Asteroids are bits and pieces leftover from the disc of gas and dust that formed around the young sun and never quite coalesced into a planet. They contain some almost pristine compounds that help tell what the early solar system was like 4.5 billion years ago.

Ryugu, as dark as coal, is a C-type, or carbonaceous, asteroid, meaning it is full of carbon molecules known as organics including possibly amino acids, the building blocks of proteins. Such molecules are not always associated with biology and can form from chemical reactions in deep space, but asteroids could have seeded Earth with the organic matter that led to life.

About three-quarters of asteroids in the solar system fall into the C-type. And the time it takes for Ryugu to rotate came as a surprise to the scientists who are studying it.

“Up to now we know several top-shaped asteroids, but all of them have a short spin period around 3 hours,” said Makoto Yoshikawa, the Hayabusa2 mission’s manager. “The spin period of Ryugu is about 7.5 hours, so this issue is quite interesting from the point of science.”

This space rock was discovered in 1999 and not given a name until 2015. Ryugu is named after Ryugu-jo, or dragon’s palace — a magical undersea palace in a Japanese folk tale.

What will it do once it gets there?

If the spacecraft is able to keep its schedule, by the end of July, Hayabusa2 will descend within 3.1 miles of Ryugu’s surface to measure the gravity field around the asteroid. In September or October, Hayabusa2 is scheduled to make its first “touchdown operation” on the asteroid.

At that point, it may deploy one or more of the three tiny rovers it is carrying. It may also deploy a European-built lander then.