Early one morning, well over a century ago, much of the population of the eastern United States was entranced, mystified and decidedly spooked by a dazzling display of shooting stars that all seemed to originate from the same point. They didn’t have modern media to get the word out; nevertheless, they did pretty well. Most provincial newspapers carried accounts of this monumental meteor storm – perhaps the most active in recorded history, with 30 or 40 meteors raining down from the sky every second. It took place in the early morning of 13 November 1833, and was documented over the following three years by Denison Olmsted, Professor of Mathematics and Natural Philosophy at Yale College.
Olmsted’s study was almost as monumental as the storm itself. His careful research was published in the American Journal of Science and Arts, and included many first-hand accounts, which were often very detailed. And it’s Olmsted we have to thank for figuring out what was going on in this event. Because the myriad ‘streams of light’ seemed to diverge from a single point, he realised that objects were entering the Earth’s atmosphere on parallel tracks. Perspective gave the impression that they originated in the constellation of Leo, which was high in the eastern sky before dawn. Olmsted interpreted this as being due to Earth passing through a dense cloud of particles, which themselves would have had a common motion through space.
We now know that this explanation is correct. The particles Olmsted surmised are specks of space dust or tiny stones, not much bigger than an orange pip, that hit the upper reaches of the Earth’s atmosphere at high speed. They are instantly vaporised by the heat generated, and shine brilliantly for a few tenths of a second as they shoot across the sky. While they shine, they are known as meteors, a word originating in the 16th century to mean any atmospheric phenomenon, but now applied specifically to what are commonly called shooting stars. So – what is a meteor before it hits the atmosphere? Ah, there’s a word for that, too: the slightly unfortunate meteoroid. And just to complete the trio, a meteorite is a meteor whose larger size allows it to survive its flight through the atmosphere, and reach the ground.
IN METEOR SHOWERS, THE PARTICLES ARE RELEASED FROM comets – ‘dirty snowballs’, a few kilometres across, which are icy remnants of the cloud of gas and dust from which the Solar System formed. They orbit the Sun in highly elongated paths, which often stretch well beyond the orbits of the planets. Comets become visible – and sometimes very prominent – when they reach perihelion, the point in their orbit when they are closest to our star. Here, the frozen gases that bind them together evaporate (or, more correctly, sublime) in the Sun’s radiation, and can form bright tails of dust and gas.
Not surprisingly, comet orbits are littered with dusty debris thrown off during their visits to the inner Solar System. Clumps of debris share the orbit of the parent comet, and, during the Earth’s annual tour around the Sun, our planet passes through a succession of these dust trails from a variety of different comets. The result is a well-established calendar of meteor showers, each of which appears to diverge from a point known as its ‘radiant’. And each shower is named after the particular constellation containing the radiant – rather comically, with ‘-ids’ stuck on the end. If the Earth’s passage through the comet’s orbit happens to coincide with a particularly dense clump of dust, then the meteor shower becomes a rare meteor storm – as happened in 1833, with the meteors known as the Leonids.
It was more than three decades after the 1833 meteor storm that the Leonids’ parent comet was discovered. Around Christmas time in 1865, two astronomers by the names of Wilhelm Tempel and Horace Parnell Tuttle independently discovered a comet with an orbital period of 33 years. It reached its perihelion in 1866. And in Europe, in the November of that year, there was another impressive display of Leonids meteors. At two or three meteors per second, it was nowhere near as spectacular as the 1833 event, but still represented a remarkably rich meteor shower. The two following Novembers also saw good displays.
I’m sure you will have quickly done the arithmetic here, and noticed that the 1833 meteor storm must also have coincided with the perihelion passage of Comet Tempel–Tuttle, when the comet was relatively close to Earth. Clearly, these showers suggested that the dust in the comet’s orbit was concentrated close to the comet itself. And sure enough, while 1899 and 1933 produced nothing out of the ordinary, in 1966 there was a meteor storm that, in the Americas, rivalled the 1833 display. Eyewitness accounts speak of ‘a blizzard of meteors’.
By then, radar observations of meteors were possible, allowing astronomers to measure details of the Leonid dust particles. They turned out to be mostly lighter than average (around 0.01 grams) and burned up rather higher than average (at heights greater than 100 kilometres). The intersection of the orbit of the meteoroid stream with that of Earth results in an atmospheric entry speed of 72 kilometres per second, the fastest known. In the 1980s and 90s, further studies of the historical Leonid showers allowed astronomers to determine that most of the Leonid meteoroids trailed behind the comet, and slightly outside its orbit.
Investigations by a number of scientists, including my former colleagues at Siding Spring Observatory, David Asher and Robert McNaught, allowed them to map the dust density in the meteoroid cloud, which meant they could predict the Leonid meteor activity in 1999 and the first few years of the new millennium. Sure enough, there were good displays in 1999, 2001 and 2002, some of which I witnessed during my routine observing at Siding Spring. And Asher and McNaught’s more recent work has refined the simplistic picture of a comet being accompanied by a few clouds of dust to discover exactly which perihelion passages of the comet have resulted in particular meteor displays. The 1833 meteor storm, for example, was caused not by that year’s perihelion passage of Comet Tempel–Tuttle, but by the dust trail ejected by the comet during its previous visit in 1800.
Today, predictions of intense meteor storms are of interest not just because of their visual appeal, but because of the risk to all that essential high-tech infrastructure orbiting our planet. How would satellite communications have fared in 1833, I wonder?
MOVING ON FROM THE LEONIDS, IT’S EASY TO FIND ONLINE information about the other meteor showers that punctuate the year, along with their progenitor comets. The Orionids, for example, which are visible from anywhere in the world in late October, originate from the dust trail of the most famous comet of all – Comet Halley. And a favourite of mine, the Geminids, peak on 14 December – the birthday of the great 16th-century Danish astronomer, Tycho Brahe (oh, and it’s mine, too). They are highly unusual in that their parent body is not a comet, but a dusty asteroid by the name of 3200 Phaethon.
Observing a meteor shower is something anyone can do. It’s interesting because, while a Leonid-like storm is unlikely, you never quite know what’s going to happen. And it’s easy because you don’t need any technical equipment other than your own patience. Shower meteors can flash across any part of the sky, so there’s no point in using binoculars or a telescope. The only thing that marks them out as members of the shower is that they seem to have come from the constellation after which they are named. Gemini in December, for example.
So, the best observing accessories are warm clothing, a cup of hot chocolate, and a comfortable chair from which to keep an eye on the whole sky for an hour or so. A clear night with little or no cloud is the main prerequisite, but if you can get away from the light pollution of cities, so much the better. It also helps if you check your calendar in advance, and select a meteor shower whose appearance that particular year isn’t going to be diluted by moonlight. The full moon brightens the sky, and reduces the number of meteors that can be seen. A lunar phase between new moon and first quarter is best to aim for, because the Moon will set during the first half of the night.
Which leads me to the one other requirement for good shower-watching I need to mention. Unfortunately, it’s rather less cheering than warm clothes and hot chocolate. It’s that you need to be out and about in the small hours of the morning to catch the shower. This is because the leading hemisphere of Earth – the one running into the dust clouds of the meteoroid stream – is the hemisphere you’re in after midnight. Before midnight, your sky is facing backwards and you’ll look in vain for shower meteors.
That’s not to say there’s no use in looking out for shooting stars in the early evening. There’s still a good chance you’ll see what are known as ‘sporadic’ meteors – ones that don’t belong to a shower. They are simply particles of interplanetary dust in the plane of the Solar System, being tidily swept up by the atmosphere of our planet. They are the leftovers of planet formation, and can come from any direction in the sky – which is what distinguishes a sporadic from a shower meteor. Scientists estimate that the sum total of meteoritic material hitting the Earth’s atmosphere every day is in the region of 50 tonnes, and possibly much more. That represents at least a billion individual meteors, which might seem hard to believe when you’re standing under a stubbornly inactive sky waiting for something to happen.
As well as being briefly brilliant, meteors deliver a number of other odds and ends to the Earth’s atmosphere as they burn up. There’s a layer of sodium, for example, located in the upper atmosphere at a height of around 90 kilometres, which comes from meteors. Surprisingly, it’s useful to astronomers, since it can be excited to glow with that familiar orange colour seen in sodium street lights. Upward-pointing high-power lasers are used to energise ‘artificial stars’ in the sodium layer, which optical instruments can then lock onto. These are designed to remove the effects of atmospheric turbulence on astronomical observations – a technique known as adaptive optics.
And burned-up meteors also leave behind their own trails of fine dust. The trails aggregate into high-altitude clouds that are occasionally made visible by the condensation of atmospheric ice onto the dust. They can be seen against the night sky when they are illuminated by the Sun long after sunset at ground level, and are known as noctilucent clouds – or ‘frosted meteor smoke’, as one poetic pundit nicely put it. These ethereal wisps of light occur predominantly in summertime at high northern and southern latitudes, although in recent years they have been seen nearer the equator and in greater numbers – perhaps as a result of climate change.
One other thing that the seasoned meteor-watcher might look out for is a fireball. In fact, you don’t really need to look out at all, because if one comes along, you won’t be able to miss it unless you’re actually indoors with the curtains drawn and the doors shut. A fireball is a very bright meteor. According to the International Astronomical Union definition, it’s one that is brighter than any of the planets. What that means, of course, is that it’s brighter than the planet Venus, since Venus is the most luminous natural celestial object after the Sun and Moon. Perhaps one in half a million meteors satisfies this definition, and often it will be bright enough to light up the landscape like a flash of lightning. The green or reddish colouring sometimes seen is characteristic of oxygen atoms in the upper atmosphere being excited by the sudden input of energy, and then releasing that energy in the form of light.
If you do happen to see a fireball, it’s worth listening out for a few minutes after the event. Occasionally, the sonic boom generated by its suicidal flight through the upper atmosphere is strong enough to reach the ground, travelling many tens of kilometres to be detectable as a dull thud.
WHEN THE METEOR ITSELF MAKES IT DOWN TO THE ground as a meteorite, it provides a valuable sample of extraterrestrial material – a free gift from the Universe. Once again, there are subtleties in terminology that are second nature to the specialists, but a bit baffling to the rest of us. A ‘meteorite fall’, for example, is one that has been tracked through the atmosphere before being recovered. Usually, it’s tracked by visual observations, but a few have been located by automated systems such as the Desert Fireball Network. A meteorite that has not been spotted as it came through the atmosphere is known, fairly predictably, as a ‘meteorite find’. Finds vastly outnumber falls in the world’s scientific meteorite collections. And one other point to note is that meteorites are named after the place where they were recovered. That’s nearly always somewhere on the Earth’s surface, but a handful of meteorites have been identified on Mars and the Moon. Where but on the planet Mars could the Meridiani Planum meteorite have been found? (Yes, by NASA’s Opportunity rover, in January 2005.)
Meteorites come in several different categories, but most of them are stony in composition, while about 5 per cent contain large amounts of iron. The stony meteorites are known as chondrites, and a large fraction of them are composed of small roundish particles that are remnants of the hot disc of dusty material in which the planets formed 4.6 billion years ago. The iron-rich meteorites, on the other hand, come from the cores of baby planets known as planetesimals – the building blocks of today’s planets. Originally molten, the iron sank to the centres of these small worlds. The meteorites were subsequently knocked out of the solidified metal cores by collisions during the Solar System’s early history, when planetesimals jostled together in the swirling disc of material surrounding the infant Sun.
Remarkably, iron meteorites have played a part in human history as well as planetary evolution. The ancient Egyptians were known to prize iron objects as long ago as 3400 BCE. In those days, iron would have been rarer than gold, because it wasn’t until the sixth century BCE that iron smelting began there, as evidenced by archaeological studies. So where did that early iron come from? It came from the sky, the home of the gods – and analysis of Egyptian iron jewellery confirms its meteoritic origin, with high levels of nickel and cobalt. No wonder these items were regarded as precious – and none more so than a funerary dagger buried with the boy king Tutankhamun (1336–1327 BCE). Surmounted with a gold handle and sheath, it has an expertly crafted iron blade, whose composition closely matches that of a meteorite that fell a few hundred kilometres away on the Red Sea coast.
Perhaps the most special – and certainly the rarest – of all meteorites are those known to have been ejected from the Moon and Mars as a result of much larger asteroids hitting their surfaces long ago. More than 300 lunar meteorites are currently known, their identification hinging on their similarity to the samples of rock and soil recovered by Apollo astronauts. Analysis of their surfaces shows that most were ejected from the Moon within the past 100 000 years. And about 220 meteorites are known to have come from Mars. Once again, their Martian origin is deduced from chemical similarities with the atmosphere and rocks of Mars, as measured by robotic spacecraft.
The Martian meteorites are subdivided into groups with differing compositions, suggesting that they came from different locations on Mars. They are called shergottites, nakhlites and chassignites – names that come from the location on Earth where the first example of each class was found, in India, Egypt and France respectively. In fact, the majority of Martian meteorites are shergottites.
All these objects have been extremely well studied, particularly a 15-centimetre-long specimen by the name of ALH84001 – or more commonly, the Allan Hills Meteorite, named after the part of Antarctica in which it was found in 1984. Formerly regarded as a shergottite, it’s now classified in a small group of its own. Famously, it contains tiny structures resembling terrestrial bacteria, which some scientists in the 1990s interpreted as fossilised Martian life-forms. Although the rock comprising ALH84001 was formed around four billion years ago when Mars was warm and wet (making ALH84001 one of the oldest known Martian meteorites), most scientists today regard the identification of fossils as speculative at most, preferring a purely chemical origin for the structures. Nevertheless, the continuing interest in ALH84001 is a bonus for the science of astrobiology, providing a useful case study.
FINALLY, WHAT DO YOU CALL A FIREBALL THAT IS INCREDibly bright and breaks up in the atmosphere? Ah, that’s a ‘bolide’, or if it’s even bigger and brighter, a ‘superbolide’. While these terms sound a lot like hyperbole, they do come with technically defined intensities that needn’t concern us here. But it’s at this level where we begin transitioning into the realm of asteroid impacts and their effects – which is a whole other story.
Right on the transition is a recent event that hit the global headlines. It was the most significant impact of an extraterrestrial body since the Tunguska superbolide of June 1908, in which 2000 square kilometres of Siberian forest were flattened by an exploding object – a small asteroid or comet – some 5 kilometres above the ground. Coincidentally, the recent event also took place over Russian territory, at wintry Chelyabinsk, in the Ural Mountains.
On the morning of 15 February 2013 at sunrise, the skies over the district lit up with a brilliance 30 times greater than the Sun itself, as the streaking superbolide detonated above the city. Unlike the Tunguska event, which no-one seems to have witnessed, Chelyabinsk saw it all. The city abounded with security cameras and vehicle dash cameras, which provided an amazingly complete record of the incoming fireball and its dramatic explosion. A few people who were outdoors reported skin burns from the intense radiation. But the flash of light illuminated the snowy landscape without a sound, bringing those indoors to their windows to see what was happening.
Then, 88 seconds later, the shock wave arrived. Doors and windows blew in, complete with their frames; free-standing walls were demolished, and the roof of a warehouse collapsed. Some 1500 people had to seek medical attention – mostly with cuts from broken glass. There are stories of heroism, like that of teacher Yulia Karbysheva, who instructed her students to duck under their desks after the flash, but sustained serious injuries herself from flying glass. Some 100 000 home-owners were affected, and everyone struggled during the following days to keep warm in windowless buildings when outdoor temperatures were below –15 °C. But, mercifully, no-one died.
Soon after the event, people located meteoritic fragments to the south and west of the city, and discovered a large hole in the 70-centimetre-thick ice of Lake Chebarkul some 70 kilometres away. And, over the ensuing months, scientists gathered all the available information from orbiting spacecraft, dashboard cameras, security cameras, damage reports, seismometers, and fragments of the meteorite. The largest of these, recovered from the muddy bed of Lake Chebarkul on 16 October, weighed in at 650 kilograms.
By November 2013, the verdict was in. Two internationally renowned journals, Science and Nature, published the details. A 20-metre body weighing around 10 000 tonnes had caused the Chelyabinsk event by entering Earth’s atmosphere at a speed of 19 kilometres per second. While its dimensions place it on the cusp of being an asteroid rather than a meteorite, it had eluded the world’s asteroid-detection cameras because of its small size and its incoming direction – which was straight out of the Sun. It reached its peak brightness at an altitude of 29.5 kilometres, but exploded a few kilometres lower with an energy of some 500 kilotons of TNT (equivalent to 30 Hiroshima blasts). Seismographs recorded a magnitude 2.7 tremor from the shock wave. And the explosion produced the largest atmospheric infrasound signal ever recorded, detected by 20 nuclear weapons monitoring stations, including one in Antarctica. Travelling at least twice around Earth, the infra-sound waves took a full day to subside.
Where did the Chelyabinsk superbolide come from? Examination of the meteoritic debris shows it to have been an ordinary stony chondrite that was once part of a larger asteroid. And its trajectory could be accurately mapped after analysing all that camera footage, revealing an elongated orbit around the Sun. Its furthest point (aphelion) was in the main asteroid belt between the orbits of Mars and Jupiter, while its perihelion was, not unexpectedly, within the orbit of Earth. Intriguingly, there are similarities between the superbolide’s orbit and that of a known Earth-crossing asteroid by the name of 1999 NC43. It is thought that this asteroid itself suffered an impact a million or so years ago, creating an accompanying clump of rubble, of which the Chelyabinsk superbolide might have been a sample. Don’t say this too loudly, but that might mean more are on the way.
DOES THIS MEAN WE SHOULD WORRY? IN FACT, NO CASES of death by meteor or asteroid impact have been recorded over the past 500 years. In bookending this chapter with the two most spectacular examples of celestial fireworks in recent history, I’ve tried to encompass the full range of what might be called ‘normal’ impact phenomena, from brilliant yet harmless cascades of milligram-sized dust particles to a decidedly dangerous object weighing thousands of tonnes.
Beyond that, though, things do get more hazardous. We know that impacts have significantly modified our planet’s history, a theory pioneered in the late 1970s by two former colleagues of mine at the Royal Observatory, Edinburgh – Victor Clube and Bill Napier. Hot on the heels of their work came the realisation that the demise of the dinosaurs was probably the result of an impact 66 million years ago by an asteroid 15 kilometres in diameter, at a place now called Chicxulub in the Gulf of Mexico. But in the four decades since then, we have made enormous strides in understanding the Earth’s environment. Today, the probabilities of objects of any given size hitting Earth are well-known. A Chelyabinsk-sized impactor might be expected somewhere in the world every 60 years; a Tunguska-sized one every thousand. And a Chicxulub-sized object will hit our planet roughly every 100 million years.
Statistics don’t tell the whole story, however. On 18 December 2018, less than six years after the Chelyabinsk superbolide, a rather smaller object – about 12 metres wide – created an airburst fireball over the remote Kamchatka peninsula in eastern Russia. With a released energy of 173 kilotons of TNT, the event went unseen by human eyes, but was picked up by infrasound detectors and imagery from two unrelated research satellites. Once again, had there been any inhabited area beneath the impact site, windows would have been broken. Statistically, this is an event you’d expect to occur every 20 to 40 years, and the fact that it happened so soon after the Chelyabinsk impact highlights the stochastic nature of such phenomena. The curious coincidence of the three largest recorded meteor events since 1900 all occurring over Russia is accounted for by its size. Russia is by far the world’s largest country by land area.
WE ARE NOW EQUIPPED WITH BATTERIES OF AUTOMATED telescopes searching for potentially hazardous asteroids. And, with the forewarning that they provide, there’s every prospect of taking counter-measures against any threatened impact. Fortunately, the larger the object, the easier it is to find. It’s estimated that 90 per cent of all hazardous asteroids bigger than 1 kilometre are already known, and search programs are now concentrating on objects down to 140 metres, of which 40 or so are discovered every month. Only a tiny fraction of asteroids are classified as potentially hazardous, and usually the level of threat from a particular object falls dramatically as its orbit becomes better characterised through ongoing observations. Small objects slipping through the net, like the Chelyabinsk superbolide, are rare occurrences, and will become rarer as the technology improves.
