Space Encyclopedia
☄️ Comets
Overview
Comets are icy relics from the birth of the Solar System. When one swings close to the Sun, the ice vaporizes and forms a glowing coma and one or more tails that always point away from the Sun — driven directly by the solar wind and radiation pressure.
Key facts
- •Composition: water ice, dust, frozen CO₂ and methane
- •Nucleus size: typically 1–10 km wide
- •Origin regions: Kuiper Belt & Oort Cloud
- •Tails: dust tail (curved) and ion tail (straight, blue)
- •Halley's Comet period: 76 years — next return 2061
Why it matters
Comet ion tails are a live probe of the solar wind — their shape and flutter reveal wind speed, density, and magnetic field structure. Comets also seeded early Earth with some of its water and organic molecules.
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Frequently asked questions
Why do comets have tails?
As a comet nears the Sun, solar heat vaporizes its icy nucleus. Solar wind and radiation pressure blow that gas and dust outward, forming tails that always point away from the Sun — not backward along the orbit.
When will Halley's Comet return?
Halley's Comet has an orbital period of about 76 years. It last passed perihelion in 1986 and is next predicted to return in mid-2061.
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