Bennu's dark twin
Ryugu
Ryugu is a carbonaceous asteroid about 900 metres across, darker than coal and shaped like a spinning top. Japan's Hayabusa2 spacecraft dropped small robots onto it, blasted an artificial crater and in 2020 brought back to Earth 5.4 grams of its material, among the oldest and least altered ever analysed.
- Size
- 1,004 × 1,004 × 876 m
- Distance from Sun
- 178 M km
- Orbital period
- 475 days
- Rotation
- 7.63 hours
- Discovered
- 1999
- Samples
- 5.4 g (2020)
Physical characteristics
Ryugu measures 1,004 × 1,004 × 876 metres. It is an extremely dark carbonaceous asteroid: it reflects about 4.5% of the light it receives. Like Bennu it is a top-shaped rubble pile, with a density of 1.19 grams per cubic centimetre, and more than half its volume is empty space.
Its surface is covered in boulders, with hardly any smooth areas: finding a spot to touch down took months. The samples showed that its rock resembles the most primitive meteorites known, the CI chondrites, whose composition mirrors that of the Sun. It originally held liquid water, which transformed its minerals; today only the water chemically bound in clays remains.
Orbit and rotation
Ryugu takes 475 days to travel an orbit lying between 0.96 and 1.42 AU from the Sun, 178 million km on average, tilted 5.9° to the ecliptic. It crosses Earth's orbit and belongs to the Apollo group. It spins once every 7.63 hours.
Its top shape suggests that in the past it spun much faster, in about 3.5 hours, enough to make material slide towards the equator. There is no known risk of impact with Earth: its next close approach, in December 2076, will take place at a safe distance.
Discovery and exploration
It was discovered on 10 May 1999 by the LINEAR programme. Its name comes from a Japanese folk tale: Ryūgū-jō is the dragon's undersea palace from which the fisherman Urashima Tarō returns carrying a mysterious casket, like the spacecraft with its capsule.
Hayabusa2, launched by the Japanese space agency in December 2014, reached Ryugu on 27 June 2018. It dropped two small hopping rovers and the French-German lander MASCOT, touched the surface on 22 February 2019, fired a copper projectile on 5 April that dug a crater about 15 metres across, and on 11 July collected the freshly exposed material. The capsule landed in Australia on 6 December 2020.
Frequently asked questions
What did Hayabusa2 find in the samples?
Material that has remained almost untouched for 4.6 billion years. In the 5.4 grams from Ryugu, scientists identified more than twenty amino acids, uracil, which is one of the bases of RNA, and vitamin B3, along with minerals formed in the presence of water. They show that molecules useful to life already existed on asteroids before Earth formed.
How does Ryugu differ from Bennu?
They are very alike: two top-shaped carbonaceous asteroids, dark and made of rubble. Ryugu is about twice the size, and its surface shows fewer signs of hydrated minerals. Comparing the two, visited in the same years by a Japanese and an American spacecraft, is one of the most useful ways of understanding how such bodies form.
Where is Hayabusa2 now?
It is still travelling. After delivering the capsule it carried on with an extended mission: on 5 July 2026 it flew past asteroid Torifune about 400 metres above its surface, and in July 2031 it is due to reach 1998 KY26, an asteroid just ten metres or so across that spins in little more than five minutes.
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