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On Monday, a NASA spacecraft will intentionally hit an asteroid named Dimorphos.
The Double Asteroid Redirection Test, or DART, mission aims to see if this type of kinetic impact can help deflect an asteroid that poses a threat to Earth.
“We’re moving an asteroid,” said Tom Statler, NASA program scientist for the DART mission. “We are changing the motion of a natural celestial body in space. Humanity has never done that before.”
Here’s what you need to know about this mission.
The DART spacecraft is about the size of a school bus. It has been traveling to reach its asteroid target since launching in November 2021. The spacecraft will arrive at the asteroid system on September 26. Impact is expected at 7:14 pm ET.
The spacecraft is aimed at a double-asteroid system, where a small “moon” asteroid, called Dimorphos, orbits a larger asteroid, Didymos.
Didymos. which means “twin” in Greek is about 2,560 feet (780 meters) in diameter. Meanwhile, Dimorphos measures 525 feet (160 meters) across, and its name means “two forms.”
At the moment of impact, Didymos and Dimorphos will be relatively close to Earth – within 6.8 million miles (11 million kilometers).
Neither Dimorphos nor Didymos is risk of collision with the earth – before or after the collision takes place.
DART goes down in a blaze of glory. It will set its sights on Dimorphos, accelerate to 13,421 miles per hour (21,600 kilometers per hour) and crash into the moon almost head-on.
The spacecraft is about 100 times smaller than Dimorphos, so it won’t obliterate the asteroid.
Instead, DART will try to change the speed and path of the asteroid in space. The mission team compared this impact to a golf cart crashing into one of the Great Pyramids – enough energy to leave an impact crater.
The impact will change the speed of Dimorphos by 1% when it revolves around Didymos. It doesn’t sound like much, but doing so will change the moon’s orbital period.
The nudge will shift Dimorphos slightly and make it more gravitationally bound to Didymos — so the collision won’t change the binary system’s path around Earth or increase its chances of becoming a threat to our planet.
The spacecraft will share its view of the double asteroid system through an instrument known as the Didymos Reconnaissance and Asteroid Camera for Optical Navigation, or DRACO.
This image, which serves as DART’s eyes, will allow the spacecraft to identify the double asteroid system and distinguish which space object it needs to hit.
This instrument is also a high-resolution camera that aims to capture images of the two asteroids that will be sent back to Earth at a rate of one image per second in what will appear roughly as a video. You can watch the live stream on NASA’s website starting at 6 a.m. ET Monday.
Didymos and Dimorphos will appear as pinpricks of light about an hour before impact, gradually growing larger and more detailed in the frame.
Dimorphos has never been observed before, so scientists can finally assume its shape and the appearance of its surface.
We should be able to see Dimorphos in exquisite detail before DART crashes into it. Given the time it takes for images to stream back to Earth, they will be visible for eight seconds before a signal loss occurs and DART’s mission ends – if it was successful.
The spaceship too has its own photojournalist along for the ride.
A briefcase-sized satellite from the Italian Space Agency has made a trip into space with DART. Called the Light Italian CubeSat for Imaging of Asteroids, or LICIACube, it detached from the spacecraft on September 11. The satellite travels behind DART to record what is happening from a safe perspective.
Three minutes after impact, LICIACube will fly by Dimorphos to capture images and video of the impact plume and maybe even spied on the impact crater. The CubeSat will rotate to keep its cameras focused on Dimorphos as it flies by.
The images and video, although not immediately available, will be sent back to Earth in the days and weeks after the collision.
The LICIACube will not be the only observer watching. The James Webb Space Telescope, the Hubble Space Telescope and NASA’s Lucy mission will observe the impact. The Didymos system may become more colorful as its dust and debris are ejected into space, said Statler, the NASA program scientist.
But ground-based telescopes will be key to determining if DART has successfully altered Dimorphos’ motion.
The Didymos system was discovered in 1996, so astronomers have many observations of the system. After the impact, observers around the world will see Dimorphos cross before and behind Didymos.
Dimorphos takes 11 hours and 55 minutes to complete a job of Didymos. If DART is successful, that time could decrease by 73 seconds, “but we actually think we’ll change it by about 10 minutes,” said Edward Reynolds, DART project manager at the Johns Hopkins University Applied Physics Laboratory.
Statler said that he would be surprised if a measurement of the period change came in less than a few days, but even more if it lasted more than three weeks.
“I am very confident that we will hit on Monday, and it will be a complete success,” said Lindley Johnson, NASA planetary defense officer.
But if DART misses its proverbial dartboard, the team will be ready to make sure the spacecraft is safe and all its information has been downloaded to find out why it didn’t hit Dimorphos.
The Applied Physics Laboratory’s Mission Operations Center will intervene if necessary, even though DART will operate autonomously for the last four hours of its journey.
It takes 38 seconds for a command to travel from Earth to the spacecraft, so the team can react quickly. The DART team has 21 contingency plans that it has rehearsed, said Elena Adams, DART mission systems engineer at the Applied Physics Lab.
Dimorphos was chosen for this mission because its size is comparable to asteroids that could pose a threat to Earth. An asteroid the size of Dimorphos could cause “regional devastation” if it hit Earth.
The asteroid system is “the perfect natural laboratory” for the test, Statler said.
The mission will give scientists a better understanding of the size and mass of each asteroid, which is crucial for understanding near-Earth objects.
Near-Earth objects are asteroids and comets with an orbit that places them within 30 million miles (48.3 million kilometers) of Earth. Detecting the threat of near-Earth objects that could cause serious damage is a primary focus of NASA and other space agencies around the world.
No asteroids are currently on a direct impact course with Earth, but more than 27,000 near-Earth asteroids exist in all shapes and sizes.
The valuable data collected by DART will contribute to planetary defense strategies, especially the understanding of what kind of force can shift the orbit of a near-Earth asteroid that could collide with our planet.
Films make fighting asteroid approach seem like a hasty scramble to protect the planet, but “That’s not the way to do planetary defense,” Johnson said. Blowing up an asteroid can be more dangerous, as its pieces can then be on a collision course with Earth.
But NASA is considering other methods to change the motion of asteroids.
The DART spacecraft is considered a kinetic impactor that can change Dimorphos’ speed and path. If DART is successful, it could be one tool for deflecting asteroids.
Another option is a gravity tractor, which relies on mutual gravitational attraction between a spacecraft and an asteroid to tug the space rock out of its impact trajectory into a more benign one, said Johnson.
Another technique is ion beam deflection, or firing an ion engine at an asteroid for long periods of time until the ions change the speed and orbit of the asteroid.
But both of these take time.
“Any technique you can imagine that changes the orbital speed of the asteroid in orbit is a viable technique,” Johnson said.
An international forum called the Space Planning Commission has brought together 18 national space agencies to assess what might be best to deflate an asteroid, depending on its size and path.
Finding populations of hazardous asteroids and determining their size are priorities of NASA and its international partners, Johnson said. The design for a space-based telescope called the Near-Earth Object Surveyor mission is currently under review.
The Didymos system will not be lonely for long. To investigate the aftermath of the impact, the European Space Agency’s Hera mission will launch in 2024. The spacecraft, along with two CubeSats, will arrive at the asteroid system two years later.
Hera will study both asteroids, measure physical properties of Dimorphos, and examine the DART impact crater and the moon’s orbit, with the goal of establishing an effective planetary defense strategy.