Spacecraft magnetometer
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
s 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, 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.
Image:Lunar Prospector orbiter.jpg|thumb|left|Lunar Prospector probe, the magnetometer is mounted on the boom-end facing toward the viewer
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.Image:MGS Fluxgate Magnetometer.png|thumb|300px|Wiring diagram and picture of the Magnetometer used on Mars Global Surveyor 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 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.Image:Search coil magnetometer.png|thumb|left|Photograph of the search coil magnetometers used on the THEMIS and Cluster/Staff mission.
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 is defined as:The Pioneer 5 mission finally managed to get a working magnetometer of this type in orbit around the Sun showing that magnetic fields existed between Earth and Venus orbits. A single magnetometer was oriented along the plane perpendicular to the spin axis of the spacecraft. Search coil magnetometers have become increasingly more common in Earth observation satellites. A commonly used instrument is the triaxial search-coil magnetometer. Orbiting Geophysical Observatory The Vela mission used this type as part of a package to determine if nuclear weapons evaluation was being conducted outside Earth's atmosphere. In September 1979 a Vela satellite collected evidence of a potential nuclear burst over the South Western Indian Ocean. In 1997 the US created the FAST that was designed to investigate aurora phenomena over the poles. And currently it is investigating magnetic fields at 10 to 30 Earth radii with the THEMIS satellites THEMIS, which stands for Time History of Events and Macroscale Interactions during Substorms is an array of five satellites which hope to gather more precise history of how magnetic storms arise and dissipate.
Ionized gas magnetometers
Heavy metal — scalar
Certain spacecraft, like Magsat, are equipped with scalar magnetometer. The output of these device, often in out frequency, is proportional to the magnetic field. The Magsat and Grm-A1 had cesium-vapor sensor heads of dual-cell design, this design left two small dead zones. Explorer 10 was equipped with a rubidium vapor magnetometer, presumably a scalar magnetometer since the spacecraft also had a fluxgate. The magnetometer was fouled accidentally which caused it to overheat, it worked for a period of time but 52 h into the mission transmission went dead and was not regained. Ranger 1 and 2 carried a rubidium vapor magnetometer, failed to reach lunar orbit.Helium
This type of magnetometer depends on the variation in helium absorptivity, when excited, polarized infrared light with an applied magnetic field. A low field vector-helium magnetometer was equipped on the Mariner 4 spacecraft to Mars like the Venus probe a year earlier, no magnetic field was detected. Mariner 5 used a similar device For this experiment a low-field helium magnetometer was used to obtain triaxial measurements of interplanetary and Venusian magnetic fields. Similar in accuracy to the triaxial flux-gated magnetometers this device produced more reliable data.Other types
Overhauser magnetometer provides extremely accurate measurements of the strength of the magnetic field. The Ørsted satellite uses this type of magnetometer to map the magnetic fields over the surface of the Earth.On the Vanguard 3 mission a proton processional magnetometer was used to measure geomagnetic fields. The proton source was hexane.