The Nysa–Polana complex (also known as the Nysa–Polana family, Nysa–Polana clan, or Nysa–Polana–Hertha complex) is a group of overlapping asteroid families in the low-inclination region of the inner main asteroid belt. In terms of proper orbital elements, asteroids in this complex orbit the Sun at semi-major axes approximately between 2.3 and 2.5 astronomical units (AU), with low orbital inclinations between 2° and 4° and eccentricities between 0.1 and 0.2. The complex was first identified in the 1950s and has undergone various interpretations until the 2000s, when astronomical surveys began distinguishing the physical properties of its members for the first time. As of 2026, the Nysa–Polana complex contains over 38,600 known asteroids. The complex is broadly divided into low-albedo and high-albedo populations, each consisting of distinct families named after their largest members. The low-albedo population, sometimes known as the Polana–Eulalia complex, comprises the New Polana and Eulalia families and contains carbonaceous asteroids with spectral types C, B, and F. The New Polana family formed from an asteroid collision 1.40±0.15 billion years ago, whereas the Eulalia family formed from a more recent asteroid collision 865 million years ago. The high-albedo population consists of S-type and X-type asteroids plausibly associated with 135 Hertha and is thought to have formed from one or two collisions less than 1 billion years ago. Although the Nysa–Polana complex bears the name of 44 Nysa, the asteroid is generally considered an interloper unrelated to the complex due to its differing composition. Over millions to billions of years, asteroids in the Nysa–Polana complex may migrate to unstable orbital resonances via the Yarkovsky effect. These resonances significantly elongate the orbits of asteroids within a few million years, allowing them to come near Earth. Because of this, the Nysa–Polana complex has been identified as a source of low-inclination near-Earth asteroids and meteoroids. The carbonaceous CI chondrites and near-Earth asteroids 101955 Bennu and 162173 Ryugu are believed to have come from the Polana–Eulalia complex, while the stony L chondrites are believed to have come from the Nysa–Polana complex's S-type population.
History of studies 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. Brouwer assigned the identification number "24" to this family, while a 1969 study by James R. Arnold assigned the label "A-74". During this time, the Palomar–Leiden Survey discovered numerous small asteroids within the vicinity of Brouwer's purported family, revealing it to be an aggregation of several neighboring unrelated families. Thus in 1971, Bertil A. Lindblad and R. B. Southworth proposed that the family should be split into the Nysa and Hertha families. However, few asteroids in these proposed families had known colors and compositions, which raised the question of whether the Nysa and Hertha families truly originated from their namesakes, and whether the two families shared the same origin. In 1982, E. F. Tedesco and colleagues analyzed the colors and albedos of asteroids in Lindblad and Southworth's proposed Nysa family and found that they were mostly F-type asteroids, unlike Nysa. This led Jeffrey F. Bell to argue in 1989 that these asteroids belonged to Polana instead of Nysa, which he labeled an interloper. In 1995, Vincenzo Zappalà and colleagues found that this family was actually two overlapping families (which does not include Hertha), which he dubbed a "complex clan" associated with Nysa. In response to this, Alberto Cellino and colleagues measured the spectra of multiple asteroids in this proposed clan and found that none matched Nysa's spectral type. They proposed in a 2001 study that the Nysa–Polana clan contains an S-type Mildred family (renamed from the Hertha family) and a F-type Polana family, neither of which share a common origin. Further observations and analyses by 2012 revealed that these families are divided into two clusters in terms of proper eccentricity and contain a mix of S-, X-, and C-type asteroids, leading various studies to reassign the families' names. The development of large asteroid surveys in the early 2000s provided the data needed to reassess the structure of the Nysa–Polana complex. Color measurements from the Sloan Digital Sky Survey and albedo measurements from the Wide-field Infrared Survey Explorer revealed a clear separation between low- and high-albedo asteroids in the region. In 2013, Kevin J. Walsh and colleagues identified the low-albedo population as two distinct collisional families associated with Polana and Eulalia. Two years later, Melissa J. Dykhuis and Richard Greenberg demonstrated that the high-albedo population could also be divided into separate S-type and X-type groups associated with Hertha, establishing the modern interpretation of the Nysa–Polana complex as a collection of orbitally overlapping but compositionally distinct asteroid families.
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