Satellites designed to observe the radiation from cold ($\mathbf{3}\mathit{K}$) dark space have sensors that are shaded from the Sun, Earth, and Moon and that are cooled to very low temperatures. Why must the sensors be at low temperatures?

Here, satellites are used to detect the radiation from a cold, dark space. Sensors fitted in this cold region are shaded from the Sun, the Moon, and the Earth. This will reduce the heat reaching the sensor, and thus the temperature won’t rise. These sensors are kept at low temperatures. Here you need to find out the reason behind this process.

The purpose of the satellite is to detect radiation, and this radiation will be affected by the heat transfer. The heat transfer through radiation is directly proportional to the change in the temperature of the object and its surroundings. Here, the surrounding temperature is very low ($\mathbf{3}\mathit{K}$).

If the temperature of the sensor is high, the difference in temperature will be greater and heat transfer by radiation will be greater. In order to prevent this, the sensor is cooled so the radiation can be observed smoothly because the heat transfer is very less.

So, sensors are kept at a low temperature to reduce the heat transfer by radiation.