The success of Nasa’s recent Dart (Double Asteroid Redirection Test) mission, which saw a spacecraft deliberately crashed into an asteroid to nudge it into a faster orbit, marked the world’s first test of a planetary defence system designed to prevent a potential doomsday meteorite collision with Earth. The relevance of the proof-of-concept mission, which was seven years in development, points directly to the fact that the Cretaceous mass extinction event which occurred about 66mn years ago, killing 78% of all species, including the remaining non-avian dinosaurs, was most likely caused by an asteroid (later named Chicxulub) hitting the Earth in what is now Mexico, potentially compounded by ongoing flood volcanism in what is now India.
The Dart mission also was a first for humanity to alter the motion of a celestial body, as the US space agency announced. Findings of telescope observations unveiled at a Nasa news briefing in Washington confirmed the suicide test flight of the Dart spacecraft on September 26 achieved its primary objective: changing the direction of an asteroid through sheer kinetic force. Astronomical measurements over two weeks after the impact showed the target asteroid was bumped slightly closer to the larger parent asteroid it orbits and that its orbital period was shortened by 32 minutes. “This is a watershed moment for planetary defence and a watershed moment for humanity,” Nasa chief Bill Nelson told reporters in announcing the results. “It felt like a movie plot, but this was not Hollywood.”
Last month’s impact, 10.9mn km from Earth, targeted an egg-shaped asteroid named Dimorphos, roughly the size of a football stadium and orbiting a parent asteroid about five times bigger called Didymos once every 11 hours, 55 minutes. The test flight concluded with the Dart impactor vehicle, no bigger than a refrigerator, slamming directly into Dimorphos at about 22,531kph. Comparison of pre- and post-impact measurements of the Dimorphos-Didymos pair as one eclipses the other shows the orbital period was shortened to 11 hours, 23 minutes, with the smaller object bumped tens of meters closer to its parent.
The outcome “demonstrated we are capable of deflecting a potentially hazardous asteroid of this size,” if it were discovered well enough in advance, said Lori Glaze, director of Nasa’s planetary science division. “The key is early detection.” Neither of the two asteroids involved, nor Dart itself, posed any actual threat to Earth. But a Dimorphos-sized asteroid, while not capable of posing a planet-wide threat, could level a major city with a direct hit. Scientists had predicted the Dart impact would shorten Dimorphos’ orbital path by at least 10 minutes but would have considered a change as small as 73 seconds a success. So the actual change of more than a half hour, with a margin of uncertainty plus or minus two minutes, exceeded expectations.
Launched by a SpaceX rocket in November 2021, Dart made most of its voyage under the guidance of flight directors on the ground, with control handed over to the craft’s autonomous on-board navigation system in the final hours of the journey. Dimorphos and Didymos are both tiny compared with the cataclysmic Chicxulub. Smaller asteroids are far more common and present a greater theoretical concern in the near term, making the Didymos pair suitable test subjects for their size, according to Nasa scientists and planetary defence experts. Also, the two asteroids’ relative proximity to Earth and dual configuration made them ideal for the Dart mission.
The Dimorphos moonlet is one of the smallest astronomical objects to receive a permanent name and is one of 27,500 known near-Earth asteroids of all sizes tracked by Nasa. Although none are known to pose a foreseeable hazard to humankind, Nasa estimates that many more asteroids remain undetected in the near-Earth vicinity.