Sources of Differential Voltage Across Bearings  The rotating flywheel is nominally electrically floating relative to the reaction wheel housing when the  wheel is spinning fast enough to develop a full EHD film. The only electrical path from housing to flywheel  is through the ball bearings and this path must flow through the lubricant film. High frequency switching of  motor currents has been shown to induce a voltage of 1-2 volts to a reaction wheel rotor relative to  ground when it is operating on an EHD lubricant film. This is a well-established and confirmed source of  differential across the bearings, which has been measured directly. Since the 1-2 volt electrical discharge  typically results in a smoother surface finish, the source of higher voltage transients is more important for  explaining friction anomalies associated with electrical discharge.  Motor-Induced Voltage  The 1-2 volts motor-induced voltage measured on a typical RWA was during steady state operation. A  higher voltage could be present when applying full torque, or with larger motors. One example of  anomalous friction occurred after a spacecraft went out of control and tumbled for a day or two. While  tumbling, the attitude control system went unstable and commanded an RWA motor to full torque in  alternate directions at a frequency of several cycles per second. When control of the spacecraft was  regained, the dry friction torque of the RWA had clearly increased. At the time, there was no known  connection between the motor and the bearings. The wheel spent some time near zero speed while  tumbling, but it had spent years of operation near zero speed with no detrimental effects. In hindsight, the  repeated high current oscillation in the motor likely resulted in higher than normal voltage electrical  discharge, which likely affected the bearing surface finish.   Ion Engine Charging  Ion engines can quickly change the ground potential of a spacecraft during on/off cycling and affect the  local plasma and charging environment. Out of 7 interstellar spacecraft using ITHACO reaction wheels,  only two had reaction wheel failures, specifically NASA’s DAWN and JAXA’s Hayabusa, and both  experienced multiple reaction wheel failures. When it is considered that only these two spacecraft had ion  engine propulsion, a Probability of Correlation of 99.8% between the ion engine and reaction wheel failure  results. While the mechanism for coupling of the ion engine operations into a voltage differential across  the bearings is not understood, the statistics cannot be ignored, and ion engine use should therefore be  considered as a potential cause of electrical discharge.  Space Charging  Space charging as a source of electrical discharge is supported in Reference 2 by a rock-solid correlation  of reaction wheel friction events with major geomagnetic storms. The failures and anomalies of the RWAs  on NASA’s Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft have a probability of correlation  greater than 99.999% to geomagnetic storms ranked in the top 20 since 1994. It is therefore nearly only a  one in a million chance that the events and the geomagnetic storms are only related randomly, or nearly  100% certainty that they there is a correlation. This strong correlation has also been confirmed in larger  data sets of RWA friction anomalies and failures, and a significant number of additional simultaneous  anomalies on different spacecraft have also been observed.   It is important to note that correlation does not prove causation. In the case of the space plasma  environment and mechanisms there are no known parallel or related potential causes other than the  potential effects of space charging or charged particle radiation. We therefore still regard space charging  as a likely source of differential voltage, and have used the correlations only to conclude that the root  cause of friction anomalies is related to the space environment, which could also include effects of  charged particle radiation upsets in the motor driver electronics producing a differential across the  bearings.  NASA/CP—2018-219887 61