Van Allen radiation belts

The belts are torus-shaped zones within Earth's magnetosphere, not solid rings. They are commonly described as an inner and an outer region, although particle populations can fill the space between them and the boundaries are not fixed. The inner zone contains energetic protons and electrons; the outer zone is dominated by energetic electrons. The particles are guided by Earth's magnetic field and can bounce between regions of stronger field near the magnetic poles.

Different processes supply and remove particles in the two zones. The inner belt's energetic protons are linked in part to cosmic rays interacting with Earth's atmosphere: secondary neutrons can travel upward and decay into protons that become trapped. The outer belt receives particles associated with the solar wind and magnetospheric processes. Solar activity and geomagnetic storms can alter the belts' particle populations; in some conditions a temporary third region of enhanced particle intensity can form.

Instruments aboard Explorer 1 detected the belts in 1958, in one of the first major discoveries of the Space Age. They were named for physicist James Van Allen, who led the scientific work interpreting the measurements. Later spacecraft, including the twin Van Allen Probes launched in 2012, studied how particles are accelerated, transported and lost.

Energetic particles can damage spacecraft electronics and contribute to radiation exposure for astronauts. Mission planners account for the belts through trajectory, transit time, shielding and operational choices. The belts are hazardous radiation environments, but they are not an impenetrable barrier: spacecraft have crossed them, and NASA reports that Apollo 8 was the first crewed mission to travel beyond them and orbit the Moon in 1968.