NASA is about to launch a spacecraft with one simple mission: Smash into an asteroid at 15,000 miles per hour.

The mission, the Double Asteroid Redirection Test, or DART, leaves Earth early on Wednesday to test whether slamming a spacecraft into an asteroid can nudge it into a different trajectory. Results from the test, if successful, will come in handy if NASA and other space agencies ever need to deflect an asteroid to save Earth and avert a catastrophic impact.

When is the launch and how can I watch it?

The DART spacecraft is scheduled to lift off atop a SpaceX Falcon 9 rocket on Wednesday at 1:20 a.m. Eastern time (or 10:20 p.m. local time) from the Vandenberg Space Force Base in California.

NASA plans to host a livestream of the launch on its YouTube channel starting at 12:30 a.m. on Wednesday.

If bad weather around the Vandenberg launch site prompts a delay, the next opportunity for liftoff would be about 24 hours later.

Why is NASA crashing into an asteroid?

NASA is crashing DART into an asteroid to test, for the first time, a method of planetary defense that could one day save a city, or maybe the whole planet, from a catastrophic asteroid impact.

DART “is something of a replay of Bruce Willis’s movie, ‘Armageddon,’ although that was totally fictional,” Bill Nelson, NASA’s administrator, said in an interview.

If all goes as planned with DART, NASA will have a confirmed weapon in its planetary defense arsenal. Should a different asteroid ever wind up on a collision course with Earth, the world’s space agencies would have confidence that an asteroid missile like DART would shoo the space rock away.

How will the mission work?

After launching to space, the spacecraft will make nearly one full orbit around the sun before it crosses paths with Dimorphos, a football-field-size asteroid that closely orbits a bigger asteroid, called Didymos, every 11 hours and 55 minutes. Astronomers call those two asteroids a binary system, where one is a mini-moon to the other. Together, the two asteroids make one full orbit around the sun every two years.

Dimorphos poses no threat to Earth, and the mission is essentially target practice. DART’s impact will happen in late September or early October next year, when the binary asteroids are at their closest point to Earth, roughly 6.8 million miles away.

Four hours before impact, the DART spacecraft, formally called a kinetic impactor, will autonomously steer itself straight toward Dimorphos for a head-on collision at 15,000 miles per hour. An onboard camera will capture and send back photos to Earth in real time until 20 seconds before impact. A tiny satellite from the Italian Space Agency, deployed 10 days before the impact, will come as close as 34 miles from the asteroid to snap images every six seconds in the moments before and after DART’s impact.

How will NASA know if DART succeeded?

Telescopes on Earth will fix their lenses on the crash site, showing the two asteroids as tiny dots of reflected sunlight. To measure whether DART’s impact changed Dimorphos’s orbit around Didymos, astronomers will track the time between one flicker of light — which indicates that Dimorphos has passed in front of Didymos — and another, which indicates that Dimorphos has orbited behind Didymos.

If Dimorphos’s orbit around Didymos is extended by at least 73 seconds, DART will have successfully performed its mission. But mission managers expect the impact to lengthen the asteroid’s orbit even more, by about 10 and 20 minutes.

Why can’t Earth just blow up asteroids that threaten the planet?

Simply striking hazardous space rocks with a nuclear weapon, like in “Armageddon” and other science fiction disaster films, could create a field of more hazardous space rocks, multiplying the dangers posed to Earth, rather than eliminating them.

Still, a nuclear device, if used the right way, is one of a few conceptual tools within NASA’s planetary defense toolbox.

For any small and distant asteroids that could threaten Earth in the next few decades, a mission like DART “has a pretty good probability of getting the job done,” said Brent Barbee, an aerospace engineer at NASA’s Goddard Space Flight Center.

“But if the asteroid is any bigger than that, or if the warning time is any shorter than that, then that’s where you transition from looking at kinetic impactors to nuclear devices,” Mr. Barbee said.

Astronomers and officials from various space agencies have simulated deflecting an asteroid away from Earth with the force of nuclear blasts.

Other asteroid-destroying simulations have shown that nuclear explosives could be used to annihilate some smaller asteroids as close as two months from impact, while posing little risk to Earth.

“There’s a lot of challenging aspects of a nuclear mission besides just the physics of the device itself, and how the device would interact with the asteroid,” Mr. Barbee said.

Treaties that ban the use of nuclear weapons, and the Outer Space Treaty, the cornerstone set of international space laws signed in the 1960s, prohibit the placement or use of nuclear weapons in space

That suggests that any countries’ emergency use of a nuclear-tipped spacecraft to fend off a killer asteroid would amount to a treaty violation. But that legal predicament could be resolved by an emergency meeting of the United Nations Security Council.