Nysa-Polana family

The Nysa-Polana family is one of the most complex and intriguing Hirayama families in the main asteroid belt; also referred to as the Nysa-Hertha family. Its members move in low-inclination orbits at a mean distance from the Sun of 2.4 to 2.5 astronomical units, a region of the belt showing an unusual abundance of the fairly rare F-type asteroids. The two largest members of the family are 44 Nysa (the largest E-type asteroid and the brightest asteroid known) and 135 Hertha. Evidence suggests that the family is actually made of two distinct groupings: the first consisting of dark asteroids, including several F-type members, headed by its least-numbered member, 142 Polana. The second group consists of S-type asteroids and should be named after its most plausible least-numbered member, 878 Mildred. Some detailed physical analysis of the two groupings can be already attempted at this stage, including a reasonable reconstruction of the original ejection velocity field for the Mildred family. For instance, it is not clear how to interpret the role of as well as the presence of several large F-type asteroids apparently not related, but very close to the clan. Moreover, recent discoveries of common spectral features indicating hydrated minerals in the spectra of Nysa and Hertha add complexity to an already puzzling scenario.


Nysa-Hertha family: key members
  Nysa Hertha Polana Mildred
asteroid number 44 135 142 878
diameter 43 km - 57 km -
spectral class E - - -
semimajor axis 2.424 AU - 2.471 AU -
eccentricity 0.148 - 0.137 -
inclination 3.70° - 2.23° -
period 3.77 years - - -