* Ice: Water ice is the most abundant component, but it also contains frozen ammonia, methane, and carbon dioxide.

* Dust: This is a mixture of rocky and metallic particles, some of which are very fine like smoke and others are larger grains.

* Organic compounds: These are complex molecules containing carbon, hydrogen, oxygen, and nitrogen.

When Halley's Comet gets close to the Sun, the ice begins to sublimate (turn directly from solid to gas), releasing dust and gas and forming the comet's iconic tail. The composition of the dust and gas in the tail can be analyzed from Earth using telescopes.

While we have a pretty good understanding of the general composition of Halley's Comet, the exact proportions of these components are still being studied and debated.