Mysteries in Science: A Journey to Ryugu Palace

Shogo Tachibana

(Professor, UTokyo Organization for Planetary and Space Science)

Launched in December 2014, the asteroid explorer Hayabusa 2 finally reached the asteroid Ryugu after three and a half years. On first inspection, Ryugu looked like two spinning tops stuck together, with its surface strewn with rocks. Having been involved in the scientific development of the sampling system, I was thrilled with the successful arrival of Hayabusa 2 and the appearance of Ryugu, but I worried whether the rocky surface might prevent the explorer from landing to collect samples.

Why do we want to collect samples from this asteroid and bring them back to Earth? Analysis of meteorites, which were witnesses to the formation of the planets, allows us to understand how the planets were created in the Solar System and how the Earth was born. From this we can estimate the age of the Solar System and catch a glimpse of the evolution of materials at its birth. Some of these meteorites, known as carbonaceous chondrites, contain organic substances and water-bearing hydrous minerals, and scientists think they may have brought the raw materials for the ocean and life to the Earth. There is no doubt that carbonaceous chondrites are extraterrestrial, but we do not know their origins in the Solar System. We still do not know which celestial objects brought water and organic substances to the Earth.

Earth-based observations using reflection spectra have revealed that some asteroids are composed of materials that are like those of carbonaceous chondrites. These are known as C-type asteroids and are typically found outside the asteroid belt between Mars and Jupiter. We think that Ryugu is a C-type asteroid that was once in the asteroid belt, but now has an orbit that brings it closer to the Earth. Analyzing samples brought back from Ryugu may answer the questions of what these C-type asteroids really are, how they contain water and organic matter, and whether they are the origin of carbonaceous chondrites. This analysis will also provide information on the evolution of materials in the earliest stages of the Solar System and on the evolution of materials in Ryugu. Rather than wait for meteorites to fall from the sky, Hayabusa 2 was sent on its journey to retrieve the optimal samples for yielding information on the Solar System's origins and on the raw materials of the Earth's ocean and life. As it might help explain the origin of our ocean, the target asteroid was named Ryugu, after the mythical Ryugu Palace which lay at the bottom of the ocean in Japanese folklore.

On arrival, Hayabusa 2 found that Ryugu was covered in rocks. Its planned October landing was postponed while alternative sites were assessed, and finally a circular site with a 3-meter radius was identified. This size was necessary because the spacecraft’s solar array paddles are about 6 meters long fully deployed. On February 22, 2019, Hayabusa 2 landed in this circle with pinpoint precision and successfully fired a projectile at the asteroid surface to collect samples. On April 5, an experiment was scheduled to create an artificial crater, and if possible, Hayabusa 2 will attempt to land in other locations and collect new samples.

The capsule holding the samples will arrive back on Earth at the end of 2020. Observations to date indicate that Ryugu is very dark and has less water than originally expected. After their return to Earth, the samples should tell us more about the significance of these findings.

― This article is from the "Mysteries in Science" series in The Rigakubu News ―

Translated by the Office of Communication

― Office of Communication ―