For those who prefer to stay a little closer to home, outer space provides lights in the sky you can see from your own back garden, provided it is not too affected by light pollution. Like the aurora, meteors are a little bit of high drama from the solar system delivered direct to the Earth for our delight.
As with the world of bow shock and magnetosphere, the study of meteors involves its own vocabulary. A meteor itself is something completely intangible. In fact, it is a streak of light in the sky, although they can also be detected by radar. Indeed, the invention of radar allowed meteors coming at us from the direction of the Sun, and hence hidden by daylight, to be observed for the first time.
A meteoroid, by contrast, is the solid object whose arrival in the Earths atmosphere actually causes the flash of light. Most are about the size of the dust motes that you might spot in the air on a sunny day. But some are far bigger and if one is big enough to get to the Earths surface in recognizable form, it is put in a collection and called a meteorite.
Meteors are an atmospheric phenomenon – the name has the same root as meteorology – that tell us about the universe around us. They come in two species. One lot are those that come in “showers”. These are defined in time, maybe lasting a few weeks, or in some cases just a few hours. The rest are called sporadic and appear without any affiliation, at a rate of perhaps ten an hour on a clear, dark night. Some astronomers argue that there are no sporadic meteors, just shower members whose affiliation has not yet been identified or whose shower has become too diffuse to be observed distinctly.
What meteors demonstrate is that the solar system is not just an ordered array of planets running on predetermined tramlines round the Sun. Instead it is packed with overlapping flows of matter. Most meteor showers are caused by leftover debris scattered along the orbits of passing comets whose paths around the Sun intersect with that of the Earth. Sometimes the meteors are so faint that they are only observable with a telescope, but other showers are prominent features of the night sky. Perhaps the best-known is the weeks-long Perseid shower, seen from the northern hemisphere in August. Another favourite is the Quadrantids, a short, sharp shower, again seen in the northern hemisphere, that may be less familiar because fewer people want to sit up in the early hours of the morning in January than in August.
It is a tribute to the reach of modern science that the comets associated with all the major meteor showers have been identified. Hailey's comet even has two, one in spring and one in autumn, since it crosses the Earths orbit on its way towards the Sun and again on its way back.
Any of these showers is worth a look, and seeing one is about the cheapest form of science you can do. The big thing you need and may not have is not a telescope or a satellite, but a dark sky. This means one free from street lights and the general murk of the big city, but also from bright moonlight. If there is a full moon in the sky, you may as well go back to bed.
If not, and the sky is dark, you can expect to see ten to twenty meteors an hour if there are no major showers going. If there are, you are in for a treat. But what is all this talk of Perseids and Quadrantids? Meteor showers are named after the part of the sky that the meteors seem to come from. Imagine (but be careful doing this for real) standing on a railway line. The lines seem to diverge out from a single point in the distance. Meteors are like that – members of a particular shower are moving in parallel and so seem to come from a point in the sky called the radiant. So the Perseids have a radiant in the constellation Perseus, and so on. (The constellation Quadrans Muralis was killed off years ago and the radiant of the Quadrantids is in Bootes.)
If there is a major shower running, just sit and look. A telescope will have too small a field of view, so go for what the military term the Mark 1 Eyeball instead. Don't stare at the radiant, but a few hand spans away from it. At the peak you may be seeing a meteor a minute and, depending on the shower, they could be slow and graceful or quick and easy to miss. It is worth staying up because as the night wears on, the Earth will be turning into the meteor stream and sweeping more up.
Even better is the treat of a meteor storm like those that come from the Leonids, a November shower, every 33 years. This is when the Earth crosses the part of the orbit that has been freshly seeded with new material, and there can be hundreds of thousands of meteors an hour. These big streams also prove that the universe we inhabit is no unchanging place. The big Leonid storms (they peak in years ending 33, 66 and 99) only began to be observed in the Middle Ages. The meteor streams in space that cause them have been mapped and it may well be that they will stop intercepting the Earth in a few hundred years, depriving our descendants of this friendliest of spectaculars.
Of course, not all arrivals from space are so benign. In the past few decades, few changes in human awareness have been so striking as our acceptance that we might be abolished by meteorite impact. Part of the cause is the knowledge that the Moon is covered in impact craters. Another factor is the recognition of an increasing number of impact craters on the Earth, mainly in old, stable rocky areas like much of northern Canada. Now about 170 such structures are recognized on the Earth, including the first good candidate in the UK, spotted by oil prospectors in the North Sea below a kilometre of rock. The most visible is Meteor Crater, Arizona, a spectacular crater which also has a reassuring quantity of meteorites scattered around it. It is the biggest extraterrestrial site on the tourist map, not least because of its strategic location just off Route 66 in Arizona, although it fails to appear in the song by Bobby Troup. The biggest suggested impact is the arc on the east side of Hudson's Bay in Canada. Take a look in the atlas – it is much the biggest exact arc of a circle anywhere on the map of the world and corresponds to a radius of about 400km.
There are various ways of finding out if a suspicious crater is the result of an impact. The most obvious is to look for bits of meteorite but this only works with the smaller specimens. Big ones are too badly vaporized to leave fragments behind.
Some possible impact craters tend to show up in areas of acute geological disturbance, which does little to enhance confidence in their authenticity. However, there are some geological structures that seem to be completely characteristic of meteorite impact, such as a distinctive rocky shape called a shatter cone, formed by the intense pressure of the impact, and a type of mineral called shistovite, which is a form of the common mineral quartz which has been altered by impact pressure. Some craters such as Mistastin Lake in Canada, and many craters on the Moon, have a central peak. Bigger ones such as the Clearwater Lakes, again in Canada, start to have a succession of ring-shaped ridges in which the rock strata have been lifted bodily by the impact like a car bonnet in a head-on collision. Other symptoms include a deep layer of shattered rock (called by the Italian word breccia) within the crater itself. Because this rock is less dense than solid rock would be, gravity is detectably less strong above one – not so you'd notice, but a sensitive instrument can detect the anomaly.
Some meteorite impacts are even associated with “tektites”, one of the oddest forms of rock found in nature. They are glassy masses formed by the impact and then flung far away, with such energy that they form a “strewn field” far from the impact itself. They tend to be called after the area in which they are strewn (moldavites, javanites, etc) and have distinctive aerodynamic shapes formed during their flight. The biggest weigh a few kilograms.
Chicxulub, a massive crater in Mexico, is deservedly a household name despite its almost complete invisibility. Its discovery brought the ominous term “mass extinction” into the public imagination. It is almost certainly the crater whose creation killed off the dinosaurs and millions of other species about 65 million years ago. Exactly how the dinosaurs died out became the subject of speculation not long after they were first given that name, by British scientist Sir Richard Owen, in the 1840s. One theory was that they had died of constipation after changes in plant life. Intense radiation from a star near the Sun going supernova was another prospect.
Things changed in 1980 when a group led by the geologist Walter Alvarez started to find traces of the metal iridium at the top of the rock sequence called the Cretaceous. Rare on Earth, iridium is often associated with meteorites. It turned up in various sites around the Earth at the same place in geological time, just the point where the dinosaurs vanish. Even then, it took time to establish that Chicxulub was the right age. This 150km crater, partly extending into the Atlantic off Mexico, had already been detected by its gravity anomaly.
This all sounds very satisfying, but just how does an impact in Central America kill off the dinosaurs in Europe? That too was an active research area in the 1980s because of fears of “nuclear winter”, the prospect that the explosion of many nuclear weapons might alter the climate fatally. The basic mechanism is that the impact hurls vast amounts of dust into the atmosphere directly, and starts fires that add to the murk. The damage is done because sunlight cannot reach the surface, the temperature plummets below freezing and edible plants die off. This explains the end of the dinosaurs but also the extinction of other species too, which makes it better science than the constipation idea.
Does all this mean that the energy now being put into detecting and even diverting asteroids and comets that might strike the Earth is worthwhile? Well, maybe, and more on this in Chapter 9. The technology to do anything about an approaching NEO (Near Earth Object) is not in place yet. Maybe the hazard is tiny compared to humanity's other problems. But there is no doubt that it is real. In 1908, a comet head or perhaps a rocky meteorite struck a region called Tunguska in Siberia and devastated an area of many square kilometres. The blast was seen and heard hundreds of kilometres away. Such an impact would have killed millions if it had occurred over a major city, but it seems that on this occasion, only one person was killed, a reindeer herder thrown into a tree.