Science & Technology

The ESA Is Launching a Mission to 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 space rock ever, and will help us avoid going the way of the dinosaurs.

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

Related Video: Asteroid Mining: Our Ticket To Living Off Earth?

It might be news to you that an asteroid can have a moon. But in fact, about 15 percent 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.

Didymos is only 780 meters (2,559 feet) across. That's tiny, but it's not the smallest one ever visited by a spacecraft. Japan's Hayabusa spacecraft visited the 350-meter (1,148-foot) Itokawa asteroid, and NASA's OSIRIS-Rex is at the 500-meter (1,640-foot) Bennu asteroid right now. But Didymos' smaller partner, Didymoon, is only 160 meters (525 feet) across, making it the smallest space rock to ever be visited.

"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 space rock ever is well-earned.

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.

But anyway, back to Hera and Didymoon, the smallest space rock 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 kilometers (7 million miles) 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 kilometers per second (4 miles per second).

The collision is designed to change Didymoon's speed on its orbit. Not a huge change, only by a fraction of 1 percent. 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."

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 sometime in 2026 and do its work.

"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 will be the smallest 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 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.

space rock

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 ratemight make mining them difficult.

This article is republished from Universe Today under a Creative Commons license. Read the original article.

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Asteroids aren't the only thing Earth has to worry about. Learn about the others in "Death from the Skies!: These Are the Ways the World Will End" by "Bad Astronomer" Phil Plait. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Evan Gough for Universe Today February 26, 2019

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