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Maya and Owen left a public science briefing with pages of hurried notes . The talk had described a collision in space , but its importance lay in a change that no human eye could see directly . Over coffee , they began to compare notes . What DART tested was whether a spacecraft could alter an asteroid's motion through a deliberate collision . NASA designed the mission as the first dedicated demonstration of this planetary defence method , known as kinetic impact . On 26 September 2022 , the DART spacecraft struck Dimorphos . This small body is a moonlet that orbits the larger asteroid Didymos , so researchers could study one body moving around the other . Nor did either asteroid pose a threat to Earth . The pair was chosen as a safe test system , not because astronomers expected it to hit our planet . Owen had pictured the collision as the entire experiment . Maya pointed to the observations around it . Ground - based telescopes had measured Dimorphos's orbit before the impact and did so again afterward . Only by comparing those observations could astronomers determine how much the orbit had changed . The impact shortened Dimorphos's orbital period around Didymos by more than half an hour . The scale of that result required some care . What changed was the time Dimorphos took to circle its companion . The announcement did not mean that a threatening asteroid had been redirected away from Earth . The test might therefore be called a proof of principle , Maya suggested . It showed that a kinetic impact could measurably alter one asteroid moonlet's motion under the conditions of this mission . It ought not to be read as a promise that every future hazard could be handled in the same way . Owen added one final caveat to his notes . A real response would depend on information about the object and on how much warning time was available . DART supplied evidence for one possible technique , while leaving many practical decisions for future missions . Their disagreement had narrowed into a useful distinction : the spacecraft delivered the impact , but measurements made the result intelligible . Without the before - and - after record , a dramatic image of a collision would not have answered the central question . Which part of the mission would you have emphasised in your notes : the collision or the measurement ? What further evidence would you want before trusting this technique in a real emergency ? Could you explain the result without suggesting that every asteroid behaves alike ?
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