Apollo programme US National Space and Aeronautic Association (NASA) initiative to land a man on the Moon, inaugurated in 1961 and comprising 17 missions in 1967–72. Apollo 11 made the first manned landing on the Moon’s surface on 20 July 1969; Apollo 17 was the programme’s final flight in December 1972. In the course of the programme there were six lunar landings in which 12 US astronauts walked on the Moon.
atmosphere Layer of gases surrounding a planet or any body of sufficient mass, including a star; the layer’s shape is maintained by gravity.
biomass Biological material that comes from living or recently living organisms.
core Central part of a planet or star.
crust The solid outer part of a planet or natural satellite.
gas giant A large planet that consists primarily of gases instead of rock. The four gas giants in our solar system are Jupiter, Saturn, Uranus and Neptune. There are other gas giants beyond the solar system, in orbit around other stars.
greenhouse effect Process through which heat radiated from a planet’s surface is absorbed and then radiated outwards in all directions (including back down towards the surface) by gases in the atmosphere. As a result, the temperature on the surface and beneath the atmospheric gases is raised. Earth has a greenhouse effect, but so do other planets, such as Venus; the greenhouse effect is far stronger on Venus than on Earth.
Low Earth Orbit An orbit of the Earth at an altitude of 145–1,000 kilometres (90–620 miles). All manned space flights, apart from those of the Apollo programme, all manned space stations and most artificial satellites are in the Low Earth Orbit.
mantle A layer about 2,900 kilometres (1,800 miles) thick, between the outer part of the Earth’s core and its surface (crust).
mare Areas of basaltic lava on the Moon’s surface. (Basalt is a grey-to-black igneous rock.) Early astronomers wrongly identified these as areas of water and they were named mare (Latin for ‘sea’ or ‘seas’). There are several such areas, including the Mare Nubium (‘Sea of Clouds’) and the Mare Serenitatis (‘Sea of Serenity’); together, they form approximately 16 per cent of the Moon’s surface. They appear as dark areas on the Moon, visible with the naked eye, and make up the patterns interpreted in some cultures as ‘the Man in the Moon’.
meteor Colloquially known as a ’shooting star’, the name given to a streak of light caused by rock or dust burning up as it falls through a planet’s atmosphere.
meteor shower The appearance of several meteors in short succession.
meteorite A meteoroid that has landed on the surface of a moon or planet.
meteoroid A rocky body, smaller than an asteroid, in our solar system.
moon Also known as a natural satellite, an astronomical body that orbits a planet (or smaller body), known as its ‘primary’. The Earth’s Moon is the fifth largest natural satellite in the solar system, after Ganymede (the largest, a moon of Jupiter); Titan (second largest, a moon of Saturn); and Callisto and Io (third and fourth largest, moons of Jupiter).
niche environments Settings specialized to suit a particular species.
outgassing The release of gas that was absorbed, frozen or otherwise trapped in a surface – for example, in an ocean or area of rock on the surface of a planet.
protoplanet The ‘embryos’ or initial formations of planets, formed in a protoplanetary disc (cloud of dust and gas surrounding a new star). They form from the collision of smaller planetesimals. Where there are several protoplanets in orbit around a star, they collide to form one or more planets.
regolith Any loose mixture, such as soil or pieces of stone, that covers solid rock, from Greek words meaning ‘blanket’ and ‘rock’. Found on the Earth, the Moon and on other planets, moons and asteroids.
ring system Also known as a ‘planetary ring’, a disc-shaped formation of dust and particles (up to several meters in size) orbiting a planet. The most celebrated ring system in our solar system is around Saturn; Neptune, Uranus and Jupiter also have planetary rings.
tectonic plates Movable pieces of a planet’s crust (outer surface) and parts of the upper mantle (the layer directly beneath the crust).
tenuous Lacking in density. Used of a planet’s atmosphere.
Mercury is the smallest of the eight planets, with a diameter of 3,032 miles (4,879 km). The closest planet to the Sun, it is the speediest in its orbit: Mercury orbits the Sun in 88 Earth days. It rotates relative to the stars once on its axis every 59 days, turning three times on its axis for every two orbits. Because of the way that the planet rotates relative to the Sun as it orbits, its calendar is bizarre: a single day on Mercury (from sunrise to sunrise) lasts two Mercurian years or 176 Earth days. Mercury has no seasons and the largest temperature range on any planet in our solar system – from 400°C (800°F) at noon on its equator to –200°C (–300°F) near its poles at night; the temperature is especially low in the perpetually shadowed bottoms of its polar craters, where there are accumulations of ice. Mercury has a cratered, solid surface, much like the Moon. Its atmosphere is tenuous (lacking in density) and consists of atoms trapped from the Sun or outgassed from its hot surface. Mercury’s craters were formed in the same way as the craters on the Moon, through bombardment by asteroids and meteors.
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Named after the messenger of the ancient Roman gods, Mercury is a fast-moving planet of extremes – very hot by day and very cold by night.
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Mercury’s orbit is the most elliptical of any planet, as well as the closest to the Sun, so it experiences a large variation in gravitational pull. This makes its orbit a test bed for the theory of gravity. Its orbit does not quite fit Isaac Newton’s theory, but Albert Einstein’s theory of gravity, known as General Relativity, solved the anomaly – and this was the first proof that General Relativity was better than Newton’s theory.
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ALBERT EINSTEIN
1879–1955
German-Swiss-American theoretical physicist
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Paul Murdin
With little atmosphere to act as an insulating blanket, Mercury’s temperature plummets by hundreds of degrees as night falls.
Venus is roughly the size of the Earth, with a diameter of 12,104 kilometres (7,521 miles). It orbits the Sun inside the Earth’s orbit, once every 224 days, and rotates every 243 days – backward. Like Earth, Venus has an atmosphere, but on Venus this is hot, dense and consists primarily of carbon dioxide, creating an intense greenhouse effect that passes on the Sun’s heat to the surface and traps it below the atmosphere. As a result, the temperature on Venus averages 480°C (890°F) – hot enough to melt zinc. Seen from outside, the atmosphere supports opaque clouds that completely obscure the surface; seen from below, the sky is sulphurous yellow, as imaged by space vehicles that have landed to record the environment. Venus has been mapped by cloud-piercing radar both from Earth and from a space satellite, Magellan (1990–94). The surface is completely dry, and made of scaly, black volcanic rocks. Venus has more than 100 volcanoes, with solidified rivers of lava on their sides. Most terrestrial volcanoes are due to upwelling magma penetrating the surface of a planet at the edges of colliding tectonic plates – Venus has no tectonic plates and its volcanoes are fed through weak surface spots.
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In some respects Earth’s twin, the planet Venus has suffered global catastrophes that have made its surface hellish – hot, black rock beneath a sulphurous sky.
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Space vehicles sent to Venus must be strengthened to withstand the atmospheric pressure (about 90 times the pressure on Earth) and proofed against sulphuric acid rain falling from the clouds. They also have to withstand the searing heat. Landing craft that have survived the descent and landed on the rocks without falling over have operated only for an hour or so. The existence of Venusian extraterrestrials seems improbable.
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CARL SAGAN
1934–96
American astronomer who identified the greenhouse effect on Venus
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Paul Murdin
A featureless black spot when silhouetted against the Sun during a transit, Venus has been revealed by space satellites to be a volcanic wasteland.
The Earth is a dense ball of iron and rock, the largest solid body in our solar system. It formed 4.5 billion years ago, from a mass of dust and gas left over from the formation of the Sun. Although the assembly of the Earth was practically completed within 10 million years, it is accurate to say that its formation is still not finished, because the Earth remains geologically active. The Earth’s crust is divided into 15 segments called tectonic plates between 5 and 50 kilometres (3 and 30 miles) thick, all slowly floating over a silicate mantle. Beneath the mantle lies an iron-nickel core. The Earth’s surface, a thin layer of a small, rocky body orbiting a typical star in the suburbs of an average galaxy, is to date the only place we know to harbour life. Life appeared on Earth within its first billion years and developed to encompass millions of species. Vegetation is the dominant life form on Earth, at least in terms of biomass and environmental impact. It has altered the atmospheric composition of the Earth and could potentially be detected from distant space by its characteristic reflectivity in infrared.
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American astronomer Carl Sagan declared of the Earth: ‘That’s here. That’s home. That’s us … a mote of dust suspended in a sunbeam.’
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Since the first satellites took photographs of the Earth, it has often been called ‘the blue planet’ on account of the dominance in these images of blue oceans. However, water represents only 0.02 per cent of the Earth’s mass; the oceans are like a thin sheet of blue paper covering a brown ball. And only 0.001 per cent of Earth’s water is accessible fresh water, on which millions of species – Homo sapiens included – are reliant.
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CARL SAGAN
1934–96
American astronomer, astrophysicist and writer
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François Fressin
Planet Earth is a ball of mud and metal, covered by a thin layer of water.
Despite being a common sight for us all, the Moon is one of the most peculiar objects in our solar system. It is the fifth largest satellite in the solar system, and the largest one relative to its host planet. The Moon likely formed as a result of the giant impact of a Mars-size body hitting the newly formed Earth. The scattered remains of both the Earth and the impactor formed the body of the Moon. After it started orbiting the Earth, the Moon slowly drifted into a more distant orbit; its rotation synchronized, which means it rotates around its axis in the same time it takes to orbit the Earth and always shows the same face to the Earth. The Moon’s most noticeable influence on the Earth is the tidal force that causes the Earth to dilate in the direction of the Moon, due to the stronger gravitational force affecting the part of the Earth that is closest to the Moon, and results in elevated ocean tides. Only 21 humans have gone further than the Low Earth Orbit, all in 1969–72 during the Apollo lunar programme, offering a glimpse of what a spatial civilization could be.
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The Moon is the natural satellite of the Earth, and the most distant place to which any man or woman has travelled.
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The Moon has no atmosphere, and its surface is covered by meteor impacts. Mostly on the side visible from the Earth, the lunar maria (‘seas’) are, in fact, basaltic plains formed by ancient volcanic eruptions. Very small particles of crust called regolith cover the Moon’s surface and make it as reflective as charcoal. The Moon’s distance from the Earth varies in the course of its orbit: the average is 384,400 kilometres (238,900 miles).
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NEIL ARMSTRONG
1930–2012
Former NASA astronaut, the first man to walk on the Moon
EDWIN EUGENE ‘BUZZ’ ALDRIN
1930–
Former NASA astronaut, the second man to walk on the Moon
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François Fressin
Buzz Aldrin, the lunar module pilot on Apollo 11, and the second man to set foot on the Moon, described its landscape as a ‘magnificent desolation’.
The next planet beyond Earth moving outwards from the Sun, Mars has a year 687 Earth days long, and it rotates in just over 24 hours. With a diameter of 6,792 kilometres (4,221 miles), Mars is smaller than Earth, but boasts both the largest mountain in the solar system – the volcano Olympus Mons, 22,000-metre (70,000-ft) high volcano Olympus Mons – and a canyon system, the Valles Marineris, with dimensions that in parts are ten times those of the Grand Canyon in Arizona. Mars has polar caps of layered water ice and dry ice (solid carbon dioxide) that wax and wane during its seasons. Frost forms on parts of its surface during cold nights, disappearing in the morning sunlight. Although its atmosphere is thin, surface winds blow up dust storms that can envelop the entire planet. In places, thin trickles of water seep down cliff faces from ice melting below the surface. In the past, water was more abundant on Mars. There were lakes in some meteor craters, and there remain flood plains of rounded boulders, scoured by a surge of water released when an ice dam collapsed. Conditions on Mars are extreme, but this planet offers hope of finding extraterrestrial life in niche environments.
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Mars is the planet most like Earth in our solar system, with ice caps, desert plains, mountain ranges, volcanoes and wide, deep canyons.
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Mars has a weak magnetic field, but – as revealed by residual magnetism in its old rocks – its magnetic field was stronger in the past. A planet’s magnetic field is caused by the circulation of its liquid iron core. A global catastrophe dried out Mars. This was triggered by the loss of its magnetic field when its small iron core froze, which allowed the solar wind to penetrate to Mars’ atmosphere and erode it.
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PERCIVAL LOWELL
1855–1916
American astronomer, founder of Lowell Observatory (Flagstaff, Arizona) to study Mars
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Paul Murdin
The Sojourner rover (11 kg/25 lb), which explored the surface of Mars in 1997, is dwarfed by the Curiosity rover (1 ton/2000 lb), which landed on the planet in 2012.
Cold Jupiter lies five times further from the Sun than does the Earth, and takes 11.86 Earth years to complete a single orbit. It contains more than twice the combined mass of all the other planetary bodies of the solar system. Despite having a volume more than 1,300 times that of Earth, Jupiter rotates once in less than 10 hours, leaving it slightly squashed at the poles. Jupiter is not a solid body, but is composed of the lightest elements in the Universe, predominantly hydrogen and helium. The ‘surface’ of Jupiter we see is only the tops of the clouds in the upper regions of its gaseous atmosphere; deeper down, the gas is progressively compressed by the weight of overlying layers to become hotter and denser, until a liquid hydrogen layer surrounds a rocky core ten times the mass of the Earth. Atmospheric motions powered by solar energy and internal heat stir up complex weather patterns in the clouds, which are wrapped around the planet by the fast rotation to form colourful bands parallel to the equator. Many storms come and go, but are dwarfed by the Great Red Spot, a hurricane large enough to swallow two Earths.
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The largest planet in our solar system, Jupiter is a world unlike our own; with its deep gaseous atmosphere, it is the archetypal gas giant.
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Jupiter has an array of more than 60 moons. The four largest – Ganymede, Callisto, Io, and Europa – were discovered in 1610 by Galileo Galilei, and his observations of their movement around the giant planet helped to convince him that the Sun instead of the Earth lies at the centre of the solar system. The four largest moons are similar to our Moon in size, but all the others are far smaller and irregular in shape.
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GALILEO GALILEI
1564–1642
Italian astronomer
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Carolin Crawford
With a diameter of 142,700 kilometres (88,700 miles) across its equator, Jupiter dwarfs planet Earth.
Mathematician, astronomer, physicist, artist, musician, teacher, physician, inventor, writer: Galileo Galilei was the late Renaissance man’s Renaissance man. The son of a musician, and no mean musician himself, he started out as a medical student in Pisa, but was soon seduced by mathematics and physics, then got sidetracked by art and design. Throughout his life, Galileo was oppressed by family financial problems and was constantly looking to invent money-spinning products – including the thermoscope, a forerunner of the thermometer and a military compass.
Although he was a major player in the seventeenth-century scientific revolution (Albert Einstein called him the father of modern science, and he is well known for his work in physics, on falling bodies) we know him best as an astronomer and mapper of the Moon. He made improvements on the telescope developed by German-Dutch lensmaker Hans Lippershey (1570–1619) and observed, identified and noted the phases of Venus and the four largest moons of Jupiter, as well as sunspots, and also discovered that the Milky Way was composed of billions of stars. These first-ever observations by telescope were recorded in his treatise The Starry Messenger (1610).
Galileo was a committed supporter of the heliocentric theories of Copernicus, which proposed that the Earth, the Moon and the planets revolved around the Sun; Galileo used his observations to support this theory after the Roman Inquisition had concluded in 1616 that the heliocentric structure was impossible. Despite being warned against taking this action, Galileo – who had a short fuse, a caustic wit, and a healthy disregard for authority – then published Dialogue on the Two Chief World Systems in 1632. He had already challenged authority and promoted experimentation in his 1623 work The Assayer (now regarded as his scientific manifesto), which had enjoyed success, but the Dialogue was seen as an insult to the increasingly paranoid Pope Urban VIII, and the wrath of the Catholic Church fell on Galileo’s head. He was tried by the Inquisition, and found ‘vehemently suspect of heresy’; under the threat of torture, he reluctantly recanted, but was put under house arrest for the rest of his life and his work added to the Index of Forbidden Books. The world had to wait until the early nineteenth century to read Galileo’s works and find him vindicated.
Born in Pisa
1581
Studied medicine at the University of Pisa
1586
Invented the hydrostatic balance
c. 1592
Invented the thermoscope
1592–1610
Taught mathematics, mechanics, and astronomy at the University of Padua
1610
Published Sidereus Nuncius (Starry Messenger), a brief treatise describing his telescopic observations
1612
Observed Neptune, but did not recognize that it was a planet
1616
First account of the motion of the tides in Discorso del Flusso e Reflusso del Mare (Dialogue on the Ebb and Flow of the Sea), the basis for the later Dialogue
1616
Observed Saturn’s rings
1616
Defended heliocentrism at the Roman Inquisition
1617
Observed the double star Mirzar in Ursa Major
1623
Published Il Saggiatore (The Assayer)
1632
Published Dialogo dei due Massimi Sistemi del Mondo (Dialogue on the Two Chief World Systems), a defense of heliocentric theories
1633
Roman Inquisition finds him guilty of heresy; he is put under house arrest
1634–38
Wrote Discorsi e Dimostrazioni Matematiche, intorno a due Nuove Scienze (Discourses and Mathematical Demonstrations Relating to Two New Sciences), a summary of his work on strength of materials and the geometry of motion
1638
Became blind
8 January 1642
Died in Florence
1718
Ban on the printing of his books is lifted
1835
His works were removed from the Catholic Church’s Index of Forbidden Books
A gas giant, and the second largest planet in the solar system, Saturn has a volume equivalent to that of more than 700 Earths, but its mass is only 95 times greater. It has the lowest density of all the planets – less dense than water on Earth. Its deep atmosphere is composed of hydrogen and helium wrapped around a small rocky core; the atmosphere is squashed outwards by fast rotation to make it 10 per cent wider at the equator than at the poles. Differences in the observed spin rate of features suggest that a day on Saturn is 25 minutes longer at the poles than at the equator. Saturn has more than 60 moons – ranging from tiny moonlets less than a kilometre across to the gigantic Titan, which has a diameter of 5,150 kilometre (3,200 miles) and is larger than the planet Mercury. Titan is comparable to Earth: it has a layered atmosphere, and is the only other known object in our solar system with stable surface liquid. Many of Saturn’s smaller moons, such as Phoebe, have orbital characteristics that suggest that they were originally asteroids that have been captured by Saturn’s gravity.
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Saturn – the furthest of the planets known to humankind since antiquity – is best known for its spectacular ring system and varied retinue of moons.
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Saturn’s rings are composed of chunks of ice and rock, the debris of a small moon shattered by gravitational forces 100 million years ago. Extending around 6,400–120,700 kilometres (4,000–75,700 miles) above Saturn’s equator, the rings are only 100 metres (325 ft) thick. During Saturn’s 29½ year journey around the Sun, the rings’ inclination changes as viewed from Earth, altering the apparent shape from wide open when face-on to almost invisible edge-on.
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GIOVANNI CASSINI
1625–1712
French-Italian astronomer who discovered four of Saturn’s moons
CHRISTIAAN HUYGENS
1629–95
Dutch astronomer who discovered Titan
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Carolin Crawford
Saturn’s rings are not solid, but are trillions of particles of rock and ice in orbit around the planet. They range from the size of sand grains to small boulders.
The most remote planets in our solar system, Uranus and Neptune orbit the Sun at distances respectively 19 and 30 times further than that of the Earth. Consequently, they are both frigid worlds, with mean temperatures in their atmospheric clouds of about –200°C (–300°F) and take, respectively, 84 and 165 Earth years to orbit the Sun once. They have been visited by one spacecraft, Voyager 2, which flew past Uranus in 1986 and Neptune in 1989. Both planets have a faint ring system, and are accompanied by a coterie of moons. Their deep, relatively featureless atmospheres are dominated by hydrogen and helium and surround a large core of rock and ice. Extra hydrocarbons, such as ammonia and methane, change the colour of the sunlight reflected by the cloud tops to give the planets their distinctive green and blue hues. Internally generated heat stirs up some of the fastest winds in the whole solar system on Neptune, reaching speeds of up to 2,000 km/h (1,250 mph). Uranus is tipped over to rotate completely on its side, an unusual orientation probably caused by a collision with another protoplanet soon after its formation.
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Uranus and Neptune are the two outermost giant gas planets of our solar system, each with a diameter about four times that of Earth.
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Uranus and Neptune are the only planets in our solar system to have been discovered in modern times, and by telescope. Uranus was discovered serendipitously in 1781 by William Herschel; from observed anomalies in its motion around the Sun astronomers deduced that it was experiencing the gravitational pull of a more distant planet. John Couch Adams and Urbain Le Verrier independently predicted its location, leading to the discovery of Neptune by Johann Galle in 1846.
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WILLIAM HERSCHEL
1738–1822
German-born British astronomer, discoverer of Uranus
URBAIN LE VERRIER
1811–77
French mathematician and astronomer
JOHANN GOTTFRIED GALLE
1812–1910
German astronomer, discoverer of Neptune
JOHN COUCH ADAMS
1819–92
British mathematician and astronomer
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Carolin Crawford
Uranus and Neptune are the outermost giant worlds of our solar system.
