- What happens when one approaches the Sun?
- The ice begins turning straight to gas and drags dust out with it. The nucleus grows a coma — a temporary atmosphere that can be wider than the Sun — and behind that the tails. Far from the Sun there is nothing to see but a dark few kilometres of ice.
- How long is a comet's orbit?
- From a few years to millions, and for some there is no answer at all. Short-period comets come from the Kuiper belt beyond Neptune and return within about two centuries; the long-period ones fall in from the Oort cloud, and one of their orbits can take millions of years. A few hundred here are on paths that never close, most of them the Sun's own comets flung outward by a close pass with a planet — and two that were never the Sun's to begin with. The third interstellar visitor found so far, 1I/'Oumuamua, passed through without ever growing a coma, so it is filed with the asteroids.
- How bright will a new comet get?
- Almost nobody can say in advance. Brightness depends on how much volatile material is left near the surface, which cannot be measured from Earth, so predictions made a year out have been wrong by several magnitudes in both directions.
- Where do meteor showers come from?
- Almost always from comets. A comet leaves a stream of dust along its orbit, and when the Earth crosses that stream the grains burn up in the atmosphere. The Perseids come from Swift-Tuttle; the Orionids and the Eta Aquariids are both Halley. The December Geminids are the exception: their parent, Phaethon, is catalogued as an asteroid — either a dead comet or a rock that sheds dust when the Sun bakes it.
- What is a comet made of?
- Water ice mostly, with carbon monoxide and dioxide, ammonia and methane, a great deal of dust, and complex organic molecules. Rosetta found that the water on comet 67P carries a different deuterium ratio from the Earth's oceans, which argues that comets did not deliver most of our water.
- How often is a comet visible without a telescope?
- Roughly one a decade becomes genuinely striking, though several a year are within reach of binoculars. Predicting brightness is notoriously unreliable, because it depends on how much ice a particular nucleus still has and how it behaves when heated — comets have both fizzled and outperformed forecasts by large factors.
- What is the difference between a comet and an asteroid?
- Composition and behaviour rather than a hard line. A comet holds enough volatile ice to produce a coma when heated; an asteroid does not. The boundary is genuinely blurred — some objects have been reclassified after unexpectedly growing a tail, and some comets appear to have exhausted their ice and now look like asteroids.
- When does Halley's Comet come back?
- In 2061. It has a period of about 76 years and last passed close to the Sun in 1986; it is currently beyond Neptune's distance and heading back in. Its returns have been recorded for more than two thousand years, and it was the first comet shown to be a repeat visitor rather than a one-off.
- Do comets run out?
- Yes. Every close pass strips away some of the ice, so a short-period comet has a limited number of returns before it goes dormant or breaks up. Several have been observed disintegrating. The debris streams they leave behind are what the Earth runs through each year as a meteor shower.