Hera probe reaches the asteroid pair Didymos and Dimorphos
· BehindLines newsroom
The Hera probe of the European Space Agency (ESA) has moved close to its targets, the asteroids Didymos and Dimorphos. According to Science News, it came within 30 kilometres of the pair on 4 November, after more than two years of travel. The ESA, which runs the mission, says the probe now sits between 25 and 30 kilometres from the system and has begun its "early characterisation phase".
An unprecedented double system
According to Science News, this is the first time scientists are exploring two asteroids that travel together: the seven other asteroids studied so far by spacecraft were all solitary. The ESA gives Didymos a diameter of about 780 metres and Dimorphos 151 metres. On 26 September 2022, NASA's DART spacecraft struck Dimorphos; according to Science News, the impact cut its orbital period around Didymos by over thirty minutes. Hera's priority is to study the aftermath of this planetary defence test.
What Hera is meant to measure
The ESA lists what remains unknown: the exact mass and internal structure of Dimorphos, the reason it was deflected far more than simulations had predicted, and how the impact transformed the asteroid, which might carry a giant crater, spin erratically or have changed shape. The agency wants to turn the kinetic impactor method into a well-understood technique.
Science News adds questions of pure science. Astronomers estimate that about 15% of asteroids are binary. Didymos spins so fast that its gravity may not hold all its rock together, and researchers expect Dimorphos to be a heap of boulders and debris. Prior to the collision, the same side of Dimorphos permanently faced Didymos; the collision probably tipped it over, but nobody knows what state it is in. Eva Lilly, of the Planetary Science Institute, stresses that it is unknown how long the system will take to settle.
Companions in January
Hera will release two small satellites in January, again according to Science News. One of them, Juventas, about the size of a bread box, carries a new kind of radar to probe the inside of Dimorphos, and will try to land on its surface. Patrick Michel, of the CNRS, hopes this landing will help connect what one sees of an asteroid with how it really behaves, after the surprise of the OSIRIS-REx probe, which sank about half a metre into the ground of Bennu.
Why it matters
The ESA presents Hera as its first planetary defence mission. Observations are only beginning: according to Science News, the probe will spend the coming weeks approaching the two asteroids, and the answers to the questions the ESA raises are not yet known.