Boulders on asteroids can be three-quarters hollow or more, a discovery that could help yield insights on how Earth and other planets formed. The earliest stage of planetary formation started with building blocks known as planetesimals, chunks of rock ranging in size from asteroids to dwarf planets. Research suggested planetesimals began as very porous, fluffy clumps of dust that heat, gravity and impacts compacted over time.
Japan’s Hayabusa2 spacecraft found that Ryugu, a diamond-shaped near-Earth asteroid, is covered with rocks that are about 30 to 50 per cent porous. Now scientists have found that those boulders may be more than 70 per cent empty space, or about as porous as prior work suggested ancient planetesimals were, suggesting the rocks may contain remnants of the early Solar System.