Spacecraft travelling beyond
low Earth orbit enter the zone of radiation of the Van Allen belts. Beyond the belts, they face additional hazards from cosmic rays and
solar particle events. A region between the inner and outer Van Allen belts lies at two to four Earth radii and is sometimes referred to as the "safe zone".
[28][29]
Solar cells,
integrated circuits, and
sensors can be damaged by radiation. Geomagnetic storms occasionally damage
electronic components on spacecraft. Miniaturization and
digitization of electronics and
logic circuits have made satellites more vulnerable to radiation, as the total
electric charge in these circuits is now small enough so as to be comparable with the charge of incoming ions. Electronics on satellites must be
hardened against radiation to operate reliably. The
Hubble Space Telescope, among other satellites, often has its sensors turned off when passing through regions of intense radiation.
[30] A satellite shielded by 3 mm of
aluminium in an elliptic orbit (200 by 20,000 miles (320 by 32,190 km)) passing the radiation belts will receive about 2,500
rem (25
Sv) per year (for comparison, a full-body dose of 5 Sv is deadly). Almost all radiation will be received while passing the inner belt.
[31]
The
Apollo missions marked the first event where humans traveled through the Van Allen belts, which was one of several radiation hazards known by mission planners.
[32] The astronauts had low exposure in the Van Allen belts due to the short period of time spent flying through them. Apollo flight trajectories bypassed the inner belts completely, passing through the thinner areas of the outer belts.
[25][33]
Astronauts' overall exposure was actually dominated by solar particles once outside Earth's magnetic field. The total radiation received by the astronauts varied from mission to mission but was measured to be between 0.16 and 1.14
rads (1.6 and 11.4
mGy), much less than the standard of 5 rem (50 mSv) per year set by the
United States Atomic Energy Commission for people who work with radioactivity.
[32]