Asteroids

Rocky and metallic remnants of planet formation, ranging from dwarf-planet Ceres down to boulder-sized bodies, mostly orbiting between Mars and Jupiter.

Definition

Asteroids are small, predominantly rocky or metallic bodies orbiting the Sun, most numerous in the main belt between the orbits of Mars and Jupiter.

Scope

This topic covers asteroids: their orbital populations, including the main belt, near-Earth objects, Trojans, and dynamical families; their spectral taxonomy and links to meteorites; their internal structure, with many being loosely bound rubble piles; and the non-gravitational forces, such as the Yarkovsky and YORP effects, that slowly change their orbits and spins. It also covers asteroid exploration and sample return, and the impact hazard from near-Earth objects.

Core questions

• How are asteroids distributed dynamically, and what maintains the belt and near-Earth populations?
• What are asteroids made of, and how do spectral classes link to meteorite types?
• Are asteroids solid rock or loosely bound rubble piles?
• How do thermal forces alter asteroid orbits and spins over time?

Key theories

Rubble-pile structure

Many asteroids are gravitationally bound aggregates of fragments rather than monolithic rock, a structure produced by collisional disruption and reaccumulation and confirmed by spacecraft visits.

Spectral taxonomy and meteorite links

Asteroids are classified into spectral types whose compositions connect them to specific meteorite groups, tracing the distribution of rocky and primitive materials across the belt.

Yarkovsky and YORP effects

Asymmetric thermal re-emission of sunlight slowly changes an asteroid's orbit (Yarkovsky) and spin state (YORP), delivering bodies into resonances that feed the near-Earth population.

Mechanisms

Collisions over the age of the Solar System fragment asteroids into families and produce gravitationally reaccumulated rubble piles. Sunlight absorbed and re-emitted with a thermal lag exerts the Yarkovsky force that drifts orbits into resonances, while the related YORP torque alters spin rates and can shed material, replenishing near-Earth and meteorite populations.

Clinical relevance

Asteroids preserve primitive materials sampled by meteorites and return missions, may hold resources, and include the near-Earth objects that pose an impact hazard and drive planetary-defense efforts.

History

Ceres was discovered in 1801, followed by a growing catalogue of minor planets and, in the 20th century, the recognition of dynamical families and the role of resonances. Spacecraft missions, including NEAR at Eros, Hayabusa and Hayabusa2, OSIRIS-REx at Bennu, and Dawn at Vesta and Ceres, plus the DART deflection test, have made asteroids targets of detailed in-situ study.

Debates

Origin and porosity of rubble-pile asteroids

How asteroids acquired their high porosity and cohesionless structure, and how this affects deflection strategies, is an active area of study following close-up imaging of Bennu and Ryugu.

Key figures

• William Bottke
• Richard Binzel
• Francesca DeMeo
• Dante Lauretta

Seminal works

• bottke2002
• demeo2013
• lauretta2019

Frequently asked questions

Where are most asteroids found?

In the main asteroid belt, a broad ring of bodies orbiting the Sun between Mars and Jupiter, though many also follow other orbits including near-Earth paths.

Could an asteroid hit Earth?

Small ones strike regularly and burn up harmlessly, and large impacts are rare; astronomers track near-Earth objects and have tested deflection to mitigate the hazard from the few that could be dangerous.