135 Hertha
Hertha (minor planet designation: 135 Hertha) is an asteroid from the inner region of the asteroid belt, approximately in diameter. Discovered on 18 February 1874 by German–American astronomer Christian Peters at the Litchfield Observatory near Clinton, New York, it was named after the Teutonic and Scandinavian goddess of fertility, Hertha, also known as Nerthus. It orbits among the Nysa asteroid family, but its classification as an metallic M-type asteroid does not match the more common F-type asteroid for this family, suggesting that it may be an interloper. Spectroscopic analysis indicates the possible presence of hydrated  silicates indicating that "Hertha" should possibly be reclassified from its present M-type to the proposed W-type. Lightcurve data from "Hertha" indicates a flattened body, and radar observations indicate that "Hertha" is non-metallic. Five occultations of stars by the asteroid have been observed between 2000 and 2015. Discovery. "Hertha" was discovered by C. H. F. Peters on 18 February 1874, in Clinton, New York. Further observations were carried out in 1883 by W. T. Sampson and communicated to Astronomische Nachrichten on his behalf by Rear Admiral R. W. Shufeldt. Physical properties. After its discovery in 1874, subsequent observations  in 1884 established "Hertha" orbit. Astronomers then began investigation of its physical properties. As early as 1904, G. W. Hill reported observations of "Hertha" brightness indicating a variation of half a magnitude and a short period. In October 1992 Dotto "et al." performed 20 hours of observations spread over 6 nights to investigate "Hertha" rotational period, approximate shape, and the coordinates of its rotational axis. They were able to confirm a rotational period of 8.398 ± .001 hours as previously measured by Harris "et al." published earlier in 1992. In the same study, Dotto "et al." measured the asteroid's  shape and rotational axis. The axes' ratios were found to be: a/b = 1.34 ± .03 and b/c = 1.22 ± .05. Two possible values were determined for the rotational axis, however further measurements at different ecliptic longitudes are required to determine which is correct. In August 2003 Torppa "et al." published their results on the shape and rotational properties of a number of asteroids, including "Hertha". Utilizing data from 42 lightcurves of "Hertha" spanning from 1978 to 2002, a more refined set of axes' ratios was obtained and a detailed shape model was obtained through inversion. New  values for the axes' ratios are: a/b = 1.1 and b/c = 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.