Nysa–Polana complex

Lore & Background

The Nysa–Polana complex was first identified as an asteroid family in 1951 by Dirk Brouwer, who noted that 44 Nysa, 135 Hertha, 142 Polana, and six other asteroids shared similar proper orbital elements. Subsequent studies in the 1970s and 1980s revealed that the group was actually an aggregation of several neighboring families, and color and albedo analyses showed that many asteroids were compositionally distinct from Nysa. By the early 2000s, large surveys such as the Sloan Digital Sky Survey and the Wide-field Infrared Survey Explorer provided data that separated the complex into low-albedo and high-albedo populations, each containing distinct collisional families.

The low-albedo population, sometimes called the Polana–Eulalia complex, comprises the New Polana and Eulalia families, which are carbonaceous (spectral types C, B, and F). The New Polana family formed from a collision 1.40 ± 0.15 billion years ago, while the Eulalia family formed more recently, 865 million years ago. The high-albedo population consists of S-type and X-type asteroids associated with 135 Hertha and is thought to have formed from one or two collisions less than 1 billion years ago. Although the complex bears the name of 44 Nysa, that asteroid is generally considered an interloper due to its differing composition.

Reader's Guide

The Nysa–Polana complex is significant as a major source of near-Earth asteroids and meteoroids. Over millions to billions of years, the Yarkovsky effect gradually alters the semi-major axes of its asteroids, causing them to drift into mean-motion resonances. Once in these resonances, gravitational perturbations elongate their orbits within a few million years, allowing them to approach Earth. The carbonaceous CI chondrites and the near-Earth asteroids 101955 Bennu and 162173 Ryugu are believed to have originated from the Polana–Eulalia complex, while the stony L chondrites are thought to come from the complex's S-type population.

The complex's nomenclature has evolved with understanding: originally named after Nysa and Polana, it is sometimes called the Nysa–Polana–Hertha complex or Hertha clan, reflecting the role of 135 Hertha as the parent of the high-albedo population. The complex remains a subject of study for understanding the delivery of material from the asteroid belt to Earth, and its overlapping families provide insight into collisional history and dynamical evolution in the inner main belt.

Did You Know?

• The Nysa–Polana complex contains over 38,600 known asteroids as of 2026.
• The low-albedo population includes the New Polana family (formed 1.40 ± 0.15 billion years ago) and the Eulalia family (formed 865 million years ago).
• 44 Nysa, for which the complex is named, is considered an interloper with an unrelated composition.