DART is NASA’s first “planetary defense” mission. The goal was to test whether this technique, called a kinetic impactor, would punch a speeding space rock enough to knock it off course significantly.
It made. Before the DART arrived, Dimorphos orbited Didymos in 11 hours and 55 minutes. So: Blame! The newly calculated orbit: 11 hours and 23 minutes.
This 32-minute change in orbital period was at the high end of a range of estimated results, NASA planetary science manager Lori Glaze said. DART exceeded the agency’s minimum benchmark for a successful mission by more than 25 times.
“We’ve shown the world that NASA is serious about defending this planet,” NASA Administrator Bill Nelson said.
The mission “felt like a movie plot”, he said. “But it wasn’t Hollywood.”
Despite the enthusiasm emanating from NASA officials, there is no fully developed system for intercepting asteroids. The key to planetary defense is finding potentially dangerous asteroids long before they cross paths with Earth. Astronomers can calculate if they are on a trajectory to hit the planet.
“You gotta know they’re coming,” Glaze said.
The idea of a kinetic impactor is to give a dangerous asteroid a boost several years before its predicted impact with Earth. This is not a last minute technique to save the world.
“We really need to have that warning time for a technique like this to be effective,” said Nancy Chabot, DART coordinator at Johns Hopkins University’s Applied Physics Laboratory, who has managed the mission under contract with NASA.
Large asteroids that could threaten Earth are easily spotted and their orbits are calculated decades into the future. But many smaller asteroids in the general size range of Dimorphos, which measures about 160 meters in diameter, are more difficult to detect.
Asteroids are not identical. Some are hard, solid bodies, while others are “piles of rubble”. The composition and form of Dimorphos were not known before the arrival of DART. It was only in the final minutes of the mission that the asteroid came into focus. The impact created a stunning plume of ejecta, and the asteroid’s dramatic motion came in part from the way it recoiled as boulders and fine particles were spewed into space.
The mission was already on the books as a technical triumph simply due to a successful collision – effectively a bull’s-eye – captured dramatically by the spacecraft’s camera in the final moments before impact.
The laws of physics dictated that there had to be an effect. And images captured by a trailing cubesat, provided by the Italian Space Agency and deployed by DART 15 days before impact, showed raw material in the space. Subsequent observations from ground-based telescopes as well as the Hubble and Webb Space Telescopes revealed a long trail of debris, creating a comet effect.
It wasn’t until Tuesday, after much analysis, that NASA revealed the precise change in Dimorphos’ orbit. The analysis continues and one question is whether the rock has become wobbly.
“We shouldn’t be too eager to say that a test on one asteroid tells us how all other asteroids would behave,” warned NASA program scientist Thomas Statler.
Even so, the bottom line is that the DART mission performed exactly as scientists and engineers had hoped.
“Let’s all take a moment to soak this in,” Glaze said. “For the first time, mankind has changed the orbit of a planetary body.”