Mission accomplished.
Two weeks ago, NASA carried out a mock trial of a planetary defense mission, smashing a cow-size spacecraft into Dimorphos, the 160-meter-wide moon of a larger asteroid. Today, the agency confirmed the mission was a success: The impact of the Double Asteroid Redirection Test (DART) spacecraft nudged the moon closer to its partner, Didymos, shortening its nearly 12-hour orbital period by 32 minutes.
The degree of deflection was far greater than anticipated; NASA said before impact that an orbital reduction of at least 73 seconds would be considered a success, most astronomers predicted a shortening closer to 10 minutes. DART’s unexpectedly strong kick, which came as a pleasant surprise, is likely thanks to the fact that Dimorphos appears to be a rubble pile rather than solid rock. When DART crashed into it, loose debris rocketed away from the impact site, thrusting the moon forward.
“We’re absolutely thrilled,” says Cristina Thomas, a planetary scientist at Northern Arizona University who led the observation team for the mission. “This really shows that kinetic impact is a viable option for planetary defense.”
NASA chose to conduct the test on a binary asteroid system for two reasons. First, even though the pair was not on a course to hit our planet, the 780-meter-wide Didymos served as a gravitational anchor during impact, ensuring that Dimorphos wasn’t inadvertently ricocheted toward Earth. And second, having a pair of space rocks locked in orbit made it easier for scientists to measure the asteroid’s deflection relative to its partner.
Both asteroids appear as a single point of light to telescopes on Earth, but when one body laps in front of the other, the brightness of that light dips by about 10%. By counting the time between flickers, scientists can calculate the duration of Dimorphos’s orbit. If the period shortened significantly after impact, that would mean the moon moved closer to its partner—and that the spacecraft succeeded in knocking it off its original trajectory.
Making those estimates through the cloud of ejecta kicked up by the crash required multiple detailed observations of the system. In addition to a few space telescopes, dozens of observatories around Earth were locked in on the event. Scientists analyzed data from three telescopes in Chile and one in South Africa to calculate the new orbital period. Two more telescopes confirmed the results with radar observations.
“I really cannot understate how fantastic these observations are,” Thomas says. “We were able to push beyond the original estimates on precision … even earlier than expected.”
All told, the early data confirm humanity’s newfound ability to defend our planet—at least from relatively small asteroids. Such space rocks are the top concern for planetary defense experts, given that they’re thousands of times more likely to strike Earth than massive dinosaur-killer asteroids. But spotting such hazards while they’re still far enough away to be intercepted with a DART-like mission will likely require greater investment in early detection systems.
“All of us have a responsibility to protect our home planet,” NASA Administrator Bill Nelson said at a press conference today. “This mission shows that NASA is trying to be ready for whatever the universe throws at us.”