  The Breakthrough Starshot Initiative aims to send a gram-scale probe to
Proxima Centuri B using a laser-accelerated lightsail traveling at relativistic
speeds. Thermal management is a key lightsail design objective because of the
intense laser powers required but has generally been considered secondary to
accelerative performance. Here, we demonstrate nanophotonic photonic crystal
slab reflectors composed of 2H-phase molybdenum disulfide and crystalline
silicon nitride, highlight the inverse relationship between the thermal band
extinction coefficient and the lightsail's maximum temperature, and examine the
trade-off between the acceleration distance and setting realistic sail thermal
limits, ultimately realizing a thermally endurable acceleration minimum
distance of 16.3~Gm. We additionally demonstrate multi-scale photonic
structures featuring thermal-wavelength-scale Mie resonant geometries, and
characterize their broadband Mie resonance-driven emissivity enhancement and
acceleration distance reduction. Our results highlight new possibilities in
simultaneously controlling optical and thermal response over broad wavelength
ranges in ultralight nanophotonic structures.
