HomeSkyStepping Out into Space – An Epic Journey through the Solar System Charles October 31, 2016 Sky The Universe is an immeasurably vast place, and our own solar system is just a minuscule part of it. Nonetheless, even our own tiny area of space boasts many fascinating worlds to explore. Today, all the eight planets that make up our solar system have been visited by robotic probes, as have all the larger moons. Nonetheless, much of the Solar System remains shrouded in mystery. Whether some primitive form of life exists in the Solar System beyond Earth remains an unanswered question, but there have been and continue to be many exciting discoveries pointing to evidence that we might not be completely alone in our planetary system. We now know more about the surface of Mars, for example, than the bottom of the Earth’s oceans. We also know that water is abundant in the Solar System. And we also know that the Solar System is really nothing that special in the greater scheme of things. In recent years, hundreds more planetary systems have already been discovered. In fact, there are probably more than 100 billion planets in our galaxy alone, only eight (or, perhaps nine?) of which happen to orbit our own star. In this guide, we’ll be taking a journey through own solar system from the Sun at its very centre to the most distant borders of our little patch of the Universe. 1 – The Sun NASAAn immense solar prominence erupts from the Sun, sending a shock wave all the way to Earth and beyond. Photographed by NASA’s Solar Dynamics Observatory in 2012. The vast fiery ball of hydrogen and helium in the sky is the lifeblood of Planet Earth. It gives us heat and light and makes life as we know it possible. Before the age of space exploration, the Sun was largely assumed to be unique and, even today, many people still think it is. However, there is nothing special about the Sun – it is a common type of star, and there are at least several billion almost identical to it elsewhere in the Milky Way Galaxy alone. The Sun is a G2 spectral class star. It’s just right for allowing life to exist within the habitable zone, of which Earth is conveniently in the middle. It is around 109 times the diameter of Earth and accounts for 99.86 percent of all the mass in the Solar System. Just as the Earth completes a trip around the Earth once per year, the Sun, along with the rest of our solar system, travels around the centre of the Milky Way galaxy in what is called a galactic year. Our galactic year is around 250 million Earth years long. The Sun is approximately half way through its nine-billion-year lifespan. Today, it continues slowly but steadily to burn hotter as it expands towards the inner planets. In billions of years’ time, it will swallow Mercury and Venus and possibly Earth and Mars as well. Humans and all life on Earth will be long extinct by the time this happens. Fun Facts about the Sun It takes light 8 minutes and 19 seconds to reach the Earth from the Sun. If the Sun were to suddenly disappear, we would still see it in the sky for that amount of time after it had physically vanished. The Sun is as deadly as it is essential to life. Fortunately, Earth’s magnetic field protects us from cosmic rays, while the ozone layer which us protects us from deadly ultraviolet radiation. The Sun hosts eight planets, at least 406 moons including 19 large ones of spherical shape, about 600,000 asteroids, at least five dwarf planets and over three-thousand comets. 2 – Mercury, the Sun-Baked World NASAThe north pole of Mercury, as seen by NASA’s Messenger probe in 2011. Mercury is the smallest planet in the Solar System and the closest to the Sun. It’s a heavily-crated world with a similar geological appearance to that of Earth’s moon. This lifeless sun-baked rock makes a full journey around the Sun every 88 Earth days, and the length of its day is 59 Earth days. Like Earth’s moon, Mercury only has a negligible atmosphere and a fraction of the gravity that Earth has. To date, the innermost planet has only been visited by two space probes, Mariner 10 in 1974 and Messenger in 2008. Known since ancient times, Mercury is sometimes visible to the naked eye but, due to its small size and proximity to the Sun, it is often hard to see. Undoubtedly the best time to see the innermost planet is during its transition, when it passes right in front of the Sun as a silhouette and may easily be seen through a good telescope. Just make sure you use a suitable solar filter, lest you risk serious eye damage! Fun Facts about Mercury Due to having no atmosphere and such a long rotational period, the surface temperatures on Mercury fluctuate more than anywhere else in the Solar System. At night, it can be -100°C while during the day, it can be 400°C. Mercury has the most eccentric (least circular) orbit of all the eight planets. It can be as little as 28.5 million miles (46 million km) from the Sun or as far as 43.5 million miles (70 million km). Recent observations have revealed evidence for water ice on Mercury, albeit only in permanently shadowed areas of the polar craters where temperatures are always frigidly cold. 3 – Venus, the Infernal World NASAA false-colour image showing the surface of Venus. The second planet from the Sun was once thought to be a lush jungle world hidden beneath a thick layer of highly reflective white cloud. Although known since humanity began, nothing was known about the true nature of the Venusian surface until the Mariner 2 robotic space probe was sent there in 1962. It was quickly discovered that, despite long being thought of as Earth’s twin, given its similar size and surface gravity, it was actually the least hospitable place in the Solar System. In fact, the Venusian surface very much resembles the traditional Biblical descriptions of Hell. Fearsomely hot due to a runaway greenhouse effect, the Venusian surface is also under intense pressures equal to those some 3,000 feet (914 metres) under the surface of the Earth’s oceans. As if that doesn’t sound bad enough, the temperature is high enough to liquidize lead and, in the upper clouds of the planet, it rains sulphuric acid. Evidence suggests that the surface is volcanically active as well. If you were to land on the surface, you would be squashed, roasted and melted down into a small carbon briquette in less than a second, making it a less-than-ideal holiday destination. Despite the Venusian surface being the epitome of horrendous, the upper-most layer of the clouds is home to the most hospitable place in the Solar System beyond Earth. Here, the temperature and air pressures are similar to those of Earth and, theoretically, it would be possible for astronauts to explore without pressure suits, provided they had oxygen supplies and some protection against the acid rain. Fun Facts about Venus Venus has the longest day of any planet, completing a full rotation every 243 Earth days. This is slightly longer than the Venusian year, which is 225 days long. It is also the only planet that rotates backwards. Venus is the only other celestial body aside from Mars, the Moon and Saturn’s Moon, Titan, which has been photographed from the surface. The Soviet robotic space probe, Venera 13 landed on the surface in 1982. Venus is the third-brightest object in the sky after the Sun and the Moon. This is due to its relative closeness to Earth and its highly reflective clouds. It is so bright that it can even cast shadows. 4 – Earth, the Life-Giving World NASAEarth seen from orbit. Earth, the fourth rock from the Sun, is our home planet and the only place in the Universe known to be home to life. However, it would be a truly remarkable and practically an impossibility for it to be completely unique in the vastness of the Universe. Earth as a planet is actually not that special. Approximately 4.54 billion years old, it has a similar geological history, size, gravity and physical composition to Venus. Therefore, despite their differences, Earth and Venus are often considered sister planets. The only fundamental difference in these terms is that Venus has no moon, and it has lost much of its magnetic field. Earth’s first geological era was the Hadean, named after the Ancient Greek Underworld. At this time, our planet was a fiery, lifeless Hell until life came into being around 3.6 billion years ago, shortly after the end of the era. Life eventually evolved from microbial sea-life into larger organisms, though it took over 3-billion years to do so. Eventually, life spread to the land, dinosaurs came into existence and then mammals and, about two-million years ago, our earliest human ancestors. As you can see, humanity’s window of existence has been incredibly short compared to the lifespan of the planet. Earth is approximately half way through its lifespan, but it will probably be completely unable to support life after another billion years or less as the Sun gets larger and hotter. Fun Facts about Earth Our moon is essential to life as we know it. Without the lunar tides, the development of life would have taken a very different course, perhaps not getting far beyond the unicellular organisms that ruled its ancient oceans. The year is not actually 365 days long. The actual orbital period of Earth is about quarter of a day longer, hence we have a leap year every four years to make up for the discrepancy. Humanity’s vast and ever-increasing population (currently over 7.4 billion), as well as its profound effect on its environment, may mean that life on Earth will cease to exist long before its natural lifespan. 5 – Mars, the Red Planet NASAThe surface of Mars, taken by the Curiosity rover in 2012. The fourth planet from the Sun is also by far the most explored object in space. Being only half the diameter of Earth, it’s a much smaller world than our own. It also has two small irregularly shaped moons: Phobos and Deimos. Currently being explored by several surface landers, more is known about the surface of Mars than the bottom of the oceans right here on Earth. Incredible high-resolution photos allow us to see the surface of the Red Planet in stunning detail as though someone were there with a high-definition camera. For many years, it was thought that life existed on Mars, perhaps even intelligent life. Although this has now been disproven, life may well once have existed in this water-rich world, and it is not beyond the realms of possibility that some form of microbial life remains. The surface of Mars appears to be dead today, and its atmosphere is also very thin. Atmospheric pressure is equal to that about three times higher than Mount Everest, so humans would need pressure suits to be able to survive on the surface. It is also bitterly cold in most areas, although temperatures as high as 20°C during the day on the equator are not uncommon. Mars also boasts huge amounts of water, with icy poles rather like those on Earth, and there’s frost in some areas, although water cannot exist as a liquid on the surface for more than mere seconds due to the low air pressure. The weather can also be hostile, with huge sandstorms ravaging parts of the planet although, in the thin atmosphere, a strong wind would feel more like a gentle breeze. Mars was not always the barren desert world that it is today. Evidence strongly suggests that, billions of years ago, the planet boasted oceans and seas just like Earth, and perhaps a magnetic field and a thick atmosphere as well. Fun Facts about Mars The Curiosity rover, currently roaming the surface of Mars, is one of the most ambitious space projects ever. Its mission is to search for evidence of the building blocks of life and learn more about the climate, history and geology. Mars is the only planet aside from Earth that has been fully mapped in detail. In fact, Google has Google Mars, the Martian equivalent to Google Earth. It even boasts full 360° panoramic photos from rover landing sites. Mars is home to the Solar System’s highest mountain, the extinct volcano of Olympus Mons. It stands some 13 miles (20 km) high, roughly 2.5 times higher than Mount Everest, and this is possible only because of Mars’s low gravity. 6 – The Asteroid Belt NASACeres, the largest asteroid in the asteroid belt. The asteroid belt is a vast stretch of space extending from near the orbit of Mars to about half way towards Jupiter. This enormous region contains tens of thousands of asteroids, the largest of which, Ceres, Vesta and Pallas, account for almost half of its total mass. Contrary to popular belief, however, the belt covers such a large area that there is a negligible danger to passing spacecraft, unlike the impression you may have got from the movies, where spacecraft tend to pass through a treacherous field of giant rocks. In fact, a total of eleven spacecraft bound for the outer solar system have safely traversed the belt without incident. The asteroids may one day be a target for mining operations due to the abundance of precious materials that they are composed of. Today, NASA’s Dawn spacecraft continues to learn more about the various asteroids and protoplanets that inhabit the region to uncover the secrets of our Solar System’s birth. Fun Facts about Asteroids If it weren’t for the great influence from Jupiter’s enormous gravitational forces on the area, it is likely that another rocky terrestrial planet could have formed instead of the Asteroid Belt. Some asteroids, such as 243 Ida and 762 Pulcova, have tiny moons orbiting around them. In fact, there are at least two-hundred asteroids and dwarf planets with moons. Ceres was the first asteroid discovered, and the largest object in the Asteroid Belt. In 1801, it was sighted by the Italian astronomer Giuseppe Piazzi. 214 years later, the Dawn spacecraft took the first ever close-up photos of it. 7 – Jupiter, the Giant World NASAThe famous crescent view photograph of Jupiter, taken by Voyager 1 in 1979 at a distance of 17 million miles (30 million km). As the brightest object in the night sky after Venus and the Moon, Jupiter has been known since ancient times, and the Romans named it after their god of sky and thunder. Like all the known outer planets, Jupiter is a gas giant world. It is also by far the largest object in the Solar System aside from the Sun. Its mass is approximately two and a half times that of all the other planets combined. The giant world is characterised by its swirling bands of cloud and its Great Red Spot, a huge storm which has been circling the planet for several hundred years. Jupiter has no solid surface to speak of, although it likely has a dense rocky core several times larger than the Earth. No spacecraft could ever land on such a surface, since it would be under incredible pressures even greater than those at the centre of our own planet. Above this rocky core is a thick layer of liquid metallic hydrogen, also under intense pressures, crushed by the immense atmosphere above it. Jupiter is a cold and stormy world, being about 3.5 times further away from the Sun than Mars. Its storms are extreme, and bolts of lightning hundreds of times more powerful than those on Earth constantly ravage the upper atmosphere of the planet. Due to its immense size and mass, the gravitational forces exerted by Jupiter play an important role in the Solar System. Sometimes described as the Solar System’s vacuum cleaner, Jupiter attracts many asteroids, keeping them away from the inner planets. Without Jupiter, it’s possible that Earth would be far more likely to get bombarded by asteroids and other space debris. Jupiter has an extensive system of natural satellites (moons), with a total of at least 67. The four largest of these, Europa, Ganymede, Callisto and Io are known as the Galilean Moons, since they were discovered by Galileo in 1610. Each of these moons has a unique set of characteristics. Io, for example, is the Solar System’s most volcanically active object, Ganymede is even larger than the planet Mercury and Callisto and Europa may have oceans under their rocky surfaces that could even be home to life. Fun Facts about Jupiter Using improved spacecraft propulsion methods, the New Horizons robotic space probe, launched in 2006, took only thirteen months to reach Jupiter. By contrast, the Galileo probe, which arrived in 1995, took six years to get there. Jupiter has the shortest day of any planet in the Solar System, spinning around on its axis in only ten hours. This unusually fast rotation is also partly responsible for its violent storms and its relatively flat polar regions. You can easily see Jupiter with a naked eye, and it looks rather like a particularly bright star. Through a small telescope or even a large pair of binoculars, however, you can even see the Galilean moons as tiny white dots. 8 – Saturn, the Ringed World NASASaturn, with its largest moon Titan in the bottom-left corner, taken by the Cassini probe in 2008. Saturn is almost twice as far away from the Sun than Jupiter and, when it is at its closest distance to Earth, it is still almost 700 million miles (1127 million km) away. Saturn is visible to the naked eye, but being much smaller and further away than Jupiter, it appears much dimmer. Like Jupiter, Saturn has been known since ancient times and it also boasts a Roman name, after the Roman deity of agriculture and wealth. Also a gas giant, Saturn probably has a dense, rocky core beneath thousands of miles of liquid hydrogen. Similarly to Jupiter, it has thick bands of cloud around its upper atmosphere and boasts many fearsome storms with some of the fastest winds in the Solar System. Saturn is best known for its stunning rings of space dust, ice particles and thousands of tiny moonlets. In addition to its rings, Saturn also has a grand portfolio of moons, counting at least 62 in total. Only one of these moons is of a significant size, however. This is the world of Titan, one of the most unique objects in the Solar System. It’s the only moon with anything more than a trace atmosphere and the only celestial body beyond Earth that boasts surface liquid. However, instead of water, Titan flows with rivers and seas of liquid methane and ethane at -179 °C. Saturn has been widely explored, and a robotic space probe even landed on Titan and took photos of its surface. Saturn itself was first visited by the Pioneer 11 probe in 1979. Since then, the ringed planet has been visited by the two Voyager probes and the Cassini-Huygens spacecraft with its landing module for Titan. Fun Facts about Saturn Saturn has the lowest density of all the planets. With a density of 0.7 grams per cubic centimetre, it is even less dense than water. If Saturn were placed in giant ocean, it would actually float! Saturn’s rings have been known since the early seventeenth century, when they were first observed by Galileo Galilei. Saturnian winds are among the most ferocious in the Solar System with wind speeds in anticyclonic storms reaching 1,118mph (1799kph)– much faster than even the speed of sound! 9 – Uranus, the Coldest World NASAUranus, compared to Earth. Uranus was once thought to be a star. However, in 1781, astronomer William Herschel confirmed that it was indeed a planet, and the first planet discovered using a telescope. Though a gas giant, Uranus is much smaller than either Jupiter or Saturn, but it is still more than 14 times more massive than Earth. Strangely, Uranus is the coldest planet in the Solar System, being even colder than the far more distant Neptune. The reason for this is still somewhat of a mystery. Uranus absorbs very little heat from the Sun, and its interior is also colder than that of the other planets, making the temperatures of the surface average at -224°C. Uranus shares many similarities with the other gas giant worlds. It has powerful storms, no solid surface and a ring system, although this is much fainter and smaller than that of Saturn. Uranus is unique in that it rolls on its side as it orbits the Sun. With an axial tilt of over ninety degrees, the seasons on Saturn are very different to those on Earth. Being so far from the Sun, Uranus also takes 84 years to orbit the Sun. Since the planet rolls on its side, only a very narrow strip around the equator experiences a night and day cycle during a full rotation, which is a bit over seventeen hours long. Uranus has at least 27 moons, but they are all very small, with the largest being Titania with a radius of less than 500 miles (804 km). Fun Facts about Uranus You can only see Uranus without a telescope or binoculars on a very clear night when there is minimal light pollution. For best results, a small telescope or powerful pair of binoculars is often necessary to even reveal a point of light. Uranus has only been visited once by a robotic space probe, and that was by Voyager 2 when it flew by the planet in 1986. No more missions are planned for the foreseeable future. The fact that Uranus rolls on its side may be due to the planet getting knocked over by an enormous impact event in its early history. 10 – Neptune, the Furthest Planet NASANeptune, seen with four of its tiny moons. The most distant planet in our Solar System, Neptune, lies thirty times further from the Sun than Earth does. The lonely gas giant is slightly smaller than Uranus and about fifty percent further away from the Sun than its twin. Despite its greater distance, however, Neptune is a few degrees warmer than its neighbour, albeit still bitterly cold with surface temperatures of -218°C. In 1846, Neptune was the last planet to be discovered, although its existence had been predicted sometime before due to observation of a strange unseen gravitational influence on the orbit of Uranus. Neptune is characterized by its beautiful deep blue hue. Like the other gas giants, it is also a stormy world boasting extremely fast winds and centuries-old storms like the Great Dark Spot. Neptune has no solid surface other than a probably rocky core about the size of Earth’s moon. Neptune has at least thirteen moons, although all but one of these is extremely small and irregularly shaped. The only moon of significant size is Triton, discovered a few weeks after the planet itself. Triton is one of the Solar System’s few geologically active worlds with ice volcanoes on its surface spewing out geysers of liquid nitrogen. Like Uranus, Neptune has been visited by only one robotic space probe. Voyager 2 flew by the planet in 1989 and took the only ever close-up photos of the blue world. Fun Facts about Neptune Neptune is enormously far away, with a minimum distance from Earth of 2.6 billion miles (4.2 million km). It took Voyager 2 twelve years of travelling to reach the planet. Wind speeds on Neptune can be up to 1,300 mph (2092 kph), markedly greater than even those on Saturn. Although Neptune is about four times bigger than Earth, it has almost the same gravity. This is because the planet is much less dense and the gravitational forces are distributed over a larger area. 11 – The Kuiper Belt and Beyond NASAThe haze of Pluto’s tenuous nitrogen atmosphere, photographed by NASA’s New Horizon’s probe in 2015. Starting from the orbit of Neptune, about 2.6 billion miles (4.2 million km) from the Sun and extending to about five billion miles (8 billion km) is the Kuiper Belt. Like the Asteroid Belt between Mars and Jupiter, the Kuiper Belt is made up of numerous asteroids and dwarf planets. The Kuiper Belt is, however, far larger than the Asteroid Belt, being about twenty times wider. Most notably, the Kuiper Belt is home to Pluto which, until 2006, was classified as the ninth planet from the Sun. Partly because Pluto is so small (significantly smaller even than Earth’s moon), it was demoted to a dwarf planet. In fact, Pluto is not even the largest dwarf planet in the Solar System. A slightly larger and more massive dwarf planet, Eris, was discovered in 2005. Eris currently lies at a distance three times further from the Sun than even Pluto, which was discovered in 1930. It has four moons, although only Charon is of any considerable size. The Kuiper Belt is home to at least two other dwarf planets: Haumea and Makemake. Eris, the Solar System’s largest dwarf planet, is one of the most distant natural objects from the Sun, currently about nine billion miles (14.5 million km) away and far beyond even the Kuiper Belt. Beyond Eris, the Voyager 1 space probe continues travelling towards interstellar space. As of October 31, 2016, Voyager 1 was 12.7 billion miles (20.5 million km) away from home, making it the furthest man-made object from Earth and the first to reach the vast gap between the stars. To put Voyager 1’s distance away from Earth in perspective, it has been travelling since its launch in 1977 at a peak speed of approximately 10.5 miles per second (17 km per second). If it were headed directly towards the nearest star (other than the Sun), Proxima Centauri, it would take about 76,000 years to reach it. This goes to show just how tiny our Solar System is when compared to the phenomenal vastness of space. While Voyager 1 is a mere 12.7 billion miles (20.5 million km) from Earth, Proxima Centauri is 4.23 light-years or 25 trillion miles (40 trillion km) away. Fun Facts about the Outer Solar System Voyager 1 will likely be around long after all traces of humanity’s existence on Earth have vanished from existence, unless, in the unlikely event it collides with something or gets picked up by an extraterrestrial civilization one day. Voyager 1 continues to transmit data back to Earth, taking several hours to do so. Its radio transmitters should continue to work until at least 2025, after which it will finally run out of power. The edge of the Solar System is known as the heliopause. This area, about 12 billion miles (19.2 billion km) away, is where the Sun’s influence fades and the unspeakably enormous void between stars that is interstellar space begins. Conclusion There you have it, a fleeting journey from the Sun to the edge of the Solar System, just a brief introduction to our little corner of the galaxy. If you were travelling at the speed of light (186,282 mph or 300,000 kph), it would still take over four years to reach our nearest stellar neighbour beyond our own Sun, 2.5 million years to reach the nearest galaxy beyond the Milky Way and about 45 billion years to reach the edge of the observable universe. 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