In the night sky: Orion
In the night sky: Orion

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

4.2.8 A comet’s tail

An image of the comet Hale-Bopp.
Figure 22 Comet Hale-Bopp seen over Joshua Tree National Park in California.

A comet actually has two tails, one of dust and the other of plasma (charged particles). The tails point away from the Sun, as the particles are pushed away by the solar wind.

The dust tail can extend for millions of kilometres away from the nucleus; the accompanying plasma tail is usually more than ten times longer. The curved dust tail is made of particles shed from the cometary nucleus. Sunlight reflected off the dust gives the tail a yellowish colour. The grains are less than one micron across, and are mainly made from carbon-rich compounds layered on top of silicates.

In contrast, the bluer plasma tail is made up of ions and electrons. It is a long, straight tail, the structure of which is controlled by magnetic fields within the tail and the interaction of these fields with the solar wind magnetic field.

A comet can lose up to 1% of its mass each time it passes close to the Sun – in the case of Halley’s comet, that was around 100 tonnes per second.

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371