Magnetic cleanliness

Why parasitic dipoles dominate 1P attitude dynamics

In a $525\text{km}$ polar orbit ($B_{\text{pole}}\approx 47\mu \text{T}$), a parasitic magnetic dipole of just $m_{\text{res}}=10^{-3}\text{A}\cdot {\text{m}}^2$ ($1\text{mA}\cdot {\text{m}}^2$) produces a disturbance torque $T_m=m_{\text{res}}B\approx 47\text{nN}\cdot \text{m}$—more than a hundred times the gravity-gradient torque on a $1\text{P}$ cube and comparable to the output of a PCB magnetorquer coil[1]. Worse, on a spacecraft with passive magnetic attitude control (PMAC), an uncompensated transverse dipole tilts the equilibrium axis by $\Delta \theta =\arctan (m_{\text{res},\perp }/m_{\text{PMAC}})$[2].

Altitude (km)	Gravity gradient (N·m)	Aerodynamic (N·m)	Solar radiation (N·m)	Magnetic residual (N·m)	Dominant source
320	2.98 × 10⁻¹¹	1.32 × 10⁻⁸	5.67 × 10⁻¹¹	7.93 × 10⁻⁸	Magnetic residual
400	2.88 × 10⁻¹¹	2.45 × 10⁻⁹	5.67 × 10⁻¹¹	7.65 × 10⁻⁸	Magnetic residual
500	2.76 × 10⁻¹¹	3.64 × 10⁻¹⁰	5.67 × 10⁻¹¹	7.32 × 10⁻⁸	Magnetic residual
525	2.73 × 10⁻¹¹	2.34 × 10⁻¹⁰	5.67 × 10⁻¹¹	7.24 × 10⁻⁸	Magnetic residual
600	2.64 × 10⁻¹¹	6.72 × 10⁻¹¹	5.67 × 10⁻¹¹	7.01 × 10⁻⁸	Magnetic residual
700	2.53 × 10⁻¹¹	1.55 × 10⁻¹¹	5.67 × 10⁻¹¹	6.72 × 10⁻⁸	Magnetic residual

Eliminating current loops and hard-magnetic hardware

1. Solar array back-wiring: A solar string carrying $I=150\text{mA}$ around the perimeter of a $42\times 42\text{mm}$ panel encloses $A=1.76\times 10^{-3}{\text{m}}^2$, generating $m=IA=0.26\text{mA}\cdot {\text{m}}^2$ per face. Routing the return trace on the inner PCB layer directly underneath the cell interconnect path cancels the enclosed area[2].
2. Battery cans and nickel tabs: Cylindrical 18650 steel cans and pure-nickel spot-weld strips retain remanent magnetization; pouch cells with aluminium–polymer laminate pouches and copper/aluminium tabs have orders of magnitude lower remanence.
3. Fasteners and RF connectors: Cold-worked 304/A2 stainless screws become partially martensitic and magnetic. Specify Ti-6Al-4V, passivated A4-80 (316) austenitic stainless steel or non-magnetic brass SMA/MMCX connectors, and screen every mechanical part with a fluxgate or 3-axis AMR magnetometer before assembly[3].