= 1.5. Measurements of the pole direction were also obtained, however like Dotto et al. they were unable to differentiate between their two possible solutions of (β=+58°, λ=96°) and (β=+53°, λ=274°). In 2017, Hanuš et al. confirmed that the correct solution is β=°, λ=°. They also calculated the first non-convex shape model, based on lightcurve and stellar occultation data. Spectral classification Although Hertha has long been classified as an M-type asteroid based on its spectral properties, observations carried out by Rivkin et al. in 1996 using the IRTF at Mauna Kea Observatory have raised the possibility of reclassification. The presence of a dip in the observed spectrum at 3 μm indicates that the surface is hydrated, suggesting that Hertha should be reclassified as a W-type (a "wet M-type") asteroid. Based on work carried out by Salisbury and Walter, the Rivkin study estimated the water content of the asteroid to be between 0.14 and 0.27 percent by mass. This estimate is based on laboratory measurements and may not be applicable to asteroids in space. A more recent study by Rivkin et al. published in 2002 examined the dependence of spectral absorption on the asteroid's rotational phase. The study looked at the 0.7 μm band, which is also associated with hydrated silicates, and found that the reflectance changes as the asteroid rotates, suggesting that the surface is heterogeneous with some hydrated areas intermixed with dry areas. Asteroid family Hertha is one of the core members of the Nysa family (405) also known as Herta family. The Nysa–Polana complex is the main-belt's largest grouping of asteroids with nearly 20,000 members. Further reading References External links Lightcurve plot of 135 Hertha, Palmer Divide Observatory, B. D. Warner (2008) 2008-Dec-11 Occultation / (2008 Asteroidal Occultation Results for North America) Asteroid Lightcurve Database (LCDB), query form (info ) Dictionary of Minor Planet Names, Google books Asteroids and comets rotation curves, CdR – Observatoire de Genève, Raoul Behrend Discovery Circumstances: Numbered Minor Planets (1)-(5000) – Minor Planet Center 000135 Discoveries by Christian Peters Named minor planets 000135 000135 000135 18740218