# Electromagnetic forces on a relativistic spacecraft in the interstellar medium

## TL;DR

This paper analyzes electromagnetic interactions affecting a relativistic interstellar spacecraft, including deflection, charge distribution, and rotation, providing insights into navigation and stability challenges for missions like Breakthrough Starshot.

## Contribution

It presents detailed calculations of electromagnetic effects on relativistic spacecraft, including deflection, charge asymmetry, and rotation, offering new insights into spacecraft stability and control in interstellar space.

## Key findings

- Interstellar magnetic fields cause measurable deflection of the spacecraft.
- The spacecraft develops an electric dipole moment due to asymmetric charging.
- Rotation induced by magnetic torques and dust impacts can be significant, with potential oscillation periods of about 0.5 hours.

## Abstract

A relativistic spacecraft of the type envisioned by the Breakthrough Starshot initiative will inevitably get charged through collisions with interstellar particles and UV photons. Interstellar magnetic fields would, therefore, deflect the trajectory of the spacecraft. We calculate the expected deflection for typical interstellar conditions. We also find that the charge distribution of the spacecraft is asymmetric, producing an electric dipole moment. The interaction between the moving electric dipole and the interstellar magnetic field is found to produce a large torque, which can result in fast oscillation of the spacecraft around the axis perpendicular to the direction of motion, with a period of $\sim$ 0.5 hr. We then study the spacecraft rotation arising from impulsive torques by dust bombardment. Finally, we discuss the effect of the spacecraft rotation and suggest several methods to mitigate it.