Three asteroids of different sizes

Creator: wise.ssl.berkeley.edu

The top of the chart shows how three asteroids of different sizes and different compositions might appear in visible light. A small asteroid that has a shiny or highly reflective surface appears just as bright in visible light as a large, dark, asteroid, so you cannot determine the size of an asteroid simply by knowing how bright it is in visible light.

The bottom of the chart shows what the same three asteroids would look like when viewed in the infrared. Now their brightness clearly depends on their size, and not on how shiny or reflective they are, because the brightness of an asteroid depends on how much heat energy they’re giving off, not on how much energy they’re reflecting. Bigger asteroids have bigger surface areas, so they give off more heat.