[URSpider94]

Specifically, it needs to absorb/emit in a frequency range called the “sky window”, where the atmosphere is transparent. In this range, the object is basically seeing the cold of space at a few Kelvin, so that almost no energy is absorbed and a lot is emitted.

The trick is to design a system where the total power absorbed across the solar irradiance spectrum is less than the power that is emitted in the sky window, so that the system is a net power sink (in this case to the tune of 100W per sq. meter or so).