Meteoroids are lumps or particles of rock or iron that orbit the sun, just like planets, asteroids, and comets do. Some meteoroids are rocky, while others are metallic or combinations of rock and metal. Meteoroids are the same as asteroids, just much smaller.

Meteoroids are also 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, and the fireballs or "shooting stars" 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 throughout Our Solar System
Meteoroids, especially the tiny particles called micrometeoroids, are extremely common throughout the solar system. They orbit the sun among the rocky inner planets, as well as the "gas giants" that make up the outer planets. Meteoroids are even found on the very edge of the solar system, in regions called the Kuiper belt and the Oort cloud.

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

Many meteoroids are formed from the collision of asteroids, which orbit 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 debris—meteoroids. The force of the asteroid collision can throw the meteoroid debris—and sometimes the asteroids themselves—out of their regular orbit. This can put the meteoroids on a collision course with a planet or moon.

Comet's Dusty Tails include Meteoroids and Micrometeoroids
Other meteoroids are the debris that comets shed as they travel through space. As a comet approaches the sun, the "dirty snowball" of ice at the comet's nucleus, or center, sheds gas and dust. The dusty tail may contain hundreds or even thousands of meteoroids and micrometeoroids. Meteoroids shed by a comet usually orbit together in a formation called a meteoroid stream.

A very small percentage of meteoroids are rocky pieces that break off from the moon and Mars after asteroids or other meteoroids impact their surfaces. Meteoroid impacts are probably the largest contributor to "space weathering." Space weathering describes the processes that act upon a space object that doesn't have an airy atmosphere. Asteroids, many moons, or the planets Mars and Mercury are examples of such space objects. Meteoroids crash into these bodies. When they do so they create craters and throw space dust (more meteoroids) back into the solar system.

Most meteoroids are made of silicon and oxygen (minerals called silicates) and heavier metals like nickel and iron. Iron and nickel-iron meteoroids are very massive and dense. Stony meteoroids are lighter and more fragile.

Assessing the Impact
Meteoroids are generally as harmless as any other space object. They're specks of dust floating around the sun. Space agencies such as NASA do monitor the movement of meteoroids, however, for two reasons: potential impact with spacecraft and potential impact with Earth.

Potential Impact for Spacecraft
The impact of even a micrometeoroid can damage spacecraft. They can damage the spacecraft's windows, thermal protection systems (which control temperature), and pressurized containers of spacecraft. This could endanger astronauts, result in the loss of valuable scientific instruments, and cost millions of dollars.

Engineers must prepare and equip spacecraft to avoid or withstand meteoroid impacts. To do this, they have classified three different "meteoroid environments:" 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 is most vulnerable to sporadic meteoroids, and prepare stronger shielding mechanisms.

The shower environment describes the threat of meteoroid streams associated with comets passing through Earth's orbit. On Earth, these debris fields are associated with meteor showers. Engineers must be able to maneuver the spacecraft in order to take its most vulnerable areas out of the path of the meteoroid stream.

The lunar environment describes the threat of meteoroids to astronauts or facilities 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 withstand meteoroid impacts.

Earth's Atmosphere Can Impact Meteoroids
When a meteoroid passes through Earth's atmosphere, it heats up due to resistance from the air around it. 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 disintegrate, or fall apart, 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 impact. Very large meteors called bolides may explode in the atmosphere with the force of 500 kilotons of TNT. These bolides, often called "fireballs," and the shock waves they produce may cause burns and even death. These can also damage buildings and crops. An actual impact—where part of the space rock actually crashes into Earth—can be even more disastrous. A single impact event about 66 million years ago, for instance, likely led to the extinction of the dinosaurs and almost every other form of life on Earth.