Electromagnetic forces on a relativistic spacecraft in the interstellar
  medium
  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.
