What is the average temperature of a comet?

The temperature on a comet will vary by its position in its orbit. The part exposed to direct sunlight can easily reach 100°C or higher when close to the Sun. When a comet is at its farthest from the Sun, its temperature falls to as low as 4 K (-269°C), the effective temperature of outer space.

Comets have been compared to "dirty snowballs" of frozen methane, nitrogen, ammonia and carbon dioxide, with some dust and rocks mixed in. Far from the Sun, which is where comets spend most of their time, their temperature cools to near 4 Kelvin (- 269° C). As the comet falls in toward the Sun, the Sun's radiation heats the comet unevenly, causing "outgassing" as the frozen gases of the comet are sublimated and blown out to form the tail of the comet. As soon as the comet surface warms to around 50 to 100 Kelvin (-223°C to -173 °C), gases begin to thaw and throw out material from the comet. A comet that approaches too closely to the Sun will not survive, as it will normally fragment or disappear completely.