TOPICAL SCIENCE August 2017
Comets and Meteor Showers
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More about Comets
Viewing the Persieds
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The Perseid Meteor Shower During the month of August each year, the Perseid Meteor Shower occurs and is visible to viewers in the Northern Hemisphere. This particular shower is named after the constellation Perseus, from which the meteors seem to originate. However, in actual fact, meteors have nothing to do with constellations. The stars, which make up the constellations, are many light-years away, while meteors occur within the atmosphere of our planet Earth. Meteors are sometimes referred to as ‘shooting stars’, or ‘falling stars’, because they do look like stars shooting across the night sky, but they are not stars at all. In fact, they are tiny particles of dirt and debris passing through the earth’s atmosphere at great speed. Some of the particles are as small as grains of sand, while others can be much larger. Because of their speed, they become very hot, as they compress and heat the air in front of them. They glow brightly for a short period, before burning up completely. Each meteor ionises molecules in the air as it goes, causing a bright streak of light for a few seconds. The brightest meteors can be seen with the naked eye and a large meteor shower can be a spectacular sight.
How to observe a meteor shower. Wait for a clear night, with few clouds. Try to find a place well away from the light pollution of street lighting. Allow time for your eyes to become dark adapted. Please note that in 2017, the moon is three-quarters full around the time of the Perseid meteor shower, so the sky will not be quite dark.
COMETS AND METEOR SHOWERS
Meteor showers occur when the Earth, in its annual orbit around the sun, enters the trail left behind by a comet. The Perseid meteor shower is caused by the debris from the passage of the comet Swift-Tuttle. Comets are usually named after the person who discovered them. The comet, which gives rise to the Perseid meteor shower, is known as Swift-Tuttle. It was discovered, independently, in July 1862, by two observers, Lewis Swift and Horace Parnell Tuttle. It has a period of 133 years. One of the most famous comets, Halley’s Comet, has a somewhat shorter period of 76 years. Lewis Swift observed Halley’s comet twice in his lifetime Following its discovery, the comet Swift-Tuttle was next observed in 1992 and is expected to be visible from Earth again in 2125. However, even though the comet itself is not often seen, the Earth enters its trail every year. It has left quite a wide trail of debris, so that it takes earth a few weeks to pass through it. Because of this, meteors are visible from around the middle of July to late August, with the peak in 2017 around August 12th. Unfortunately, the waning gibbous moon will ensure that the sky is not quite dark this year at the time of the peak meteor activity. Nevertheless, it should be possible to see some meteors if the sky is clear and you are well away from the ‘light pollution’ of street lighting. 2
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More About Comets Comets are rather strange members of the solar system. They travel in extremely elongated elliptical orbits, coming from far beyond the outer planets and then zooming in to swing around the sun before making their way out into the distant reaches of the solar system again. A typical comet is much smaller than a planet, maybe about the size of a large mountain. It is made mainly of ice, with rocks and stones embedded in it. The frozen water may be mixed with other frozen substances such as methane or ammonia, which would be gases at normal temperatures on earth, but which exist in the solid state in the extremely low temperatures of the outer solar system. As a comet approaches the sun, it reflects the sun’s light and becomes visible as a hazy blob. Some of the ice begins to melt and vaporize, releasing streams of gas, dust and debris that form a long tail, which may be visible to the naked eye. The tail of a comet is made up of glowing gas and it becomes visible when the comet is near the sun. It appears to be ‘blown’ away from the sun by the charged particles in the so-called ‘solar wind’ and always appears on the side of the comet farthest from the sun. Each time a comet swings by the sun, some of the dust, pebbles and rocks that are embedded in the ice are released. These particles of solid debris are called meteoroids and they form a long trail (not to be confused with the luminous ‘tail’) that remains in the wake of the comet long after the comet itself has disappeared. When the earth’s orbit intersects this trail, meteoroids enter earth’s atmosphere and begin to glow and become visible. It is at this point that they are referred to as meteors. Most meteors are minute grains of sand that burn up very quickly in earth’s atmosphere. A smaller proportion is made up of larger pebbles, which take a little longer to burn. Only a very small number of the larger chunks of rock from a comet may survive the transit through the atmosphere and land on the surface of the earth. Those that do land are called meteorites and they can vary in size.
Meteorites Most meteorites are made of silicate rock, but they are not easy to find. Only about 6% of meteorites are made of iron, but these are the kind more often found, as they can be located with the help of metal detectors. They are also relatively easy to spot, even without a detector, as they can be distinguished from local rocks. Some of the largest meteorites make an impact crater on landing. The Barringer crater in Arizona has a diameter of 1.4 kilometres and a depth of 170 metres, It was probably caused by a meteorite of about 150,000 tons. There is no trace of the meteorite itself, which would have been vaporised by the enormous heat generated by the impact. 3
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Meteor Showers and Stray Meteors. Meteor showers can be a wonderful sight on a clear night. But not all meteors come from comets. There is quite a lot of random dust and rock flying around in interplanetary space and if any of these isolated bodies enters the earth’s atmosphere, it will become a sporadic meteor. The length of a meteor’s path across the sky depends on a number of factors, including the speed at which it travels and the angle at which it enters the earth’s atmosphere. If a meteoroid comes in at a low angle, it heats up more slowly and travels a greater distance before being burnt up than if it comes in at a steep angle. The size, composition and density of the particle also have an effect. The fastest meteors are the Leonids, which occur in November, arriving at speeds of about 70 kilometres per second. The Perseids travel at about 60 kilometres per second. The Lyrids, which occur in April, arrive at a more moderate speed of about 40 kilometres per second.
Sky Map showing Perseid radiant
Where to look to see the Persieds As they come from a distance, the meteors travel in parallel paths, but because of perspective, they appear to originate from a point in the sky, known as the radiant. The radiant of the Perseids is located near the double cluster, at the northern tip of the Perseus constellation. The ‘W’ shaped Cassiopeia is also close by, so if you can spot that, it may help you to find the radiant. But you don’t really need to look anywhere in particular, as it is possible to see meteors if you simply look up towards the zenith. About the Author Margaret Franklin. M.Sc., FICI, is the author of this ‘Topical Science’ newsletter. She spent most of her career at Athlone Institute of Technology, in the Republic of Ireland, where she taught chemistry for over 40 years. Margaret now writes popular science articles for the information of the general public. 4