NASA’s DART spacecraft crashes into the harmless asteroid Dimorphos in the world’s first planetary defense test.
The moment the DART spacecraft collided with the asteroid Dimorphos. (Video: NASA)
The Double Asteroid Redirection Test (DART) spacecraft from NASA crashed into a small asteroid 11 million kilometers away from Earth at 7:14 PM on September 26 (U.S. time), which is 6:14 AM on September 27 in Hanoi. NASA describes this as the world’s first planetary defense test. The mission’s goal was to change the orbit of Dimorphos, which orbits a larger body known as Didymos, enough to demonstrate that humanity can deflect a hazardous asteroid heading towards Earth.
The DART spacecraft, the size of a golf cart, impacted Dimorphos while traveling at a speed of 22,500 km/h. While not as large as other spacecraft, NASA hopes its weight of 600 kg is sufficient to alter the motion of the 163-meter-long asteroid Dimorphos on its orbit. “The spacecraft is very small,” said Nancy Chabot, the planetary scientist leading the DART coordination team at the Johns Hopkins University Applied Physics Laboratory (JHUAPL), who oversees the mission. “Sometimes we describe it as driving a golf cart straight into the Great Pyramid.”
The DART mission control center at JHUAPL faced significant pressure as the spacecraft approached its target. Most of DART’s final hours were automated, with the spacecraft’s navigation system locking onto Dimorphos in the last moments. DART’s main camera transmitted an image back to Earth every second until the screen went black at the moment of impact with the asteroid.
The DART mission, costing $313 million, was launched on November 23, 2021. DART is the first mission to test the “kinetic impactor” technique for planetary defense: crashing a spacecraft into an asteroid to change its orbit. This is a fundamental method for protecting Earth if a hazardous asteroid is detected about 5 to 10 years before a potential collision.
“We are changing the motion of a natural celestial body in the universe. Humanity has never done this before,” said Tom Statler, a scientist working on NASA’s DART program.
An image of the asteroid Dimorphos a few seconds after being struck by the DART spacecraft. (Photo: NASA).
The risk of an asteroid colliding with Earth is very distant but real, according to NASA scientists. NASA estimates that about 40% of asteroids larger than 140 meters pose a threat to Earth and continuously scans the skies for more. NASA is also developing a new space telescope named Near Earth Object Surveyor, specifically designed to scan for dangerous asteroids in the Solar System. This mission is set to launch in 2026. However, humanity also needs methods to deflect asteroids if they are detected. This is why the DART mission was created.
NASA chose Dimorphos, the moon of Didymos, for the DART collision for several reasons. First, Dimorphos is part of a binary system and orbits its host every 11 hours and 55 minutes, a short enough time to detect changes in its orbit using ground-based telescopes in subsequent observations.
Didymos and Dimorphos were discovered in 1996 and 2003, respectively, and are the first detailed binary asteroid system studied. By using a binary asteroid system instead of a single asteroid, NASA can utilize one spacecraft with the help of ground-based telescopes to measure the deflection rather than requiring additional spacecraft.
Although classified as “potentially hazardous asteroids,” Didymos and Dimorphos pose no threat of collision with Earth in the near future. The DART mission will cause Dimorphos to orbit Didymos 10 minutes faster but will not alter the orbit of the binary system.
At a distance of 11 million kilometers, Didymos and Dimorphos are closest to Earth. Signals take only 38 seconds to travel from DART to Earth, making it a timely target.
Dimorphos is also a perfect choice for astronomers because its size is similar to the asteroids NASA is most concerned about colliding with Earth. It also has an S-shaped structure, one of the most common types of asteroids in the Solar System.
DART is the first mission of its kind, and the team responsible for it did not know exactly what the result would be for Dimorphos. Angela Stickle, lead impact scientist for the DART mission at JHUAPL, noted that their simulations and models indicated the spacecraft would likely create a crater approximately 20 meters wide.
Scientists monitor the spacecraft’s impact on the asteroid.
“The impact on Dimorphos is a significant technical achievement,” according to NASA. The DART spacecraft transmitted an image every second as it approached its target. The spacecraft also witnessed its own outcome. In the weeks leading up to the collision, DART deployed a small satellite named LICIACube to observe the impact with the asteroid. Images from this satellite will reach Earth in a few days following the collision, revealing close-up pictures of the impact.
The James Webb Space Telescope, Hubble Space Telescope, and Lucy spacecraft all monitored the collision from operational points in the Solar System. On Earth, a vast network of telescopes was coordinated to observe the event and the Didymos-Dimorphos binary over time to see how much faster Dimorphos moved around its orbit. NASA stated that it will take time to determine if the DART collision is a successful planetary defense test.
More than 30 telescopes worldwide, including at least one telescope on every continent, will monitor the Didymos-Dimorphos asteroid duo over the next six months to assess the effectiveness of the test. The first radar observations of the collision will yield results on September 27, according to Cristina Thomas, a planetary scientist at Northern Arizona University and lead observer for DART.
The European Space Agency is planning its own mission to the Didymos-Dimorphos asteroid system to further study the DART collision. This mission, named Hera, will launch a spacecraft in 2024 and reach the binary system’s orbit in 2027 to investigate the two asteroids and the crater created by DART on Dimorphos.
