Halley's Tail — Comet Tail Prediction Game
Why does a comet's tail always point away from the Sun?
This mini-game places Halley's Comet on its real, highly elliptical orbit (semi-major axis 17.8 AU, eccentricity 0.967, period about 76 years) and asks you to aim its tail. Contrary to intuition, a comet's tail does not trail behind it like a rocket's exhaust. The solar wind and the Sun's radiation pressure blow the tail straight away from the Sun, no matter which way the comet is travelling.
Because of that, the tail behaves differently on each part of the orbit. On the inbound leg toward the Sun the tail trails behind the comet's motion; near perihelion (closest approach, about 0.59 AU) the comet is most active and the tail is longest and brightest; and on the outbound leg the tail actually leads the comet, pointing ahead of its direction of travel.
- Ion tail: straight and blue, pushed anti-sunward by the solar wind.
- Dust tail: yellow-white and gently curved, lagging along the orbit under radiation pressure.
- Perihelion distance: about 0.59 AU — inside the orbit of Venus.
- Aphelion distance: about 35 AU — beyond the orbit of Neptune.
- Halley's orbit is retrograde: it circles the Sun opposite to the planets.