In 2022, the Hubble Space Telescope focused on an asteroid struck by a half-ton NASA robotic probe, which was traveling more than seven times faster than a bullet, and discovered a cluster of 37 boulders created by the impact.
On September 26, 2022, NASA’s Double Asteroid Redirection Test (DART) deep space probe deliberately collided with the asteroid Dimorphos at approximately 22,500 km/h (14,000 mph) as part of a demonstration test to check whether it is possible to change the orbit of a dangerous asteroid so that it does not hit Earth.
Dimorphos was chosen because it is a small asteroid, about the size of the Great Pyramid of Giza, and orbits a much larger asteroid, Didymos. The idea is that this impact will alter the orbital trajectory of the twin form, allowing scientists to calculate the effectiveness of such deflection missions.
However, the DART is more than just playing pool with the Cosmic Ball. A true deflection task has to deal with many considerations. One of these is the composition of the asteroid, since a solid asteroid responds differently than a ball of asteroid debris.
When Hubble observed Dimofors, it discovered that the asteroid has a bright tail of dust pushed away by the pressure of sunlight, like a comet’s tail. That was expected, but the telescope also saw 37 jet boulders ranging in diameter from 1 meter to 6.7 m (3 to 22 feet) that looked like a small constellation. According to ESA, these boulders make up about 0.1 percent of Dimorphos’ total mass.
The space agency said the boulders were unlikely to be shrapnel shattered by the impact. More likely, they had dispersed across the asteroid’s surface and were spotted by DART at an altitude of 11 kilometers (7 miles) from the surface two seconds before impact. An estimated 2 percent of the asteroid’s boulders were dislodged by the collision, either by seismic waves loosening them or by direct impact.
This suggests that Dimorphos may be composed of fragments left over from the formation of Didymos. It formed a ring that later merged into a mass with a structure similar to a bunch of grapes loosely held together by gravity.
Whatever the answer, this raises more questions for ESA’s upcoming Didymos and Dimofors missions to resolve.