Relativistic Gas Drag on Dust Grains and Implications

TL;DR

This paper derives a new formula for relativistic gas drag on dust grains, applies it to astrophysical and spacecraft contexts, and finds that relativistic effects significantly reduce drag, allowing high-speed travel with minimal deceleration or damage.

Contribution

It introduces a novel analytical expression for relativistic gas drag and demonstrates its implications for dust acceleration, spacecraft deceleration, and damage assessment at high velocities.

Findings

Relativistic gas drag is significantly lower than classical predictions.

Relativistic grains can be accelerated by luminous AGNs.

Spacecraft experience negligible deceleration due to gas drag at relativistic speeds.

Abstract

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.