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Hayabusa 2 finds Asteroid Ryugu is young, dark and dry



Hayabusa 2's asteroid Ryugu has some unexpected features. Especially its dry surface could spell problems for the current theories of how the Earth got its water.

  Ryugu close up

Hayabusa 2's optical navigation camera captured this image by Ryugu on June 26, 2018.
JAXA / Tokyo University and Affiliates

It's the weeks with the asteroids! During the Lunar and Planetary Science Conference held in Texas, the two mission teams – NASA's Osiris-REX and the Japanese aviation exploration agency – discovered the Hayabusa 2 community about their explorations of near-earth asteroids. The presentations were accompanied by pictures and data as well as a lot of papers in the journals Nature, Nature Astronomy, and Nature Geoscience .

We have already reviewed NASA's Osiris-REX find on the asteroid 101955 Bennu. Now let's look at updates from the team behind Hayabusa 2, in circulation around 162173 Ryugu. The Japanese team is ahead of their American counterparts arriving about six months earlier. After the rapprochement, the team found the same kind of unexpected uneven terrain that NASA ultimately found on Bennu: Ryugu is peppered with rocks and cuts in diameter, threatening the spacecraft as it gets samples from the surface.

While the Osiris-REX team is still planning the spacecraft's descent to Bennu, Hayabusa 2 has already successfully exploited three landers floating to the asteroid's surface and snapping images along the way. The Morseskibet has already completed one of the three expected touchdown maneuvers.

During these touchdowns, the spacecraft expands a sampling mechanism resembling a 1 meter long limb ending in a receptacle for collecting samples. When the container is close enough to the surface, a special mechanism in the limb shoots a ballpoint pen into the asteroid surface and agitates the regolith in the hope that some of the surface material will end up in the container. Earlier this month, the team released recordings of the first attempt. The scientists expect to have more material than they originally planned to collect, as the ball created a more lively response than they had predicted – the scattered nearby rock up to 1 meter in diameter.

] However, there is no way to know how much they have achieved since the spacecraft has no mechanism to weigh or evaluate what is inside the test chamber. "This is keeping things interesting until the earth returns," joked mission manager Yuichi-Tsuda (JAXA's Department of Aerospace and Astronautics) during the press conference. "It's like a souvenir present box."

Ryugu: Like and not like Bennu

Ryugu and Bennu are so similar in appearance, it's hard to tell them in pictures. Both have been called "rubble-pile asteroids", collections of a variety of sizes held together by gravity. Their low densities mean that their interiors are filled with hollow spaces, such as Swiss cheese. These objects probably came back from the pieces from larger asteroids, which may be separated during impact in the early solar system. Both objects are formed as spinning plates, as their rapid rotation causes a ridge to be formed along the equator. But while Bennu's rotation accelerates, Ryugu seems to have slowed down and it's unclear why. The current rotation period is 7.6 hours, but scientists estimate that at some point in the past it should have rotated faster, with a period shorter than 3.5 hours, to acquire its current form.

  Ryugu

Ryugu is about 900 meters wide.
© 2019 Seiji Sugita et al., Science

There is also a big difference in age. While Bennu is old, with an estimated age between 100 million and 1 billion years, Ryugu seems to be much younger, less than 100 million years old.

Ryugu is also darker than Bennu and reflects on average half the light Bennu does. a reflectivity or albedo of 2%, it is one of the darkest objects known in the solar system. Scientists have matched Ryuguas observed surface color to two asteroid families in the main asteroid band, called Polana and Eulalia. Equality refers to one of these groups as Ryugue's most likely family of origin.

Another unexpected difference is the amount of water scientists have found on the two asteroids. Although there are hydrated minerals on both items, Ryugu seems to be much dry than scientists expected, suggesting that the mother's body didn't have much water either. This is in contrast to Bennu, which has more abundant hydroxyls. These molecules contain hydrogen and oxygen atoms linked together and are probably associated with clay minerals suggested by previous water interactions.

Cleaning up why the two asteroids have different water contents will be important in understanding how the Earth won its water. Solar system formation models often assume that most of the water found on Earth today arrived aboard meteorites and comets from both the asteroid band and the outside of the solar system. The existence of dry asteroids in the Earth's neighborhood may mean that these models need to be adjusted.

  Ryugu in false color

A false-colored picture of Ryugu shows that it is surprisingly missing water.
© 2019 Seiji Sugita et al., Science

Next Step: Fire the cannon!

The next step for the Hayabusa 2 mission doesn't sound very high tech, but it will be spectacular. On April 5, the spacecraft burns a 2.5 kilogram (5.5 pound) projectile on Ryugu to create an artificial crater on the surface. The team has already identified which point on the surface they will blow and aims to make a hole of at least 10 meters wide and 1 meter deep.

With this maneuver, the team expects to learn more about how asteroids respond to effects that will help scientists better understand Ryugu's history and evolution. Impact information can also be useful if one of these things ever comes too close to Earth and we have to blow it out of the way.

Creating the crater will also allow scientists to look at what lies beneath the surface. Two weeks after shooting the cannon, Hayabusa 2 will attempt another sampling aimed at the bottom of the crater. So in November or December of this year, the spacecraft begins its one-year return trip to Earth.

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