Science & Technology

Hayabusa2 reveals asteroid secrets

The Hayabusa2 probe in orbit around asteroid Ryugu - reveals the clues about the object's formation and conditions in the early solar system
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The first batch of data collected by the Hayabusa2 probe — in orbit around asteroid Ryugu — reveals the clues towards the object’s formation and early history as well as conditions in the early solar system. 

The Hayabusa2 space probe has gathered vast amounts of images and other data which gives researchers clues about Ryugu’s formation from a larger parent body. These details should, in turn, allow researchers to better estimate quantities and types of materials essential for life that were present as Earth formed.

The Hayabusa2 probe launched on Wednesday, 3rd December 2014, atop an orange and white rocket over 50m tall weighing almost 300 tons from Tanegashima Space Center in South West Japan.

Ryugu is a C-type asteroid — rich in carbon — about 900m wide ( © 2019 Seiji Sugita et al., Science)

Professor Seiji Sugita of the University of Tokyo’s Department of Earth and Planetary Science oversaw the successful launch: “I’ve never felt so excited and nervous at the same time, that wasn’t just another science experiment on top of that rocket. That was the culmination of my life’s work and the hopes and dreams of my entire team.”

Its carefully calculated trajectory swung Hayabusa2 round the Earth to pick up speed so it could reach its destination in the asteroid belt between Mars and Jupiter, arriving on schedule at its target — the asteroid Ryugu — on Wednesday 27th June 2018.

Since then the spacecraft has used a wide range of cameras and instruments to collect images and data about Ryugu which it continually sends to researchers back on Earth. Hayabusa2 has even made a brief soft-landing on the asteroid and is being prepared for a second in which it will collect loose surface material — regolith — to return to Earth. Even though it will be another year before that sample returns, the team are far from idle in the meantime.

Sugita describes the progress the probe has made thus far: “Just a few months after we received the first data we have already made some tantalising discoveries.

“The primary one being the amount of water or lack of it, Ryugu seems to possess. It’s far dryer than we expected, and given Ryugu is quite young (by asteroid standards) at around 100 million years old, this suggests its parent body was much largely devoid of water too.”

Ryugu has an unusually low albedo, or reflectance, of 2 per cent, so to our eyes, it is blacker than coal. Hayabusa2’s cameras are especially sensitive in order to resolve fine details (© 2019 Seiji Sugita et al., Science)

Colleagues of Sugita state in a companion paper that various instruments on Hayabusa2 including a visible-light camera and a near-infrared spectrometer confirm this lack of water. 

This fact could be very significant, as it is currently believed that all of Earth’s water came from local asteroids, distant comets and the nebula or dust cloud that became our sun. The presence of dry asteroids in the asteroid belt would change models used to describe the chemical composition of the early solar system. 

Sugita explains: “This has implications for finding life. There are uncountably many solar systems out there and the search for life beyond ours needs direction. Our findings can refine models that could help limit which kinds of solar systems the search for life should target.”

But there’s more to this than water; other compounds crucial to life exist in asteroids and Ryugu has some surprises here too. To understand why it’s important to know that Hayabusa2 is not the only terrestrial robot out there exploring asteroids right now. In 2016 NASA launched OSIRIS-REx which arrived at its target asteroid Bennu on 3rd December 2018, four years to the day from the launch of Hayabusa2.

The two projects are not in competition but actively share information and data which could help one another. Researchers compare their asteroids to learn even more than would be possible if they could only probe one. Although alike in most ways, Bennu and Ryugu differ significantly in some areas. They are both extremely dark, have spinning-top-like shapes and are covered in large boulders, but Ryugu contains far less water. 

This discrepancy has researchers scratching their heads.

It is unclear how Ryugu’s parent body became so dehydrated. It’s possible it was internally heated by radioactive materials, or suffered prolonged bombardments by other rocky bodies (© 2019 Seiji Sugita et al., Science)

Sugita continues: “I hoped the surface of Ryugu had more variety as previous ground-based observations had suggested. But every surface feature and boulder on Ryugu seems to be like every other, showing the same scarcity of water.

“However, what felt limiting is now enlightening; Ryugu’s homogeneity demonstrates the capacity of our instruments to capture nuanced data. It also serves as a necessary constant to compare subsequent data against. So much of science is about controlling variables and Ryugu does this for us.”

As Hayabusa2 continues to explore our little rocky neighbour researchers gradually piece together its history, which is entwined with our own. Sugita and his colleagues believe Ryugu comes from a parent asteroid several tens of kilometres wide, most likely in the asteroid families Polana or Eulalia.

Sugita concludes: “Thanks to the parallel missions of Hayabusa2 and OSIRIS-REx, we can finally address the question of how these two asteroids came to be.

 “That Bennu and Ryugu may be siblings yet exhibit some strikingly different traits implies there must be many exciting and mysterious astronomical processes we have yet to explore.”

Original research: http://dx.doi.org/10.1126/science.aaw0422

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