No other satellite varies in brightness as spectacularly as an Iridium one, but most satellites change in brightness as you watch. If you observe carefully, you might see that the variations occur with a repeated pattern, on time scales from several times a second up to once a minute or even longer.

What's going on here? The satellite is tumbling, or spinning, and as different parts of the satellite face the sun, they reflect different amounts of light, thus the satellite brightness varies.

The period of fluctuations is the time it takes the satellite to spin once. The rate is influenced by Earth's magnetic field. Unexpected changes in this period might be a warning of unusual developments, such as a fuel leak.