First light: Hera resolves two asteroids instead of one

2 min read · reported by ESA

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

Hera imaged Didymos and Dimorphos with its monochromatic Asteroid Framing Camera on the evening of Friday 18 October, at a distance of 1280 km away from the asteroid pair.

“With this image the smaller Dimorphos asteroid becomes visible for the 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.”

This first image came following the success of Hera’s braking manoeuvres earlier this month. A pair of thruster firings a week apart slowed the spacecraft to just 20 m/s relative to its two target asteroids, allowing it to come gradually closer on a fine-tuned basis.

At 151 m across Dimorphos is about the size of the Great Pyramid of Giza, while the 780-m diameter Didymos asteroid is a little smaller than Dubai’s Burj Khalifa skyscraper, the tallest building in the world. The pair orbit an average 1.15 km apart from each other, with Dimorphos orbiting its parent at about 18 cm/s.

“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,” adds 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.”

On 26 September 2022 Dimorphos became the subject of a grand-scale 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 while also slowing the asteroid’s orbit by around 33 minutes.

But key unknowns remain to turn this ‘kinetic impact’ method into a well-understood planetary defence technique: what did the impact do to the asteroid itself? Did it leave a crater, or reshape the asteroid body entirely? What is the precise makeup, structure and mass of Dimorphos – and which properties explain a deflection so much greater than impact simulations predicted?

Over the coming months Hera will perform a close-up ‘crash site investigation’ of Dimorphos, harnessing multiple instruments and deploying a pair of shoebox-sized ‘CubeSats’ for a still closer survey, eventually progressing to landing.

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Published by the European Space Agency. View the original →