Contact binary (small Solar System body)

A contact binary (small Solar System body) refers to a specific type of binary asteroid or Kuiper Belt Object (KBO) where the two constituent bodies are physically touching, sharing a common surface. This distinguishes them from binary systems where the objects orbit each other with a defined separation. Contact binaries are formed when two originally separate bodies, gravitationally attracted to each other, come into close proximity and their mutual gravitational forces cause them to merge. This merger is typically a low-velocity collision, preserving much of the original shapes of the individual components.

The overall shape of a contact binary often resembles a peanut, a dumbbell, or a kidney bean, reflecting the joined nature of the two original bodies. The connecting "neck" region can be quite narrow.

Contact binaries provide valuable insights into the formation and evolution of small Solar System bodies. Their existence suggests a significant role for gentle collisions and accretion in building larger objects in the early Solar System. Studying their composition, structure, and rotational properties can help scientists understand the physical processes that govern the aggregation of planetesimals.

Observations of contact binaries are primarily conducted through radar, lightcurve analysis, and direct imaging by spacecraft missions. Analyzing the light reflected from these objects as they rotate can reveal information about their shape and surface properties. Spacecraft flybys, such as those of the New Horizons mission to Arrokoth in the Kuiper Belt and the Rosetta mission to comet 67P/Churyumov–Gerasimenko, have provided detailed images and data of these intriguing celestial objects, greatly enhancing our understanding of their formation and composition.