Endnotes

1. Historically, we find that a general breakdown of the natural order is attributed to a comet's appearance. Comets were often said to predict the end of the world. In 1066 Halley's comet was so bright that it terrified millions of Europeans and was credited with the Norman victory at the Battle of Hastings. This battle is depicted in the famous Bayeaux tapestry with the comet overhead. On a more trivial note, many citizens reported unusual animal behaviour, including enormous hens' eggs that bore the image of the comet or were covered with stars. More serious and plausible is the current theory that a comet's or a meteor's encounter with Earth caused the extinction of the dinosaurs.

3. Most comets have highly eccentric orbits which take them far beyond the orbit of Pluto; these are seen once and then seemingly disappear, perhaps for milennia. Others, the short-and-intermediate-periodic comets (like Comet Halley), stay within the orbit of Pluto for a significant fraction of their orbits and are seen more frequently.

4. In 1950 Jan Oort, a Dutch astronomer, proposed that there was a huge reservoir of comets which could orbit the sun but whose orbits took them far beyond those of the planets, to the edge of the solar system. This Oort cloud may consist of billions of small, solid nuclei of comets.

5. The Roche limit is the closest distance a body can come to a planet without being pulled apart by the planet's tidal (gravitational) force.

6. Members of the Heaven's Gate cult believed that the arrival of Comet Hale-Bopp
signalled the end of the world, and that in its wake was a spaceship that would scoop up their souls to take them to the " real Kingdom of Heaven", situated out in the galaxy.
Thirty-nine American cult members committed suicide. Interestingly, this resembles some early explanations that a comet was really an angel coming to gather up the souls of the dead so that they could be reincarnated.

7. The general consensus is that after numerous passes near the sun, a comet will
evaporate. One theory is that after most of a comet's ice and gas is lost, a rocky object, much like an asteroid, is left behind. So, many asteroids might be " dead comets".

8. One scientist predicts that Halley's will disappear through evaporation after about 3000 more revolutions. Giotto managed to come within 600 kilometres of Halley's nucleus.

9. The mission is named after the Rosetta Stone, a second century B.C. stela inscribed in three scripts. The great significance of the Stone is that it provided the key to deciphering Egyptian hieroglyphs.

10. This comet was discovered by Carl A. Wirtanen of the Lick Observatory, California. It is a periodic comet and has been observed at intervals of 5-7 years.

11. Gravity assists involve the use of a planet's gravity during a spaceship or satellite flyby to help " slingshot" it further into space. As the spacecraft approaches the planet, its velocity increases, while that of the planet decreases slightly (by a billionth of an inch). The assist saves fuel. Some spacecrafts, such as Galileo, looped around the Earth twice to receive an additional boost.