When NASA crashed a 610 kg (1,340 lb) impactor into tiny Dimorphos, a moon of the asteroid Didymos, it was all part of an effort to defend Earth. The impact showed how asteroids respond to impacts, and the data is helping NASA prepare for the day when we have to redirect an asteroid away from an eventual impact with Earth.
NASA’s DART (Double Asteroid Redirection Test) smashed into Dimorphos on the 26th of September, 2022, and ground telescopes watched the result.
DART was a success. The impact lowered Dimorphos’ orbital speed and reduced its orbital radius. DART also changed Didymos’ trajectory and excavated a crater on the Dimorphos’ surface that ejected more than 900,000 kg (990 US tons) of debris into space.
The Hubble Space Telescope got in on the action and captured images several hours apart until about 18 days after the impact.
There’s a little bit of mystery in the debris. At first, debris moves away from the impact in straight lines. It’s travelling at about 6.5 kph (4 mph.) That’s fast enough to escape Dimorphos’ weak gravity and prevent it from falling back to the surface. The debris takes on a cone shape with long filaments. (The straight spikes coming from the center of the images are Hubble optic artifacts.)
17 hours after the impact, the debris changes. In this second stage, the gravitational interactions between Didymos and Dimorphos start to distort the ejecta’s cone shape. Rotating, pin-wheeled features form, their rotation anchored to Didymos’ gravitational pull.
Next, the debris is swept back into a shape like a comet’s tail by the Sun’s radiative pressure on the tiny particles in the ejecta. The debris stretches out further, with the lightest particles the furthest from the asteroid. Then, mysteriously, the debris tail breaks into two tails.
The Hubble people created a time-lapse movie of the impact and the aftermath. The movie begins about 1.5 hours before the impact and ended 18 days post-impact.
NASA and the ESA aren’t done with DART, Didymos, and Dimorphos yet. In 2024, the ESA will launch their Hera mission, which will arrive at Didymos in December 2026. Hera will undertake a detailed study of Dimorphos to understand more deeply how the impact affected it. Hera will help turn the DART mission into data we can use to protect ourselves from asteroid impacts.
Hera is also a technology demonstration mission. It’ll test things like autonomous navigation around an asteroid and low-gravity proximity operations. The mission will also be the first to rendezvous with a double asteroid.