by Paul Murdin
In mid-November of each year occurs the Leonid meteor shower (known by this name because its radiant is in the constellation Leo). The Leonid meteor shower of November 1833 was particularly spectacular, peaking at 1,000 meteors per minute. It was best seen from North America, and was recorded by Native Americans; the calendars (‘winter counts’) kept by the Sioux tribes name each year after a notable event, and 1833–34 was called ‘stars all falling down year’, adding: ‘They feared the Great Spirit had lost control over his creation.’ The shower was referred to by Abraham Lincoln in an anecdote recorded by Walt Whitman, and it inspired the jazz standard ‘Stars fell on Alabama’.
This Leonid shower of 1833 was analysed by Denison Olmsted and Catlin Twining of what became Yale University, who discovered the radiant of the shower and realized that the radiant was actually the orbital path of the meteor stream. Later, Hubert Newton, also of Yale, calculated the orbital period of the meteoroids at thirty-three years and identified appearances of the shower dating back to 902 CE. After Tempel’s Comet was discovered in 1866, it became clear that it was the parent body of the Leonids, because its orbit was identical with the orbit of the meteors.
In 1836 a Belgian astronomer and statistician, Adolphe Quetelet, discovered a second shower, the Perseids, which occur in mid-August each year. The 1834 shower had been seen by John Locke, the headmaster of a girls’ school, who published a letter about it in the Cincinnati Daily Gazette. His account, which also mentioned that he had discovered the shower’s radiant in Perseus, went unnoticed by astronomers, most of whom would not have read this local newspaper. Quetelet predicted a display of meteors in August 1837. Edward Herrick, a bookseller–librarian in New Haven, Connecticut, observed the shower on 9 August and identified seven occasions in the past millennium when August meteors had been seen before, ranging from 1029 in Egypt to 1833 in England. He found a reference to a European superstition that the ‘burning tears’ of St Lawrence are seen in the sky on the night of the 10th of August, this day being the anniversary of the saint’s martyrdom in Rome in 258 CE. Evidently the Perseid meteor shower had been known for centuries. In 1867 Italian astronomer Giovanni Schiaparelli discovered that the Perseid meteors came from a stream whose orbit was the same as the bright comet of 1862, Comet Swift–Tuttle, named after the two American astronomers who had discovered it.
Meteor showers can only be seen at night (unless the meteor is exceptionally bright). If a shower occurs at a time of the year when the Sun lies near to the direction of the radiant then the shower remains invisible, because you can’t see meteors during daytime. However, in 1944 radio engineer James Hey discovered that meteors generate radar echoes, which can be detected by day as well as by night. In September of that year, during the Second World War, V-2 missiles were being launched on London from Germany. As a member of the British Army Operational Research Group, Hey developed a radar system to detect incoming missiles in the hope that the civilian population could be given warning of an attack to minimize casualties. Hey’s system could indeed detect incoming V-2 rockets but also gave many false alarms, detecting launches of rockets when none was reported by spies and predicting attacks that never materialized.
After the war had finished, Hey set out to discover what had caused the false echoes. He discovered that his spurious radar echoes occurred when a meteor trail passed through the radar beam. The radar reflection was from the ionized air produced in the meteor trail. Final proof came from an organized campaign, when Hey and colleagues at other stations coordinated to look for radar echoes during the Giacobinid meteor shower of October 1946, a shower associated with Comet Giacobini–Zinner. They saw ten thousand radar echoes per hour rather than the usual two or three. Using radar echoes, Hey and his team quickly discovered several new daytime meteor showers: the Arietids, the Zeta Perseids and the Beta Taurids.
The Earth’s Magnetosphere
Our defence against the Sun
And now the Northern Lights begin to burn, faintly at first, like sunbeams playing in the waters of the blue sea. Then a soft crimson glow tinges the heavens. There is a blush on the cheek of night. The colours come and go; and change from crimson to gold, from gold to crimson. The snow is stained with rosy light. Twofold from the zenith, east and west, flames a fiery sword; and a broad band passes athwart the heavens, like a summer sunset. Soft purple clouds come sailing over the sky, and through their vapoury folds the winking stars shine white as silver.
Henry Wadsworth Longfellow, Driftwood, Frithyol’s Saga, 1837
The Earth is a giant magnet. Its liquid-iron core generates a magnetic field around the planet that extends outward as far as the Moon. This magnetic field not only causes magnetized compass needles to point north, but also shields the Earth from lethal doses of solar radiation. Without its magnetic field, the surface of the Earth would resemble the desolate landscape of Mars.
The Earth’s magnetic field is caused by the circulatory motions of the liquid-iron core of the Earth, which generates currents and a magnetic field much in the same way that dynamos and generators do. The region that is subject to the magnetic field’s direct influence is called the magnetosphere (the term was coined by Cornell University scientist Tommy Gold in 1959) and extends into space towards the Moon. The magnetosphere was the first major scientific discovery of the space age.
For centuries, sailors in many cultures knew of the lodestone, which indicated the direction north. If freely suspended – for example, floating on cork on the surface of water – some minerals, like magnetite, constitute a magnetic compass, by which it is possible to navigate if the shore or the stars are not visible. The magnetic properties of the lodestone can be transferred to an iron needle for greater clarity of direction.
In 1576 a ship’s instrument maker, Robert Norman, noticed that in London a magnetized needle not only turned to point north, but also tended to dip down below the horizontal by about 70 degrees. In 1600 the English physicist William Gilbert realized that this was because the needle was following lines of magnetic force that converged down towards the surface of the Earth, and that the Earth is itself a magnet whose effects extend out above the surface and must continue into space. The Earth has two concentrations of the lines of force at its magnetic poles, where, as Gilbert realized, a magnetic compass needle would point vertically downwards.
Between 1698 and 1700 the astronomer Edmond Halley combined his own magnetic survey of the Atlantic Ocean with other people’s measurements, producing the first map of the world showing the direction in which a magnet pointed at any given location. Magnets always point a little off the true north because the north magnetic pole is not identical with the North Pole of the axis of rotation of the Earth. There were repeated surveys in the centuries since then, but in 1957–58 there was a major, coordinated global effort to study geomagnetism, called the International Geophysical Year (IGY). Its scientific role was usurped when it became the public arena for the military objectives of the Cold War between the USA and the USSR.
American scientist James van Allen became a key figure in the execution of the IGY. He had worked on high-altitude experiments, using V-2 rockets at first. He had then developed small so-called ‘sounding rockets’ and a hybrid vehicle called a rockoon, a rocket taken to altitude and launched from a balloon. He used all these to ‘sound’ the upper atmosphere.
On 4 October 1957 the USSR reached beyond the confines of the Earth and launched Sputnik 1, the first artificial satellite to orbit the Earth. This event is reckoned as the start of the ‘Space Race’ between the USSR and the USA. Early in 1958 the USA responded to Sputnik by launching its first space probes, Explorer 1 and Explorer 3, both carrying experimental equipment with which van Allen measured the density of charged particles in space.
Charged particles from the Sun and from outer space are caught by the Earth’s magnetic field and funnelled through the magnetosphere into the magnetic polar regions, as discovered by the Norwegian physicist Kristian Birkeland in 1895. Birkeland
put a magnetized iron sphere – a terrella, or ‘little Earth’ – in a vacuum chamber and aimed a beam of electrons towards it. He saw that the electrons were steered by the magnetic field to the terrella’s magnetic poles. This experiment replicated in miniature the phenomenon of the aurora, or Northern and Southern Lights. When electrons strike the Earth’s atmosphere, they produce a colourful, shimmering aurora that can be readily seen at polar latitudes. Birkeland’s terrella was an experimental way to simulate the magnetosphere, which is nowadays carried out by numerical simulations by computers.
Van Allen discovered that the Earth was encircled by a doughnut-shaped region of charged particle radiation. This radiation was trapped within ‘magnetic bottles’ that were enclosed by the Earth’s magnetosphere. The doughnut-shaped region of radiation was named the ‘Van Allen Belt’ in honour of its discoverer. A second, outer belt was identified in 1958 by the Pioneer 3 lunar probe (which failed to get to the Moon but nevertheless reached an altitude of 101,000 kilometres), and also detected by the Sputnik 2 and 3 satellites. The Van Allen Belts were the first major scientific discovery to be made as a result of space exploration.
The Van Allen Belts reach from the top of the Earth’s atmosphere out to a distance of about 3.5 times the diameter of the Earth. They are major components of a system of electric currents and high-radiation regions that have been mapped by a succession of space satellites. The satellites sent into the high-radiation regions have to be especially robust because the particles deleteriously affect the environmental conditions for spaceflight. The particles also affect the transmission of power and electrical signals at the Earth’s surface. All these effects are known as ‘space weather’. They are caused by storms produced by the Sun and are associated with sunspots.
The first signs that the Sun causes changes in the magnetosphere were noticed in the nineteenth century. In 1843, Heinrich Schwabe, a German apothecary and amateur astronomer, discovered that sunspots came and went on a ten-year cycle (now more accurately reckoned at eleven years). The explorer Alexander von Humboldt popularized Schwabe’s work. Consequently, when an army officer named Edward Sabine discovered that magnetic storms (violent vibrations of a compass needle) were more frequent at intervals of ten years, he suggested that this was related to the solar cycle. Sabine’s theory was dramatically confirmed when English amateur astronomer Richard Carrington discovered a large flare on the Sun in 1859, which was immediately followed by a violent magnetic storm and aurora.
Solar wind – the second major discovery of the space programme – is the actual mechanism in the Sun that affects the Earth’s magnetosphere. This is a constantly flowing but erratic stream of charged particles that emanates from the Sun and impinges on the Earth – or rather onto the Earth’s magnetic field. The behaviour of comet tails – which always point away from the Sun, regardless of the direction in which the comet is travelling – gave early indications that something was flowing outwards to sweep the tails away, like the loose ends of a scarf blowing in the wind. Physicists Eugene Parker, Ludwig Biermann and Hannes Alfvén independently came to the conclusion in the 1950s that there existed a relentless outward flow of charged particles from the Sun: the solar wind. The solar wind was confirmed by the Soviet lunar probes Luna 1, 2 and 3 as they transited from the Earth to the Moon in 1959 and by the US Mariner 2 probe as it travelled to Venus in 1962. Except at the poles, the magnetic field of the Earth defends the Earth’s atmosphere and surface from the solar wind. If the magnetosphere did not exist, the Earth’s atmosphere would be blown off by the solar wind and its surface would be exposed to lethal radiation. This seems to be what happened to the planet Mars when its magnetic field died away to almost nothing. By contrast, the Earth’s magnetosphere is unusually large because the iron core of the Earth is unusually large, being the amalgamation of two iron cores in the collision that formed the Moon.
The magnetic field of the Earth is always shifting, because it is sustained by the circulatory motions in the Earth’s liquid-iron core, which are erratic and oscillate. This causes the magnetic field of the Earth to drift and tilt, as English astronomer Henry Gellibrand discovered in 1635. As a result, the magnetic poles wander. The North Magnetic Pole has been moving quickly northwards from Hudson’s Bay since 1990 and lies in the Arctic Ocean, near the North Pole. In 2020 it is expected to cross from the sea north of Canada and Alaska into the sea north of Siberia. The South Magnetic Pole is in the Ross Sea just off the continent of Antarctica, towards Australia. The Earth’s magnetic field also changes strength, and even reverses polarity (the North Pole changes to the South Pole and vice versa) every 250,000 years or so. It isn’t known how quickly this happens, or whether for some periods of time the Earth’s magnetic field becomes so weak that the magnetosphere is turned off. If so, the atmosphere and the surface of the Earth would be exposed to the solar wind. It is not known what happens temporarily to the natural environment at that time.
Comets
Dirty snowballs?
He, first of men, with awful wing pursued
The comet through the long elliptic curve,
As round innumerous worlds he wound his way,
Till, to the forehead of our evening sky
Returned, the blazing wonder glares anew,
And o’er the trembling nations shakes dismay.
James Thomson, ‘To the Memory of Sir Isaac Newton’, 1727
Although comets are no longer superstitiously associated with bad fortune, the mystery of where they come from and what they are made of continues to intrigue astronomers. Recent images from space probes show that comets are made of ice and fine dust. It is possible that water in our oceans and perhaps even the molecular seeds of early life were brought to Earth by comets.
Comets are small bodies that orbit in the Solar System, like planets. However, unlike planets, whose orbits are nearly circular and confined largely to one plane (the ‘ecliptic’), comets’ orbits are highly eccentric and may be inclined upward or downward at any angle. Aristotle considered comets to be atmospheric phenomena, but in 1577 Tycho Brahe discovered by measuring the parallax of a comet that it was located beyond the Moon, and astronomical in origin.
Kirch’s Comet of 1680 was the first comet discovered by telescope. Isaac Newton discovered that the comet was following a near-parabolic orbit around the Sun and conformed to Kepler’s laws. It was the comet that proved Newton’s law of gravitation. Newton’s colleague Edmond Halley used Newton’s laws to discover that the orbits of three comets, which appeared in 1531, 1607 and 1682, were very similar. Halley suggested that all three comets were actually the same object, revisiting every seventy-six years on an elliptical orbit, not a parabolic one, and predicted the next appearance in 1758 or early 1759. This happened after his death, when what is now called Halley’s Comet was rediscovered by Johann Georg Palitzch, a farmer from Saxony and an amateur astronomer. During the 1986 reappearance, two Russian Vega spacecraft surveyed Halley’s Comet from a distance and two Japanese probes investigated its plasma tail, while the European Space Agency probe, Giotto, passed within 600 kilometres of its nucleus – so close that the spacecraft was damaged, hitting a large particle emitted from the comet.
American astronomer Fred Whipple proposed the ‘dirty snowball’ model for the composition of comets. He suggested that a comet’s nucleus is made of dust grains cemented together by ices, such as water, ammonia and methane. The ices sublimate to gas and are released from the comet as it warms on approaching the Sun. The vaporization of the ices lets loose the dust grains, which are dragged from the nucleus by outflowing gas. The comet develops a bright, dusty atmosphere, called the ‘coma’, and ‘tails’ of gas and dust. But in the early 1950s, Giotto found that Halley’s Comet was not really a ‘dirty snowball’, more a ‘snowy dirtball’ – its nucleus was a coal-black, peanut-shaped body, about 15 kilometres long and between 7 and 10 kilometres wide; its structural arrangement was dictated by the physical properties of dust rather than ice.
Dust consolidates on a comet’s surface as a crusty skin. The action of sunlight on the compounds of which the dust is made creates a sticky substance that coats everything with a black tar: comets are as black as coal. Among the chemical compounds created in this way are organic molecules like amino acids: perhaps these molecules, delivered to Earth in cometary collisions, are materials that helped life to develop here. The European Space Agency’s space probe Rosetta orbited Comet 67P/Churyumov–Gerasimenko for two years between 2014 and 2016. The comet was structured like two snowballs consolidated together, with spectacular cliffs and chasms. It may be the outcome of a soft, low-speed collision between two comets that fused together. Unusual, knobbly humps on the comet’s surface indicate it is the result of the accumulation of many smaller comets. In 2014 Rosetta loosened a lander, Philae, onto the comet’s surface. Unfortunately, it made a bad landing and fell into shade, so scientific results from its mission were limited.
The tails of comets always point away from the Sun. The dust of a comet tail is pushed away from the comet by solar radiation pressure, as was discovered in 1900–1 by the Swedish chemist Svante Arrhenius and the German physicist Karl Schwarzschild. The dust trails of some comets (Halley’s Comet, for one) are associated with meteor showers. In 2004 NASA’s Stardust probe flew through the tail of Comet Wild 2 and retrieved some of its dust. Some of it was crystalline, ‘born in fire’ – presumably in the hot inner parts of the nebula that formed the Solar System. It was similar to the material that makes up some asteroids.