Ryugu is a heap of space rubble that might unlock the mysteries of water on Earth


We traveled millions of miles from Earth to visit a pile of rubble in space. Luckily, Ryugu, the near-Earth asteroid visited by the Japanese Hayabusa2 probe, is far more interesting than that sounds. Not only is it giving us unique insights into how space rocks form, but it’s teaching us more about how water may have appeared on Earth.

The Hayabusa2 mission launched in 2014 and has taken a number of pictures and scans of the asteroid. It has also deployed hopping rovers on its surface and shot bullets into its rocks, which told us more about the geology of its surface. Now, three papers published in Science today have used this data to measure the asteroid’s density, mass, shape, and spin. The findings should help scientists better understand the rock samples Hayabusa2 is slated to bring back to Earth in 2020.

Here’s some of what we know so far:

  • The asteroid has a low density. This suggests it has a porous rubble-like interior.
  • It’s about 1 kilometer wide at its equator, with an approximate mass of 450 billion kilograms.
  • Ryugu was probably created from a much larger parent body.
  • The rocks that make up the asteroid created the shape of a spinning top during a time when the object was rotating at about twice its current rate.
  • A near-infrared spectrometer found hydrated minerals—minerals that have water as part of their chemical structure—on the surface of the rock, but less water than researchers expected. Ryugu has significantly less water than Bennu, a similar near-Earth asteroid currently being studied by NASA.

Other surprises emerged as well. “The biggest surprise to me was the fact that Ryugu’s surface is covered by the same color of boulders,” says one of the paper’s lead authors, Seiji Sugita. This probably means that Ryugu’s parent asteroid—the larger body from which it was created—had a uniform interior.

(Those surface rocks took other teams by surprise—but for other reasons. The craft’s landing on Ryugu was supposed to happen in October 2018, but gravel on the asteroid’s surface was bigger than expected. That called for some new…



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