NASA now hopes the collision will change the trajectory of the asteroid, in a first-of-its-kind planetary defence test.
A NASA spacecraft successfully crashed into an asteroid on Monday in an unprecedented dress rehearsal for the day a killer rock actually threatens Earth.
The galactic slam occurred 11.3 million km away, with the spacecraft named DART - short for Double Asteroid Redirection Test - ramming a harmless asteroid at 22,500 km/h in an attempt to alter its orbit.
Telescopes around the world and in space were aimed at the same point in the sky to capture the moment.
Though the impact was immediately obvious - DART’s radio signal abruptly stopped - it will take a couple of months to determine how much the asteroid’s path was changed by the collision.
The $325 million (€333 million) mission is the first attempt by any space agency to shift the position of an asteroid or any other natural object in space.
"As far as we can tell, our first planetary defence test was a success," Mission Control's Elena Adams told a news conference, the room filling with applause. "I think Earthlings should sleep better. Definitely, I will".
NASA Administrator Bill Nelson reminded people earlier in the day on Twitter that this was not a movie plot.
"We’ve all seen it on movies like 'Armageddon,' but the real-life stakes are high," he added in a prerecorded video.
DART's target: Dimorphos
The misson's target is a 160-metre asteroid named Dimorphos. It’s a moonlet of Didymos, Greek for twin, a fast-spinning asteroid five times bigger that flung off the material that formed the junior partner.
The pair have been orbiting the sun for eons without threatening Earth, making them ideal save-the-world test candidates.
The fact that DART, which is only the size of a vending machine, travelled through space for 10 months and successfully hit its target in space is in itself "a fantastic achievement," said Professor Alan Fitzsimmons, of the Astrophysics Research Centre at Queen’s University Belfast.
"It is even more incredible when you realise that we had not set eyes on the target asteroid Dimorphos before last night. It was too small and faint to be pictured by telescopes from Earth, and astronomers had to use other methods to measure exactly where it was so the DART interception was timed precisely right," he told Euronews Next.
Launched last November, DART navigated towards the asteroid using new technology developed by Johns Hopkins University’s Applied Physics Laboratory, the spacecraft's builder and mission manager.
Cheers, hugs and high fives
DART’s on-board camera, a key part of this smart navigation system, caught sight of Dimorphos barely an hour before impact. “Woo hoo!” exclaimed Adams, a mission systems engineer at Johns Hopkins.
With an image beaming back to Earth every second, Adams and other ground controllers in Laurel, Maryland, watched with growing excitement as Dimorphos loomed larger and larger in the field of view alongside its bigger companion.
Within minutes, Dimorphos was alone in the pictures; it looked like a giant gray lemon, but with boulders and rubble on the surface. The last image froze on the screen as the radio transmission ended.
Flight controllers cheered, hugged one another and exchanged high fives. Their mission complete, the DART team went straight into celebration mode. There was little sorrow over the spacecraft's demise.
"Normally, losing signal from a spacecraft is a very bad thing. But in this case, it was the ideal outcome," said NASA programme scientist Tom Statler.
Johns Hopkins scientist Carolyn Ernst said the spacecraft was definitely "kaput," with remnants possibly in the fresh crater or cascading into space with the asteroid's ejected material.
What happens next?
"DART has already successfully shown we can target and hit a small asteroid. The next stage will be to calculate how efficiently DART moved Dimorphos, by measuring how much its orbit around Didymos was altered," Fitzsimmons explained.
"This will be done by astronomers over the next few weeks and should allow us to start checking our calculations for planetary defence for the first time," he added.
Scientists insisted DART would not shatter Dimorphos. The spacecraft packed a scant 570 kg, compared with the asteroid’s 5 billion kg. But that should be plenty to shrink its 11-hour, 55-minute orbit around Didymos.
The impact should pare 10 minutes off that. The anticipated orbital shift of 1 per cent might not sound like much, scientists noted. But they stressed it would amount to a significant change over years.
"Now is when the science starts," said NASA’s Lori Glaze, planetary science division director. "Now we’re going to see for real how effective we were".
Planetary defence experts prefer nudging a threatening asteroid or comet out of the way, given enough lead time, rather than blowing it up and creating multiple pieces that could rain down on Earth.
Multiple impactors might be needed for big space rocks or a combination of impactors and so-called gravity tractors, not-yet-invented devices that would use their own gravity to pull an asteroid into a safer orbit.
"The dinosaurs didn’t have a space programme to help them know what was coming, but we do," NASA’s senior climate adviser Katherine Calvin said, referring to the mass extinction 66 million years ago believed to have been caused by a major asteroid impact, volcanic eruptions or both.
Space rocks lurking
The non-profit B612 Foundation, dedicated to protecting Earth from asteroid strikes, has been pushing for impact tests like DART ever since it was founded by astronauts and physicists 20 years ago.
Monday’s feat aside, the world must do a better job of identifying the countless space rocks lurking out there, warned the foundation’s executive director, Ed Lu, a former astronaut.
Significantly less than half of the estimated 25,000 near-Earth objects in the deadly 140-metre range have been discovered, according to NASA. And fewer than 1 per cent of the millions of smaller asteroids, capable of widespread injuries, are known.
The Vera Rubin Observatory, nearing completion in Chile by the National Science Foundation and US Energy Department, promises to revolutionise the field of asteroid discovery, Lu noted.
Finding and tracking asteroids, "that’s still the name of the game here. That’s the thing that has to happen in order to protect the Earth," he said.