Meteoroids are lumps of rock or iron that orbit the sun, just as planets, asteroids, and comets do. 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 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.

Other meteoroids are the debris that comets shed as they travel through space. As a comet approaches the sun, the “dirty snowball” of the comet’s nucleus 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 celestial bodies—often 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 celestial body that doesn’t have an airy atmosphere, such as asteroids, many moons, or the planets Mars and Mercury. Meteoroids crash into these bodies, creating craters and throwing 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 massive and dense, while stony meteoroids are lighter and more fragile.

Assessing the Impact
Meteoroids are generally as harmless as any other celestial body—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.

Spacecraft
The impact of even a micrometeoroid can damage the windows, thermal protection systems, 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 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. Although there have been no long-term astronaut stays on the moon, engineers have designed space suits, vehicles, and habitats that can withstand meteoroid impacts.

Earth
When a meteoroid passes through Earth’s atmosphere, it heats up due to air 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 disintegrate before they reach the ground. The pieces that do strike Earth's surface are called meteorites.

Both meteors and meteorites can become natural hazards to the communities they impact. Very large meteors called bolides may explode in the atmosphere with the force of 500 kilotons of TNT. These meteors and the shock waves they produce may cause burns and even death, as well as damage to buildings and crops. An actual impact—where part of the space rock actually crashes into Earth—can be even more catastrophic. 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.