What is Ryugu and how did it come about?
Ryugu is a near-Earth asteroid first discovered in 1999. It is named after the dragon god’s underwater palace in Japanese mythology, from which a fisherman retrieved a chest containing a great secret.
This name reflects that it is the target of JAXA’s Hayabusa2 probe, which was only the second mission to return samples from an asteroid. Over five grams of Ryugu were collected from the asteroid’s surface, and the probe subsequently returned the samples to Earth in pristine condition.
Even before those samples returned, Ryugu was already beginning to change our understanding of the structure of asteroids.
“Both Ryugu and Bennu, an asteroid visited by NASA, turned out to be debris,” explains Ashley. “These are bodies formed by the accumulation of smaller rocks under their own gravity into larger aggregates.”
“We now believe that this is a really common type of asteroid, formed from the remnants of the Solar System’s first generation of asteroids.”
Researchers believe this first generation formed within two million years of the beginning of the solar system, which is estimated to have occurred around 4.6 billion years ago.
Because the samples contain only finite amounts of inclusions known as chondrules, which form under high temperatures, scientists believe Ryugu’s mother body was collected more than 600 million kilometers from the Sun, in a region of the early Solar System where water and carbon dioxide would have turned to ice.
Radioactive elements inside the asteroid gradually heated it until the ice began to melt. The rock chemically reacted with the meltwater to form hydrous minerals, with the asteroid reaching its peak temperature some five million years later.
A billion years ago, a catastrophic impact shattered Ryugu’s mother asteroid. Using simulations based on the available evidence, the researchers calculated that the parent, probably about 50 kilometers in radius, was struck by a smaller asteroid moving at about five kilometers per second.
Some of the parent’s fragments that were on the opposite side of the impact site were hurled together and later fused into Ryugu. The asteroid’s rotation resulted in its top-like shape, with centrifugal forces creating a bulge in the center.
Other fragments could be part of the Eulalia or Polana asteroid families found in the asteroid belt.