Hayabusa-2: Asteroid mission exploring a 'rubble pile'
The asteroid being explored by the Japanese mission Hayabusa-2 is a "rubble pile" formed when rocks were blasted off a bigger asteroid and came back together again.
The discovery means that asteroid Ryugu has a parent body out there somewhere, and scientists already have two candidates.
They have also found a chemical signature across the asteroid that can indicate the presence of water, but this needs confirmation.
Ryugu's unusual shape is also a sign that it must have been spinning much faster in the past.
Scientists from the Japanese Space Agency (Jaxa) mission and from Nasa's Osiris-Rex spacecraft, which is exploring a different asteroid called Bennu, have been presenting their latest findings at the 50th Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas.
The Hayabusa-2 team has also published its results over three papers in Science journal.
Meanwhile, the team behind the Osiris-Rex mission has made the first close-up observations of particle plumes erupting from an asteroid's surface. These findings are published in a suite of papers in the Nature journals.
What have they learnt?
Bennu and Ryugu have many similarities. They are comparable in size, rich in carbon and shaped like spinning tops. Both missions aim to deliver samples from these objects to Earth.
Both asteroids are primitive objects, made of the same basic material that went into building rocky planets like Earth. Studying samples in laboratories could shed light on how our own world came to be.
The identification of Ryugu as a rubble pile asteroid comes from an assessment of its density. Project scientist Sei-ichiro Watanabe said the asteroid's porosity - a measure of the voids, or spaces, present in the object - was 50%.
The large number of rough boulders on Ryugu's surface support this idea, he added. These boulders are probably fragments that joined up after the disruption of its parent body.
What's the significance of shape?
The spinning top shape, Dr Watanabe said, "was formed from a past rapid rotation".
He added: "Most of the known top shapes are rapid rotators, but Ryugu is rather slow."
In fact, the scientists think that Ryugu once spun at twice its current rotation period of once every 7.6 hours. At some point in its history, the object slowed down, though what happened to cause this remains unclear.
Team-member Seiji Sugita, from the University of Tokyo, said: "The size of Ryugu is very small - 800m or 900m across. It's too small to survive the entire Solar System evolution of 4.6 billion years. Ryugu must have been born from a much older and larger parent body in relatively recent times - several hundred million years."
Analysis of the reflected sunlight from Ryugu shows it is a close match to two larger asteroids, known as Polana and Eulalia. These are good potential candidates for the asteroid's parent body.
What have they discovered about water?
Ryugu is surprisingly dark, much darker than any carbonaceous chondrite meteorites, which could partly be due to exposure of the rocks to the space environment.
"The surface of Ryugu is extremely dark," said Ralph Milliken, from Brown University in Rhode Island and a co-investigator on the near-infrared spectrometer instrument (NIRS3).
He held up a 3D-printed model of Ryugu, saying that he suspected the jet-black plastic used to make it was brighter than the real thing.
Data from NIRS3 has also revealed the presence of minerals with hydroxyl groups (OH), which can indicate the presence of water.
"There is evidence for water on Ryugu, but we do not have any strong evidence yet for the presence of molecular water, H2O," said Ralph Milliken.
The particular hydroxyl groups found on Ryugu appear to be associated with the element magnesium, which is often associated with clay minerals in meteorites.
What are the next steps in sample collection?
At Bennu, the team behind Osiris-Rex detected plumes of material erupting from the asteroid on 6 January this year. The immediate cause isn't clear, but it could be related to volatile gases that escape from the rocks when sunlight heats them up. This would push the dust out into space.
Bennu also appears to be a rubble pile asteroid, and, like Ryugu, was much more rugged than expected - posing a hazard for sample collection.
Hayabusa-2 has just finished a touchdown operation to collect a sample of rock and cache it for return to Earth.
Although there was no way to confirm if Hayabusa-2 had collected a sample, project manager Yuichi Tsuda said the team was confident it had, judging from the large amount of material kicked up after the spacecraft fired a 5g tantalum "bullet" into Ryugu's surface.
During the touchdown operation, Hayabusa-2's thrusters shifted 50cm-1m rocks, Yuichi Tsuda said. The thrusters also blew away the top layer of regolith, revealing darker material underneath.
Mission scientists have also set a date for Hayabusa-2's next set piece: the kinetic impact experiment. This will involve the spacecraft detonating an explosive charge near the surface of Ryugu - generating an artificial crater.
The spacecraft will move to the other side of Ryugu for safety when the charge goes off, returning later to grab a sample of rock from within the crater. The idea is for Hayabusa-2 to get at pristine samples from below the surface, samples that haven't been altered by aeons of exposure to space.
The operation will take place on 5 April, said Dr Tsuda.
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