Glossary

Achondrites: Stony meteorites that do not contain chondrules, indicating they have undergone melting and differentiation within their parent body.

Angular momentum transfer: When two bodies interact gravitationally, they can exchange angular momentum without changing the total amount in the system. During a gravity-assist manoeuvre, for example, a spacecraft gains or loses angular momentum at the expense of a planet’s orbit. Similar exchanges between the giant planets and countless planetesimals are thought to have driven the early migration of the Jovian planets.

Asteroid belt: The main ring of asteroids located between Mars and Jupiter, containing remnants of planetesimals that never coalesced into a planet because of Jupiter’s gravitational influence.

Asteroids: Small, rocky bodies that orbit the Sun, mostly within the asteroid belt, but also as near-Earth objects, Trojans, or trans-Neptunian bodies.

Chondrites: Stony meteorites containing chondrules, showing little or no evidence of melting or differentiation. They are among the most primitive materials in the Solar System.

Chondrules: Small, spherical grains that formed as molten droplets in the early solar nebula. They are the original seeds that aggregated into planetesimals and are preserved in chondrites.

Coma: The diffuse envelope of gas and dust that forms around a comet’s nucleus when it is heated by the Sun.

Comets: Small, icy bodies that orbit the Sun on elongated paths. When they approach the Sun, their ices sublimate, producing a glowing coma and one or more tails.

Conservation of angular momentum: A principle of physics stating that angular momentum — the rotational equivalent of linear momentum — remains constant in a closed system unless acted upon by an external torque.

Co-orbital asteroids: Asteroids that share a planet’s orbit in a stable resonance, such as the Trojan asteroids of Jupiter and Neptune.

Dawn mission: NASA mission (2007–2018) that orbited and studied the large asteroids Vesta and Ceres, revealing evidence of internal differentiation and past water activity.

Differentiation: The process by which heavier materials sink toward a body’s core while lighter materials rise toward the surface, producing distinct layers such as core, mantle, and crust.

Dust tail: One of a comet’s tails, formed from larger dust grains that are weakly affected by the solar wind and thus trail behind the comet along its orbit.

Fireball: A very bright meteor, typically one that ends in an explosion or produces audible sounds.

Gas tail: Also called the ion tail. A comet’s tail formed from ionized gas that interacts strongly with the solar wind, always pointing directly away from the Sun.

Ice: In planetary science, any volatile compound that is solid under space conditions — such as water, carbon dioxide, ammonia, or methane.

Iron meteorites: Meteorites composed primarily of iron and nickel, often derived from the cores of differentiated planetesimals.

Kuiper Belt: A region of icy bodies beyond Neptune, including dwarf planets like Pluto, Haumea, and Eris. It is the source of most short-period comets.

Late Heavy Bombardment: A period roughly 3.8–4.0 billion years ago when the inner Solar System experienced an intense spike or extended phase of impacts, possibly triggered by the migration of the giant planets as described by the Nice model.

Long-period comets: Comets with orbital periods longer than 200 years, believed to originate in the distant Oort Cloud.

Mars-crossing asteroids: Asteroids whose orbits intersect that of Mars; some can evolve into near-Earth objects.

Meteor: The flash of light produced when a meteoroid enters Earth’s atmosphere and vaporizes due to frictional heating.

Meteorite: A fragment of a meteoroid that survives passage through Earth’s atmosphere and reaches the ground.

Meteoroid: A small piece of rocky or metallic debris traveling through space, smaller than an asteroid.

Meteor showers: Events that occur when Earth passes through debris left by a comet, producing numerous meteors that appear to radiate from a common point in the sky.

Near-Earth objects (NEOs): Asteroids or comets whose orbits bring them within 1.3 astronomical units of the Sun, close enough to pose potential impact risks to Earth.

Nebula: A diffuse cloud of gas and dust in space; the solar nebula was the birth cloud of our Solar System.

Nebular hypothesis: The idea that the Sun and planets formed from the gravitational collapse of a rotating nebula of gas and dust.

Nice model: A model of Solar System evolution proposing that the giant planets migrated from their original orbits, triggering widespread scattering of planetesimals and shaping the current distribution of small bodies.

Nucleus: The solid, central part of a comet, made of rock, dust, and frozen gases.

OSIRIS-REx / OSIRIS-APEX mission: NASA mission that launched in 2016, collected samples from the asteroid Bennu in 2020, and returned them to Earth in 2023