(Originally printed in c.2000)
MERCURY FACTS:
Fact One:
Before the first space-crafts flew to Mercury to take pictures of its surface, some astronomers believed that the planet would have a smooth surface. When Mariner 10 took pictures of Mercury during its flyby of Venus and Mercury in November 1973, scientists discovered that Mercury is the most cratered planet in the Solar System with a surface similar to Earth's Moon.
Fact Two:
Mercury has no atmosphere at all. Sunlight reflects off its surface, similar to how light is reflected directly off the Moon's surface. The lack of atmosphere means that sunlight cannot be spread through the atmosphere. The planet's sky is dark, just like the Moon's and, if you were able to stand on the side of the planet not facing the Sun, you would be able to see billions of stars in the sky.
Fact Three:
A year on Mercury is only 88 days long, but a full day (from sunset to sunrise) takes 176 days, even though the planet takes 59 days to rotate on its axis. This causes the Sun to rise above the horizon at the beginning of a Mercurian day in the East (as on Earth), then move higher into the sky, move back down towards the horizon at the East, and then quickly move over to the Western horizon to set, after about 60 days!
Fact Four:
Mercury (4,878km wide) is slightly bigger than Earth's Moon (3475.6km wide) and is smaller than Ganymede (5,270km) and Callisto (4,890km), two of Jupiter's moons, and Titan (5,150km wide), a moon of Saturn.
Fact Five:
Mercury can only be seen just after the Sun has risen and just before the sun sets.
Fact Six:
If you were to stand on Mercury, the Sun would appear two and a half times bigger than it appears on Earth.
Fact Seven:
Mercury is about 100 times closer to the Sun than Pluto, the most distant planet in the Solar System.
Fact Eight:
Mercury's speed is caused by the Sun's strong gravitational pull. Hold a short piece of string with something tied to the end of it and spin it around. Try it again with a longer piece of string. Notice how much longer it takes for the object tied to the end of it to spin round. Your finger acts like the Sun, and the object acts like a planet orbiting the Sun.
Fact Nine:
Despite being the closest planet to the Sun, Mercury is not the warmest. Venus is warmer. In fact, Mercury can be one of the coldest planets in the Solar System
Fact Ten:
By the time one year has passed on Pluto (after 248 years), 1028 years have happened on Mercury!
VENUS FACTS:
Fact One:
Venus and Mercury are the only two planets in the Solar System not to have moons orbiting them.
Fact Two:
If we were able to stand on the surface of Venus, it would feel like being 1 kilometre under the sea on Earth, a depth deep enough to sink a submarine. A person or a creature would immediately be crushed by Venus' amazingly strong pressure.
Fact Three:
Venus may have such a thick atmosphere because it spins so slowly. In fact, a year on Venus is shorter than its day. It takes the planet longer to turn on its axis than it takes it to orbit the Sun. The slow rotation means that atmosphere does not have a force to spin off into space.
Fact Four:
Venus is the only planet in the Solar System to turn clockwise. All other planets turn anti-clockwise. It rotates clockwise on its axis extremely slowly, suggesting that something might have once collided with it to disrupt its regular rotation.
Fact Five:
People once believed Venus to be two different stars known as the Morning Star and the Evening Star, because it can be seen in the morning and the evening.
Fact Six:
After the Sun and the Moon, Venus is the brightest object in the night-sky from Earth
Fact Seven:
The Americans have only ever landed one probe on Venus. This was Pioneer Venus 2, launched on 8th August 1978 which was to probe the planet's atmosphere, not examine its surface. All other landings on Venus were made by the Russians. There are no plans for missions to return to Venus in the near future.
Fact Eight:
Venus' axis hardly has any tilt at all, unlike Mars and Earth. This means that, if it had a thin atmosphere, the planet would not have seasons.
Fact Nine:
There are more volcanoes on Venus than on any other planet in the Solar System, although it is not yet known whether any of these volcanoes are still active.
Fact Ten:
Venus may now resemble what Earth will become in millions of years time, when the Sun expands, heats the Earth, turning all of its surface water into a vapour which will trap sunlight and heat in its atmosphere, causing suffocating conditions like those on Venus.
MARS FACTS:
Fact One:
Of all the planets in the Solar System, Mars is the one people believe is most likely to contain or to have contained life. In 1900, a prize was offered to the first person to be contact an extra-terrestrial being. However, this extra-terrestrial being was not allowed to come from Mars because that would make the competition too easy! In 1938, a radio broadcast of The War of the Worlds by H.G. Wells (a story about an invasion of Earth by Martians) caused a near panic in America because so many people believed it to be true.
Fact Two:
The largest volcano in the Solar System is on Mars. It is called Olympus Mons.
Fact Three:
Mars has polar caps like Earth, containing frozen carbon dioxide (and small amount of water).
Fact Four:
Mars is believed to have had water flowing around it like Earth once. It may have had a blue sky too. However, it is unlikely that it had grass, trees and plants like Earth has now.
Fact Five:
The Valles Marineris, the greatest gorge on any planet in the Solar System, was caused when volcanoes erupting around it tore up the land, leaving a huge valley.
Fact Six:
We can see how long Mars has been a 'dead' planet by the number of meteorite impacts on its surface. In comparison, Earth and Venus have fewer impact craters because they have newer surfaces formed by recent geological activity.
Fact Seven:
Mars has seasons like Earth. This is caused by the tilt of the planet's axis, at a similar angle to the tilt of Earth's axis.
Fact Eight:
The Sun appears about half the size on Mars as it does from Earth.
Fact Nine:
The first space probe to take pictures of Mars' surface (Mariner 4 in 1964) is still in space, orbiting the Sun. Still orbiting Mars itself are Mariner 9 (launched in 1971), Viking 1 and 2 (launched in 1975), the Mars Global Surveyor (launched in 1996, which is currently looking for the Mars Polar Lander, launched in 1998) and the Mars Climate Orbitor (launched in 1998 to orbit Mars in 1999). These are American space probes. Russian probes orbiting Mars are Mars 2 and Mars 3 (launched in 1971), Mars 5 (launched in 1973) and Phobos 1 (launched in 1988). Japan has Nozomi orbiting Mars (launched in 1998).
Fact Ten:
Rocks from Mars have landed on Earth from meteorite impacts blasting debris through space.
JUPITER FACTS:
Fact One:
Jupiter has a liquid metal ocean (metallic hydrogen) at its centre, surrounding by thousands of kilometres of hydrogen and helium gas.
Fact Two:
Jupiter's famous Red Spot is in fact a great storm that has raged for at least four hundred years.
Fact Three:
Jupiter has a thin set of rings, hardly visible from Earth.
Fact Four:
Jupiter is the vacuum cleaner of the Solar System. It sucks in comets, asteroids and meteorites which could be on a collision course for Earth. For example, Shoemaker-Levy 9 in 1994.
Fact Five:
Jupiter takes only 9 hours and 55 minutes to spin on its axis. This means a day on Jupiter is less than 10 hours long. Its fast rotation causes the planet to be squashed, being wider at the equator than from North to South.
Fact Six:
Jupiter is the planet with the strongest pull of gravity in the Solar System. If we were able to stand on the surface of Jupiter, we would weigh three times as much as we would way on Earth. The only other object in the Solar System with a stronger pull of gravity is the Sun.
Fact Seven:
The mass of Jupiter is 318 greater than the mass of Earth.
Fact Eight:
Jupiter's moon, Ganymede, is the biggest moon in the Solar System.
Fact Nine:
If we were able to see Jupiter's radiation belt from Earth, Jupiter would appear as big as the Sun.
Fact Ten:
Jupiter's gravity is used to catapult space-craft on deep space missions further away. This is how the Voyager missions of 1975 managed to succeed.
SATURN FACTS:
Fact One:
Saturn has the most moons in the Solar System, including Titan, a moon with an atmosphere possibly similar to Earth's billions of years ago.
Fact Two:
Saturn is a slightly smaller version of Jupiter, with similar, but less distinctive, surface patterns. Its only main difference is its amazing rings.
Fact Three:
Saturn's rings are believed to be the particles of a old moon orbiting the planet, smashed apart in a collision about 50 million years ago.
Fact Four:
Saturn's rings orbit the planet at different tilts. Sometimes, they can appear like 'ears' sticking out of the planet. At other times, they are flat on when seen from Earth and are hardly visible. This shows how thin they can be.
Fact Five:
It is believed that Saturn's rings will one day disappear. They will either disperse (spread out) into space or get sucked into the planet by its pull of gravity. This will happen in about 50 million years.
Fact Six:
Despite its similarities to Jupiter, there is no great spot on Saturn, although the planet does have stormy weather.
Fact Seven:
Saturn is twice as far away from the Sun as Jupiter is.
Fact Eight:
Saturn is the second largest planet in the Solar System after Jupiter. It is so big that Earth could fit into it 755 times.
Fact Nine:
Saturn has such a low density (meaning that its particles are far apart) that, if there was an ocean big enough, Saturn would float on it. In comparison, Earth and Mercury would sink to the bottom quickest.
Fact Ten:
A year on Saturn would take almost thirty Earth years.
URANUS FACTS:
Fact One:
After Saturn, a space-craft would have to travel 1,500,000,000 kilometres to reach Uranus. This means the planet is almost twice the distance from the Sun than Saturn is.
Fact Two:
Uranus orbits the Sun on its side. Its South Pole is pointed towards Earth. The angle of the tilt of the axis of Uranus is 97 degrees.
Fact Three:
When the first pictures of Uranus from Voyager 2 were received on Earth in January 1986, scientists were disappointed to see a planet with a featureless, pale blue surface.
Fact Four:
Uranus' pale blue colour is caused by the methane in its atmosphere which filters out red light.
Fact Five:
If we were able to see Uranus' moons orbiting the planet, they would go over and under the planet like lights on a ferris wheel.
Fact Six:
Uranus has rings of ice and small rock particles which are so faint they appear as black as charcoal.
Fact Seven:
Uranus has 15 moons (so far discovered) orbiting the planet. Ten of these were discovered in 1986 by the Voyager 2 mission.
Fact Eight:
A year on Uranus would last 84 years.
Fact Nine:
A day on Uranus is only slightly shorter than a day on Earth, at about 17 hours long.
Fact Ten:
Because of Uranus' unique tilt, a night at one of its poles lasts for 21 Earth years, during which it will receive no light or heat at all from the Sun.
NEPTUNE FACTS:
Fact One:
Neptune's moon, Triton, is slowly getting closer to Neptune. Eventually, it will get so close that it will get torn apart by Neptune's gravity and possibly form rings more spectacular than Saturn's.
Fact Two:
The strongest winds in the Solar System have been recorded on Neptune, at speeds of up to 2,000 kilometres per hour.
Fact Three:
Neptune has dark spots similar to the Great Red Spot on Jupiter. These are areas of high atmospheric pressure which force clouds of methane gas high up into the atmosphere, appearing like cirrus (thin, whispy) clouds on Earth. However, these spots disappear and reappear on different parts of the planet, unlike Jupiter's spot.
Fact Four:
Neptune sometimes orbits the Sun further away than Pluto making it the most distant planet in the Solar System. It returned to its usual position as the second furthest planet away from the Sun in December 1999.
Fact Five:
Triton orbits Neptune in the opposite direction to the planet's rotation. It is the only large moon in the Solar System to do this.
Fact Six:
Neptune has four faint rings. Some parts of these rings are brighter in areas than others and appear like arcs orbiting the planet. Maybe they are still forming.
Fact Seven:
Since its discovery in 1846, Neptune has not yet completed a full orbit. In fact, it takes 165 years for the planet to go around the Sun.
Fact Eight:
Neptune was discovered when scientists noticed something peculiar about Uranus' orbit, believing something to be pulling it in the opposite direction to the Sun's pull of gravity. They calculated the position of a planet and soon afterwards, discovered Neptune.
Fact Nine:
The coldest temperatures measured in the Solar System (-230°c) have been recorded on Neptune's moon, Triton.
Fact Ten:
Pluto, a planet which has an orbit which sometimes crosses Neptune's, may have been a moon of Neptune which escaped the planet's pull of gravity, but being prevented from escaping the Solar System by the gravitational pull of the Sun.
PLUTO FACTS:
Fact One:
Pluto is the smallest planet in the Solar System, smaller than Earth's Moon, and half the width of Jupiter's moon, Ganymede.
Fact Two:
Pluto orbits the Sun on a different plain than the other 8 planets, going over them and below them.
Fact Three:
Pluto's orbit is elliptical, meaning that it can come closer to the Sun than Neptune, but then go almost two billion kilometres further away from Neptune's orbit. In the picture below, you can see Pluto's orbit (in grey) and Neptune's orbit (in blue)
Fact Four:
Pluto has one moon, Charon, which is not much smaller than Pluto itself. No other moon is as close to the size of its planet as Charon is to Pluto. (Pluto is 2,280 kilometres wide, Charon is 1,212 kilometres wide).
Fact Five:
Pluto's journey around the Sun takes 248 Earth years. This means that, since its discovery in 1930, it still has 177 years to go until it has made a complete orbit around the Sun.
Fact Six:
Pluto is the only planet in the Solar System not to have been visited by a space probe.
Fact Seven:
Pluto is the planet with the lowest pull of gravity in the Solar System. This will explain why its moon, Charon, orbits the planet so closely (at a distance of 19,640 kilometres).
Fact Eight:
Pluto was the only planet to have been discovered in the Twentieth Century.
Fact Nine:
A day on Pluto lasts for 6 days and 9 hours, meaning that it has the second slowest speed of rotation in the Solar System (after Venus, which takes 243 days to turn on its axis).
Fact Ten:
Nobody knows what Pluto's atmosphere contains, or even if it has an atmosphere. Any atmosphere is most likely to contain nitrogen.
