The European Space Agency's Hera spacecraft has resolved its two target asteroids separately for the very first time, revealing the binary nature of the Didymos system, currently positioned some 197 million km away from Earth.

Hera imaged Didymos and Dimorphos using its monochromatic Asteroid Framing Camera on the evening of Friday 18 October, at a distance of 1,280 km from the asteroid pair. This first image came following the success of Hera's braking manoeuvres earlier in October.

First Glimpse of Dimorphos Asteroid

Prior to this image acquisition, the Hera spacecraft had been resolving the Didymos system as a single dot. Now, for the first time, Dimorphos appears as a distinct object separated from its parent body.

"With this image the smaller Dimorphos asteroid becomes visible for the very first time," explains Michael Kueppers. "Before this acquisition the spacecraft was still resolving the Didymos system as a single dot. But now, even though Dimorphos is made up of only around one pixel in this initial image, it is clearly well separated from its parent body."

To put the scale in perspective: Dimorphos measures approximately 151 m across—about the size of the Great Pyramid of Giza—while Didymos spans roughly 780 m in diameter, slightly smaller than Dubai's Burj Khalifa. The pair orbit an average 1.15 km apart, with Dimorphos orbiting its parent at approximately 18 cm/s.

Successful Braking Manoeuvres Enable Close Approach

A pair of thruster firings, spaced a week apart, slowed the spacecraft to just 20 m/s relative to its two target asteroids, allowing for a gradual, fine-tuned approach.

"For the very first time we are seeing through Hera's own eyes the very double asteroid system that the mission was designed and built for," says mission manager Ian Carnelli. "Our views will much grow sharper as Hera comes closer. We will unveil the first detailed images of Dimorphos – the first object in the Solar System to have had its orbit measurably shifted by human action – on 25 November."

Context: The DART Impact and Key Unknowns

On 26 September 2022, Dimorphos became the subject of a landmark planetary defence experiment when NASA's DART spacecraft impacted the smaller asteroid at a velocity of 6.1 km/s. The resulting collision sent out a plume of debris across 10,000 km of space and slowed the asteroid's orbit by around 33 minutes.

However, significant questions remain unanswered about the impact's effects. Key unknowns include what structural changes the collision inflicted on Dimorphos itself, whether it created a crater or reshaped the asteroid body entirely, and the precise makeup, structure, and mass of Dimorphos—data that will help explain why the deflection proved substantially greater than impact simulations had predicted.

Hera's Mission Ahead

Over the coming months, Hera will conduct a close-up investigation of the impact site, deploying multiple instruments and a pair of shoebox-sized CubeSats for even closer survey work, eventually progressing to landing operations. These investigations will address remaining unknowns and help turn the kinetic impact method into a well-understood planetary defence technique.