Panel a: apparent bolometric magnitude (left ordinate) and observed energy flux (right ordinate) of a swarm of spherical solar sails with different radii as a function of distance from the Sun. The observer is supposed to be sitting 1 AU from the Sun. The 10σ sensitivity limit of a 10 ks (2.78 h) exposure with the MIRI instrument of JWST is indicated at 10⁻²⁰ W m⁻². Panel b: peak emission wavelength (left ordinate) and sail temperature (right ordinate) as a function of distance from the Sun. The wavelength coverage of the NIRCam and MIRI instruments of JWST are shown along the left ordinate together with the JHKL′ standard filters.