Scientists have found microorganisms crawling over a sample retrieved from the 200 million-mile-distant asteroid Ryugu. But they almost certainly came from Earth.

A sample of the asteroid Ryugu returned to Earth by the Hayabusa2 mission was rapidly colonized by terrestrial microorganisms, new research has revealed.

Asteroid Ryugu was altered by water and contains organic molecules older than the Sun. The building blocks of RNA and Vitamin B3 have been found in its samples, together with polycyclic aromatic hydrocarbons (PAHs) such as naphthalene, a molecule used in mothballs. Earthly bacteria don’t seem to care where the molecules originated from.

A Ryugu asteroid sample has been found contaminated by terrestrial microbes, raising questions about planetary protection.

Last year, researchers excitedly announced that they had found two organic compounds essential for living organisms in samples retrieved from an asteroid called Ryugu. The Japanese Space Agency's Hayabusa2 spacecraft obtained the samples from the space rock in 2019.