ANTIPROTONS appear to ring the Earth, confined by the planet’s magnetic field lines. The antimatter, which may persist for minutes or hours before annihilating with normal matter, could in theory be used to fuel ultra-efficient rockets of the future.

Charged particles called cosmic rays constantly rain in from space, creating a spray of new particles - including antiparticles - when they collide with particles in the atmosphere. Many of these become trapped inside the Van Allen radiation belts, two doughnut-shaped zones around the planet where charged particles spiral around the Earth’s magnetic field lines.

Satellites had already discovered positrons - the antimatter partners of electrons - in the radiation belts. Now a spacecraft has detected antiprotons, which are nearly 2000 times as massive.