## 4 Magnetic sensors for space missions. The implementation of magnetic COTS in spacecrafts  
As it has been introduced, the history of the magnetic sensors for space starts with the Soviet Sputnik-3. Sputnik-3 was a 1.3 ton satellite devoted to researching the upper atmosphere and near space. The onboard instrumentation contained the first vector magnetometer: a three axis fluxgate magnetometer, which however did not succeed in the determination of the direction of the Earth magnetic field due to the uncertainty in the attitude of the spacecraft. Triaxial fluxgate magnetometers have been widely used for monitoring the magnetic fields of the Earth and Moon (Luna 1, Luna 2, Pioneer Venus, Mariner 2, Venera 1, Explorer 12, Explorer 14, and Explorer 15). Explorer 33 already foresaw a boom for the magnetometer to avoid in part the contribution of the spacecraft to the total magnetic field in the orbit of the Moon. But this purpose was not achieved until Explorer 35 entered a stable orbit around the Moon. Fluxgates have evolved with magnetic space missions [3]. The fluxgate technology has been improving simultaneously with the techniques of magnetic field sensing and now fluxgates are still the sensors that present the best magnetic properties for the measurement of the geomagnetic field. It has to be taken into account that an increase in the knowledge about the magnetic field of the Earth only can be achieved by means of very fine measurements since actual models of the field already reproduce the contribution of the core, the crust and the ionospheric phenomena to the global field, thanks to the data provided by POGO-Polar Orbiting Geophysical Observation missions (1967-1971), Magsat (1979), Astrid-2 (1998) [56], SAC-A (1998), \(\O\)rsted (1999), Komsat (1999), CHAMP (2000), ST-5 (2006) [15], etc. It cannot be said that geomagnetic missions have generally followed the trend of the modern concept of small satellites, as almost all of these missions were medium-to-large satellites with Astrid-2 (with a mass of 30 kg) being the smallest amongst them. Besides, apart from the weight of the sensor, often it has to be taken into account the weight of the deployment system. MAGION-1, the first Czech MAGetopheric and IONospheric satellite launched in 1978, and devoted to doing research on the magnetosphere and the ionosphere could have been considered an exception due to its smaller weight of 15 kg. This prism-shaped satellite of 300 x 300 x 150 mm\({}^{3}\) also carried a vector fluxgate magnetometer for Earth magnetic field measurements. Regrettably, this mission was not very successful since a failure in the power system resulted in limited data for a certain period. It re-entered the atmosphere on September 11\({}^{\mathrm{th}}\), 1981 [57]. Nevertheless, the magnetometers for other on-board applications have been deeply affected by this trend towards miniaturization. In many missions the big box or boxes for the fluxgates (in most of the cases the sensor head is located in a different box than the front-end electronics) with a power consumption in the order of the Watt, are being replaced by smaller sensors that comply with the magnetic requirements. Actually, in some applications small sensors may be the better choice since they can be easily disposed in array [42] with temperature sensors for temperature compensation, thereby reaching a better trade-off between magnetic performance and spatial resolution [58]. This is illustrated for instance by the case of the LISA (Laser Interferometer Space Antenna) mission of ESA-European Space Agency. LISA is aimed at detecting gravitational waves generated by black holes. The measurement is to be performed by two test-masses floating inside a cavity in one of the Lagrange points, where the gravitational attraction by the Earth and the Sun are compensated and there is a nearly perfect gravitational free-fall. To do so LISA needs to be capable to distinguish among gravitational forces and other type of forces.