Children will love the cool, crazy, strange, funny, weird, odd, bizarre and wacky information.
The Sun is over 300000 times larger than earth. More Sun facts. Halley’s Comet was last seen in the inner Solar System in 1986, it will be visible again from Earth sometime in 2061 (get your camera ready).
More comet facts. Venus is the hottest planet in our solar system with a surface temperature of over 450 degrees celcius. Many scientists believe that an asteroid impact caused the extinction of the dinosaurs around 65 million years ago.
More asteroid facts. The Solar System formed around 4.6 billion years ago. More Solar System facts. The Moon appears to have more craters and scars than Earth because it has a lot less natural activity going on, the Earth is constantly reforming its surface through earthquakes, erosion, rain, wind and plants growing on the surface, while the moon has very little weather to alter its appearance. More Moon facts.
Saturn isn’t the only ringed planet, other gas giants such as Jupiter, Uranus and Neptune also have rings, they are just less obvious. Footprints and tyre tracks left behind by astronauts on the moon will stay there forever as there is no wind to blow them away.
In 2006, astronomers changed the definition of a planet. This means that Pluto is now referred to as a dwarf planet. Learn more dwarf planet facts. Because of lower gravity, a person who weighs 200 pounds on earth would only weigh 76 pounds on the surface of Mars.
The only planet that rotates on its side like a barrel is Uranus. The only planet that spins backwards relative to the others is Venus. Some of the fastest meteoroids can travel through the solar system at a speed of around 42 kilometres per second (26 miles per second).
Check out more meteoroid facts or learn the difference between comets, asteroids and meteoroids. The first man made object sent into space was in 1957 when the Russian satellite named Sputnik was launched.
Jupiter’s 4 biggest moons are named Europa, Ganymede, Callisto and Io. More Solar System moon facts. It is because of the Sun & Moons gravity that we have high & low tides. For a list of important space definitions take a look at our glossary of easy space and astronomy definitions for kids.
Easy Space Definitions
Asteroid: Asteroids are small solar system bodies that orbit the Sun. Made of rock and metal, they can also contain organic compounds. Asteroids are similar to comets but do not have a visible coma (fuzzy outline and tail) like comets do. Asteroid Belt: The asteroid belt lies roughly between the orbits of Mars and Jupiter in the Solar System. It is home to a large amount of irregular shaped asteroids that range in size from dust through to the dwarf planet Ceres. Astronaut: An astronaut (also known as cosmonaut) is someone trained to be a crew member of a spacecraft. While the word astronaut usually refers to space travel professionals it can also include normal people who have the privilege of traveling into space. Comet: A comet is a relatively small solar system body that orbits the sun. When close enough to the Sun they display a visible coma (a fuzzy outline or atmosphere due to solar radiation) and sometimes a tail. Dwarf planet: An object orbiting the Sun that is large enough to be rounded by its own gravity but is not gravitationally dominant in its orbital area and is not a moon. There are currently five recognized dwarf planets: Pluto, Ceres, Eris, Makemake & Haumea. Earth: Earth is the fifth largest planet in the Solar System and third from the Sun. It was formed around four and a half billion years ago and is the only place in the Universe where life is known to exist. Galaxy: A galaxy is a large group of stars, dust, gas and dark matter held together by gravity. They vary in size with some containing millions of stars while others could contain as many as a trillion. They can also form in different shapes such as elliptical galaxies and spiral galaxies. Halley’s Comet: Halley’s Comet (or Comet Halley as it is also known) is the most well known comet in the Solar System. It orbits the Sun and can be seen with the naked eye from Earth around every 75 years, returning for its next visit sometime in 2061. Jupiter: Jupiter is the largest planet in our solar system. It features the famous ‘Red Spot’ and a large number of orbiting moons. Mars: Mars, or the 'Red Planet' as it is sometimes known, is the fourth planet from the Sun. It features a dusty, rocky surface, relatively calm conditions and a thin atmosphere. Mercury: Mercury is the closest planet to the Sun in the Solar System. As well as being very hot, it features a barren, crater covered surface which looks similar to the Moon. Meteor: A meteoroid that burns up as it passes through the Earth’s atmosphere is known as a meteor. If you’ve ever looked up at the sky at night and seen a streak of light or ‘shooting star’ what you are actually seeing is a meteor. Meteorite: A meteoroid that survives falling through the Earth’s atmosphere and colliding with the Earth’ surface is known as a meteorite. Meteoroid: A meteoroid is a small rock or particle of debris in our solar system. They range in size from dust to around 10 metres in diameter (larger objects are usually referred to as asteroids). Milky Way: The Milky Way is a spiral galaxy where our Solar System and Earth are located. Moon: The Moon is a natural satellite which orbits the Earth. It is around a quarter the size of Earth and can be easily seen in the night sky. While other planets in the Solar System have ‘moons’, they are usually referred to by name, such as Jupiter’s Ganymede, or as natural satellites. Neptune: Neptune is the eighth planet from the Sun and is nearly four times the size of Earth. It features strong winds and violent weather. Planet: A planet is an object orbiting a star that is large enough to be rounded by its own gravity. It is also gravitationally dominant in its orbital area but not large enough to cause thermonuclear fusion (like stars do). There are eight planets in the Solar System. Pluto: Pluto was the furthest planet from the Sun until it was demoted from a planet to a dwarf planet in 2006. Saturn: Saturn is the second largest planet in our solar system, the sixth planet from the Sun and features an impressive system of rings. Small Solar System Body: Objects that orbit the Sun but aren’t planets or dwarf planets are known as small solar system bodies, these include comets, asteroids and other small bodies. Solar System: The solar system includes the Sun and all the objects that orbit around it due to its gravity, including Earth. Star: A star is a huge, bright ball of burning gas that is held together by gravity. Stars contain mostly hydrogen as well as helium and smaller amounts of other elements. The Sun is the closest star to Earth. Sun: The Sun is a star and the biggest object in the Solar System, it burns brightly in the center as planets and other objects orbit around it. It has a diameter around 110 times bigger than the Earth’s and is located around 150 million kilometres (93 million miles) away. Universe: The Universe is made up of everything that exists, including planets, stars, galaxies and all forms of matter and energy. Uranus: Uranus is the third largest planet in the Solar System and seventh planet from the Sun. Uranus rolls like a barrel rather than spinning like Earth and was the first planet discovered by telescope. Venus: Venus is the second closest planet to the Sun. It is similar in size to Earth and features thick a thick atmosphere which locks in heat as the surface rages with active volcanoes.
Asteroids are small Solar System bodies that orbit the Sun. Made of rock and metal, they can also contain organic compounds (some scientists suggest that asteroids could have brought they necessary chemicals to start life on Earth). Asteroids are similar to comets but do not have a visible coma (fuzzy outline and tail) like comets do. Asteroids are also known as planetoids or minor planets. Asteroids vary greatly in size, some feature diameters as small as ten metres while others stretch out over hundreds of kilometres. Note that objects under ten metres in diameter are generally regarded as meteoroids. The first asteroid was discovered in 1801 by Italian astronomer Giuseppe Piazzi. Named Ceres, it features a diameter of around 950 kilometres and is now regarded as a dwarf planet. Ceres was given dwarf planet status in 2006, along with Pluto, Eris, Makemake and Haumea. The asteroid belt lies roughly between the orbits of Mars and Jupiter in the Solar System. It is home to a large amount of irregular shaped asteroids that range in size from dust through to the dwarf planet Ceres. The technology used for discovering asteroids has improved dramatically since original discoveries and astronomers now have access to a range of powerful telescopes to aid in their research and discoveries. It is believed by many scientists and researchers that an asteroid impact was the cause behind the extinction of the dinosaurs around 65 million years ago. The possibility of an asteroid colliding with Earth has received increased attention over recent years. The Shoemaker-Levy comet that collided with Jupiter in 1994 were given widespread media coverage and Hollywood also played its part with moves such as Deep Impact (1998) and Armageddon (1998). While these movies sometimes featured dubious science they certainly increased public awareness of the topic. There are now many groups and organizations that use automated systems to discover near Earth asteroids. While many are discovered, they rarely have the potential to cross paths with Earth. There have been many ideas suggested as ways to avoid the unlikely but potentially devastating impact of an asteroid collision with Earth, these include using nuclear explosions to break the asteroid into smaller pieces or other weapons to deflect it off course.
An astronaut or cosmonaut is a person trained by a spaceflight program to lead, pilot or be a crew member of an expedition to space on a spacecraft. As well as being used to describe a professional space traveller, the term astronaut or cosmonaut is now often used for anyone who travels into space, such as scientists, politicians, journalists, and tourists. The World Air Sports Federation (FAI) recognizes only those flights that reach an altitude of more than 100 km (62 mi) as space flights. However, the USA awards astronaut wings to any astronauts who travel above 80 km (50 miles). The "edge of space" is scientifically accepted to be 100 km (62 mi) above sea level at the 'Karman line'. As of June 2013, a total of 532 people representing 36 nations have been above the Karman Line and therefore have reached outer space. The term "astronaut” is derived from the Greek words astron, meaning "star", and nautes, meaning "sailor". An astronaut employed by the Russian Federal Space Agency is called a kosmonavt (cosmonaut in English), derived from the Greek words kosmos, meaning "universe", and nautes, meaning "sailor". The first person to reach space was Soviet, Yuri Gagarin, in 1961, on board the spacecraft Vostok 1, he orbited Earth for 108 minutes. The first woman in space was Soviet Valentina Tereshkova, in 1963. She orbited Earth for nearly 3 days aboard Vostok 6. Russian, Sergei Krikalev, has been to space 6 times including on two ISS expeditions and has spent a total of 803 days, 9 hours and 39 minutes (or 2.2 years) in space, more than any other human who has ever lived (as at 2013). Including Neil Armstrong and Buzz Aldrin, 12 men have walked on the Moon, two from each of the six different Apollo missions. Cosmonaut Alexei Leonov was the first person to carry out an extra-vehicular activity (EVA) or a "spacewalk", on March 18, 1965, on the Voskhod 2 mission. Cosmonaut Anatoly Solovyev holds the record for the most EVAs or spacewalks with 16, over a total of 82 hours and 22 minutes (as at 2013). Astronauts that undertake an EVA or spacewalk usually have to use 70 to 110 tools to complete the tasks or fixes required on a spacecraft. Americans Jerry L. Ross and Franklin Chang-Diaz have been into space a record seven times each (as at 2013). The farthest an astronaut has travelled from Earth was 401,056 km (249,205 mi), by Jim Lovell, Jack Swigert, and Fred Haise, aboard Apollo 13. Throughout spaceflight history 18 astronauts have lost their lives during four tragic space flights. 11 other people have lost their lives training for spaceflight. In early space programs, trainee astronauts needed previous military jet test piloting and engineering experience. Today, high achieving students of engineering, biological science, physical science or mathematics can all make it. To become a NASA trainee, you are required to be a US citizen, you must pass a strict physical examination, have 20/20 vision, and good blood pressure. Once selected, NASA astronauts spend 20 months training in a variety of areas. To simulate spacewalks and test equipment astronauts often have to train underwater in swimming pools here on Earth.
A comet is a relatively small solar system body that orbits the Sun. When close enough to the Sun they display a visible coma (a fuzzy outline or atmosphere due to solar radiation) and sometimes a tail. The coma is created as the comet gets closer to the Sun, causing water, carbon dioxide and other compounds to sublime (quickly changing from solid to gas) from its surface. Comets are made of ice, dust and small rocky particles. The name comet comes from the Greek word meaning ‘hair of the head’, it came from the Greek philosopher Aristotle who observed comets as ‘stars with hair’. Short term comets (also known as periodic comets) have orbital periods of less than 200 years while long term comets have orbital periods of over 200 years.
Halley’s Comet (or Comet Halley as it is also known) is the most well known comet. It is known as a periodic comet (or short term comet) because the time it takes to orbit the Sun is less than 200 years. Records of humans observing Halley’s Comet go back thousands of years, with appearances noted by Babylonian, Chinese and European star gazers. It can be seen with the naked eye from Earth every 75 to 76 years (although the time period has between 74 and 79 years in the past). It last appeared in the inner Solar System in 1986 and will return again sometime in 2061 (start charging your camera battery). Halley’s Comet is named after English astronomer Edmond Halley who first determined its period of orbit. It was the first comet to be recognized as having a periodic orbit. Halley’s Comet appearance in 1986 allowed researchers to investigate its make up more closely using spacecraft. While some previous theories were proven correct, other models were altered with the new information. For example, while earlier models predicted the comet to feature many volatile ices, the actual amount was less than first expected. The tail and fuzzy glow you see around Halley’s Comet is known as a coma. It occurs when the comet gets close to the Sun and compounds such as frozen water and carbon dioxide sublime (rapidly change from solid to gas) from its surface. While the coma over Halley’s Comet can stretch up to 100,000 km across, the nucleus is actually small, only around 15km (9.3 miles) long, 8km (5 miles) wide and 8km (5 miles) thick.
Comet Shoemaker-Levy 9
In July 1994, the Shoemaker-Levy 9 comet broke apart and collided with Jupiter. This event gave astronomers a unique opportunity to observe what happens when such a collision occurs. The largest fragments were 2 km (1.2 mi) in diameter and struck Jupiter at a speed of around 60 km/s (37 mi/s). The impact scars were clearly visible for months after the impact. Shoemaker-Levy 9 was originally located by astronomers Eugene M. and Carolyn Shoemaker and David Levy in March 1993.
What are dwarf planets? Why is Pluto no longer a planet?
In 2006 the International Astronomical Union adopted the term ‘dwarf planet’ for solar system objects that were bigger than small solar system bodies such as comets and asteroids but not quite planets. The definition of a dwarf planet is an object orbiting the Sun that is large enough to be rounded by its own gravity but is not gravitationally dominant in its orbital area and is not a moon. As of 2008, there are five recognized dwarf planets: Pluto, Ceres, Eris, Makemake & Haumea. There are a number of other candidates for the status of dwarf planet. Some of these classifications could be resolved as NASA’s Dawn and Horizon missions venture towards Pluto in the coming years.
Pluto hit the headlines in 2006 when it was demoted from a planet to a dwarf planet by the International Astronomical Union. While most astronomers agreed with the new classifications, some disagreed and still refer to Pluto as the ninth planet.
Ceres is located in the asteroid belt between the orbits of Mars and Jupiter. It was discovered in 1801, well before Pluto and 45 years before Neptune. Ceres was considered a planet for around 50 years before being reclassified as an asteroid and once again in 2006 as a dwarf planet.
Eris was discovered in 2005 and was referred to as the tenth planet until it was reclassified in 2006. It is the largest of the dwarf planets.
Makemake was discovered in 2005 and the third largest dwarf planet behind Eris and Pluto.
Haumea was discovered in 2004 and named a dwarf planet in 2008.
A galaxy is a massive group of stars, star clusters, interstellar gas and dust, and dark matter which is all gravitationally bound together. The word 'galaxy' is derived from the Greek word galaxias which means "milky", it is a reference to our own galaxy the Milky Way. There are potentially more than 170 billion galaxies in the observable universe. Some, called dwarf galaxies, are very small with about 10 million stars, while others are huge containing an estimated 100 trillion stars. Based on shape astronomers have identified various kinds of galaxies including, elliptical galaxies, spiral galaxies, lenticular galaxies and irregular galaxies. Spiral galaxies are rotating flattened disk-shapes with at least two spiral arms of newer stars extending out from a central bulge of older stars. The dense molecular clouds of hydrogen gas and dust in the spiral arms of spiral galaxies are areas of intense star formation. Barred spiral galaxies (like our Milky Way) contain a long bar in the middle with spirals arms coming off the ends. Around two-thirds of spiral galaxies contain a barred structure in their center. The Hubble classification scheme, lists spiral galaxies as type S, with an 'a', 'b', or 'c' depending on how tight the spiral arms are and the size of the center bulge. Barred spiral galaxies have the symbol SB. Spiral galaxies are believed to be younger than elliptical galaxies, as spiral galaxies burn through their gas and dust star formation slows, they lose their spiral shape and slowly evolve into elliptical galaxies. Elliptical galaxies are a mass of stars bunched together in the shape of an elliptical disk. Elliptical galaxies are often larger, very old and contain little gas and dust, so therefore form very few new stars. The Hubble classification scheme identifies elliptical galaxies with the letter E, followed by a number representing the degree of ellipticity. Lenticular galaxies (S0 symbol) have a bright central bulge with a disk-like structure but, unlike spiral galaxies, the disks have no spiral structure and are not actively forming many stars. Any galaxy that has no obvious spiral or elliptical structure are called irregular galaxies. Some irregular galaxies would have just formed that way while others are the result of other galaxy types crashing into each other. Our Milky Way Galaxy is a barred spiral galaxy about 120,000 light-years in diameter containing up too 400 billion stars and possibly just as many planets. Our Solar System is located within the disk of the Milky Way Galaxy, around 27,000 light-years from the Galactic Center of the galaxy. Supermassive black holes are believed to sit at the center of most galaxies. Our Milky Way Galaxy is part of a 'Local Group' of galaxies in which the galaxies move relative to each other. The Andromeda Galaxy is the largest galaxy of the Local Group, followed by the Milky Way and the Triangulum Galaxy, there are around 30 other smaller galaxies in the group. Galaxies often collide with each other. Usually, the stars within each will move past one and other due to the vast space between them. Any gas clouds and dust will interact, forming new stars. Gravity can pull the galaxies into new shapes, two spiral galaxies might join into a new elliptical, others produce bars, rings, or tails. Starburst is a name for galaxies that form a lot of new stars at a fast rate, usually after much molecular cloud is produced as two galaxies merge.
A meteoroid is a small rock or particle of debris in our solar system. They range in size from dust to around 10 metres in diameter (larger objects are usually referred to as asteroids). A meteoroid that burns up as it passes through the Earth’s atmosphere is known as a meteor. If you’ve ever looked up at the sky at night and seen a streak of light or ‘shooting star’ what you are actually seeing is a meteor. A meteoroid that survives falling through the Earth’s atmosphere and colliding with the Earth’s surface is known as a meteorite. The fastest meteoroids travel through the solar system at a speed of around 42 kilometres per second (26 miles per second). The Earth’s atmosphere experiences millions of meteors every day. Meteors are easier to see during the lower light conditions of night. A small percentage of meteoroids fly on a path that goes into the Earth’s atmosphere and then back out again, they are known as Earth grazing fireballs. When many meteors occur in a close time frame in the same part of the sky it is called a meteor shower. Around 500 meteorites reach the Earth’s surface every year but of those only around 5 ever make it to scientists for study. Meteorites that are observed as they fall through the Earth’s atmosphere and later recovered are called ‘falls’, all others are called ‘finds’. To this date there have been around 1000 collected ‘falls’ and 40000 ‘finds’.
The Moon is the Earth’s only natural satellite. A natural satellite is a space body that orbits a planet, a planet like object or an asteroid. It is the fifth largest moon in the Solar System. Learn more about the other moons in the Solar System. The average distance from the Moon to the Earth is 384403 kilometres (238857 miles). The Moon orbits the Earth every 27.3 days. Mons Huygens is the tallest mountain on the Moon, it is 4700 metres tall, just over half the height of Mt Everest (8848m). The Moon rotates on its axis in around the same length of time it takes to orbit the Earth. This means that from Earth we only ever see around 60% of its surface (50% at any one time). The side that we can see from Earth is called the near side while the other side is called the far side (it is sometimes called the dark side despite the fact that it illuminated by the Sun just as much as the near side). The effect of gravity is only about one fifth (17%) as strong on the surface of the Moon compared to the strength of gravity on the surface of the Earth. The Soviet Union’s Luna program featured the first successful landing of an unmanned spacecraft on the surface of the Moon in 1966. The USA’s NASA Apollo 11 mission in 1969 was the first manned Moon landing. The first person to set foot on the Moon was Neil Armstrong. The far side of the Moon looks quite different due to its lack of maria (ancient pools of solidified lava). The surface of the Moon features a huge number of impact craters from comets and asteroids that have collided with the surface over time. Because the Moon lacks an atmosphere or weather these craters remain well preserved. Although research is continuing, most scientists agree that the Moon features small amounts of water. The Moon is very hot during the day but very cold at night. The average surface temperature of the Moon is 107 degrees Celsius during the day and -153 degrees Celsius at night. The Earth’s tides are largely caused by the gravitational pull of the Moon. The phases of the Moon are: New Moon, Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, Crescent, New Moon…. A lunar eclipse occurs when the Earth is between the Sun and the Moon.
A satellite is an artificial object that is deliberately sent into orbit in space, usually in order to send, receive or bounce back information to different areas of Earth. Artificial satellites are man-made satellites, they differ to natural satellites such as the Moon which orbits Earth and Earth itself which orbits the Sun. Satellites are usually launched on rockets into space to reach their orbits. In October 1957, the Soviet Union launched Sputnik 1 which was the world's first artificial satellite to be sent into space orbit. This event lead to the start of the 'Space Race' between the Soviet Union and the United States which would continue for many years. Sputnik 2, launched in November 1957, carried the first living creature into orbit, a dog named Laika. Just three months after the Soviet Unions first successful satellite launch, the USA launched its first satellite, Explorer 1, in January 1958. Since the first satellite was launched in 1957, over 6,500 satellites have been sent into orbit (as at October 2010). An estimated 3,600 of these remain in orbit. With about 1,000 still operational, the rest are now classed as space debris. The International Space Station (ISS) is the largest artificial satellite currently orbiting Earth. There's 3 main groups of satellites. Fixed satellite services handle billions of voice, data, and video transmissions. Mobile satellite systems used for navigation (GPS) and to connect remote ships, aircraft, etc. Scientific research satellites for meteorological data, land survey images, and other scientific research functions. A geostationary satellite orbits Earth from west to east over the equator. It moves in the same direction and speed as Earth is spinning, therefore from Earth, these satellites don't appear to be moving. Polar-orbiting satellites orbit Earth in a north-south direction from pole to pole, so as Earth spins underneath, these satellites scan the entire globe strip by strip. Low Earth orbit (LEO) is an altitude classification for any satellites that are in orbit less than 2000 km (1240 mi) above Earth, which has about 500 active satellites. Medium Earth orbit (MEO) is any orbit higher than LEO but still below the altitude for geostationary orbit which is 35,786 km (22,236 mi), about 50 active satellites. High Earth orbit (HEO) is any orbit higher than geostationary orbit at 35,786 km (22,236 mi), which has over 400 active satellites. 'Geocentric orbit' is the term for an orbit around Earth. 'Heliocentric orbit' is a term for an orbit of the Sun. 'Areocentric orbit' is the term for an orbit of Mars. The two main components of an artificial satellite is an antenna to send and receive information and a power source such as a solar panel or battery. The speed a satellite must travel to stay in space is called its orbital velocity. It usually needs to be more than 17,500 mph (28,200 km/h). Space debris can be anything from large obsolete satellites to natural or artificial fragments of space materials. Since 1957, the Space Surveillance Network SSN has tracked more than 26,000 objects above 10 cm in diameter. As more satellites are launched the chances of a crash increases, in 2009, two communications satellites - one American and one Russian - collided in space. Space probes have been put into orbit around other planets and moons in our solar system.
The solar system includes the Sun and all the objects that orbit around it due to its gravity. This includes things such as planets, comets, asteroids, meteoroids and moons. The Solar System formed around 4.6 billion years ago. There are eight planets in the Solar System. The four inner planets are Mercury, Venus, Earth and Mars while the four outer planets are Jupiter, Saturn, Uranus and Neptune. The inner planets (also known as terrestrial planets) are smaller and made mostly of rock and metal. The outer planets (also known as gas giants) are much larger and made mostly of hydrogen, helium and other gases. As of 2008, there are also five dwarf planets: Pluto, Ceres, Eris, Makemake & Haumea. There is an asteroid belt which lies between the orbits or Mars and Jupiter, it features a large number of irregular shaped asteroids. For thousands of years humans were unaware of the Solar System and believed that Earth was at the center of the Universe. Astronomers such as Nicolaus Copernicus, Galileo Galilei, Johannes Kepler and Isaac Newton helped develop a new model that explained the movement of the planets with the Sun at the center of the Solar System. The Sun contains 99.86 percent of the Solar System's known mass, with Jupiter and Saturn making up making up most of the rest. The small inner planets which include Mercury, Venus, Earth and Mars make up a very small percentage of the Solar System’s mass.
Solar System Moon
While we usually think of the Earth’s Moon there are many other moons in the Solar System. As of 2009, there were 336 moons in the Solar System. 168 of these orbit planets, 6 orbit dwarf planets, while the rest orbit asteroids and other Solar System objects (many yet to be classified). Jupiter’s four main moons are named the Galilean moons (after Galileo Galilei). Their names are Io, Europa, Ganymede and Callisto. Io features over 400 active volcanoes. Some scientists believe that water could exist below the surface of Europa. Ganymede is the largest moon in the Solar System. Callisto has a similar diameter to the planet Mercury but only one third of its mass. Saturn’s largest moon is named Titan, it is the only moon known to have a dense atmosphere. Neptune’s largest moon, Triton, is similar in size to Earth’s moon. Discovered in 1846 it is the seventh largest moon in the Solar System. The largest moon of the dwarf planet Pluto is named Charon, it was discovered in 1978 and has a diameter that is around half as wide as Pluto’s. Mars has two moons named Phobos and Deimos, both were discovered in 1877. Venus and Mercury have no moons.