My Science School

Halley’s Comet

The comet we call Halley’s Comet was first seen more than 2,000 years ago. It was sighted by Chinese astronomers in 240 B.C. It appeared again and again over the centuries, but most people did not realize it was the same comet. They believed that comets appeared by chance and travelled through space in no set path.

An English astronomer named Edmund Halley proved them wrong. He discovered that the comet travelled in a set path around the sun.

Halley knew that the paths of comets seen in 1531 and 1607 were exactly the same as the path of a comet observed in 1682. And he realized that these were all the same comet! Halley also noted that 76 years had passed between 1531 and 1607. And between 1607 and 1682, 75 years had passed. He predicted that people would see that comet again 76 years later and at fairly regular intervals after that.

Sure enough, the comet was sighted on Christmas Day in 1758. Halley’s Comet became the most famous comet in history.

 

A “Star” with a Tail

For thousands of years, whenever a bright star with a glowing tail appeared in the sky, people were afraid. If it seemed to grow bigger and brighter, some people thought the end of the world was near! Now we know that these “stars” are actually just comets.

Billions of comets travel around and around the sun. Most are balls of frozen gas, like snowballs mixed with dust. They come from two groups of comets near the edge of the solar system. Just beyond Neptune is the group in the Kuiper belt. Beyond that lies the ball-shaped Oort cloud.

At first, comets have no tails. But when a comet gets near the sun, the sun’s heat melts some of the frozen gas. Gas and dust stream off into space, forming a tail. The tail glows because sunlight shines on the gas and dust and also releases energy from the gas.

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Meteoroids, the shooting stars

If you ever wished on a shooting star, you were really wishing on a burning rock. Sometimes a big lump of rock and metal called a meteoroid zooms through the blackness of space. For billions of years, this meteoroid has been zipping around and around the sun. But now it is heading straight for Earth.

The lump of rock enters Earth’s atmosphere. It is travelling so fast that Earth’s air rubs hard against it. This makes the meteoroid very hot.

Bits of the rock burn up, leaving a trail of glowing gas. Seen from Earth, it looks like a bright streak flashing across the sky. Some people call the streak a “shooting star” or a “falling star”.

As many as 200 million meteoroids come into Earth’s atmosphere every day! Most of them are small and burn up completely. But if a meteoroid is big enough, it may not burn up before it hits the ground. Instead, it may come smashing to Earth and make a crater - a big hole in the ground.

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The Stars

Stars are huge balls of glowing gas in the sky. The sun is the only star close enough to Earth to look like a large ball to us. The other stars are so far away that they look like tiny dots of light.

Stars may look close, but they are actually very far away - much further from Earth than the planets are. The nearest star to us, after the sun, is so far away that it takes more than four years for its light to reach us! Some stars are so far away that their light takes billions of years to reach us. That twinkle of light you see in the sky tonight may have been given off before Earth existed!

How many stars are there? Scientists believe that there are about 10 billion trillion stars in the universe. To understand how large a number this is, imagine that all the people in the world had to count an equal number of stars one by one. Each person would have to count more than 1 1/2 trillion stars. And, even if you could count 1,000 stars per second for 24 hours a day, you would need 50 years to count 1 1/2 trillion stars!

Where do stars go during the day? They don’t go anywhere. Our daytime sky is so bright, we just can’t see them.

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Asteroids

The sun’s family also includes small masses of rock or metal called asteroids. Most asteroids circle the sun in a ring between Mars and Jupiter.

There are millions of asteroids. Most are less than 1.6 kilometres across, but some are several hundred kilometres across. The largest asteroid, named Ceres, is also considered a dwarf planet.

The larger asteroids are nearly ball-shaped, like the bigger planets. The smaller asteroids are bumpy and jagged.

Asteroids often bump into one another and break into smaller bits. Sometimes they are knocked out of their orbits and begin travelling in another direction. These asteroids become meteoroids that sometimes collide with other planets. Collisions like this made craters on Mars, Mercury, and Earth’s moon. Bits of asteroids also become meteors, the “shooting stars” that we see from Earth.

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The Dwarf Planets

Some objects in the solar system look like planets, but they are smaller than planets. Astronomers call these objects dwarf planets. For many years Pluto was considered a planet. But in 2006, astronomers decided it should be called a dwarf planet. It lies in the Kuiper belt.

The Kuiper belt includes other dwarf planets, such as Eris, Haumea, and Makemake. In 2006, the United States launched New Horizons, the first space probe sent to study Pluto and other Kuiper belt objects. New Horizons should reach Pluto in 2015.

Not all dwarf planets are in the Kuiper belt. The dwarf planet named Ceres orbits the sun in the asteroid belt between Mars and Jupiter.

Neptune, a distant planet

Neptune is the eighth planet from the sun. It is about the same size as Uranus and is also about four times as wide as Earth. The planet is mostly made up of gases, but Neptune’s centre may be a mixture of slush and rocks.

Scientists are still learning about Neptune. Before 1989, it was thought that Neptune had only two moons. But that year the U.S. space probe Voyager 2 flew past and discovered six smaller, dark moons. In 2002 and 2003, five additional moons were discovered, bringing the total to 13. Neptune also has four faint rings that scientist think are made of dust.

Uranus helped scientists discover Neptune. As Uranus travels around the sun, it slows down at certain points. Scientists realized that what slows Uranus down is the gravity of another planet pulling on it. They began to look for that other planet and found Neptune!

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Uranus, the tilting planet

In England, in 1781, an amateur astronomer named William Herschel looked through a homemade telescope he set up in his garden. He found a new planet, Uranus, which became the first planet to be discovered by using a telescope.

Uranus is the seventh planet from the sun. It is a blue-green planet nearly four times wider than Earth. Scientists believe that Uranus has a rocky core covered by a deep ocean and thick clouds of gas. Uranus has at least 27 moons and at least 13 rings.

When Earth spins around its imaginary line called an axis, it is tilted like a spinning top. But the axis of Uranus is tilted even more than that - more than any other planet. At times, Uranus’s North Pole is pointed almost straight toward the sun. About 42 Earth years later, it is pointed away from the sun. Scientists learned a lot about Uranus from Voyager 2, which flew past the planet in 1986.

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Saturn and its famous rings

Saturn, the sixth planet from the sun, is famous for its magnificent set of gleaming rings. Saturn has seven major rings and many ringlets. Saturn’s rings are formed of billions of pieces of ice, from tiny specks to very large “snowballs”. These bits of ice travel around Saturn just as the moon travels around Earth.

Saturn is the second largest planet. It is nearly 10 times bigger than Earth. It has a rocky centre, which is probably covered by a thin layer of liquid. And it is surrounded by thick layers of gas.

Big as it is, Saturn weighs less than a ball of water its size would weigh. If there were an ocean big enough to hold it, Saturn would float!

Scientists used the space probes Voyager 1, and Voyager 2 to study Saturn. In 1997, the U.S. spacecraft Cassini explored Saturn’s rings and its moons. Cassini launched a probe named Huygens to Saturn’s largest moon, Titan, in 2005.

Jupiter, the giant planet

Jupiter is the biggest planet in our solar system. Just how big is it? It would take more than 1,000 Earths to fill up Jupiter.

Jupiter is the fifth planet from the sun. It has a small, rocky core, but most of it is surrounded by thick clouds of gas. The clouds form coloured bands around the planet. Jupiter also has three thin rings of dust that sometimes look like one.

Wild windstorms are always raging in Jupiter’s thick clouds. But can you imagine a huge hurricane that whirls for 300 years? Scientists believe that a hurricane on Jupiter called the Great Red Spot has been whirling for at least that long. This hurricane is more than three times as wide as Earth!

In 1979, two U.S. probes called Voyager 1 and Voyager 2 flew past Jupiter. The U.S. space probes Cassini and New Horizons also flew by Jupiter, in 2000 and 2007. From 1995 to 2003, the U.S. spacecraft Galileo orbited the planet. In 2003, Galileo sent a probe into Jupiter’s atmosphere.

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People need the kind of atmosphere we have on Earth to survive. The air must contain oxygen. Also, it must be the right temperature - not too hot or too cold. If Mars had more oxygen in its air, and if it were a little warmer, people might be able to live there.

Scientists have considered ways to warm up the atmosphere on Mars and add oxygen to it. For example, they may be able to warm Mars with huge mirrors that reflect sunlight onto the planet. And adding oxygen would make the air on Mars breathable. If they could do these things, the atmosphere would change a lot. Then people would be able to breathe the air on Mars and go outside without wearing spacesuits.

NASA scientists have thought about sending people to Mars. Right now, it would take astronauts about a year to reach the planet. The spaceship would have to be large enough to hold fuel, food, and other supplies for the crew. Visiting Mars will have to wait a while, until scientists develop the technology we will need.

Mars is more like Earth than any other planet. It is farther from the sun than Mercury or Venus, so it is not as hot as they are. Its atmosphere is thinner than the air on Earth, but some kinds of living things could survive in it. That’s why some scientists believe that life may once have existed on Mars.

In 1976, two U.S. space probes named Viking 1 and Viking 2 landed on the red planet. Neither of these two probes found any evidence of life.

In 1984, however, a small meteorite was found in Antarctica. A meteorite is a meteoroid that lands on Earth. Scientists believed that this meteorite came from the planet Mars. Material on it seemed similar to substances that come from tiny living things on Earth.

In 2008, the U.S. Phoenix spacecraft landed on Mars and found frozen water near the surface. Water is necessary for life on Earth. Water on Mars could also mean life on Mars has existed or could evolve in the future.

Mars, the Red Planet

Mars is the fourth planet from the sun. It is also known as the red planet because it has a reddish tint. What makes Mars red? Its dry, desert-like regions are covered by rust-coloured dust, sand, and rocks. Fierce windstorms whirl the orange-coloured sand up from the plains and fill the air with dust. This dust gives the Martian sky a reddish colour as well.

There are many canyons, craters, and volcanoes on Mars. One volcano, called Olympus Mons, is nearly three times as high as Mount Everest, the highest mountain on Earth. In fact, Olympus Mons is the largest volcano in the entire solar system!

Mars is only about half the size of Earth and has two small moons. The largest, called Phobos is about 27 kilometres wide. The other, Deimos, is about 15 kilometres wide.

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Venus – Earth’s “Twin”

Venus is nearly the same size as Earth, so it is often called Earth’s “twin”. But it is nothing at all likes our world. Venus’s atmosphere is full of poisonous gases. Its clouds contain a chemical strong enough to dissolve metal! And the clouds on Venus are so thick that cameras can’t see the planet’s surface.

High in Venus’s atmosphere, powerful windstorms are raging. Venus’s windstorms are much worse than storms on Earth. Lightning flashes in the sky as often as 20 times a minute.

Venus is the second closest planet to the sun. This makes it extremely hot and dry. As seen from Earth, Venus is brighter than all the other planets and stars. It is so bright that it can sometimes be seen in the daytime! A year on Venus is as long as 225 Earth days. Like Mercury, Venus has no moons.

Scientists once thought the planet Venus would be much like Earth, but warmer. They were wrong. Venus is extremely hot, and its atmosphere is very heavy. Scientists have used many space probes to study Venus, including the Soviet Vanera probes, the U.S. Mariner and Magellan probes, and a European Space Agency probe named Venus Express.

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Earth

Earth is a watery planet. More than two-thirds of Earth’s surface is covered with water. That’s good for all the living things in our world because animals and plants need water to live. Animals and plants live almost everywhere on Earth.

Earth is the third planet from the sun, and air surrounds it. The air is made up of gases, such as oxygen, nitrogen, and carbon dioxide. These gases are needed for almost all living things to survive. Human beings breathe in oxygen. Plants need carbon dioxide.

If you look at a globe, you will find the North Pole on one end and the South Pole on the other. Earth looks like a ball, but it is actually a little flatter at the North and South poles.

Earth travels 958 million kilometres on its journey around the sun. It takes about 365 days for Earth to orbit the sun once. That’s why there are about 365 days in an Earth year.

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