Satellites in low Earth orbit (LEO) move through a plasma environment made up of electrons and ions, while also being exposed to solar particles and cosmic radiation.
These interactions can cause spacecraft surfaces to accumulate charge, creating a voltage difference (ΔV) between the spacecraft and the surrounding plasma.
Because most metallic spacecraft components are tied to the negative terminal of solar panels, ΔV is typically negative. When its magnitude exceeds roughly −100 volts, the likelihood of electrostatic discharge increases.
Such electrostatic discharge events are thought to be a leading cause of satellite electronics failures. Despite this, direct measurements of spacecraft charging remain limited, especially for small satellites.
Gaps to Address in Existing Spacecraft Sensors
Existing spacecraft potential instruments frequently achieve over 1 V accuracy. However, such systems, like floating potential measurement units and double-probe instruments, require metre-scale structures or extended wire booms. These bulky structural parts make them impractical for CubeSat-class spacecraft.
Smaller surface electrometers do exist, but they rely on semiconductor conduction, a mechanism that can degrade under space radiation and become less reliable as they are further miniaturized.
The newly demonstrated sensor takes a different approach by reading voltage optically.
Optical Voltage Measurement
The sensor is built around a silicon photonic crystal waveguide, fabricated via CMOS-compatible processes onto a 225-nanometre-thick silicon layer.
When the spacecraft develops a negative ΔV, positive ions from the surrounding plasma are drawn toward the exposed silicon surface.
These ions generate holes – positive charge carriers – in the silicon. The carrier density increases in proportion to the magnitude of ΔV.
As broadband light centred at 1550 nanometres passes through the waveguide, some of the signal is absorbed by free carriers via free-carrier absorption. The resulting drop in transmitted light intensity provides a direct measure of ΔV.
The sensor's optical response closely followed theoretical predictions in vacuum chamber experiments that simulated LEO-like conditions. With a plasma density of around 5.15 × 1011 m-3 and plasma potential near 17 V, |ΔV| values were approximately 67 V.
At −200 volts, the device showed about 25 % attenuation of transmitted light, corresponding to a hole density of approximately 5.6 × 1017 cm-3. That is well below silicon’s structural integrity limit of roughly 1 × 1020 cm-3, suggesting the material can tolerate quite substantial charging without degradation.
Withstanding Discharge Events
During repeated −200-volt tests, brief discharge spikes occurred inside the chamber, but the module remained undamaged.
Because the sensing mechanism depends solely on optical absorption, not changes in electrical conduction, the silicon waveguide contains no p-n junctions, which are typically vulnerable to radiation damage.
This design is expected to improve resistance to both electrostatic discharge and space radiation.
The projected total system power requirement is about 0.1 watts, with each sensor module needing less than 0.1 milliwatts of excitation light. In principle, multiple modules could share a single light source through fibre splitting.
Optical Measurement is Still Far from Reality
The researchers stress that conventional probe systems still offer superior absolute voltage precision.
Large instruments can measure spacecraft potential to better than 1 volt, while the photonic sensor prioritizes compactness and strength over high-precision accuracy.
The experiments also revealed response asymmetry. As voltage steps moved toward more positive values, recovery was slower due to charge redistribution and Coulomb repulsion.
Module-to-module baseline variations were also observed, revealing the need for calibration.
Temperature presents another constraint. Free-carrier absorption increases significantly with chip temperature, meaning in-orbit deployment would require thermal monitoring and compensation.
Based on xenon and argon plasma tests, the team estimates that performance in real LEO conditions could reproduce laboratory behaviour with roughly 70 % agreement.
The Sensor Will be Useful in Smaller Satellites
The device was also shown to detect positive ΔV conditions, where free-carrier absorption is driven by electrons rather than holes.
Future refinements may include surface passivation, adjustments to improve plasma interaction geometry, and longer waveguides to enhance sensitivity below the ~67 V regime where theoretical agreement weakens.
The photonic sensor is a compact alternative that could be deployed on small satellites or distributed across larger spacecraft, but it is unlikely to replace high-precision spacecraft charging instruments.
As satellite constellations expand rapidly in low Earth orbit, improved monitoring of charging conditions may help reduce electrostatic discharge-related failures and strengthen the reliability of growing space-based infrastructure.
Journal Reference
Otsuka, K. et al. (2026). Compact potential sensor for spacecraft based on a silicon photonic waveguide. Npj Nanophotonics, 3(1), 10. DOI: 10.1038/s44310-025-00100-6
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