Richard Mahoney, of Dunedin, asks :-

Why doesn't the constant shedding of debris from a comet (giving it a tail) mean that they 'wear out' in a few years, or decades, or centuries?

Duncan Steel (duncansteel.com), a Wellington-based space scientist whose first book, "Rogue Asteroids and Doomsday Comets", spawned the Hollywood movies "Armageddon" and "Deep Impact", both of which were about comets hitting the Earth, responded.

In a single passage through the inner solar system a typical comet loses mass equivalent to about a one-metre surface layer. For a large comet perhaps ten kilometres in size, like the well-known Comet Halley, a consistent reduction of mass at that rate would imply that the comet might make 10,000 such passages before it was all gone. Comet Halley’s orbit last for about 76 years, and so it could continue to follow its present path for almost a million years before it decays away to nothing.

Such a timescale estimate we call a physical lifetime. We also sometimes see comets break into fragments, and those may soon disappear, marking the comet’s death.

In fact the lifetimes of comets in the inner solar system are mostly limited by close encounters with the planets. A comet may hit a planet, or (more likely) one of the giant outer planets may gravitationally throw it onto a path taking it out of the solar system and into interstellar space, never to return. This we term a dynamical lifetime.

The closest observed passage near the Earth by any comet in history was by Comet Lexell in the late 18th century. It was diverted by Jupiter into a trajectory bringing it near our planet, but on its next approach to Jupiter it was thrown onto a much larger orbit, and we’ve not seen it since.

Comets have multiple tails. There is generally a bluish gaseous tail pointing away from the Sun, and that is called the ion tail. There is also a fan-shaped tail of small solid particles spread behind the comet’s path, and that is the dust tail. Meteor showers occur when our planet passes through such elongated dust tails. For example, each year in the first week of May and the third week of October we witness meteor showers due to tiny grains that were once part of Comet Halley.

Comets appear quite bright due to the vast clouds of vapour that surround their solid nuclei. Such a cloud we call the coma. Typically a cometary coma is about 100,000 km across, and so can reflect a lot of sunlight back to our telescopes.

Often comets are first spotted when they pass the distance from the Sun that is three times as great as the Earth-Sun separation, because at that point the flux of sunlight is sufficient to cause the ice on the surface of the nucleus to start vaporizing. The coma then rapidly develops, and the comet is easy to find. However, there are other ‘ices’ in comets – such an solid ammonia, carbon monoxide and dioxide, methane, and various other organic chemicals – which will vaporize further from the Sun, making it feasible to discover a comet when it is perhaps out near the orbits of Jupiter or Saturn.

Humankind has recorded apparitions of Comet Halley 30 times so far, since at least 240 BC. Another object named Comet Encke was not discovered until 1786, but has returned a record number of times because it takes only 40 months to orbit the Sun, never venturing outwards as far as Jupiter.