NASA slams DART into asteroid in a test of planetary defense

In the world’s first demonstration of planetary defense technology, NASA’s Double Asteroid Redirection Test (DART) successfully slammed into the asteroid Dimorphos on Sept. 26.

This mission, whose spacecraft launched in November 2021 and spent 10 months in space before intercepting the asteroid, was intended to practice asteroid deflection methods and technology before a need arises from a risky asteroid in Earth's orbit. While the DART’s target asteroid doesn’t threaten Earth, it gives us a chance to test if intentionally crashing a spacecraft into an incoming asteroid will be enough to redirect its course and defend the planet, one of the leading ideas for planetary defense from incoming asteroids. While DART reports that there is “no known asteroid larger than 140 meters in size (that) has a significant chance to hit Earth in the next 100 years,” they acknowledge that our knowledge could be incomplete and preparation is better than complacency.

The asteroid Dimorphos was purposefully chosen for this test because it was located in an eclipsing binary system called Didymos, where Dimorphos passes in front of and behind the larger asteroid Didymos, meaning that Earth-based telescopes can measure changes easily. By looking at the regular variation of brightness from this binary asteroid system, scientists can observe the orbit before and after impact with relative ease. Andrew Rivkin of "The Planetary Science Journal" explains other benefits, like how the system is also the closest of the roughly 60 identified binary near-Earth asteroids, is close to Earth, and has an orbit that eclipses often enough for scientists to glean useful data. Additionally, Didymos is in a cycle where in 2022, the distance between Earth and Didymos was minimized to roughly seven million miles, making telescope observations higher quality. This combination of factors means that measurements can be taken efficiently and affordably, with contributions from scientists across the globe.

Beyond careful asteroid selection, the DART mission was meticulously designed. The DART program notes that the Didymos system orbit doesn’t intersect Earth’s in current predictions, and that the energy from the crash is low enough to not disrupt the asteroid to the point where it would be a threat to Earth. Scientists completed complicated projections and created geometric models to determine the best trajectory for the spacecraft to have a successful journey and impact, and achieve mission goals. Rivkin explains these goals for a successful mission as four-fold: (1) intercept the Didymos asteroid, (2) cause at least a 73 second change in the binary orbital period, (3) have successful ground observations of the system to determine the orbital period change, and (4) measure momentum transfer of the impact, location and surface characteristics of the impact site, and estimate resulting changes to the Didymos system using ground-based or spacecraft-based observations.

The initial results of this collision were photographed by both the Hubble Space telescope and the James Webb Space Telescope, with simultaneous pictures taken of the asteroid both before and after the crash. As Leah Crane of NewScientist describes, this marks the first time that the two most powerful telescopes in service have simultaneously observed the same object. She furthers that such images and other readings from after the collision reveal the huge plumes of dust and debris that came off the asteroid, and that scientists plan to observe “how much Dimorphos’ orbit around Didymos changed, as well as the material properties of of the asteroid” to determine how successful the technology was at redirecting the asteroid from orbit.

This also is crucial for application of this technology at home, because additional readings on how much material was blasted off Dimorphos as well as the direction and speed it hurtled away will help us prepare for similar impacts when the debris is near Earth. The Hubble Space Telescope and James Webb Space Telescope will continue to view Dimorphos over the next few months to monitor such changes and impacts.

Looking to the future, the DART program furthers that the Hera mission by the European Space Agency is the next launch to the Didymos system. With a planned launch in 2024, the main spacecraft and two companion cubesats are expected to rendezvous with the Didymos system in 2026. While there, they will “conduct detailed surveys on both asteroids, with particular focuses on the crater left by DART’s collision and a precise determination of the mass of Dimorphos.” Such readings will give us more insight into the outcomes of this test, and help us plan for uses of this technology for planetary defense. This collaboration comes from AIDA, the Asteroid Impact and Deflection Assessment, which plans to use both missions to “extract the best possible information for planetary defense and Solar System science from these groundbreaking space missions.”

Hopefully, thanks to work by DART, if an asteroid did happen to come dangerously close to Earth, we’d know how to respond.