A small NASA spacecraft closed in on a tiny asteroid Monday, on course for a suicidal 14,000-mph head-on collision, the first real-world test of humanity's ability to nudge a threatening body off course before it could crash into Earth.
The asteroid in question, a 500-foot-wide body known as Dimorphos, is actually a moon orbiting a 2,500-foot-wide asteroid named Didymos. Neither poses any threat to Earth, either before or after the impact of NASA's 1,260-pound Double Asteroid Redirection Test, or DART, spacecraft.
But the double-asteroid system offers an ideal target for the $330-million DART mission because the effects of the probe's impact can be measured from Earth by precisely timing how the moonlet's orbital period around Didymos changes as a result of the collision.
Beaming back images of Dimorphos once per second, the DART probe should give scientists a thrilling on-board view as the spacecraft races toward its quarry, showing the target growing larger and larger until it fills the entire field of view seconds before impact.
"At two and a half minutes before impact, SMART Nav, which is the autonomous algorithms that have brought us to that point, is going to turn off and we're just going to point the camera and take the most amazing pictures of this asteroid," said Elena Adams, DART mission systems engineer at the Johns Hopkins University Applied Physics Laboratory.
NASA plans to stream the images live on YouTube with mission coverage starting at 6 p.m. EDT. The impact is expected at 7:14 p.m., with streaming, one-per-second images from the spacecraft starting a few minutes earlier.
Dimorphos has never been directly observed, and DART's camera won't detect it as a separate body from Didymos until an hour before impact, at a distance of about 15,000 miles. At that point, it will show up as a point of light just 1.4 pixels across. But it will quickly grow to 21 pixels across by the time it reaches a distance of 1,000 miles just 4 minutes before impact.
"Our last image is probably going to be from about two and a half seconds prior to impact," Adams said. "So the DRACO (camera's) field of view is actually going to be completely filled with this beautiful image of Dimorphos."
Transmissions will cease at the moment of impact. But because the collision will happen 7 million miles from Earth, the final few images will need 45 seconds or so to cross the gulf and make it into computers and onto NASA's live stream.
"You're going to hear that we've lost radio contact and there are still going to be images coming through and being displayed once per second for about eight seconds," said Edward Reynolds, the DART project manager at APL.
"Those are images that are traveling through space. They've hit the ground, we've detected that we've lost radio contact, but those images are still working themselves through the pipeline and being displayed."
A small Italian hitchhiker spacecraft known as LICIACube, released from DART earlier this month, will attempt to photograph the collision and the debris blasted back out into space. Those images will be stored on board and related back to Earth later.
The Didymos-Dimorphos double asteroid system offers an ideal planetary defense test bed because the moonlet's orbit carries it directly in front of and then behind Didymos as viewed from Earth, allowing scientists on Earth to precisely measure slight changes in the combined reflected light from both asteroids.
By timing how the light dims and brightens, researchers have calculated how long it takes Dimorphos to complete one orbit - 11 hours and 55 minutes - and post-impact observations will allow them to determine what effect DART might have had.
Researchers expect the crash to shorten the asteroid's orbital period by about 10 minutes, but it could take a few days to weeks for telescopes around the world and in space, including the Hubble and James Webb space observatories, to make the measurements needed to nail down the number.
"The double asteroid redirection test is a test," said Tom Statler, a DART program scientist. "We're doing this test when we don't need to, on an asteroid that isn't a danger, just in case we ever do need to and we discover an asteroid that is a danger."
He said the DART mission has two primary objectives, the first being a test "of our ability to build an autonomously guided spacecraft that will actually achieve the kinetic impact on the asteroid."
"The second is a test of how the actual asteroid responds to the kinetic impact," he said. "Because at the end of the day, the real question is, how effectively did we move the asteroid, and can this technique of kinetic impact be used in the future if we ever needed to?"
Unlike Hollywood thrillers like "Armageddon" and "Deep Impact," which imagined piloted flights carrying nuclear bombs to deflect or destroy their targets, DART's goal is much simpler and much less destructive.
While nuclear devices might be a last resort in some future Armageddon-class scenario, deflection, not destruction, would still be the goal.
"You just don't want to blow it up, because that doesn't change the direction of all the material," Lindley Johnson, NASA's "planetary defense" officer, told CBS News before DART's launch last November. "It's still coming at you, it's just buckshot instead of a rifle ball."
"What you want to do is just change the speed at which this is all moving by just a bit. Over time, that will change the position of the asteroid and its orbit."
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