Astronauts on a space shuttle mission can see planet Earth below. They see different parts of Earth as the shuttle goes around the planet. The shuttle is in orbit above Earth. An orbit is a curved path going around something in space.

WHAT KINDS OF THINGS ORBIT IN SPACE?

The Moon orbits Earth. Space shuttles and artificial satellites also orbit Earth.

Spacecraft sent to explore distant planets sometimes go into orbit. A spacecraft orbits a planet and takes pictures and measurements.

Everything in our solar system orbits around the Sun. All the planets orbit the Sun. Pieces of rock called asteroids orbit the Sun. Balls of ice and rock called comets orbit the Sun. Moons orbit planets, which orbit the Sun.

Our Sun is a star in the Milky Way Galaxy. A galaxy is a group of stars. All the stars in the Milky Way orbit the center of the Galaxy.

HOW ARE ORBITS SHAPED?

An orbit can be shaped like a circle. Most orbits are oval-shaped like a racetrack. Planets, asteroids, and comets in our solar system have oval-shaped orbits. Another name for the oval shapes of orbits is ellipse.

Some orbits are bigger ovals than others. The bigger the oval, the longer it takes something to orbit the Sun. It takes Earth one year, or 365 days, to go around the Sun. It takes Pluto 248 years to orbit the Sun. Pluto has an orbit that takes it very far from the Sun.

Most comets orbit the Sun in huge oval paths. Sometimes the orbital paths of these comets take them close to the Sun in the center of our solar system. Sometimes their paths take them out to the edge of our solar system.

HOW DO SATELLITES ORBIT EARTH?

Weather satellites, communications satellites, and other types of satellites take different paths around Earth. Some satellites orbit directly above Earth’s equator. The equator is an imaginary line around the middle of the planet. The orbits of other satellites take them over the North and South poles.

Some communications satellites orbit at the same speed that Earth turns. They look like they are not moving. A satellite has to move at just the right speed to stay in its orbit above Earth.

WHAT KEEPS THINGS IN ORBIT?

The force of gravity holds satellites, moons, planets, and stars in orbit. Gravity is a force that pulls one thing toward another. When you throw a ball up in the air, Earth’s gravity pulls it back down to the ground. The bigger something is, the stronger is its force of gravity.

The Sun is very large and its gravity pulls on planets and everything else in the solar system. The Sun’s gravity makes the planets travel in a curved path around the Sun. Earth’s gravity holds the Moon and artificial satellites in curved paths around Earth.

When a spacecraft goes fast enough, it can overcome the pull of Earth’s gravity. Rockets can make spacecraft move fast enough to head toward distant planets.

Sometimes satellites in low orbits around Earth slow down. This happens when the satellites brush against faint wisps of air. Earth’s gravity pulls slow-moving satellites down. The satellites fall toward Earth. Sometimes the satellites burn up in the air. Sometimes the satellites crash into the ocean.

Source: Microsoft ® Encarta

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