We calculate the surface temperature and the resulting brightness of sub-relativistic objects moving through the Solar system due to collisional heating by gas and radiative heating by solar radiation .
The thermal emission from objects of size \gtrsim 100 m and speed of \gtrsim 0.1 c , can be detected by the upcoming James Webb Space Telescope out to a distance of \sim 100 au .
Future surveys could therefore set interesting limits on the abundance of fast-moving interstellar objects or spacecraft .