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 mapping the Van Allen radiation belts around Earth after its discovery by Explorer 1, and have detailed the magnetic fields of the Earth, Moon, Sun, Mars, Venus and other planets and moons. There are ongoing missions using magnetometers, 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.
Magnetometer types Magnetometers for non-space use evolved from the 19th to mid-20th centuries, and were first employed in spaceflight by Sputnik 3 in 1958. A main constraint on magnetometers in space is the availability of power and mass. Magnetometers fall into 3 major categories: the fluxgate type, search coil and the ionized vapor magnetometers. The newest type is the Overhauser type based on nuclear magnetic resonance technology.
Fluxgate magnetometers
Fluxgate magnetometers are used for their electronic simplicity and low weight. There have been several types of fluxgate used in spacecraft, which vary in two regards. Primarily better readings are obtained with three magnetometers, each pointing in a different direction. Some spacecraft have instead achieved this by rotating the craft and taking readings at 120° intervals, but this creates other issues. The other difference is in the configuration, which is simple and circular. Magnetometers of this type were equipped on the "Pioneer 0"/Able 1, "Pioneer 1"/Able 2, Ye1.1, Ye1.2, and Ye1.3 missions that failed in 1958 due to launch problems. The Pioneer 1 however did collect data on the Van Allen belts. In 1959 the Soviet "Luna 1"/Ye1.4 carried a three-component magnetometer that passed the Moon en route to a heliocentric orbit at a distance of 6,400 miles (10,300 km), but the magnetic field could not be accurately assessed. Eventually the USSR managed a lunar impact with "Luna 2", a three component magnetometer, finding no significant magnetic field in close approach to the surface. Explorer 10 had an abbreviated 52 hr mission with two fluxgate magnetometers on board. During 1958 and 1959 failure tended to characterize missions carrying magnetometers: 2 instruments were lost on Able IVB alone. In early 1966 the USSR finally placed Luna 10 in orbit around the Moon carrying a magnetometer and was able to confirm the weak nature of the Moon's magnetic field. Venera 4, 5, and 6 also carried magnetometers on their trips to Venus, although they were not placed on the landing craft.
Vector sensors The majority of early fluxgate magnetometers on spacecraft were made as vector sensors. However, the magnetometer electronics created harmonics which interfered with readings. Properly designed sensors had feedback electronics to the detector that effectively neutralized the harmonics. Mariner 1 and Mariner 2 carried fluxgate-vector sensor devices. Only Mariner 2 survived launch and as it passed Venus on December 14, 1962 it failed to detect a magnetic field around the planet. This was in part due to the distance of the spacecraft from the planet, noise within the magnetometer, and a very weak Venusian magnetic field. Pioneer 6, launched in 1965, is one of 4 Pioneer satellites circling the Sun and relaying information to Earth about solar winds. This spacecraft was equipped with a single vector-fluxgate magnetometer.
Ring core and spherical Ring core sensor fluxgate magnetometers began replacing vector sensor magnetometers with the Apollo 16 mission in 1972, where a three axis magnetometer was placed on the Moon. These sensors were used on a number of satellites including Magsat, Voyager, Ulysses, Giotto, AMPTE. The Lunar Prospector-1 uses ring-coil made of these alloys extended away from each other and its spacecraft to look for remnant magnetism in the Moons 'non-magnetic' surface. Properly configured, the magnetometers are capable of measuring magnetic field differences of 1 nT. These devices, with cores about 1 cm in size, were of lower weight than vector sensors. However, these devices were found to have non-linear output with magnetic fields greater than >5000 nT. Later it was discovered that creating a spherical structure with feedback loops wire transverse to the ring in the sphere could negate this effect. These later magnetometers were called spherical fluxgate or compact spherical core (CSC) magnetometers used in the Ørsted satellite. The metal alloys that form the core of these magnetometers has also improved since Apollo-16 mission with latest using advanced molybdenum-permalloy alloys, producing lower noise with more stable output.
Search-coil magnetometer
Search-coil magnetometers, also called induction magnetometers, are wound coils around a core of high magnetic permeability. Search coils concentrate magnetic field lines inside the core along with fluctuations. The benefit of these magnetometers is that they measure alternating magnetic field and so can resolve changes in magnetic fields quickly, many times per second. Following Lenz's law, the voltage is proportional to the time derivative of magnetic flux. The voltage will be amplified by the apparent permeability of the core. This apparent permeability (μa) is defined as:
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