Millions of rocks are flying through space as you read this. Meteoroids are one example of them. They are like asteroids, but much smaller.

Some meteoroids are rocky, while others are metallic. Some are combinations of rock and metal. They orbit the sun, just like planets, asteroids, and comets do.

Meteoroids are related to meteors and meteorites. When meteoroids enter Earth's atmosphere (or that of another planet, like Mars) they come in at high speed. Many burn up. The fireballs or "shooting stars" they become are called meteors.

Sometimes a meteoroid survives a trip through the atmosphere and hits the ground. In that case, it's called a meteorite.

Meteoroids Are Common in Our Solar System
Meteoroids are extremely common throughout the solar system. Most are tiny. Many float around the planets, but some are even found on the very edge of the solar system.

Different meteoroids travel around the sun at different speeds and in different orbits, or paths. The fastest meteoroids travel through the solar system at a speed of around 42 kilometers (26 miles) per second.

Many meteoroids are formed when asteroids crash together. Asteroids circle the sun between the paths of Mars and Jupiter in a region called the asteroid belt. As asteroids smash into each other, they produce crumbly bits—meteoroids. The force of the asteroid crash can put the meteoroids on track to strike a planet or moon.

Meteoroids Are often Shed from Comets
Others meteoroids are the bits that comets shed as they travel through space. A comet is a different space object, made up of frozen gases, rock, and dust. As a comet approaches the sun, the comet's center sheds gas and dust. Its dusty tail may contain thousands of meteoroids. Meteoroids shed by a comet usually orbit together in a formation called a meteoroid stream.

Meteoroid impacts are probably the largest cause for "space weathering." Some space objects don't have an airy atmosphere, like Earth. The air around Earth is like a shield from meteors. Surfaces without this shield are more likely to be worn away, or weathered. Asteroids, many moons, or the planets Mars and Mercury are examples. Meteoroids crash into these bodies. They create craters and throw space dust—more meteoroids—back into the solar system.

Most meteoroids are made of silicon and oxygen, and heavier metals like nickel and iron. Iron and nickel-iron meteoroids are very massive and dense. Stony meteoroids are lighter and more fragile.

NASA Assesses the Impact of Meteors
Meteoroids are generally harmless. They're specks of dust floating around the sun. Space agencies such as NASA do follow the movement of meteoroids, however, for two reasons: possible crashes with spacecraft and possible impact with Earth.

Even Small Meteoroids Can Harm Spacecraft
Even a tiny meteoroid can damage spacecraft. This could endanger astronauts. It could destroy valuable scientific instruments and cost millions of dollars.

Engineers must prepare spacecraft for this. To help this, they have classified three different "meteoroid environments." These are the sporadic environment, the shower environment, and the lunar environment.

The sporadic environment describes the threat of meteoroids created by asteroids or comets. Engineers must determine what area of the spacecraft could be hit by sporadic meteoroids. In response, they prepare stronger shield equipment.

The shower environment describes the threat of meteoroid streams that come with comets passing through Earth's orbit. On Earth, a meteor shower is an example of this. Engineers must steer spacecraft out of the way. The hope is they can avoid meteoroids hitting parts of the craft that are most in danger.

The lunar environment describes the threat of meteoroids to astronauts or gear on the moon. There have been no long-term astronaut stays on the moon. Still, engineers have designed space suits, vehicles, and habitats that can handle being hit by meteoroids.

Meteoroids close to Earth Can Become Shooting Stars
When a meteoroid passes through Earth's atmosphere, it heats up. This is caused by air pushing back against it, which is also called resistance. The heat causes gases around the meteoroid to glow brightly. This glowing meteoroid is called a meteor, sometimes nicknamed a shooting star. Most meteoroids that enter Earth's atmosphere fall into bits before they reach the ground. The pieces that do strike Earth's surface are called meteorites.

Both meteors and meteorites can become natural dangers to the communities they hit. Very large meteors called bolides explode in the atmosphere. They are also known as "fireballs." Bolides and the shock waves they produce may cause burns and even death. These can also damage buildings and crops. When parts of the space rock actually crash into Earth, they can cause a disaster. A single meteor event about 66 million years ago might be the most famous example. It likely caused the extinction of the dinosaurs and almost every other form of life on Earth.