An asteroid is a rocky body orbiting the Sun that is smaller than a planet and is often called a minor planet. These remnants from the early solar system are essentially leftover building blocks that never fully coalesced into a larger world. While asteroids are distributed throughout the solar system, the majority of these objects are clustered into a few highly specific orbital regions where gravity has allowed them to collect and remain stable.
The Main Asteroid Belt
The single largest reservoir of asteroids is the Main Asteroid Belt, a vast, torus-shaped region located between the orbits of Mars and Jupiter. This belt is home to the vast majority of the solar system’s known asteroids, with estimates suggesting over a million bodies larger than one kilometer in diameter, alongside countless smaller fragments. This region is believed to be the site where a planet might have formed, but the immense gravitational influence of Jupiter prevented the material from gathering into a single large body. Jupiter’s gravity stirred the planetesimals, causing them to collide at high speeds and fragment rather than gently accrete.
The Main Belt is not uniformly populated, and the powerful pull of Jupiter creates distinct zones of depletion known as Kirkwood Gaps. These gaps occur at specific distances from the Sun where an asteroid’s orbital period would be a simple fraction of Jupiter’s orbital period. At these resonant distances, Jupiter’s repeated gravitational tugs at the same point in the asteroid’s orbit destabilize the path, effectively clearing out the region. The belt’s largest object is the dwarf planet Ceres, which is about 950 kilometers in diameter, but the population also includes bodies as small as dust particles.
Asteroid Populations Sharing Orbits
Asteroids are also found in gravitationally stable locations outside the Main Belt, most notably in groups that share a planet’s orbit. These objects are called Trojan asteroids, and they congregate near special points in a planet’s orbit known as Lagrange points, specifically L4 and L5. These points form an equilateral triangle with the Sun and the planet, providing pockets of stability. The L4 point leads the planet, and the L5 point trails it.
The most numerous co-orbital population by far are the Jupiter Trojans, which are thought to number in the hundreds of thousands. Trojans have also been found sharing the orbital paths of other planets, including Mars and Neptune. The stable and relatively pristine nature of these populations makes them valuable targets for scientific study as samples of the early solar system.
Near-Earth Asteroid Groups
A third distinct location for asteroids is the inner solar system, populated by Near-Earth Asteroids (NEAs). These are asteroids whose orbits bring them into close proximity with Earth, generally defined as having a closest approach to the Sun of less than 1.3 Astronomical Units. The vast majority of NEAs are thought to originate from the Main Belt, having been nudged toward the inner solar system primarily by Jupiter’s gravity and subsequent orbital changes caused by the Kirkwood Gaps.
NEAs are categorized into groups based on their orbital relationship to Earth. For instance, Apollo asteroids cross Earth’s orbit and have a semi-major axis larger than Earth’s, while Aten asteroids also cross Earth’s orbit but have a smaller semi-major axis. A third group, the Amors, approach Earth’s orbit from the outside, crossing the orbit of Mars but not Earth’s. While they are small in number compared to the Main Belt, this population is the focus of intense observation due to the potential for collision with our planet.