NASA Crashed a Spacecraft Into an Asteroid on Purpose. It Worked Better Than Anyone Expected.

On 26 September 2022, NASA's Double Asteroid Redirection Test spacecraft — DART — struck the asteroid Dimorphos at approximately 6.1 kilometres per second. Dimorphos is a moonlet orbiting the larger asteroid Didymos, roughly 11 million kilometres from Earth. Neither asteroid posed any threat to Earth. The mission was a proof of concept: could a spacecraft, launched from Earth, deliberately alter the orbit of a celestial body? The answer, it turned out, was yes — and by considerably more than the models predicted.

Before impact, Dimorphos completed one orbit of Didymos every 11 hours and 55 minutes. The pre-mission models predicted that a successful impact would shorten this orbital period by approximately 73 seconds — a change detectable by ground-based telescopes. The actual result, confirmed by observations from multiple observatories over the weeks following impact, was a reduction of 33 minutes. The spacecraft moved the asteroid more than 25 times more effectively than the minimum threshold for mission success.

The reason for the larger-than-expected deflection was ejecta. When DART struck Dimorphos, the impact excavated a crater and launched a plume of material — rock, dust, and debris — into space. This ejecta acted as a natural thruster: the momentum of the expelled material pushed back against Dimorphos in the opposite direction, amplifying the deflection effect of the impact itself. The plume was observed by the James Webb Space Telescope and the Hubble Space Telescope simultaneously — the first time both instruments had observed the same event.

The ESA's Hera mission, launched in October 2024, is currently en route to the Didymos system to conduct a detailed post-impact survey. It will map the crater left by DART, measure the mass and composition of Dimorphos, and assess the long-term effects of the impact on the asteroid's structure and orbit. The data will be used to refine the models used for future planetary defence planning.

The implications of DART's success are significant. The mission demonstrated that kinetic impactor technology — crashing a spacecraft into an asteroid — is a viable planetary defence strategy, at least for asteroids of Dimorphos's size. It also demonstrated that the ejecta effect is a major factor in deflection calculations, and that current models may be systematically underestimating the effectiveness of kinetic impacts.

There are no known asteroids on a collision course with Earth in the foreseeable future. The key word is 'known.' The DART mission was not a response to a specific threat. It was preparation for a threat that may or may not materialise in the coming centuries. The fact that it worked — and worked better than expected — is one of the more quietly reassuring scientific results of recent years.