The asteroid, Dimorphos, is the size of a stadium — or the Great Pyramid of Giza, as one scientist put it Monday — and is about 7 million miles from Earth at the moment. It orbits a larger asteroid named Didymos. Neither poses a threat to our planet now or anytime in the foreseeable future.
This 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 blow to alter its orbital trajectory.
Launched last November from California, the spacecraft was small, roughly the size of a vending machine or golf cart. Dimorphos is rather big — roughly 500 feet or so in diameter, although its precise shape and composition were unknown before the final approach. Scientists anticipated a plume of debris from the asteroid upon impact but no significant structural change. This is more akin to a bug splattering on a windshield.
“This isn’t just bowling-ball physics,” Applied Physics Laboratory planetary scientist Nancy Chabot told reporters. “The spacecraft’s gonna lose.”
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But even small effects on an asteroid’s movement could provide a planet-saver. An early collision with an asteroid, if done early enough — say, 5 to 10 years in advance of its projected encounter with Earth — could be just enough to slow it down and make it miss.
As engineers conceived of an asteroid deflection mission, they seized on an ingenious idea that would greatly reduce the costs: Hit an asteroid “moonlet” that’s 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 is moving at tremendous speed, and the impact from a small spacecraft would result in a minimal, hard -to-detect change. A second spacecraft would have to be present to scrutinize the effect.
But a moonlet, like Dimorphos, orbits its larger twin at a stately pace. The effect of the impact should be more easily detected — including by telescopes on Earth and in space. No second spacecraft is necessary.
It will take at least a couple of days to tell if the DART mission succeeded in slowing down the targeted asteroid, and to what degree it did so. Telescopes on Earth and in space observed the collision, as did a small instrument, called a cubesat, that was deployed 15 days before impact.
This is an unusual mission in that it does not involve a spacecraft trying to survive a hazardous landing on an alien world or proving itself operational in the rough environment of outer space, noted Robert Braun, head of the space exploration sector at the Applied Physics Laboratory .
NASA spacecraft will slam into an asteroid Monday — if all goes right
“Here, what we’re looking for is loss of signal,” he told reporters before the collision. “What we’re cheering for is a loss of the spacecraft.”
By Monday afternoon the engineers in Laurel had sent their final course corrections to the DART spacecraft, and from that point it was on their own, making final navigational adjustments autonomously. The vehicle was aimed directly at the larger, brighter asteroid, but programmed to fire thrusters that would pivot it toward the smaller asteroid when it came into view.
Some bizarre scenarios could not be ruled out because the asteroid’s shape wouldn’t be determined until the final hour before impact. Indeed, only the larger asteroid — not Dimorphos — could be seen in the live feed from the spacecraft’s camera 90 minutes before impact.
“If we were right on course, and it was shaped like a donut, we’d fly right through it,” Braun said.
There are thousands of potentially hazardous asteroids that come close to, or cross, the Earth’s orbital path around the sun. None is currently known to be on a trajectory to hit the planet.