Quite a number of satellites that can be observed are tumbling,
and this causes their brightness to vary with time. This
variation can often be seen with the naked eye, and the
frequency of the brightness peaks can range between several per
second, to once every minute or so. A good example is the
recently launched SL-16 rocket, which is currently rotating at 2
or 3 times a second. The tumbling can be started by several
mechanisms, but the most common is probably out-gasing. This
happens often to spent rockets which have some fuel left over in
their tanks once the engine shuts down. Sometimes this fuel is
vented deliberately by ground controllers to reduce the danger
of an explosion which would create many pieces of orbital debris
and pose a hazard to other satellites. Other times the fuels
leaks out gradually. Occasionally, objects will explode in
orbit, and the force of the explosion will start the resulting
pieces spinning.