Physics – Countdown to DART Impact


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In a unique test of planetary defense, NASA’s DART spacecraft is scheduled to crash into an asteroid next week and change the celestial body’s course.

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NASA/Johns Hopkins APL/Steve Gribben

An illustration of NASA’s DART spacecraft approaching its target, the asteroid Didymous.

If all goes according to plan, NASA’s DART spacecraft will meet a fiery end at 7:14 p.m. Eastern Daylight Time on September 26. DART, whose name stands for Double Asteroid Redirection Test, is poised to intentionally collide with an asteroid that will be 11 million kilometers from Earth at the time of impact. The aim of the mission is to change the speed and trajectory of the space chunk hit. The technology developed for the mission could one day help shift the orbit of an asteroid that, unlike this one, is on a collision course with Earth.

“Our DART spacecraft will impact an asteroid in mankind’s first attempt to alter the motion of a natural celestial body,” said Tom Statler, a scientist on NASA’s planetary defense team, at a recent press briefing about the mission. “This will be a truly historic moment for the whole world.”

Launched in late November 2021, the DART spacecraft has spent the last 10 months flying through space toward a pair of asteroids named Didymous and Dimorphous. Dimorphous – the smaller of the two – is 160 m in diameter, about the length of a football field and a half. Didymous is about 5 times larger. Little else is known about the shape, mass, or composition of these giant chunks of space.

Dimorphous orbits Didymous in a path that occasionally aligns the two from an Earth perspective. From the timing of these eclipse-like alignments, astronomers have determined the orbital period to be 11 hours and 55 minutes. The DART team expects the impact to shorten that period by a few minutes, as the force of the collision pushes the lunar-asteroid toward its planetoid, reducing separation by about 1%. (At the time of impact, DART will have a mass of 570 kg and will be moving at 6.1 km/s relative to Dimorphous.) “It will only make a small change in the way Dimorphous drives around Didymous,” said Nancy Chalbot, one of the DART team leaders. “It makes it a very safe and efficient way to do this initial test.”

For the DART team, which includes scientists from space agencies and research institutions around the world, the test consists of two parts: hitting Dimorphous and then deflecting his path. In order to hit its target, DART must autonomously identify the moonless asteroid and then adjust its trajectory itself to stay on target. “[DART] will not be able to distinguish between Didymous and Dimorphous until the last hour of the mission,” Chalbot said. At this point, the spacecraft is too far from Earth for any course adjustment signals to reach it before the countdown ends. Instead, DART must continuously capture images of the asteroids and then use algorithms onboard its computer to interpret those images and make the course adjustments needed to intercept.

Images captured by DART are also sent back to Earth. They will be broadcast live on NASA television beginning at T-minus 1 hour and 14 minutes, when Didymous and Dimorphous should both be visible as tiny bright dots. The dots then grow larger and gradually turn into blobs of gray before becoming recognizable as asteroids. The view then switches to Dimorphous, giving viewers a close-up of this never-before-imaged asteroid. It’s going to be “absolutely amazing,” said Patrick Michel, chief scientist of the European Space Agency’s space security program. “Pictures will come gradually, and scientists and the public alike will share in the excitement of discovering a new area.”

While the world will know by 7:14 p.m. EDT if the first part of the mission was successful — if cameras continue rolling, DART missed its target — it will be several days before data on the impact of the impact arrives. LICIACube – the lightweight Italian CubeSat for asteroid imaging accompanying DART – is designed to pass through Dimorphous at the time of impact and record what happens when DART impacts. Meanwhile, telescopes on every continent will track Dimorphous’ orbit to assess the second part of the mission – whether the impact changed the asteroid’s path.

These orbit observations will take some time. Statler explained the situation using a wristwatch analogy. If your watch starts to tick a second too fast – or too slow – you may not realize the timekeeping is off until a few weeks later. Scientists expect the same delay in accurately measuring Didymous’s acceleration. “Personally, I wouldn’t be surprised if a month went by and we still didn’t have a clear detection of this change,” Statler said. He noted that with so many unknowns about the asteroid, it’s “difficult” to give a more accurate time frame.

Meanwhile, Statler, Chalbot, and others emphasize that this mission is just a test — Didymous isn’t a killer asteroid headed for Earth. “In fact, there are no known asteroid threats to Earth for the foreseeable future,” Chalbot said. Scientists have traced the orbits of 98% of the largest near-Earth objects, and none of them are in imminent danger of hitting Earth. However, astronomers continue to find new objects with diameters similar to Didymous, and one of these asteroids could eventually pose a threat. “If one were to hit Earth, it could have regionally devastating effects[over] Effects that would affect everyone on the planet,” Statler said.

—Katherine Wright

Katherine Wright is Associate Editor of Physics Magazine.


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