WASHINGTON-The asteroid being explored by the Japanese mission Hayabusa-2 is a “rubble pile” formed when rocks were blasted off a bigger asteroid and came back together again. The discovery means that asteroid Ryugu has a parent body out there somewhere, and scientists already have two candidates.

They have also found a chemical signature across the asteroid that can indicate the presence of water, but this needs confirmation. Ryugu’s unusual shape is also a sign that it must have been spinning much faster in the past.

Scientists from the Japanese Space Agency (Jaxa) mission and from Nasa’s Osiris-Rex spacecraft, which is exploring a different asteroid called Bennu, have been presenting their latest findings at the 50th Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas. The Hayabusa-2 team has also published its results over three papers in Science journal.

Meanwhile, the team behind the Osiris-Rex mission has made the first close-up observations of particle plumes erupting from an asteroid’s surface. These findings are published in a suite of papers in the Nature journals.

Bennu and Ryugu have many similarities. They are comparable in size, rich in carbon and shaped like spinning tops. Both missions aim to deliver samples from these objects to Earth.

Both asteroids are primitive objects, made of the same basic material that went into building rocky planets like Earth. Studying samples in laboratories could shed light on how our own world came to be.

The identification of Ryugu as a rubble pile asteroid comes from an assessment of its density. Project scientist Sei-ichiro Watanabe said the asteroid’s porosity - a measure of the voids, or spaces, present in the object - was 50%.

The large number of rough boulders on Ryugu’s surface support this idea, he added. These boulders are probably fragments