NASA is about to launch an unprecedented mission to knock an asteroid slightly off course.

In the first real-world test of a technique that could someday be used to protect Earth from a threatening space rock, a spacecraft is scheduled to blast off from Vandenberg Space Force Base in California on Tuesday at 10:20 p.m. PST.

The golf-cart-size spacecraft will travel to an asteroid that's more than 6 million miles away — and poses no danger to Earth — and ram into it. Scientists will then watch to see how the asteroid's trajectory changes.

NASA has identified and tracked almost all of the nearby asteroids of a size that would cause world-altering damage if they ever struck Earth. For the foreseeable future, none that big are headed our way. But there are plenty of smaller asteroids, the size that could take out a city, that still haven't been found and tracked.

It's a space rock of that smaller size that the DART mission — short for Double Asteroid Redirection Test — will take head-on.

It might sound like a movie plot, but it's not

"A lot of times when I tell people that NASA is actually doing this mission, they kind of don't believe it at first, maybe because it has been the thing of movies," says Nancy Chabot, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.

Movies like Armageddon or Meteor typically feature a surprise, imminent killer asteroid, and saving humanity invariably requires blowing it to pieces with a nuclear bomb.

In reality, messy and unpredictable nuclear weapons aren't the preferred choice of planetary defense experts, who would much rather identify dangerous space rocks way in advance of any possible collision and use more controlled methods to alter its path.

"The right time to deflect an asteroid is as far away from the Earth as we can," says Lindley Johnson, NASA's planetary defense officer. "The strategy is to find these objects not only years but decades before they are any kind of an impact hazard to the Earth."

With enough advance warning, NASA could send out a spacecraft that would simply give an asteroid a little push, changing its course so that it no longer posed a problem. That's the approach that NASA is testing out with DART.

"DART is demonstrating asteroid deflection. It is absolutely not asteroid disruption, which is how it goes a lot of times in the movies," says Chabot, who serves as DART's coordination lead.

The target asteroid is not a threat

The asteroid targeted by DART isn't a danger to the planet now, and Chabot says there's no chance this mission could make it one. "There is absolutely no way that the DART test is a threat to the Earth," she says.

After the DART spacecraft launches, it will spend about 10 months traveling out toward an asteroid called Didymos, which is about 2,500 feet across. This asteroid is orbited by a smaller asteroid called Dimorphos, which is about 525 feet across.

It's the small asteroid, Dimorphos, that will get smacked by the spacecraft. "So it's like a small golf cart running into a Great Pyramid," says Chabot — only this "golf cart" will be going 15,000 miles per hour.

In the last few hours of the mission, managers back on Earth will only be able to watch as the spacecraft flies itself and homes in on its target.

"It is four hours of watching paint dry, but kind of terrifying at the same time because the spacecraft is completely autonomous," says Elena Adams, the mission systems engineer at the Johns Hopkins University Applied Physics Laboratory.

Initially, the spacecraft will orient itself by aiming for the larger asteroid, explains Adams. Then, in the final hour or so, it will detect the smaller one and switch to that target.

Images sent back by the doomed spacecraft in the last seconds before the crash will give scientists their first good look at Dimorphos. No one knows what shape this asteroid has or whether its surface is smooth or rugged. In fact, this pair of asteroids is so small and far away that telescopes see them as little more than a point of light.

Changes in brightness, however, tell scientists when the orbiting Dimorphos passes in front of its companion. Right now, Dimorphos goes around every 11 hours and 55 minutes. The impact should shorten that time period, but scientists don't know by how many minutes.

Adams says that telescopes will be watching closely in the weeks and months after the impact to "see how does it react to being pushed."

A tiny satellite jettisoned from the spacecraft 10 days before the impact should send back images of the collision itself and the resulting blast of debris.

This test is just the beginning

What's more, the European Space Agency is expected to launch a mission in 2024 that will travel to these two asteroids and be able to observe the crater on Dimorphos and determine the mass of this asteroid.

All of these results should help NASA and other space agencies understand what could be possible to do with this kind of "kinetic impact deflection" approach in the future if an asteroid is headed toward Earth.

"Assuming we don't have any major surprises, of course, we think that this technique would be then available, would be a part of the toolbox that we are starting to build of capabilities to deflect an asteroid," says Johnson.

After this test, he says, NASA will want to try out other asteroid deflection techniques like the so called "gravity tractor" approach, which involves stationing a spacecraft near an asteroid to apply a small tug of gravity.

Worrisome asteroids, of course, can only be deflected once NASA knows they exist. Astronomers believe they have located and tracked at least 90% of the largest nearby asteroids, meaning ones that measure more than 3,280 feet across.

"None of them are a threat for the foreseeable future," says Chabot. "That's the good news."

But a hit from a smaller asteroid, about 450 feet in diameter, could take out a city and devastate a region. And scientists currently know of only about 40% of such objects that are believed to be out there.

That's why NASA's planetary defense office is supporting the development of a new space telescope to hunt for more in that size range.

"Although there isn't a currently known asteroid that's on an impact course with the Earth," says Johnson, "we do know that there is a large population of near-Earth asteroids out there."

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