A solar system is a group of planets and one or more stars. Our solar system has just one gigantic star, the Sun. It also has eight planets and three dwarf planets. Many planets have their own moons. There are also thousands of asteroids, meteoroids, and comets. Gravity from our huge Sun holds the solar system together. Planets, asteroids, and comets revolve around (orbit), the Sun. moons orbit some of the planets. A year for a planet is the amount of time it takes the planet to make one complete revolution around the Sun. The planets also rotate on an axis. One day on a planet is the time it takes for the planet to make one complete rotation (360 degrees) on its axis. It takes Earth about 24 hours to complete one rotation.
There are inner and outer planets. Mercury, Venus, Earth, and Mars are the inner planets. They orbit close to the Sun. The inner planets are very rocky. They are smaller than the outer planets. An asteroid belt separates the inner planets from the outer planets.
Jupiter, Saturn, Uranus, and Neptune are the outer planets. They orbit far from the Sun. The outer planets are all much larger than the inner planets. We call the outer planets the gas giants because they are made mostly from gasses.
Are you wondering about Pluto? In the past, we thought Pluto was a planet but astronomers now classify it as one of the dwarf planets. The other dwarf planets are Ceres and Eris. Ceres and Eris are both large asteroids. Astronomers discovered Eris in 2005. Eris is larger than Pluto and is even farther away from the Sun.
The Origins of Our Solar System
Where did the solar system come from? Many astronomers believe it began as a cloud of dust and gas. Nearly 5 billion years ago, a nearby star may have exploded. The explosion caused the cloud to collapse and spin into a disk. Gravity made the dust and gas pull together into the center of the disk. The force of gravity caused great heat and pressure. Nuclear reactions occurred. These reactions caused the Sun to shine for the first time.
The rest of the solar system also came from that cloud. The dust and gas formed into hot clumps. The clumps began orbiting the Sun. As they grew bigger, they became planets and moons.
Many planets and moons still have scars from the formation of the early solar system. Impact craters are holes in the ground that form when material from space slams into a planet, or sometimes even a moon.
All elements that make up the Sun and planets came from the ancient dust and gas. This includes all the plants and animals on the Earth. Scientists believe that even the atoms that make up your body are as old as the stars.
Even though it is 93 million miles away, the Sun is the nearest star to Earth. In comparison to other stars, the Sun is a medium sized star. All stars, including the Sun, are balls of exploding gas. You might think of stars as having points like the ones we draw but stars are shaped more like a ball (sphere).
The Sun is the center of our solar system. Another name for the Sun is Sol. Most of the light and heat on Earth comes from the Sun. Without that heat source, life on Earth could not exist. Scientists believe that eventually changes in the Sun will make life on Earth, as we now know it, impossible.
Compared to the Earth, the Sun is huge. It is the largest object in our solar system. The Sun’s diameter is over 864,432 miles (1,392,000 kilometers). That is nearly 109 Earths across. It would take 1,300,000 Earths to fill up the Sun.
Because it is so huge, the Sun exerts a lot of gravity. This force holds the planets in orbit.
The Sun’s Core
The center (core), of the Sun creates solar energy. In the core, temperatures and pressures are very high. This makes nuclear reactions occur. It takes a million years for energy made in the core to reach the surface. The Sun releases its energy as heat and light.
The Sun’s Outer Layers
The photosphere is the top layer of the Sun’s surface. This is where we find sunspots. Sunspots appear as dark green areas in the photosphere. Solar flares come from the sunspots. These bright arcs of hot gas can interfere with radio communications on Earth. The chromosphere is above the photosphere. Light and solar flares pass through the chromosphere on their way out into space. The corona is the outermost part of the Sun’s atmosphere.
During a solar eclipse, the corona is the part of the Sun that is seen shining around the Moon. A solar eclipse occurs when the Earth’s Moon blocks out most of the light from the Sun. Many people are tempted to look directly at the Sun during an eclipse. This is very dangerous. Even if you don’t feel pain in your eyes, looking directly at the Sun can cause permanent eye damage or blindness.
Mercury is the closest planet to the Sun. It is a small planet, just a little bigger than Earth’s Moon. Mercury has a diameter of about 3,029 miles (4,878 kilometers). It revolves, or orbits, around the Sun once every 88 Earth days. It has an elliptical orbit. An elliptical orbit is like a stretched out circle (oval shape). Its average distance from the Sun is about 36 million miles (58 kilometers).
Days are very long on Mercury. This is because the planet has a very slow spin. Mercury rotates once every 59 Earth days. One day on Mercury would be 1,416 hours long!
Temperatures get very hot during the daytime. Surface temperatures can reach over 752 degrees Fahrenheit (400 degrees Celsius). At night it gets very cold. It can reach below -279.4 degrees Fahrenheit (-173 degrees Celsius). Mercury has some of the highest and lowest temperatures in the solar system.
There is almost no atmosphere on Mercury. Because of the heat, any gasses on Mercury would burn up.
Mercury has no moons.
The surface of Mercury is hard and rocky. Impact craters cover the surface. Mercury’s craters formed a long time ago during the early history of the solar system. Some craters are small. Others are very large. Mercury has cliffs up to 1.8 miles (3 kilometers) high and hundreds of miles long.
The first spacecraft to visit Mercury was Mariner 10, in 1974. It took photographs of the planet. It found that Mercury has a weak Welding goggles are the best eye protection magnetic field.
Venus is the second planet from the Sun. Because it is so close to Earth, you can often see Venus in the sky at dusk or dawn, without the use of a telescope. Sometimes we call Venus our sister planet because it is a lot like Earth. It is about the same size as Earth. Both planets have a near circular orbit around the Sun. A lack of craters shows that both planets formed about the same time.
Although they are alike in many ways, Venus and Earth are different in some very important ways. Venus rotates in the opposite direction as Earth. Its spin is retrograde. That means it spins backward from the rotation of Earth and the other planets. On Venus, the Sun rises in the west and sets in the east. Venus rotates much slower than Earth. It takes about 243 Earth days for Venus to rotate one time. Another difference is that Venus has a very weak magnetic field.
The diameter of Venus is 7,516 miles (12,103 kilometers). Its average distance from the Sun is about 67 million miles (108 million kilometers). It takes Venus 225 Earth days to orbit the Sun. It might be a little confusing to live on Venus because one planetary year is shorter than one planetary day!
The surface of Venus is very hot. Temperatures on the surface can reach 880 degrees Fahrenheit (470 degrees Celsius) making it the hottest planet. Venus is far too hot to support life.
The atmosphere of Venus is mostly carbon dioxide. Because the carbon dioxide is so thick, the pressure on Venus is 90 times that of Earth. The clouds in the upper atmosphere are made of tiny drops of sulfuric acid. The sulfuric acid forms thick clouds that reflect the Sun’s rays making Venus shine brightly like a star.
Venus has no moons.
The surface of Venus is rocky. It has many inactive volcanos. There are also many mountains, some of which are higher than the mountains on Earth.
The first spacecraft to successfully orbit Venus was Mariner 2, in 1962. Since then, many other spacecraft have flown by, orbited, or landed on its surface. The Magellan probe has provided maps of Venus.
Earth, our home, is the third planet from the Sun. It is the only planet we know of that contains life. The diameter of the Earth is about 7,920 miles (12,754 kilometers). The Earth rotates one time every 24 hours. A planetary day on Earth is 24 hours. Earth’s planetary year is about 365 days. It takes the Earth 1 year to revolve, or orbit, the Sun. Earth’s average distance from the Sun is about 93 million miles (150 million kilometers). We call this distance 1 astronomical unit, or AU. We sometimes measure distances to other planets in astronomical units.
The coldest parts of Earth can reach -129 degrees Fahrenheit (-90 degrees Celsius). The hottest parts can reach 136 degrees Fahrenheit (58 degrees Celsius). Most life on Earth will survive in places with temperatures somewhere in between.
The Earth’s atmosphere is made mainly of nitrogen and oxygen. Our atmosphere has more oxygen than any other planet’s atmosphere. When people talk about the air, they are really talking about the atmosphere. It is the perfect mix of gasses to support life. The atmosphere keeps temperatures from getting too high or too low to support life. It also helps keep out harmful radiation from the Sun.
The Earth is the only planet with lots of water. Water covers two thirds (2/3) of the Earth. The amount of water on Earth stays about the same. Other planets may have only tiny amounts of ice or steam. Without water, there would be no life on Earth.
The Earth orbits the Sun in a near perfect circle. This helps keep temperatures from getting too high or too low. About 1.5 million types of animals and plants live in this perfect environment.
Even though we live on Earth and explore it every day, there are things about Earth that we can learn from space. Space exploration began by looking at our own planet. In 1957, Sputnik was the first spacecraft to orbit the Earth. Since then, thousands of spacecraft have studied the Earth from space. Today satellites send information back to Earth that can help us in our daily lives. Meteorologists, more often called weather forecasters, use information from satellites to study and predict the weather. In 1969, the Apollo 11 spacecraft was the first manned mission to the Moon. It carried astronauts Michael Collins, Edwin “Buzz” Aldrin, and Neil Armstrong. Collins orbited the Moon during Armstrong and Aldrin’s historic Moon walk.
The Earth has one Moon. It is about 240,000 miles (384,000 kilometers) from Earth. Its diameter is about 2,159 miles (3,476 kilometers). The Moon is about one fourth (1/4) as big as the Earth.
The force of gravity ties the Moon to the Earth. The Moon revolves around (orbits), the Earth once every 27.3 days. It also takes the Moon 27.3 days to rotate one time. This means that the same side of the Moon always faces the Earth. Astronauts orbiting the Moon are the only humans to see the other side of the Moon in person. We only know what it looks like from photographs. High tides are caused by the gravitational pull of the Moon and the Sun. The pull stretches the Earth's oceans into an ellipse with Earth in the center. The oceans of the Earth seem to bulge out to make the oval shape. Areas on the Earth near the bulge will experience a high tide. The high tide moves from west to east as the Earth rotates. High tides occur twice a day. This is because the water on the opposite side of the Earth from the Moon also bulges.
Craters (holes), cover the Moon. Meteorites striking the surface of the Moon created these bowlshaped holes.
Find out more... There’s Water on the Moon
Millions of tons of ice are inside some of the craters on the Moon’s poles. In the future, space missions might use the water for fuel, or to support life. Would you like to live in a space station on the Moon?
Mars is the fourth planet from the Sun. Sometimes we call it the Red Planet because of its red coloring. Mars looks red because of the pinkish color of its rocks,soil, and sky.
Mars is about half the size of Earth. It has a diameter of 4,219 miles (6,794 kilometers). Its average distance from the Sun is about 142 million miles (228 million kilometers). Mars orbits the Sun once every 687 Earth days. You can see Mars without a telescope. It appears as a bright red object in the night sky.
Mars is more like Earth than any other planet. The length of a day on Mars is almost 24 Earth hours. Like Earth, Mars has polar ice caps.
During the winter on Mars, one of the poles is covered in snow and ice. At the same time, it is summer in the other hemisphere of the planet. When one ice cap gets bigger, the other ice cap gets smaller.
It is always cold on Mars. The highest temperature is about 32 degrees Fahrenheit (0 degrees Celsius). But Mars can get much colder than freezing. It can get as cold as -200 degrees Fahrenheit (-144 degrees Celsius). Mars has an elliptical orbit. As it orbits, the planet gets colder as it travels farther away from the Sun.
The atmosphere of Mars is 95% carbon dioxide. The atmosphere is very thin. Humans would not be able to breathe the air on Mars. The surface of Mars is also very dry. Sometimes, giant dust storms cover the entire planet. If you lived on Mars, the sky would not look blue like the sky on Earth does. It would be reddish orange in color.
Mars has two moons named Phobos and Deimos. They are both odd shaped and were probably asteroids at one time. Phobos is about twice as large as Deimos, but both are tiny compared to Earth’s Moon. Phobos is only 17 miles (27 kilometers across).
There are many volcanos on the surface of Mars. Most of them are in the northern regions of the planet. The biggest volcano is Olympus Mons. It is the largest volcano in the solar system. It is 342 miles (550 kilometers) wide and 17 miles (27 kilometers) high.
Scientists believe that water once existed on the surface of Mars. A huge canyon called the Valles Marinares is evidence of water in Mar’s past. It is over 5 miles (8 kilometers) long. Dried up flood plains and riverbeds also cover parts of the surface.
The first probe to go to Mars was Mariner 4, in 1965. Since then, many other spacecraft have visited the planet. Viking 1 and 2 were the first to land on the surface.
Jupiter is the fifth planet from the Sun. It is the largest of all the planets in the solar system. It is also the first of the gas giants, or outer planets. All the outer planets are huge balls of gas. The other gas giants are Saturn, Uranus, and Neptune.
Jupiter is a huge planet. It has a diameter of over 88,679 miles (142,000 kilometers). Jupiter is 11 times larger than Earth. This giant planet also has a great mass. Jupiter has much more mass (matter) than all the other planets put together. If Jupiter were 13 times bigger, it would be classified as a star called a brown dwarf instead of a planet. That would make it another star in the solar system.
Jupiter takes more than 11 Earth years to orbit once around the Sun. The distance from the Sun to Jupiter is over 5 astronomical units or five times the distance of the Sun to the Earth. Jupiter spins very fast. It takes a little less than 10 Earth hours for Jupiter to rotate once.
You might think Jupiter would be a frozen planet because it is so far away from the Sun but it’s not. Jupiter’s core is very hot. It can reach temperatures of 43,000 degrees Fahrenheit (24,000 degrees Celsius). In contrast, Jupiter’s atmosphere is cold. The clouds average -193 degrees Fahrenheit (-125 degrees Celsius).
The atmosphere of Jupiter is very deep. It is made mostly of hydrogen and helium. Layers of clouds cover the planet. These layers are thin bands made of hydrogen sulfide, ammonia, and water vapor. Clouds near the top of the atmosphere move very quickly. As bands pass each other, they sometimes create a storm. Giant lightning bolts come from the clouds during a storm.
A thin ring of dust orbits the planet far above the clouds. We cannot see the ring from Earth because it is so faint.
Jupiter creates a huge and powerful magnetic field that surrounds the planet and its moons. It is unlikely that there will ever be a manned space mission to Jupiter. The pressure on Jupiter is so great that it turns gas into liquid. That is enough pressure to crush a spaceship!
Jupiter has sixty-three known moons. That is the highest number of moons identified for any planet. Four of these moons are very large. Galileo discovered them in 1610. We know them as the Galilean satellites. They are named Ganymede, Callisto, Io, and Europa.
Jupiter’s Four Largest Moons:
- Ganymede is the largest moon in the solar system. Its diameter is 3,280 miles (5,262 km).
- Callisto is covered in ancient craters. It is about the same size as Mercury.
- Io is about the size of Earth’s Moon. It has a violent surface with active volcanos.
- Europa is the smallest of the four large moons. It has a smooth and icy surface. It is possible that below the ice there is a layer of liquid water.
The other moons of Jupiter are much smaller. Many of them are recent discoveries. Astronomers discovered more than half of Jupiter’s moons after 1999.
There is a “Great Red Spot” on Jupiter. This spot is actually a giant storm. It is red from the gases that swirl within it. The spot has existed for hundreds of years. The storm is wider than three Earths!
The first spacecraft to visit Jupiter was Pioneer 10 in 1972. Since then, Voyager 1 and 2 have flown past the planet. The Galileo probe began to orbit Jupiter in 1996. It studied Jupiter and its moons. We learned from data sent back to Earth from the Galileo probe that the winds are stronger on Jupiter than the winds on Earth.
Saturn is the sixth planet from the Sun. It is the second largest planet in the solar system. Saturn is one of the four gas giant planets. It is the farthest planet that we can see without the help of a telescope. Saturn is a lot like its neighbor planet, Jupiter. Both planets are made mostly of gases and neither one has a solid surface.
Saturn has a diameter of about 74,085 miles (119,300 kilometers). It is nine times wider than the Earth. Saturn takes over 29 Earth years to revolve around (orbit), the Sun. Saturn’s shape is like a ball squeezed flat on the top and bottom. Saturn spins very fast causing the flattened shape. It takes just over 10 Earth hours for the planet to rotate once.
The average temperature on the surface of Saturn is very cold. It is about -290 degrees Fahrenheit (-179 degrees Celsius).
The atmosphere on Saturn is mostly hydrogen. There are also small amounts of helium and methane.
Strong winds blow across the surface of Saturn. These winds move very fast. Its clouds can move at about 1,100 miles per hour (1,800 kilometers per hour). There are wide bands of clouds. These clouds contain storms like those on Jupiter. A layer of haze covers the upper atmosphere. This haze makes Saturn look yellow.
At least 56 moons orbit Saturn. Some are a few kilometers across. Others are the size of small planets. Titan is the largest moon of Saturn. It has a thick atmosphere covered in a brownish orange haze. Probes have detected nitrogen and methane on Titan. Many of Saturn’s other moons have icy surfaces covered with craters. Some scientists believe that the moons of Saturn and Jupiter are where we might find evidence of simple life forms.
Many people call Saturn the jewel of the solar system or the ringed planet because of its giant, beautiful rings. Galileo discovered the rings in the 17th century. From Earth, we can see only three big rings. Saturn actually has thousands of small rings called ringlets. The rings are made mostly of ice particles, but also contain dust and rocks. Some particles are as small as dust. Others are the size of a house. Saturn’s rings are about .6 miles (one kilometer) thick. The gravitational pull of Saturn’s moons gives the rings their shape.
The first spacecraft to visit Saturn was Pioneer 11, in 1979. Since then, Voyager 1 and 2 have flown past and studied the planet. In 2004, the Cassini space probe landed on the surface of Saturn’s moon, Titan.
Uranus is the seventh planet from the Sun. It is the third largest planet in the solar system. Like Jupiter and Saturn, it is one of the gas giants. The diameter of Uranus is 32,168 miles (51,800 kilometers). It takes Uranus 84 Earth years to orbit the Sun. The planet rotates once every 17 Earth hours.
Uranus does not spin around the Sun like the other planets. It is tipped on its side and rolls around the Sun like a barrel. We can’t be sure, but scientists think Uranus is tipped because it was hit by a small planet or comet.
Because it is so far from the Sun, Uranus is very cold. The average temperature is about -355 degrees Fahrenheit (-215 degrees Celsius). Scientists do think that there is a layer of very hot water, ammonia, and methane on Uranus. Like Earth, Uranus experiences changing seasons. Because Uranus is titled on its side, a season lasts a very long time. One season on Uranus can last more than 20 years!
The atmosphere on Uranus is made mostly of hydrogen, helium, and methane. Methane gives Uranus its blue-green color. Wind speeds on Uranus are much slower than on Saturn and Jupiter.
Uranus has at least 27 moons. Most of them are very small. The five large moons are mostly made of rock and ice. Miranda is the most unusual moon orbiting Uranus. It looks like a combination of many different pieces. Scientists think it may have been shattered as many as five times. The surface has craters and deep grooves unlike any other moon.
In 1977, scientists discovered Uranus’ 11 rings. Scientists observed a star blinking on and off as it passed behind the planet. The blinking was caused by the rings blocking the starlight. Pictures taken in 1986 from the Voyager 2 space probe showed that Uranus’ rings are made mostly of dark dust.
Even though the Voyager 2 space probe launched in 1977, it did not reach Uranus until 1986. This was the first space probe to reach this distant planet. Since that time, we have learned more about Uranus from the Hubble Space Telescope.
Neptune is the eighth planet from the Sun. It is the smallest gas giant in the solar system. Neptune has a diameter of 30,740 miles (49,500 kilometers).
Neptune is very far away from the Sun. Its average distance from the Sun is 2.8 billion miles (4.5 billion kilometers). It orbits the Sun once every 165 Earth years. In 2011, Neptune will complete its first orbit around the Sun since scientists discovered it in 1846. A day on Neptune is just over 16 Earth hours. If you want to see Neptune from Earth, you must use a telescope. Even with a telescope, Neptune can be hard to see. It looks like a very small bluish ball.
Neptune has recorded some of the coldest temperatures in the solar system. It has an average temperature of -355 degrees Fahrenheit (-214 degrees Celsius).
The atmosphere on Neptune is made of hydrogen, helium, and methane. The methane in the upper layers gives the planet its blue color.
A good nickname for Neptune would be the windy planet. The strong wind rips through the atmosphere at 1,200 miles per hour (2,000 kilometers per hour). These winds are the strongest of any planet in the solar system. Thin white clouds move around the upper atmosphere. A giant storm, the Great Dark Spot continues to move across the surface. The storm is about the size of the Earth.
At least thirteen moons orbit Neptune. Triton is the largest moon. It orbits in the opposite direction of Neptune’s other moons. This is a retrograde orbit. Triton has a thin atmosphere. The surface temperature of Triton is the coldest of any known moon or planet in the solar system. The other moons of Neptune are much smaller than Triton.
Neptune has rings like the other gas giants, Jupiter, Saturn, and Uranus. There are four rings in all. They are very faint and difficult to see.
Neptune was the first planet identified by using mathematical predictions rather than by observation. Neptune was not actually seen until 1846, by astronomers J.G. Galle and Heinrich Louis d’Arrest. In 1989, Voyager 2 became the first space probe to visit Neptune giving us our first chance to learn more about this distant planet.
Pluto and the Dwarf Planets
In 1930, when astronomer Clyde Tombaugh discovered Pluto, he identified it as a very small planet. Astronomers classified Pluto as a planet for 76 years. Then on August 24, 2006, the International Astronomical Union (IAU) changed Pluto’s label from a planet to a dwarf planet.
Scientists reclassified Pluto as a dwarf planet because there are other objects in its orbital path. Two other dwarf planets are Ceres and Eris.
Pluto’s diameter is smaller than the Earth’s Moon. It has an elliptical orbit. Pluto’s distance from the Sun ranges between 2.7 and 4.6 billion miles (4.4 and 7.4 billion kilometers). From 1979 to 1999, Pluto was closer to the Sun
than Neptune. It takes Pluto 249 Earth years to orbit the Sun once. Pluto orbits in what we call the Kuiper Belt.
Pluto has three moons. They are Charon, Nix, and Hydra.
Ceres is the smallest dwarf planet. It is about the same size as Texas. It is the only dwarf planet in the main asteroid belt. Unlike most objects in the belt, Ceres is a sphere.
Scientists discovered Ceres in 1801. They first thought it was a planet, but then changed the classification to an asteroid. For another 150 years, scientists considered Ceres to be the largest identified asteroid. In 2006, scientists upgraded Ceres to be a Dwarf planet.
Although Eris is called a dwarf planet, it is the ninth largest object orbiting the Sun. It is just slightly larger than Pluto. It has its own moon, Dysnomia. Scientists discovered Eris in 2005. Some astronomers initially called it the tenth planet. However, based on the definition developed by the IAU, it is recognized as a dwarf planet along with Pluto and Ceres. Eris is located in the outer regions of the Kuiper Belt, which is a band of small bodies orbiting the Sun beyond Neptune.
Asteroids, Meteoroids, and Comets
Asteroids are objects that orbit the Sun. They might also be called planetoids, minor planets or small solar system bodies. They are usually made of rock or iron, similar to the makeup of the four inner planets. Asteroids come in many different sizes and shapes. Some are large, but others are as small as pebbles. Most are in the asteroid belt between the orbits of Mars and Jupiter.
Most scientists think asteroids are pieces of material left over from the formation of the solar system. Asteroids could be material that never formed into a planet.
Meteoroids are large sand to boulder size pieces of debris in the solar system. Many meteoroids form when asteroids break apart. A meteoroid that hits the Earth’s atmosphere usually burns up in a streak of light. This visible streak of light is a meteor. People sometimes call a meteor a shooting star. A meteorite is a meteor that strikes the Earth.
A ball of ice and dust particles that orbits the Sun is a comet. A comet is like a giant, dirty snowball. The orbit of a comet is constantly changing. Sometimes, a comet orbits close to the Sun and then will be thrown far out into space. Most comets are too faint to be seen without a telescope. We can see comets only when they are heated and illuminated as they near the Sun. As the comet gets closer to the Sun, the nucleus (center) of the comet begins to melt. The melting particles following behind the comet are its tail. The tail can extend for millions of miles or kilometers away from the Sun.
Between 1995 and 1997 the Comet Hale-Bopp was the clearest comet people on Earth had seen for over 100 years. It was so bright it could be seen without using a telescope. Unless you plan to live a very long time, you will not see Hale-Bopp again. Hale-Bopp won’t be visible again from Earth for about 2,380 years!
Find out more... Halley’s Comet
Perhaps the most famous comet of all is Halley’s Comet. It was named after Edmond Halley, an astronomer and mathematician. Halley correctly predicted in 1705 that a certain comet would return in 1758. Halley’s Comet is visible every 75-76 years. The last time it was visible from Earth was 1986. Scientists predict Halley’s Comet will be visible again in the year 2061. How old will you be the next time Halley’s Comet passes near the Earth? Because it looks like a huge fireball in the sky, many people used to think that Halley’s Comet was a sign of bad luck. Some people even thought it was the end of the world!