Spacecraft magnetometers are magnetometers used aboard spacecraft and satellites, mostly for scientific investigations, plus attitude sensing. Magnetometers are among the most widely used scientific instruments in exploratory and observation satellites. These instruments were instrumental in the discovery of the Van Allen radiation belts around Earth by Explorer 1, and have detailed the magnetic fields of the Earth, Moon, Sun, Mars, Venus and other planets. There are ongoing missions using magnetometers,Template:Such as? including attempts to define the shape and activity of Saturn's core. The first spacecraft-borne magnetometer was placed on the Sputnik 3 spacecraft in 1958 and the most detailed magnetic observations of the Earth have been performed by the Magsat and Ørsted satellites. Magnetometers were taken to the Moon during the later Apollo missions. Many instruments have been used to measure the strength and direction of magnetic field lines around Earth and the solar system. Spacecraft magnetometers basically fall into three categories: fluxgate, search-coil and ionized gas magnetometers. The most accurate magnetometer complexes on spacecraft contain two separate instruments, with a helium ionized gas magnetometer used to calibrate the fluxgate instrument for more accurate readings. Many later magnetometers contain small ring-coils oriented at 90° in two dimensions relative to each other forming a triaxial framework for indicating direction of magnetic field.