A meteoroid is a small, solid body traveling through outer space, typically ranging in size from a grain of sand to about a meter across. These rocky or metallic fragments exist in a chaotic orbital environment within the solar system. The term applies only while the object is still in space, orbiting the Sun. If it enters a planet’s atmosphere and burns up due to friction, the resulting streak of light is called a meteor. Should the fragment survive and land on the ground, it is designated a meteorite. Meteoroids are created through three primary mechanisms: the fragmentation of asteroids, the shedding of debris from comets, and the ejection of material from planetary impacts.
Creation Through Asteroid Fragmentation
The vast majority of meteoroids originate in the main Asteroid Belt, a crowded region between the orbits of Mars and Jupiter. Larger asteroids occasionally collide at high relative velocities, often around 5 kilometers per second. These catastrophic impacts shatter the parent bodies into countless smaller fragments, which become meteoroids.
These fragments, grouped into asteroid families, are initially confined to the main belt. Over long timescales, a subtle force known as the Yarkovsky effect modifies their orbits. This effect is a thermal radiation force caused by the uneven heating and cooling of a rotating object’s surface. The delayed re-emission of solar energy creates a tiny, continuous thrust that slowly changes the meteoroid’s orbital path.
For meteoroids smaller than about 20 kilometers, this thermal push causes their orbits to slowly drift inward or outward. This gradual drift can push the fragments into gravitational resonances with Jupiter. Once in these chaotic resonance zones, the meteoroid’s orbit is rapidly destabilized, flinging it out of the main belt and into the inner solar system. The Yarkovsky effect is the primary delivery mechanism transporting asteroid-derived meteoroids to Earth-crossing orbits.
Debris Shed from Comets
Meteoroids also form from comets, which are composed of frozen volatiles, dust, and rock. This process differs fundamentally from the violent collisions in the asteroid belt. As a comet travels toward the Sun, solar heat causes its icy components to change directly into gas, a process called sublimation.
This outgassing carries embedded dust and rock particles away from the comet’s nucleus. These released solid fragments travel along the comet’s orbital path, forming a diffuse trail of debris. When Earth’s orbit intersects one of these dust trails, the particles enter the atmosphere, creating a meteor shower.
Cometary meteoroids are typically less dense and more loosely bound than those from asteroids. They are often referred to as “dustballs” because they are porous agglomerations of grains held together by weak forces. This thermal-release mechanism supplies material to the interplanetary dust cloud, which is the source of many annual meteor showers, such as the Perseids and Leonids.
Ejecta from Planetary Impacts
The rarest type of meteoroid forms from the surfaces of planets and moons through a process called impact ejection. This occurs when a large, high-velocity impactor, such as an asteroid, strikes a solid body like Mars or the Moon. The immense energy generates a shock wave that travels through the target rock.
If the impact is sufficiently powerful, the shock wave accelerates fragments of the planet’s surface material to speeds exceeding the body’s escape velocity. For example, material ejected from Mars must achieve a velocity greater than 5 kilometers per second to escape gravity. This material is launched into space and begins an independent journey as a meteoroid.
These meteoroids are identified as planetary in origin because they have the exact same chemical and isotopic signatures as their parent body. The existence of Martian and Lunar meteorites found on Earth is direct evidence that material can be transferred between celestial bodies.