Astronomers still have their eyes on that asteroid NASA whacked

Astronomers are still watching that asteroid that NASA whacked with a spacecraft back in September, in the first-ever test of whether an asteroid could be deliberately pushed off-course.

Almost immediately after NASA's Double Asteroid Redirection Test (DART) mission sent a golf cart-sized spacecraft crashing into an asteroid called Dimorphos, scientists hailed it as a huge success – and a powerful demonstration that an asteroid's trajectory can be altered.

"We know this process is really very effective – it's even more effective than a lot of people had originally expected," says Andy Cheng with the Johns Hopkins University Applied Physics Laboratory.

The experiment has boosted scientists' confidence, he says, that this kind of deflection technique could really work to protect the planet if Earth ever got menaced by a dangerous incoming space rock.

The collision altered the path of Dimorphos through space, shortening the time it takes to orbit another, larger asteroid by 33 minutes, according a new analysis in the journal Nature. The journal published a study detailing the results this week, alongside four additional scientific reports on this unprecedented asteroid deflection experiment.

Dimorphos is millions of miles away and about the size of the Great Pyramid of Giza in Egypt. Astronomers got their first good look at it in the final moments of the mission, as the DART spacecraft drew ever closer, sending back images of a gray, egg-shaped asteroid strewn with rubble.

Once the spacecraft hit the surface, it was obliterated, and its stream of pictures stopped. But telescopes watching the pair of asteroids saw that the impact kicked up a huge amount of dust and rocky debris, brightening the scene.

"It's just a bright cloud. A lot of dust came off. And we were just amazed. We knew right then, we can do some good science with this," says Ariel Graykowski with the SETI Institute, who works with a global network of telescope enthusiasts.

All the material ejected out of the asteroid by the impact gave the asteroid an extra kick, says Cheng, in the same way that shooting a bullet out of a gun makes the gun kick back.

"That's the recoil force, an extra force that's pushing against the asteroid," says Cheng, adding that this extra force was actually a lot bigger than the push that the spacecraft delivered by hitting the asteroid and embedding itself inside.

The orbiting Hubble Space Telescope was on the wrong side of Earth when the collision happened, so it couldn't watch the event, but it gazed at the asteroid soon after and watched the debris cloud change over time.

"That's something really exciting to see," says Jian-Yang Li, with the Planetary Science Institute, who says that eventually, a comet-like tail formed and got longer and longer.

It looked remarkably like tails that occasionally are seen on other asteroids, he says. It had never been clear what created those so-called 'active' asteroids, although some astronomers suspected impacts played a role.

"DART is the first experiment that actually demonstrated that impact can indeed generate a tail," he says.

The tail streaming off of Dimorphos can still be detected by telescopes. "We are still observing," says Cristina Thomas with Northern Arizona University, who says observations should wrap up this month.

Next year, the European Space Agency will send out a mission called Hera that should take close-up images of the asteroid, revealing the size of any crater left behind. It should also be able to determine the asteroid's mass. All of this should help astronomers understand even more about how to push asteroids around.

While astronomers say no large space rocks are currently known to threaten Earth, many small-but-still-dangerous asteroids have not yet been tracked, and planetary defenders say it's good to be prepared, just in case.

"We've shown now that we have a method to move an asteroid," says Graykowski. "It makes me feel a lot better to see that it worked, and that it worked so well."

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