BAN #287: An asteroid comes to Earth… on purpose

11 January 2021   Issue #287

January 12, 2021

[The planetary nebula M 2-9, winds from a dying star. Credit: NASA / ESA / Hubble Legacy Archive / Judy Schmidt]

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What I’ve recently written on the blog, ICYMI

[Dark matter is thought to have formed a huge web in the early Universe, like this model from a computer simulation, allowing galaxies to form along the filaments. From Tuesday’s article. Credit: Springel et al. / The Millennium Simulation Project]

Friday 8 January, 2021: Mars. Is. Weird.

Space news

Space is big. That’s why we call it “space”

On 5 December, 2020, the Japanese Space Agency successfully dropped an asteroid on Earth. Well, pieces of one.

The Hayabusa2 spacecraft dropped off a sample return container with about 5 grams of grains and pebbles from the surface of the asteroid Ryugu. The mission launched in 2014 and arrived at the 900-meter wide asteroid in June 2018. It stayed near the asteroid for the next year and a half, mapping it and studying it in incredible detail.

That included dropping small rovers onto the surface — though these weren’t like the Mars rovers, like golf carts with wheels. There isn’t enough gravity on the wee rock for that, so these used mechanisms inside them to change their center of gravity, allowing them to, well, kinda flop around on the surface.

A major mission objective was to collect samples from the surface and subsurface and return them to Earth. It fired a small projectile into the surface to blast out debris, which it collected, and then some time later fired a much larger projectile to carve a crater into the surface and eject material from deeper in. Both attempts were successful.

[The surface material from the asteroid Ryugu returned by the Hayabusa2 spacecraft. Asteroids can be extremely dark, reflecting less light than asphalt, as you can see here. Credit: JAXA]

The spacecraft left the asteroid in November 2019, beginning its long voyage back to Earth. However, it did not land. Instead, the sample return container was ejected, which then came screaming in to the Earth’s atmosphere on 5 December 2020, ramming the air to slow down, then using a parachute to ride the rest of the way down into the Australian desert. Scientists have confirmed the mission was a success, and returned 54 grams of extremely dark carbon-rich material from the surface of the asteroid. This is only the second time in history this has been done; the first was from the original Hayabusa space mission to the asteroid Itokawa, but that returned a small amount of material, about 1,500 tiny particles in total. But it was enough to analyze and produce many interesting scientific results.

Scientists will analyze the Hayabua2 material from Ryugu, which is considered “primitive”, meaning relatively unchanged chemically since the early days of the solar system. This will be a huge boon for people studying the origin of the solar system, and the origins of the planets themselves. These asteroids were, in part, the building blocks of Earth!

As the for the main Hayabusa2 spacecraft, after deploying the sample return container it fired its thrusters to change its trajectory to miss Earth. That went smoothly, and it’s flying off, back into interplanetary space.

[An animation from Subaru telescope images shows the asteroid 1998 KY26’s motion as it orbits the Sun. Credit: NAOJ]

But not aimlessly! In July 2026 it will approach the asteroid 2001 CC21, another small near-Earth asteroid (probably ~1 km in diameter). That’s a flyby, just passing through. But in July 2031 it will rendezvous with the tiny asteroid 1998 KY26, which is only about 30 meters across. It’s an interesting rock; observations indicate it’s roughly spherical and a fast rotator, spinning once every 10 minutes or so.

[The tiny 30-meter asteroid 1998 KY26 (indicated by the lines) was imaged by the huge Subaru telescope on 10 Dec. 2020. The stars appear as streaks as the ‘scope tracked the asteroid. These images will help scientists improve the accuracy of the orbital determination for the asteroid. Credit: NAOJ]

We don’t know much about asteroids of this size; they’re so faint even when close to us that they’re difficult to observe. We’re pretty sure they’re all rubble piles, loosely bound collections of, well, rubble. Something 2/3rds this size is what exploded over Russia in 2013, and is a much more likely impactor than something much larger and more solid. Knowing better how to deflect such an asteroid is pretty important, obviously.

The science will be interesting too, of course. Small asteroids are affected strongly by sunlight; they absorb that energy and reradiate in a different direction, which very slowly changes their trajectory. This is called the YORP effect, and while it’s been observed many times, it depends strongly on the thermal properties of the asteroid. Being able to see one up close, measure its thermal emission and also track its orbit will be a huge boon for understanding this effect.

Mind you, a small rock changes its orbit rapidly enough that tracking them into the future becomes difficult. Being able to understand how to do that better means being better able to predict possible future impacts.

So Hayabusa2’s mission is far from over. Congratulations to JAXA and everyone involved for the amazing things accomplished so far, and here’s to looking to more amazement in the future!

Et alia

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