space

No one knows what caused the big bang, but scientists think they know what happened all the way back to the first seconds after the big bang.

The brand new universe was very hot and very small. It blew outwards very fast. In the first three minutes, matter started to form. Hundreds of years later, the universe looked like a big ball of fire.

You can picture the universe as something like a black balloon with white dots painted on it. The black represents space and the white dots are galaxies. Blowing air into the balloon makes it bigger. The spaces between each dot get farther apart as the balloon expands.

As it got bigger, the universe got cooler. Hydrogen gas formed. The gas broke into clumps. The clumps came together to make galaxies and stars. Other kinds of matter formed in the stars. Finally, planets like Earth formed around some stars.

The expansion of the universe is evidence for the big bang. American scientist Edwin Hubble studied light coming from galaxies far out in the universe. In 1929, he found that the galaxies were speeding away from Earth and from each other in all directions. Scientists tracked the paths of the galaxies back to their starting place. They saw that all the galaxies must have started from the same place. Packing all that matter into a small area would make a very dense, searing hot ball—the big bang.

Scientists use math to describe how the universe behaves. In the early 1900s, German American scientist Albert Einstein came up with equations that predict an expanding universe. These equations have correctly predicted the motions

of stars, planets, and light.

More proof came in the 1990s from a spacecraft called the Cosmic Background Explorer (COBE). COBE saw rays coming from far off in the universe. The rays are left over from the early days of the universe. They could only have been created in a much smaller and hotter universe long ago.

Scientists are not sure what will happen to the universe. They currently think it will keep expanding forever. They even think the expansion is speeding up. But scientists are still studying this question.

In the 1600s, the famous Italian scientist Galileo was the first person to look at the Moon through a telescope. He saw dark spots that he thought were oceans. He called them maria, the Latin word for “seas.” Galileo thought the light areas were large landmasses called continents.

Today, we know a lot more about the Moon. We know that nothing lives on the Moon, and there are no oceans. The maria are dry, flat plains covered with rocks. The Moon is the only place in space that human beings have visited.

The first astronauts landed on the Moon in 1969. They traveled in a United States spacecraft named Apollo 11. The astronauts set up experiments on the Moon and brought some moon rocks back to Earth. Later, five more Apollo missions explored different parts of the Moon. The astronauts on these missions brought back more rocks and soil.

Scientists learned many things about the Moon from the Apollo space missions. They also learned from other spacecraft that orbited (went around) the Moon. Some of these spacecraft sent robot landers down to the surface of the Moon.

The dry, gray Moon might seem like a boring place now. But you should have seen it several billion years ago.

Many times over the past two or three billion years, chunks of rock and ice have come whizzing toward the Moon. The space rocks and ice are asteroids and comets. They slam into the Moon’s surface. The biggest ones came just after Earth and the other planets were formed. When they hit the Moon, these large objects threw up tons of rock and dust. There are billions of big and small pits on the Moon made by the space rocks. These pits are called craters.

If you went to the Moon, you’d see the dark-colored maria. Scientists think the dark gray rock is lava (melted rock). They believe that billions of years ago, red-hot rock gushed up from volcanoes on the Moon. The lava flowed over the Moon’s surface. It filled in low places, including some of the big craters. Then the lava cooled to make the Moon’s gray rocks.

The lava also left round hills on the Moon called domes and carved grooves called rilles.

There are rough and mountainous places all over the Moon. Scientists call these places highlands.

There are highlands on the far side of the Moon but almost no maria. Only one side of the Moon faces Earth, so you can never see the far side of the Moon. Scientists learned what the far side looks like from pictures taken by orbiting spacecraft.

The astronauts who walked on the Moon had to wear big space suits. The space suits provided air for the astronauts to breathe, because there is no air on the Moon. The suits also kept the astronauts cool during hot Moon days and warm during cold Moon nights.

With no atmosphere to protect it, Moon temperatures can be very high and very low. It can be 261° Fahrenheit (127° Celsius) at noon during a Moon day—hotter than boiling water! It can be as cold as -279° Fahrenheit (-173° Celsius) on a Moon night. Days and nights on the Moon each last about two weeks.

Days and nights are long because the Moon turns very slowly. It takes the Moon about 27 days to make one turn. Earth turns once every 24 hours.

There is no water on the Moon, but scientists think that there may be ice. Two spacecraft in the 1990s saw signs of the ice. If there is ice on the Moon, it could help future explorers stay there longer.

The signs of ice were found in deep craters at the north and south poles of the Moon. Because these craters are always in shadow, it stays very cold there—about -364° Fahrenheit (-220° Celsius).

The Moon always seems to change shape. Sometimes it looks like a round ball in the sky. Sometimes it is a thin sliver. But the Moon does not really change shape. What happens to it?

The Moon reflects light from the Sun. How you see the reflected sunlight depends on where the Moon is. The Moon orbits (goes around) Earth. Sometimes it is between the Sun and Earth, and you can’t see any reflected sunlight. This is called the new moon.

Sometimes Earth is between the Moon and the Sun. You can see all of the reflected sunlight. The Moon looks round. This is called a full moon.

The rest of the time, you see only part of the reflected sunlight from the Moon. The reflected sunlight looks like slivers of Moon. It takes about 27 da

ys to go from a new moon to a full moon and back to a new moon again.

No one knows for sure how the Moon was formed. By testing moon rocks, scientists have learned that the Moon is about 4.6 billion years old. This is the same age as the solar system.

Scientists think that at that time something as big as a planet crashed into Earth. The collision blasted huge pieces of Earth into space. Some of the pieces came together to make the Moon.

Scientists continue to study moon rocks for clues. There is still much to learn about the Moon.

HOW THE MOON FORMED ?

There are thousands of asteroids, and they come in all sizes. The biggest asteroid ever found is called Ceres. Ceres is more than 600 miles (1,000 kilometers) wide. Astronomers have found about 200 asteroids that are more than 60 miles (100 kilometers) across. All the other asteroids are much smaller. Some are only a few feet wide.

Astronomers wonder if once there were just a few big asteroids. The big asteroids may have crashed into each other. The crashes would have broken them into smaller pieces, making all the asteroids we see today.

Some asteroids are round. Some asteroids are long and bumpy. Some asteroids even have tiny moons going around them.

Asteroids go around, or orbit, the Sun just like planets. Most asteroids orbit in the asteroid belt. The asteroid belt is farther out from the Sun than Earth’s orbit. It lies between the orbits of the planets Mars and Jupiter.

Sometimes asteroids change orbits and move out of the asteroid belt. These asteroids cross the orbits of planets as the planets go around the Sun. A few cross Earth’s orbit.

Asteroids are normally too small and far away to see with your eyes. Astronomers study asteroids with telescopes. They have also sent spacecraft for close-up looks at several asteroids. A spacecraft named Near-Earth Asteroid Rendezvous (NEAR) Shoemaker actually landed on an asteroid in 2001.

Astronomers have found that most asteroids are made mainly of stone. Some asteroids are made mostly of metals. Astronomers think that asteroids made of metal must have melted at some time in the past. The liquid metal clumped together at the center of the asteroid. Most of the rocky part later broke off from the asteroid, leaving the metal behind. What melted these metal asteroids is still a mystery.

Astronomers think that several thousand asteroids have orbits that might one day make them strike Earth. Asteroids have certainly hit Earth in the past. People have found thousands of meteorites (stones from space) that have crashed into Earth. Most meteorites are pieces of asteroids. There is a giant crater (hole in the ground) in Arizona that is more than half a mile (about a kilometer) wide! The crater was formed when a meteor crashed into Earth.

An asteroid crashing into Earth may have killed off the dinosaurs 65 million years ago. Scientists have found a big meteorite crater around Mexico’s Yucatán Peninsula. They think the asteroid that created this crater may have killed the dinosaurs. After the asteroid crashed, a huge cloud of dust would have darkened Earth. It could have become very cold on Earth, and plants that dinosaurs ate might have died. As the plant-eating dinosaurs died from lack of food, meat-eating dinosaurs would have run out of food and died as well.

Scientists are setting up a system to warn us of asteroids coming toward Earth. If they find one they might be able to blow up the asteroid. They might be able to attach a rocket to the asteroid and push it just enough to miss Earth.

Asteroid Close-Up

Most asteroids are far away from Earth. But spacecraft have taken close-up pictures of asteroids, including the asteroid Eros, shown here. In 2001, a spacecraft actually landed on Eros!

Asteroid Impacts

Asteroids and comets sometimes hit Earth. In 1908, an asteroid or comet flattened trees, left, over more than 400 square miles (1,000 square kilometers) in Russia. About 50,000 years ago, an asteroid carved out a huge crater, right, in what is now Arizona.

Asteroid Hitting Earth

If a large asteroid hit Earth, as shown in this artist’s sketch, the results could be disastrous. Many scientists believe that such an impact occurred about 65 million years ago, changing the Earth’s climate enough to kill off the dinosaurs.