ESA is Planning a Mission to the Smallest Spacerock Ever Visited: the Moon of an Asteroid

For some small minority of humans, Death By Asteroid is a desirable fate. The idea probably satisfies their wonky Doomsday thinking. But for the rest of us, going out the same way the dinosaurs did would just be embarrassing. Thankfully, the ESA’s Hera mission will visit the smallest spacerock ever, and will help us avoid going the way of the dinosaurs.

For added kicks, it will forestall the happiness of any over-earger doomsday cultists, and the rest of us can revel in their existential anguish.

It might be news to you that an asteroid can have a moon. But in fact, about 15% of asteroids are in binary systems, and the smaller of these pairs is easily referred to as a moon. Hera’s target is the asteroid Didymos, and its tiny moonlet, dubbed “Didymoon.”

The Hera mission is part of a one-two punch of asteroid investigation, with the overall goal of helping us deflect any dangerous asteroids onto a harmless trajectory. Together with NASA’s DART mission, Hera will visit the smallest space-rock ever, the aforementioned “Didymoon.” Hera and its partner DART are part of the overall effort to understand how to protect Earth from asteroids.

When an asteroid struck the Yucatan region about 66 million years ago, it triggered the extinction of the dinosaurs. ESA’s Hera mission is visiting the smallest spacerock ever as part of our effort to not get creamed by an asteroid. Credit: NASA/Don Davis

Didymos is only 780 m across. That’s tiny, but it’s not the smallest one ever visited by a spacecraft. Japan’s Hayabusa spacecraft visited the 350 m Itokawa asteroid, and NASA’s OSIRIS-Rex is at the 500 m Bennu asteroid right now. But Didymos’ smaller partner, Didymoon, is only 160 m across, making it the smallest spacerock to ever be visited.

“Didymoon’s miniscule size really becomes clear when you look at other asteroids.”

Patrick Michel, CNRS Director of Research of France’s Côte d’Azur Observatory.

“Didymoon’s miniscule size really becomes clear when you look at other asteroids,” commented Hera’s lead scientist Patrick Michel, CNRS Director of Research of France’s Côte d’Azur Observatory.

Didymoon’s size can’t really be understated. It’s just a tiny rock hurtling through space. A chart from Emily Lakdawalla at the Planetary Society showing the comets and asteroids visited by spacecraft makes it clear how small Didymoon really is. It’s so small, it wouldn’t even show up on their chart. Its title of smallest spacerock ever is well-earned.

Didymoon is so small it wouldn’t even show up on the Planetary Society’s chart of comets and asteroids visited by spacecraft. If this is hard to see, check out the full-size chart at Planetary Society. They have lots of cool stuff to look at there. (Just Google it.) Image Credit: Emily Lakdawalla/Planetary Society.

The asteroid Ryugu, which is the target of Japan’s Hayabusa 2 mission, helps us understand Didymoon’s size. A notable feature on Ryugu is a boulder at the asteroid’s northern pole name Otohime Saxum. (It’s named after a Princess in a Japanese fairy tale.) O Saxum is about the same size as Didymoon.

In this Hayabusa 2 image of the asteroid Ryugu, the boulder Otohime Saxum is clearly visible at the top. That little boulder is the same size as Didymoon. In fact, Didymoon at one time may have been a rock on the surface of Didymos, before it detached and started orbiting Didymos. Image Credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST

But anyway, back to Hera and Didymoon, the smallest spacerock ever visited.

Before Hera can make its contribution, NASA’s DART mission has to do its thing. DART will launch sometime between December 2020 and May 2021. In October 2022, it will intercept the Didymos system, and at that point the asteroid will be close enough to Earth, about 11 million kms away, for ground-based telescopes to monitor what’s going on. DART will crash itself into the little moonlet known as Didymoon, at a speed of 6 km/second.

“This isn’t the first spacecraft impact into a planetary body.”

Patrick Michel, CNRS Director of Research of France’s Côte d’Azur Observatory.

The collision is designed to change Didymoon’s speed on its orbit. Not a huge change, only by a fraction of 1%. But that’s enough to be detected by our telescopes, and that’s all that DART needs to do.

“This isn’t the first spacecraft impact into a planetary body,” adds Patrick Michel. “NASA’s Deep Impact crashed into comet Tempel 1 in 2005, but not to try and deflect it, instead it was to expose subsurface material – the 6-km diameter body was much too large. But Didymoon is small enough, and in a tight enough 12-hour orbit around its parent, that its orbital period can indeed be shifted in a measurable way.”

We will better understand whether this technique can be used even for larger asteroids, giving us certainty we could protect our home planet if needed.”

Michael Küppers, ESA’s Hera Project Scientist

Then comes Hera.

Hera’s job is to perform a detailed post-impact study of Didymoon. From its close-in viewpoint, it will determine Didymoon’s mass, its surface properties and the shape of DART’s crater. It will arrive there some time in 2026 and do its work.

Asteroids compared to Didymoon. If this is hard to see, visit Image Credit:
Ian Carnelli adapting Planetary Society – E. Lakdawalla image.

“This will give us a good estimate of the impact’s momentum transfer, and hence its efficiency as a deflection technique,” explains ESA’s Hera project scientist, Michael Küppers. “These are fundamental parameters to enable the validation of numerical impact models necessary to design future deflection missions. We will better understand whether this technique can be used even for larger asteroids, giving us certainty we could protect our home planet if needed.”

Even though Didymoon will be the smallest spacerock ever visited by a spacecraft, it’s actually in a class of asteroids that poses the greatest threat to Earth. Larger asteroids are easier to track, and so easier to detect earlier to see if they pose a threat. Smaller rocks would likely burn up on entry into Earth’s atmosphere, or else would cause minimal damage if they did impact. But an impactor the size of Didymoon could devastate a large region of Earth.

Beyond planetary defense aspect of Hera and DART are the bonus science objectives. These binary asteroid systems aren’t well understood, and these tiny moonlets are difficult to observe from Earth.

“Didymos is spinning very swiftly, rotating once every two hours,” says Michel. “Around its equator, its weak pull of gravity could be overcome by centrifugal force, potentially leading to material rising from the surface – the leading theory of where Didymoon came from. So landing on the equator would be impossible; you would have to touch down near its poles instead.”

“Didymoon’s small size means we know little about it, but we assume it would be spin-locked around its parent like Earth’s moon, implying a slower spin equal to its orbital period. The plan is to land at least one CubeSat there, although it will require precise navigation to achieve this. The asteroid will have something like one millionth of Earth’s gravity, with an estimated escape velocity of just 6 cm per second, so one danger might be bouncing back out to space.”

According to Patrick Michel, these tiny asteroids could also be targets for asteroid mining. While larger bodies are much rarer, small ones like Didymoon are much more plentiful, even though their rapid spin rate might make mining them difficult.

A nice little video that explains the overall Hera-DART mission.

So there you have it. We may have to wait until 2026 for Hera to visit the smallest spacerock ever, and for science to strike another blow against doom and gloom fervour. If science can delete another existential threat to humanity, and one with a track record of catastrophic extinction, then the doom-and-gloom types will have to re-think their greatest hopes for nihilistic ecstasy.

Of course, knowing them, they’ll just move on and find something else to obsess over.


Evan Gough

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