 Lightcurve-base 3D-model ofHertha on the top with an image of the asteroid on the bottom. | |
| Discovery[1] | |
|---|---|
| Discovered by | C. H. F. Peters |
| Discovery site | Litchfield Obs. |
| Discovery date | 18 February 1874 |
| Designations | |
| (135) Hertha | |
| Pronunciation | /ˈhɜːrθə/ |
Named after | Nerthus[2] (Norse mythology) |
| A874 DA[3][a] | |
| main-belt[1][3] · (inner) Nysa (Hertha)[4][5] | |
| Orbital characteristics[3] | |
| Epoch 23 March 2018 (JD 2458200.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 133.34yr (48,701 d) |
| Aphelion | 2.9320AU |
| Perihelion | 1.9239 AU |
| 2.4279 AU | |
| Eccentricity | 0.2076 |
| 3.78 yr (1,382 d) | |
| 233.89° | |
| 0° 15m 37.8s / day | |
| Inclination | 2.3052° |
| 343.65° | |
| 340.16° | |
| Physical characteristics | |
| 76.12 ± 3.29 km[6] 79.24±2.0 km[3] | |
| Mass | (1.21 ± 0.16) × 1018 kg[6] |
Meandensity | 5.23 ± 0.96 g/cm3[6] |
| 8.40061 h[7] | |
| 0.1436[3] | |
| M[8] | |
| 8.23[1][3] | |
135 Hertha is anasteroid from the inner region of theasteroid belt, approximately 77 kilometers (48 miles) in diameter. Discovered on 18 February 1874 by German–American astronomerChristian Peters at theLitchfield Observatory near Clinton, New York,[1] it was named after the Teutonic and Scandinavian goddess of fertility, Hertha, also known asNerthus.[2] It orbits among theNysa asteroid family, but its classification as a metallicM-type asteroid does not match the more commonF-type asteroid for this family, suggesting that it may be aninterloper.[9] Spectroscopic analysis indicates the possible presence of hydratedsilicates indicating thatHertha should possibly be reclassified from its present M-type to the proposed W-type.[10]
Lightcurve data fromHertha indicates a flattened body,[7] andradar observations indicate thatHertha is non-metallic.[11] Fiveoccultations ofstars by the asteroid have been observed between 2000 and 2015.
Hertha was discovered byC. H. F. Peters on 18 February 1874, inClinton, New York.[12] Further observations were carried out in 1883 byW. T. Sampson and communicated toAstronomische Nachrichten on his behalf by Rear AdmiralR. W. Shufeldt.[13]
After its discovery in 1874, subsequent observations in 1884 establishedHertha's orbit. Astronomers then began investigation of its physical properties. As early as 1904,G. W. Hill reported observations ofHertha's brightness indicating a variation of half a magnitude and a short period.[14]
In October 1992 Dottoet al. performed 20 hours of observations spread over 6 nights to investigateHertha's 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 Harriset al. published earlier in 1992.[8][15] In the same study, Dottoet 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 differentecliptic longitudes are required to determine which is correct.[8]
In August 2003 Torppaet al. published their results on the shape and rotational properties of a number of asteroids, includingHertha. Utilizing data from 42 lightcurves ofHertha 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 Dottoet al. they were unable to differentiate between their two possible solutions of (β=+58°, λ=96°) and (β=+53°, λ=274°).[7]
In 2017, Hanušet al. confirmed that the correct solution is β=53±3°, λ=277±3°. They also calculated the first non-convex shape model, based on lightcurve andstellar occultation data.[16]
AlthoughHertha has long been classified as an M-type asteroid based on its spectral properties, observations carried out by Rivkinet al. in 1996 using theIRTF atMauna 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 thatHertha should be reclassified as a W-type (a "wet M-type") asteroid.[17] 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.[17]
A more recent study by Rivkinet 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.[18]
Hertha is one of the core members of theNysa family (405) also known as Herta family.[4][5] TheNysa–Polana complex is the main-belt's largest grouping of asteroids with nearly 20,000 members.[19]: 23
