-Earth asteroids remains incomplete.No known asteroid poses a threat to Earth for at least the next century, but the catalogue of near-Earth asteroids is incomplete for objects whose impacts would produce regional devastation.
The research also raised unexpected questions.The impact sent boulders ranging from 1 to 7 metres in diameter flying off the loosely bound asteroid, with ejected boulders generally perpendicular to the direction of DART, potentially tilting the asteroid's orbital plane by up to one degree. These subtleties matter enormously for future missions.The European Space Agency's Hera mission will arrive at Dimorphos in 2026 to further analyse the impact.
For planetary defence, this mission represents a proof of concept. It shows that early warning systems combined with kinetic deflection could give humanity a tool to prevent catastrophe. The real work now lies in detection: finding asteroids before they find us. NASA's detailed mission information provides ongoing analysis of the DART impact and its implications for future planetary protection strategies.