  We study the drag force on dust grains moving at relativistic velocities
through interstellar gas and explore its application. First, we derive a new
analytical formula of the drag force at high energies and find that it is
significantly reduced compared to the classical model. Second, we apply the
obtained drag force to calculate the terminal velocities of interstellar grains
by strong radiation sources such as supernovae and active galactic nuclei
(AGNs). We find that grains can be accelerated to relativistic velocities by
very luminous AGNs. We then quantify the deceleration of relativistic
spacecraft proposed by the Breakthrough Starshot initiative due to gas drag on
a relativistic lightsail. We find that the spacecraft's slowing down is
negligible because of the suppression of gas drag at relativistic velocities,
suggesting that the lightsail may be open for communication during its journey
to $\alpha$ Centauri without causing a considerable delay. Finally, we show
that the damage to relativistic thin lightsails by interstellar dust is a minor
effect.
