The Mainichi Shimbun answers common questions readers may have about the asteroid Ryugu, which Japan's Hayabusa 2 space probe arrived at on June 27.
Question: What kind of asteroid is Ryugu?
Answer: Ryugu is a C-type asteroid. This means it may have organic substances and water that are essential for life. The plan is for Hayabusa 2 probe to collect rock samples from Ryugu and bring them back to Earth by 2020.
Q: What other traits does a C-type asteroid have?
A: C-type asteroids contain a comparatively large amount of carbon, thus the letter "C," which is the chemical symbol for carbon. Itokawa, from which the first Hayabusa space probe took samples and brought back to Earth 10 years ago, is an S-type asteroid, which is mainly composed of rocks. Among other types is M, which refers to an asteroid with abundant metals. The composition of the substances on the surface can be calculated from the characteristics of reflected sunlight, and this is how asteroids are classified. For example, Ryugu is a darker color than Itokawa.
Q: Which type is the most common?
A: C-type. Many asteroids revolve around the sun in the asteroid belt between Mars and Jupiter. There are many S-type asteroids in the inner part of the belt closer to Earth, so most of the asteroids that come close to Earth are S-type. Ryugu was chosen for investigation because it has an orbit that is unusually close to Earth for a C-type asteroid.
Q: Are there any other ways to classify asteroids?
A: There are also groups based on the radius of asteroids' orbits around the sun. Ryugu and Itokawa belong to the Apollo group, which may enter Earth's orbit and come close enough to Earth for a possible collision with the planet.
Q: There are really a lot of ways to classify asteroids, aren't there?
A: There is also a classification called "family" for cases when a celestial body is broken into many parts that travel in similar orbits.
Astronomer Kiyotsugu Hirayama, who served as a professor at Tokyo Imperial University, now the University of Tokyo, proposed this classification for the first time in 1918. Because Ryugu has such an unusual orbit for a C-type asteroid, its family is unclear.
(Answers by Tomohiro Ikeda, Science and Environment News Department)