The asteroid, Dimorphos, is the size of a stadium – or the Great Pyramid of Giza, as one scientist put it on Monday – and is currently about 7 million kilometers from Earth. It orbits a larger asteroid named Didymos. Neither poses a threat to our planet now or at any time for the foreseeable future.
It was just a test, NASA’s first demonstration of a potential planetary defense technique, called a kinetic impactor. The idea is to give a hypothetically dangerous asteroid just enough of a hit to alter its orbital path.
Launched last November from California, the spacecraft was small, about the size of a vending machine or golf cart. Dimorphos is rather large – approximately 500 feet in diameter, although its precise shape and composition are unknown until final approach. Scientists anticipated a plume of debris from the asteroid upon impact but no significant structural change. It looks more like a bug splattering on a windshield.
“It’s not just bowling ball physics,” Nancy Chabot, a planetary scientist at the Applied Physics Laboratory, told reporters. “The spaceship will lose.”
But even small effects on an asteroid’s motion could save the planet. An early collision with an asteroid, if done early enough – say, 5 to 10 years before its scheduled encounter with Earth – could be enough to slow it down and cause it to miss.
There are thousands of potentially dangerous asteroids approaching or crossing Earth’s orbital path around the sun. None are currently known to be on a trajectory to strike the planet.
As engineers were designing an asteroid deflection mission, they came up with an ingenious idea that would dramatically reduce costs: hitting a “moonlet” of asteroids orbiting a larger asteroid.
To detect the effect of a collision with a single asteroid orbiting the sun would have required two spacecraft, engineer Andrew Cheng told reporters, because such an asteroid travels at tremendous speed and the impact of a small spacecraft would result in minimal, hard-detect change. A second spacecraft should be present to peer into the effect.
But a moon, like Dimorphos, orbits its larger twin at a majestic rate. The effect of the impact should be more easily detected, including by telescopes on Earth and in space. No second spacecraft is needed.
It will take at least a few days to tell if the DART mission was successful in slowing down the targeted asteroid, and how well it did. Telescopes on Earth and in space observed the collision, as did a small instrument, called Cubesat, which was deployed 15 days before impact.
This is an unusual mission in that it does not involve a spacecraft attempting to survive a hazardous landing on an alien world or proving operational in the harsh environment of outer space, a noted Robert Braun, head of the space exploration sector at the Applied Physics Laboratory. .
“Here what we’re looking for is loss of signal,” he told reporters before the collision. “What we applaud is the loss of the spacecraft.”
By Monday afternoon, Laurel engineers had sent their final course corrections to the DART spacecraft, and from then on it was on its own, making final navigation adjustments autonomously. The vehicle was aimed directly at the larger, brighter asteroid, but programmed to fire thrusters that would rotate it towards the smaller asteroid when it came into view.
Some weird scenarios couldn’t be ruled out as the shape of the asteroid would not be determined until the last hour before impact. Indeed, only the largest asteroid – not Dimorphos – could be seen in the spacecraft’s live camera feed 90 minutes before impact.
“If we were on track and it was shaped like a donut, we would fly through it,” Braun said.
It wasn’t until the last minutes of DART’s journey that the spacecraft or its human operators back on Earth were able to observe Dimorphos. It was not visible at all until about an hour before impact. Even then, it was just a small, barely noticeable speck next to its brighter twin.
There was joy in the Mission Operations Center as the asteroid appeared larger on the screen. About 50 minutes before impact, project manager Edward Reynolds began uttering the same phrase: “It’s nominal, it’s nominal” – aerospace engineering jargon for “everything is going exactly as planned” , he said later.
“We locked Dimorphos,” engineer Elena Adams said 27 minutes before impact.
The camera on board kept clicking. The point turned into a clearly spherical rock with a rough surface covered in boulders, looking like something you’d keep in the garage as a scrubbing tool. At the mission operations center, engineers stood and cheered the final moments, too excited to sit at their consoles.
In the last image, Dimorphos completely filled the frame. DART hit the bullseye.
Then came a blank screen. DART had succeeded and ceased to exist.
“Confirmed impact for the world’s first planetary defense test mission,” the NASA live stream announced.
On the NASA wire, agency administrator Bill Nelson said the mission had demonstrated the technology “to save our planet.” Ralph Semmel, director of the Applied Physics Laboratory, said he felt a rush of adrenaline as DART hit the target directly: “Never before have I been so excited to see a signal moving away.”
