User:Pdeitiker/Spacecraft magnetometers



Magnetometers are one of the most widely used scientific instruments used in exploratory and observation satellites. The first spacecraft born magnetometer was placed on the Sputnik-3 spacecraft and the most detailed observations of the Earth have been done on Magsat. and Ørsted. There are magnetometers on the Moon during the later Apollo missions. Many various instruments have been used to measure the strength, direction, of magnetic field lines that exist around Earth or the 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 both, the helium magnetometer used to calibrate the fluxgate instrument for the most accurate readings. Many later magnetometers contain small ring-coils oriented 90° in two dimensions relative to each other forming a triaxial framework for indicating direction of magnetic field.

Magnetometer types
Magnetometers for non-space used evolved from the 19th to mid 20th century where they were employed in space flight. A main constraint on magnetometers used in space is the availability of energy and weight. Magnetometers have fallen into 3 major categories, the fluxgate type, search coil and the ionized vapor magnetometers.

Fluxgate Magnetometers
Fluxgate magnetometers are used for their electronic simplicity and low weight. There have been several type of fluxgate used in space craft. These instruments vary in two regards. Primarily better readings are obtain with three magnetometers, each pointing in a different dimensions. Some space craft have achieved this by rotating the space craft and taking readings at 120' Rotation, but this creates other issues. The other difference is the configuration, simple very circular.

Magnetometers of this type were equipped on the "Pioneer 0"/Able 1, "Pioneer 1"/Able 2, Ye1.1, Ye1.2, and Ye1.3 each missions that failed in 1958 due to launch problems, the pioneer 1; however did collect data on the Van Allen belts. The Soviet "Luna 1"/Ye1.4 carried a three-component magnetometer that passed the moon en-route to a heliocentric orbit in 1959 at a distance of 6400 miles 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. 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 moons magnetic field. Venera 4, 5, and 6 also carried magnetometers on their trip to Venus, although they were not placed on the landing craft.

Vector Sensor
The majority of early fluxgate magnetometers on spacecraft where made as vector sensor. The magnetometer electronics created, however, 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 14th, 1962 it failed to detect a magnetic field around Venus. 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, were a three axis magnetometer was placed on the moon. These sensors were used on a number of satellites including Magsat, Voyager, Ulysses, Giotto, AMPTE. Properly configured the magnetometers are capable of measuring magnetic field difference of 1 nT. These devices, with cores about 1 cm in size, were of lower weight than vector sensor. However, these devices were found to have non-linear output with magnetic feilds greater than >5000 nT. Later is 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 (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-pemalloy alloys, producing lower noise with more stable output.

Search Coil Magnetometer
Search coil magnetometers are wound coils around core of high magnetic permeability. Search coils concentrates magnetic field lines inside the core along with fluctuations. . 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 space craft.

Scalar
Certain spacecraft, like Magsat are equiiped with scalar magnetometer. The output of these device, often in out frequency, is proportional to the magnetic field. The Magsat cesium-vapor (Cesium-133) sensor heads of dual-cell design, this design left two small dead zones.

Heavy Cation
Ranger 1 and 2 carried a rubidum vapor magnetometer, failed to reach lunar orbit.

Ce-133, Ru-85, Ru-87 Explorer 10, Calibrating MAGSAT Satellite 1964 83C

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 ealier, 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.

Overhauser magnetometer
Overhauser magnetometer provides extremely accurate measurements of the strength of the magnetic field. The Orsted (satellite) uses this type of magnetometer to map the magnetic fields over the surface of the earth.

Configurations of Magnetometers
Unlike ground based magnetometers that can be oriented by the user to determine the direction of magnetic field, in space the user is linked by telecommunications to a satellite traveling at 25,000 km per hour. The magnetometers used need to give an accurate reading quickly to be able to deduce magnetic fields. Several strategies can be employed, it is easier to rotate a space craft about its axis than to carry the weight of an additional magnetometer. Other strategy is to increase the size of the rocket, or make the magnetometer lighter and more effective. One of the problems, for example in studying planets with low magnetic fields like venus, does require more sensitive equipment. The equipment has neccesarily needed to evolve for todays moder task. Ironically satellites launched more the 20 years ago still have working magnetometers in places where it would take decades to reach today, at the same time the latest equipment is being used to analyze changes in the Earth here at home.

Uniaxial
These simple fluxgate magnetometers were used on many missions. On Pioneer 6 the magnetometers were mounted to a bracket external to the space craft and readings were taken as the spacecraft rotated every 120°. Pioneer 7 and Pioneer 8 are configured similarly. Search coil magnetmeters were used on Pioneer I, Explorer VI, Pioneer V, and Deep Space 1.

Diaxial
A two axis magnetometer was mounted to the ATS-1 (Applications Technology Satellite). One sensor was on a  15 cm boom and the other on the spacecrafts spin axis (Spin stabilized satellite). The sun was used to sense the position of the boom mounted device, and triaxial vector measurements could be calculated. Compared to other boom mounted magnetometers, this configuration had considerable interference. Interestingly with this spacecraft, the sun induce magnetic oscillations and this allowed the continued use of the magnetometer after the sun sensor failed.

Triaxial
The Sputnik-3 had a vector fluxgate magnetometer, however because the orientation of the spacecraft could not be determined the direction vector for the magnetic field could not be determined. Three axis magnetometers were used on Luna 1, Luna 2, Pioneer Venus, Mariner 2. Explorer 33 was 'to be' the first US spacecraft to enter stable orbit around the moon was equipped with the most advanced magnetometer, a boom-mounted triaxial fluxgate (GFSC) magnetometer of the early-vector type. It had a small range but was accurate to a resolution of 0.25 nT. However after a rocket failure it was left in an highly elliptical orbit around Earth that orbited through the electro/magnetic tail. The Pioneer 9 and Explorer 34 used a configuration similar to Explorer 33 to survey the magnetic field within Earth's solar orbit. Explorer 35 was the first of its type to enter stable orbit around the moon, this proved important because with the sensitive triaxial magnetometer on board, it was found the moon effectively had no magnetic field, no radiation belt, and solar winds directly impacted the moon. With Apollo 12 improved magnetometers were placed on the moon as part of the Lunar Module/Apollo Lunar Surface Experiments Package (ALSEP). The magnetometer continued to work several months after that return module departed. As part of the Apollo 14 ALSEP, there was a portable magnetometer.

The first use of the three axis ring-coil magnetometer was on the Apollo 16 moon mission. Subsequently is was used on the Magsat. The MESSENGER mission has triaxial ring-coil magnetometer with a range of +/- 1000 mT and a sensitivity of 0.02 mT, still in progress, the mission is designed to get detailed information about Mercurian magnetosphere. The first use of spherical magnetometer in three axis configuration was on the Orsted (satellite).



Dual Technique
Each type of magnetometer has its own built in 'weakness'. This can result from the design of the magnetometer to the way the magnetometer interacts with the spacecraft, radiation from the sun, resonances, etc. Using completely different design is a way to measure which readings are the result of natural magnetic fields and the sum of magnetic fields altered by spacecraft systems. In addition each type has its strengths. The fluxgate type is relatively good at providing data that finds magnetic sources, whereas the vector helium is better at tracking magnetic field lines. Cassini spacecraft used a Dual Technique Magnetometer. One of these devices is the ring-coil vector fluxgate magnetometer (RCFGM). The other device is a vector/scalar helium magnetometer. The RCFGM is mounted 5.5m out on an 11m boom with the helium device at the end.

Magsat Earth geological satellite was also Dual Technique. This satellite carried a scalor cesium vapor magnetometer and vector fluxgate magnetometers. A satellite for similar purposes, theØrsted satellite, also used a dual technique system. The Overhauser magnetometer is situated at the end of an 8 meter long boom, in order to minimize disturbances from the satellite's electrical systems. The CSC fluxgate magnetometer is located inside the body and associated with a star tracking device. One of the greater accomplishments of the two missions, the Magsat and Orsted missions happen to capture a period of great magnetic field change, with the potential of a loss of dipole, or pole reversal.