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There are literally millions of asteroids that a mission could choose as its target. Why did Hayabusa2 choose asteroid #Ryugu? A thread.
Asteroid missions are still relatively new. The first time that an asteroid was imaged close up was when NASA's Galileo mission imaged two large asteroids, 951 Gaspra (1991) and 243 Ida (1993). The discovery that Ida, an asteroid, had a "moon" was mind-blowing (and still is!)
NASA's Near-Earth Asteroid Rendezvous (NEAR) mission (later NEAR-Shoemaker after planetary geologist Dr. Gene Shoemaker) launched in 1996 to study a near-Earth asteroid for the first time. It targeted asteroid 433 Eros.
A near-Earth asteroid is one whose orbit has been nudged over time to take it into Earth's neighborhood. Most of these asteroids aren't dangerous to Earth, though some are. This is a nice breakdown of the types of near-Earth asteroids (and other objects): https://cneos.jpl.nasa.gov/about/neo_groups.html
The NEAR-Shoemaker mission was the first mission ever to LAND on an asteroid. From what I've heard from people who worked on it, the mission wasn't designed to land, but the engineers wanted to see if it could. And so, they did! PC: @NASA, 2003
This got people wondering about what could happen if we intentionally landed on an asteroid. Even further, what if we could bring some of the asteroid back to Earth? This material would be untouched by Earth's atmosphere, unlike meteorites. How would it differ?
The next asteroid mission was the Hayabusa mission by @JAXA_en. It launched in 2003 and targeted a tiny (it's only 535 m long!) asteroid called 25143 Itokawa. It's my favorite asteroid. Look at it! It looks like an otter, right? PC: JAXA, 2005
Hayabusa collected ~1500 individual grains of material from the smooth areas of Itokawa. It also had serious mechanical failures on its way back to Earth and almost didn't make it. It was so dramatic there's actually a Japanese movie about the mission: https://www.imdb.com/title/tt1915563/
The thing that 433 Eros and 25143 Itokawa have in common is they are both S-type asteroids. This means their compositions are very rich in silicate materials, much like stones on Earth. 17% of asteroids are in this class and it is the second-most common type.
Why does this matter? 162173 Ryugu, the target of the @haya2e_jaxa mission, is a C-type asteroid. We estimate that around 75% of asteroids have this compositional type, so it's very common. Now we'll have the unaltered material from TWO different kinds of asteroids!
But why pick a C-type asteroid in general? For one thing, the meteorites that we classify as carbonaceous (the C in C-type) only make up 4.6% of all meteorites. They are rare, so going to get fresh material is important to know more about them.
The other big reason is because carbonaceous chondrite meteorites contain water-bearing minerals in relatively high amounts (3-22% by weight). Water! On an asteroid! This suggests that these asteroids initially formed far from the sun (or else all the water would have evaporated)
We are piecing together what the early solar system's composition was at different distances from the Sun. Getting different material directly from the source is the best way to get the best data on what was going on billions of years ago.
@NASA also has a sample return mission to a C-type asteroid going on, which I think of as a cousin to the Hayabusa2 mission. This is, of course, the @OSIRISREx mission, which this October acquired samples of C-type #asteroid 101955 #Bennu. Check it out! https://www.asteroidmission.org/
OSIRIS-REx will officially leave Bennu in March 2021 and won't return to Earth until September 2023. Today, though, we can welcome back the first samples of a C-type asteroid. This is an amazing accomplishment for science and for humanity. Congratulations, @haya2e_jaxa!
