Some 4.6 billion years ago, a spinning cloud of cosmic dust and gas contracted to form a young star and its retinue of planets. What emerged was the Solar System of which the Earth is a part.
Our Sun weighs almost 1000 times as much as the rest of the system put together. Its massive gravitational force controls and guides the nine planets, dozens of satellites and comets, and many hundred thousands of asteroids.
The path of each Planet around the Sun is determined by a balance between the inward pull of the Suns gravity, and the outward centrifugal force of each planets orbital speed. Planets closer to the Sun where the inward pull is stronger move faster than those further out.
the Sun Signs
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The influence of the Sun is all important. As well as controlling the orbital velocities of the planets, it has shaped their characters, so that their chemical composition and mass vary according to their distance from the Sun.
Despite this subjection to the Sun, the individual planets have their own idiosyncrasies. Unlike the Earth and the other planets that rotate on their axes from west to east, Venus and Uranus spin in the opposite direction. In addition Uranus spins like a skidding wheel around the Sun, whereas every other planet spins like a top while rotating around the Sun.
It is only 70 years or so since Pluto the ninth planet was discovered. There is still much speculation about a potential tenth planet in our Solar System. Some Astronomers are convinced that a slight wiggle in the orbits of Uranus and Neptune is caused by the gravitational pull of a very dim and distant planet way beyond Pluto. Interestingly there is also some speculation within Astrological circles of a similar nature.
It is almost certain that our Solar system is not unique. The same system for detecting the presence of planets used to detect a possible tenth planet in our Solar System has successfully detected planets in orbit around distant stars.
The Sun is the most massive body in the Solar System, and provides two essential things without which life would not have developed, and could not exist: light and heat. It is our star, and is only significant because we are so much closer to it than the other 2 billion odd stars in the Galaxy.
Composition It is a great ball of searingly-hot gas, 860,000 miles in diameter (109 times the diameter of the earth), held together by its own gravity. It is roughly 75% hydrogen, 25% helium, with a tiny fraction of heavier elements. The temperature at the surface is over 6,000 degC, hot enough to vaporise iron. Inside, it is a different story. At its core, temperatures soar to over 10 million degC. This causes the nuclei of hydrogen atoms to fuse to form helium - a process known as nuclear fusion. As a result of this process, heat is released in vast quantities - driving the process of fusion in other atoms. Every second, 1038 reactions occur, destroying 5 million tonnes of matter. This huge release of energy prevents the star from collapsing under its own gravity.
Einstein in Action Despite the gargantuan number of reactions, and loss of mass (behaving according to Einstein & Rsquos famous equation: E=mc2), the Sun will continue much as it is for another 5 billion years. At the end of this time, the hydrogen in its core will be exhausted. However, the layer of helium that will have built up over the aeons will begin fusing to form even heavier elements, causing the star to swell in size to beyond the orbit of mars; destroying all the inner planets in the process. It will have become a red giant, but its surface temperature will have fallen to a mere 2,000 degC.
The Solar Spectrum Newton discovered that the Einstein & Rsquos light is not pure, but is made up of all the colours of the rainbow. Years later, it was discovered that not all the colours seem to be there!
If you produce the spectrum carefully, you will see thousands of black lines, knows as Fraunhofer absorption lines. These give away the presence of chemicals in the Sun, and that's how we know so much about it.
Under Study The Sun has been under study for years, but not until late 1995 did major discoveries start to be made with the launch of the joint NASA/ESA probe, SOHO. In April 1998, the British built and operated CDS spectrometer on board discovered tornadoes wider than Africa! The Sun has even been seen to have regular quakes. In June 1998, SOHO observed two comets crash into the Sun in quick succession.
End in Fire Red giants are smaller versions of red supergiants, like the star Betelgeuse. After a mere few million years, the Sun will expel its outer layers until only a small, but dense, core remains: it will have become a white dwarf. In size, it will be no bigger than the earth is today; but its density on earth would make a teaspoonful would weigh over 15 tonnes!
Mercury is the closest planet to the Sun. Its orbit is said to be "inferior", as it lies between the Sun and the Earth, and, consequentialy, it always appears in the sky comparatively near to the Sun, being visible a little before sunrise or a little after sunset. It takes 87 deg9 days to complete its orbit.
The planet has no appreciable atmosphere, and the temperature on the surface facing the Sun is as high as 430 degC. On its night side, it falls to below -170 degC since it has no atmosphere. In many ways, Mercury's surface resembles that of the Moon. Key differences are fewer mid-size craters; no mountain ranges; many shallow, scalloped cliffs, called scarps; fewer basins and large lava flows; and more relatively uncratered plains. The scarps vary in length from 20 to 500 km and have heights from a few hundred metres up to 2 km. They result from surface shrinkage in radius of about 2 km, from cooling of either its core or its crust billions of years ago.
The planet rotates once every 58.6 Earth days, and all of its surface sees the Sun as a result. Interestingly, its rotation is tidally coupled to the Sun, rotating 1.5 times every orbit, in much the same way that the Moon always keeps one face towards the Earth. Simple maths show that its solar day is just twice the length of its year; 176 days.
Probing the inner limits Much of the information on Mercury was found by the NASA 1973 to 1975 Mariner 10 mission launched from Cape Canaveral. Cameras on board scanned 50% of the surface to increase our resolution of its detail by 5000 times. Shortly after the year 2000 a new probe will enter orbit around Mercury.
Venus is the second planet from the Sun. Its orbit is said to be inferior as it lies between the Sun and the Earth, and, as a consequence, it always appears in the sky comparatively near to the Sun, being visible a little before sunrise or a little after sunset.
The planet's atmosphere is composed mainly of carbon dioxide, (CO2) amounting to 97% of the total mass. As a result of the carbon dioxide's greenhouse effect, the surface temperature is as high as 490 degC, slightly hotter than Mercury!
Curious Rotation The planet rotates from east to west once every 243 Earth days, in stark contrast to all other planets. Yet its orbit takes only 225 days. The upper clouds have been observed to rotate once every four days indicating winds of 100m/s. These clouds extend to 70km above the planet's surface, and obscure all of the surface details from telescopes on Earth. They are largely composed of sulphuric acid (H2SO4), and make conditions at the surface highly corrosive.
Probes to Venus The Russians launched a number of Venera series probes to send back information from Venus, Venera 1 being launched in 1961. However, most of the early probes failed for various reasons, not in the least because of the hostile conditions found on the planet's surface.
It wasn't until 1975 that the first pictures were returned by Veneras 9 and 10. In 1985, the Soviet Vega probes visited both Venus and Halley's Comet. More information.
On August 10th, 1990, the NASA Magellan probe entered orbit - the first probe ever to become an artificial satellite of another planet. It was named after the sixteenth-century Portuguese explorer who was the first to circumnavigate the earth. The surface of Venus is normally completely shrouded by the dense clouds. However, by using RADAR, Magellan was able to complete a very successful mission in October, 1994, having mapped 98% of its surface in highly intricate detail. The image below is from a mosaic of RADAR images, with a resolution 10ã better than the earlier Soviet Venera missions.
The RADAR used, operated at a frequency of 2 deg385 GHz, peak power of 325W, and pulse length of 26.5 microsec. During its extended mission, completed in September 1992, two further mapping cycles brought coverage to 98%, showing detail of about 100m. Magellan allowed the first global geological understanding of Venus, the planet most like Earth in the Solar System.
Venus in Transit About once or twice a century, Venus passes directly in front of the Sun, as viewed from the Earth (much like the Moon does during a Solar Eclipse). This last happened in 1874 and 1882, but will next occur in 2004 and 2012. The great Edmund Halley never saw a transit, but recognised their importance in measuring the size of the solar system. Through his work, explorers including Captain Cook were to make observations of transits in 1761 and 1769, after Halley & Rsquos death.
Earth is the third planet from the Sun. It is the first planet from the Sun to have a moon, a common feature of all of the other planets beyond our orbit. The Earths atmosphere reaches to approximately 80km above the surface, but this is only about one hundredth of the planets radius, and so occupies a relatively thin band around the planet. It is composed mainly of nitrogen (78%) and oxygen (21%), with the remainder built up from small traces of other gases. These relative proportions remain fairly constant up to around 80km, but the heavy gases become rare at greater heights and only hydrogen and helium are found in the outermost regions. At the height of the stratosphere, there is a concentrated layer of ozone, at about 25km. This ozone layer is vital to life on the planet, as it stops nearly all of the Suns harmful ultraviolet radiation from reaching the surface.
Atmosphere & Surface The Earth's atmosphere also contains a large amount of water vapour, although its concentration varies in different places, and is subject to seasonal variations. Most of it is visible as clouds, and most of these extend only to the top of the troposphere - up to 13km. It is in this region that the dominant weather systems are found. Above this lies a layer of warm air which holds cooler air below containing most of the water vapour, and the clouds that we see.
The surface of the planet is subject to much geological activity, and is shaped by continental drifting (plate tectonics). This makes the planet's surface relatively young, and taken in conjunction with other factors, such as the existence of an atmosphere and weathering, explains why few visible impact craters are to be found. The surface of the moon, which is heavily cratered, gives a more accurate a more accurate guide as to the quantity of collisions with debris making its way through the solar system. Here, the surface features date back to the formation of the solar system, and have remained undisturbed due to the moon's geological inactivity.
The Moon is the Earth's sole companion, orbiting at an average distance of 384,400km. It follows us on our path around the Sun, making one orbit of us (as seen against the background of stars) every 27 days, 7 hours, 43 minutes. This period is referred to as the sidereal month. The Moon's equatorial diameter measures 3,476km making it less than one third the diameter of the Earth. The tides in the Earth-Moon system have slowed the Moon's rotation and locked it to Earth, this is tidal coupling and occurs throughout the Solar System.
Fleeting Eclipses As the orbital plane of the Moon lies within 5 deg of the apparent orbit of the Sun as seen from Earth, occasionally their positions in the sky coincide, giving rise to a solar eclipse.
At other times the Moon's orbit takes it into the shadow cast by the Earth into space, causing a lunar eclipse. Unlike solar eclipses, Lunar eclipses are readily observed from large areas of the Earth's surface, due to the greater extent of the Earth's shadow than that of the Moon's.
Changing Shape As we see the Moon only in reflected light, it shows distinct phases, depending on the angle made between itself, the Sun and the Earth, as the diagram shows.
At New Moon (1), all three bodies are aligned, with the Moon between the Earth and the Sun. During this phase, despite the absence of direct illumination by the Sun, the lunar surface is just visible in light reflected from the Earth. Full Moon (5) occurs when the Earth lies directly between the Sun and the Moon, with first (3) and last (7) quarters occurring at the 90 deg and 270 deg positions. The time taken from New Moon to New Moon, called the synodic month, is 29 days, 12 hours, 44 minutes.
Mars is the fourth planet from the Sun. It is the first of the 'superior' planets, indicating that its orbit is further from the Sun than that of the Earth's. It has two very small moons, Phobos and Deimos. Like Earth, Venus, and Mercury, Mars is a rocky planet, but is relatively small, being between Mercury and Venus in size. It has many striking surface features, such as Olympus Mons, a giant volcano, three times the height of Everest, and with a base 600km in diameter. An extensive valley complex, named Valles Marineris, can be followed for a total length of over 4,500km, and, in places, descends to 7km below the rim.
Atmosphere The Martian atmosphere is composed mainly of carbon dioxide (95%), with the rest being made up from nitrogen and argon. The surface pressure is very low at around 10 mbar (200Pa), due to the lack of depth of atmosphere. Under these conditions, liquid water cannot exist on the planet's surface. However, there is plenty of H2O on Mars, locked up for example in the polar caps, and probably underground. The range of temperatures experienced on the planet's surface is very wide. Away from the equator, maximum daytime temperatures reach only -30 degC, while, on the equator, this can rise to over 22 degC. The thin atmosphere is a poor heat retainer, and night-time temperatures fall to around -100 degC in even the warmest of places.
The wide range of temperatures are the cause of extremely high winds across the planet, producing fearsome dust storms which can be seen with telescopes from the Earth. Coupled with seasonal changes at the poles, the surface appearance is constantly changing.
Martian Explorers The first accurate information gained about the planet came from the American Mariner missions conducted during the 1960s and culminating in the Mariner 9 launch in 1971. Viking 1 and 2 were to land on the planet in 1976 and provided the first views from the planet surface.
Many recent missions by both the USA and Russia have been unsuccesful
Relentless Pursuit In 1997, NASA began an intensive campaign of exploration of the planet, starting with the Pathfinder lander. The rover, called Sojourner (below) underwent a controlled crash landing! In the picture, you can see the now-deflated air bags that cushioned its landing!
A total of eight missions are planned or under way. In late 1997, an Orbiter arrived to map the surface in intricate detail. In 1999, the Climate Orbiter will arrive to study its climate and surface. In the same year, the Polar Lander will arrive. On-board are two small microprobes (Deep Space 2) which will penetrate into the surface to detect water ice. All the scientific missions being conducted will ultimately lead to the first manned flights to the red planet, the forerunner to the setting up of permanent bases on its surface.
A Rocky Return NASA intends returning samples of Martian rock as soon as practicable. However, we already have over a dozen samples of rocks from Mars that have come crashing to the Earth as meteorites!
This includes the rock found in the Artic, that was first suspected of harbouring the first evidence of life, other than on Earth.
Jupiter is the fifth planet from the Sun, and is easily the largest planet in the solar system. It is orbited by sixteen moons, only four of which have significant size: Ganymede, Callisto, Io, and Europa. These are the Galilean Satellites, discovered by Galileo Galilei (1564-1642) in 1610!
Jupiter is referred to as a 'gas giant', meaning that it is nearly all atmosphere, with a relatively small solid core. This very deep atmosphere is composed mainly of hydrogen and helium, with the upper clouds being formed from droplets of ammonia. The visible surface of the atmosphere is very vivid, showing distinct features and weather systems. One such feature is the Great Red Spot, which has been identified as a whirling storm, rotating clockwise once every 12 days. Clearly visible, too, are bands, representing different currents occurring in the cool Jovian atmosphere. These can be seen to rotate, with the polar regions moving more slowly than the equatorial regions.
A Faint Ring Jupiter has a faint ring system, not visible from Earth. It was first discovered when the Voyager probes flew by, photographing it in the shadow behind the planet.
Inside a Giant The planet's size creates enormous pressure at its core, and, as such, produces large amounts of radiation and heat. The core temperature may be as high as 30,000 degC. The radiation belts extend far beyond the planet into space, and provide a lethal environment for spacefaring craft that venture too close. The planet also produces a powerful magnetic field. It is interesting to note that, had the planet been somewhat more massive than it is, it would be classified as a star. There would have been enough pressure, due to gravity, to start nuclear fusion in its core. It would have shone with its own light, giving us a solar system with two stars (a commonly observed occurrence in other parts of the Galaxy). Had this happened, life could not have existed on Earth.
Comet Collision On the 18th July, 1994, Jupiter was hit by 19 fragments of comet Shoemaker-Levy that it had pulled apart the year before.
Despite striking the far side of the planet, as seen from Earth, the impacts caused explosion after explosion - and it was to be recorded by every telescope on Earth, the H.S.T. in orbit, and Galileo probe, still 18 months away from the great planet.
The collisions were more powerful than atomic bombs, and left impact bruises twice the size of the Earth in Jupiter's atmosphere!
Probes to Jupiter Close up details of Jupiter and its moons have been received by various probes launched since 1972.
The first to arrive was Pioneer 10 in 1973, followed by Pioneer 11 and Voyagers 1 and 2 in 1979.
In December 1995, NASA's Galileo probe finally arrived, beginning a two year tour of the Jovian system, and launching its own probe into the turbulent atmosphere. In 1997, it began an extended mission concentrating on Europa. The probe is named after Galileo Galilei.
Jupiters Moons Jupiter has such a stong gravitational field that it produces gaps in the Asteroid belt. No asteroids have ever been found with an orbital period that is 1/3, 1/2, 2/5 or 3/7 that of Jupiter. This effect is similar to the Cassini division in Saturn's rings.
Saturn is the sixth planet from the Sun. It is the second largest of the four gas giants in the solar system, but is also the lightest, having only one half the density of Jupiter. There are many moons orbiting the planet, only nine of which are particularly significant, Titan being the largest with a diameter of 5,150km and possessing an atmosphere of its own. Titan is the only planetary satellite known to have an atmosphere, the largest component of which is nitrogen. Other moons are Rhea (1,530km), Iapetus (1,460km), Dione (1,120km), Tethys (1,000km), Enceladus (500km), Mimas (392km), Phoebe (220km) and Hyperion, which is elongated, and measures 205km by 110km.
Rings The planet has an extensive system of rings which are composed of icy particles ranging in size from pebbles to beach balls. These rings appear from Earth as distinct bands with divisions between them, the main rings being designated a letter to identify them, while the gaps are named after the astronomers who first discovered them. Spacecraft data taken close to the rings reveals many more such bands with finer detail. This data also indicated that the ring thickness was no more than 150m, with their temperature in sunlight at around -180 degC and as low as -200 degC in shadow. A few of the smaller moons lie within the ring system and are seen to influence their surroundings. For example, two moons, Prometheus and Pandora, are seen to 'shepherd' the particles of a ring held between them.
Composition The composition of the planet is very similar to that of Jupiter, with a silicate core surrounded by liquid hydrogen, and a deep gaseous hydrogen atmosphere. This atmosphere is cooler than Jupiter's, and, while displaying similar banding, the colouring is less vivid, due to a haze of ammonia crystals at its outer reaches. Wind speeds of up to 1,400km/h have been detected, and give an indication of extreme meteorological activity.
Probes to Saturn After a brief fly-pass by Pioneer 11 in 1979, Voyagers 1 and 2 sent back much important information about the planet, its satellites and rings during their encounters in 1980 and 1981.
The Cassini probe, launched in October 1997, will rendez-vous with Saturn in 2003, entering a permanent orbit. It is named after the Italian-born French Astronomer Giovanni Cassini (1625-1712), who discovered the major gap (see photo above) in Saturn's rings; the Cassini Division. Cassini's 12 instruments will measure magnetic fields, plasmas, dust particle densities, Saturn&rsquos weather and the geology of several of Saturn's moons.
A Titanic Visit The ESA-built Huygens probe, on board Cassini, will detach and parachute to the surface of Titan in 2004. The tour selected also allows superior, long-term studies of Saturn's rings, detailed mapping of Saturn, and measurements of Saturn's magnetic field. In addition, the tour includes three specially targeted close flybys of the moon Enceladus, and one each of Dione, Rhea, Hyperion and Iapetus.
Uranus is the seventh planet from the Sun. It is the third largest of the gas giants, although less than half the diameter of Saturn. One feature which puts the planet in a class of its own is the tilt of its rotational axis at 98 deg to its orbital plane. Uranus has five main satellites orbiting in the plane of its equator. The largest is Oberon with a diameter of 1,630km, Titania (1,600km), Ariel (1,330km), Umbriel (1,110km) and Miranda, a mere few hundred km across.
Composition Like Saturn, Uranus possesses a ring system. They have a completely different appearance however, being black as coal and are considerably thinner than Saturn's. Minor moons have been discovered on either side of the outermost rings, and perform a shepherding action similar to that seen in the Saturnian ring system.
Due to its great distance from the Sun, and the fact that it has very little internal heating, the planet's temperature is very low. At the top of the atmosphere the temperature is so low that methane condenses above the clouds. There is a considerable magnetic field around the planet which is offset from the centre, and is inclined at 60 deg to the rotational axis. This magnetic field extends far out into space and envelopes all the Uranian satellites.
Probes to Uranus Little was known about Uranus until its encounter with Voyager 2 in 1986. Voyager was able to return to Earth highly detailed images of each of the major satellites, and also revealed the presence of ten which were previously unknown.
Usually Neptune is the eighth planet from the Sun, but is occasionally further away from the Sun than Pluto, which is in a highly elliptical orbit. Neptune is the smallest of the four gas giants, just smaller than Uranus, but much more massive. Faint dark rings have been detected around the blue planet, but are much less substantial than those of Saturn or Uranus. The planet has two main moons: Triton with a diameter of 2,720 km, and Nereid only a few hundred km across, and in a highly elliptical orbit. Triton is a very dense moon and very cold, with a surface temperature of -236 degC. Its surface is shrouded in nitrogen and methane ice, and has a polar cap covered with pink snow. Six more moons were discovered to lie close to the planet within its rings, by Voyager 2.
Composition Despite being so far away from the Sun, Neptune has considerable internal heating due to its mass. This provides enough energy to drive 1,100km/h winds in the planet&rsquos atmosphere, which is mainly composed of hydrogen (85%), helium (13%) and methane which is visible as white cirrus like clouds high up in the atmosphere. Other features can be seen such as a large dark spot rotating in the southern hemisphere. This feature is similar in nature to the Great Red Spot on Jupiter. Like Uranus, Neptune has a magnetic field inclined to the rotational axis by a surprising amount, in this case 50 deg, and also offset from the planet's centre by some 10,000 km.
Probes to Neptune The only spacecraft to visit Neptune so far has been Voyager 2 in 1989, when it made its last encounter with a planet in our solar system. Despite a journey lasting 12 years, Voyager returned data and high resolution colour images of excellent quality, revealing Neptune and its moons as magnificent sights at the boundary of our place in the Galaxy.
Pluto is usually the furthest planet from the Sun, but due to its highly elliptical orbit, it is sometimes closer than the planet Neptune. It is easily the smallest planet, only half the size of Mercury. It is also a rocky planet, unlike it nearest neighbours and due to its extreme distance from the Sun, the surface temperature is only -230 degC. Pluto's surface remained invisible to observors on Earth until mid-1994 when the Hubble Space Telescope (H.S.T.)showed highly-contrasting surface features, including a northern polar cap. These features undoubtedly alter with time as frosts migrate across its surface.
A Lucky Find The method used to find Pluto was the same as that which lead to the discovery of Neptune. Neptune's orbit, like Uranus's, was found to depart slightly from where it should be. Using the Lowell Observatory, in 1931 Clyde Tombaugh (1906-97) found what he was looking for. Although smaller than our Moon, and about a sixth of its mass, Pluto is not the perturbing body that was predicted. This means there could well be a tenth planet.
Controversy! In early 1999, just as Pluto edged its way back to being the furthest planet from the Sun, arguments ignited as to whether it should even be classed a planet.
Companion at the Edge Pluto has one known satellite, Charon, which with a diameter of 1,199km is nearly half the size of Pluto. Charon orbits Pluto at exactly the same speed as Pluto rotates. This tidal coupling means that it 'hangs' stationary in the sky, seemingly motionless.
Annabel Burton . Astrology
the Astrologer : www.annabelburton.com