Comets do not give off their own light but reflect light from other sources, such as the Sun  Scientists estimate the mass of comets by their brightness: the greater a comet' s mass, the more light that comet will reflect
A satellite probe, however, has revealed that the material of which Halley' s comet is composed reflects 60 times less light per unit of mass than had been previously thought
Previous estimates of the mass of Halley's comet which were based on its brightness were too low.