Europe
ESA’s Hera spacecraft sees the asteroids Didymos and Dimorphos as two separate bodies for the first time
ESA announced on October 27 that its Hera planetary-defence mission has for the first time seen the asteroids Didymos and Dimorphos as two separate bodies, from 1,280 km away.
Until recently the Hera spacecraft saw the asteroid system of Didymos and Dimorphos as a single bright point. Now the point has become two. The European Space Agency (ESA) announced on Tuesday, October 27 that Hera, its first planetary-defence mission, has for the first time resolved its two target asteroids separately, proving visually that it is a double system. Didymos is currently about 197 million kilometres from Earth, according to the agency.
The image was taken on the evening of October 18 with the spacecraft’s monochrome Asteroid Framing Camera, from a distance of 1,280 kilometres from the pair. The smaller body, Dimorphos, takes up only about one pixel in the first frame, but it can be clearly distinguished from its parent asteroid. “With this image the smaller asteroid Dimorphos becomes visible for the first time,” explained Michael Kueppers. “Before this shot the spacecraft still saw the Didymos system as a single dot.”
The image came after the success of the braking manoeuvres Hera carried out earlier this month. Two thruster burns a week apart cut the spacecraft’s speed relative to the asteroids to just 20 metres per second, so that it approaches gradually and precisely. “For the first time we are seeing through Hera’s own eyes the double asteroid system for which the mission was designed and built,” said mission manager Ian Carnelli.
The sizes help convey the scale. Didymos, 780 metres across, falls only slightly short of the height of the world’s tallest building, the Burj Khalifa in Dubai, while Dimorphos, at 151 metres, is about the size of the Great Pyramid of Giza. The small asteroid moves around its parent at about 18 centimetres per second, at an average distance of 1.15 km from it.
The mission follows up the 2022 experiment. On September 26, 2022, NASA’s DART spacecraft struck Dimorphos at 6.1 kilometres per second. The collision threw out a cloud of debris to a distance of 10,000 km and shortened the asteroid’s orbit by about 33 minutes. The method, known as kinetic impact, is not yet fully understood well enough to be used as a reliable planetary-defence technique. Open questions remain: what the impact did to the asteroid itself, whether it opened a crater or changed its whole shape, what its exact composition, structure and mass are, and which properties explain a deflection much larger than the simulations predicted.
NASA’s NEO Surveyor telescope, which is being built specifically to detect asteroids and comets, is part of the same effort to protect Earth. We covered its latest milestone in NASA completes the solar array on NEO Surveyor’s sunshade.
“Our images will get much sharper as Hera gets closer,” Carnelli added. The first detailed images of Dimorphos, the first body in the Solar System whose orbit has been measurably changed by human intervention, are expected on November 25. In the coming months Hera will carry out a close-up “investigation of the impact site” with several instruments, and will deploy two CubeSat microsatellites, the size of a shoebox, for even closer study. The mission ultimately includes a landing.