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NASA's DART Spacecraft to Crash Into Asteroid Tonight in Planetary Defense Test

The Wall Street Journal. 9/26/2022 Aylin Woodward
© Johns Hopkins APL/NASA

An uncrewed spacecraft is on track to smash into and deflect a distant asteroid on Monday, the dramatic climax of a National Aeronautics and Space Administration mission to test whether the technique could one day be used to protect Earth.

“For the first time ever we will measurably change the orbit of a celestial body in the universe,” said Bobby Braun, head of the space exploration sector at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. The research center built the Double Asteroid Redirection Test mission spacecraft and is responsible for its operation at the direction of NASA’s Planetary Defense Coordination Office.

The 1,300-pound DART spacecraft will be traveling at more than 14,000 miles an hour when it hits the asteroid Dimorphos, a much more massive, 525-foot-wide space rock that orbits a larger one known as Didymos. The asteroid pair—what astronomers call a binary asteroid—will be about 7 million miles from Earth at the time of impact and poses no threat to our planet.

“Regardless of what the DART spacecraft does, there’s a zero percent chance that this asteroid will come towards the Earth,” Dr. Braun said of Dimorphos at a recent press briefing.

NASA will live-stream the event using imagery from a camera aboard the spacecraft. The stream starts about an hour and 15 minutes before the impact, which is expected to come at 7:14 p.m. ET on Monday.

Orchestrating a collision millions of miles away with a relatively small asteroid that scientists know little about is tricky business, according to mission systems engineers. Round-trip radio signals between the DART spacecraft and mission team on Earth can take more than a minute. That time lag means the team members can’t effectively steer the craft. Instead, DART steers itself using its onboard navigation system, said Andrew Cheng, a Johns Hopkins APL planetary scientist and a DART investigation team lead.

The craft has been aiming at light from Didymos and won’t be able to see Dimorphos until about an hour before impact. So the spacecraft has scant time to lock on to its target and use its onboard thrusters to make course corrections.

“I’m very tense, very anxious and having trouble sleeping,” Dr. Cheng said. “We’re doing something that’s never been done before.”

NASA’s James Webb and Hubble space telescopes will help scientists analyze the results of the celestial smashup. So will nearly 40 Earth-based observatories across all seven continents.

“As far as telescopes here on the Earth are concerned, this is going to be the best data they’ve ever gotten for this system,” said Nancy Chabot, another Johns Hopkins APL planetary scientist and DART coordination lead. “They’re going to be able to take observations through all of 2022 and into even 2023,” she added.

Observations have shown that Dimorphos takes 11 hours and 55 minutes to complete one orbit around Didymos. The DART team expects the impact to shorten this so-called orbital period.

Dimorphos’ orbital period “could go down anywhere between, you know, seven-ish minutes to maybe even up to an hour,” said Harrison Agrusa, a University of Maryland astrophysicist and DART investigation team member. Dr. Agrusa said it is hard to know just how much the impact will affect the movements of Dimorphos because “we don’t know exactly what it’s made of, and what its density is, whether or not it’s one solid rock, or it’s like a rubble pile or just like an aggregate of a bunch of boulders.”

Those variables will help determine how much momentum the spacecraft transfers to the asteroid, which Dr. Chabot estimated to be about 11 billion pounds.

On Sept. 11, the DART craft released a toaster-size spacecraft known as a cubesat that will observe the impact as it flies by the binary asteroid Monday. Images from the Italian-built cubesat should help scientists determine the size of any impact crater made on Dimorphos as well as how much debris—what astronomers call ejecta—is sent careening into space.

The DART spacecraft launched in November from Vandenberg Space Force Base in California aboard a SpaceX Falcon 9 rocket. Since then it has been orbiting the sun on a trajectory that draws it closer to that of the asteroid system. Mission scientists chose the impact date because it comes at a time when the system is relatively close to Earth, according to Dr. Chabot.

A future mission named Hera, helmed by the European Space Agency, will launch another spacecraft toward the asteroid system in about two years. That spacecraft will reach the system in 2026 with the goal of landing on Dimorphos and further characterizing any impact crater, as well as determining the asteroid’s composition and mass.

Although it dwarfs the DART spacecraft, Dimorphos is tiny compared with the six-mile-wide space rock that scientists believe doomed the dinosaurs. But even an asteroid of Dimorphos’ size would be big enough to cause regional devastation and cause “tens of billions of dollars in damage, not to mention lost lives,” if it were to collide with Earth, according to NASA’s Planetary Defense Officer Lindley Johnson.

Mr. Johnson said the $325-million DART mission represents “a large step forward in showing that we do now have the knowledge and the technology to protect the Earth” from an asteroid like Dimorphos.

Write to Aylin Woodward at aylin.woodward@wsj.com

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