135 Hertha

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135 Hertha
Discovery[1]
Discovered by Christian Heinrich Friedrich Peters
Discovery site Litchfield Observatory, Clinton, New York
Discovery date February 18, 1874
Designations
Minor planet category Main belt
Nysa
Orbital characteristics[2]
Epoch 2008-05-14 (JD 2454600.5)
Aphelion 2.9287124854 ± 4.995×10−9 AU
Perihelion 1.92827931 ± 2.0509×10−7 AU
Semi-major axis 2.4284958975 ± 4.1419×10−9 AU
Eccentricity 0.205977942 ± 8.4222×10−8
Orbital period 3.78 ± 9.682×10−9 y
Average orbital speed 18.91 km/s
Mean anomaly 16.444645 ± 2.1094×10−5°
Inclination 2.3051442 ± 9.8708×10−6°
Longitude of ascending node 343.84267 ± 1.901×10−4°
Argument of perihelion 339.91983 ± 1.9113×10−4°
Physical characteristics
Dimensions 79.24 km[2]
76.12 ± 3.29[3] km
Mass (1.21 ± 0.16) × 1018[3] kg
Mean density 5.23 ± 0.96[3] g/cm3
Rotation period 8.40061[4]
Albedo 0.1436[2]
Spectral type M[5]
Absolute magnitude (H) 8.23[2]

    135 Hertha is a large main-belt asteroid. Named Hertha, another name for Nerthus, a Germanic fertility goddess. It orbits among the Nysa asteroid family but its classification as an M-type asteroid does not match the more common F-type asteroid for this family, suggesting that it may be an interloper.[6] Spectroscopic analysis indicates the possible presence of hydrated silicates indicating that 135 Hertha should possibly be reclassified from its present M-type to the proposed W-type.[7]

    Lightcurve data from Hertha indicates a flattened body,[4] and radar observations indicate that Hertha is non-metallic.[8] One occultation of a star by the asteroid has been observed, in 2000.

    Discovery

    Hertha was discovered by C. H. F. Peters on February 18, 1874, in Clinton, New York.[1] 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.[9]

    Physical properties

    After its discovery in 1874 and subsequent observations in 1884 had established Hertha's orbit, astronomers began investigation of its physical properties. As early as 1904 G. W. Hill reported observations of Hertha's brightness indicating a variation of half a magnitude and a short period.[10]

    In October 1992 Dotto et al. performed 20 hours of observations spread over 6 nights to investigate 135 Hertha'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 Harris et al. published earlier in 1992.[5][11] 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.[5]

    In August 2003 Torppa et al. published their results on the shape and rotational properties of a number of asteroids, including 135 Hertha. Utilizing data from 42 lightcurves of 135 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°).[4]

    Spectral classification

    Although 135 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 135 Hertha should be reclassified as a W-type (a "wet M-type") asteroid.[12] 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; it should be noted, however, that this estimate is based on laboratory measurements and may not be applicable to asteroids in space.[12]

    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.[13]

    Further reading

    • Hardersen, P.S.; Gaffey, M.J.; Abell, P.A. (May 2005). "Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids". Icarus 175 (1): 141–158. Bibcode:2005Icar..175..141H. doi:10.1016/j.icarus.2004.10.017. 
    • Hardersen, P.S.; Gaffey, M.J.; Abell, P.A. (September 2006). "Near-infrared Reflectance Spectra Of 135 Hertha, 224 Oceana, 516 Amherstia, And 872 Holda". Bulletin of the American Astronomical Society 38: 626. Bibcode:2006DPS....38.7103H. 

    References

    1. ↑ 1.0 1.1 Peters, C.A.F. (1874). "Observations of the Planet Hertha (135), made at the Litchfield Observatory of Hamilton College". Astronomische Nachrichten 84 (2001): 129. Bibcode:1874AN.....84..129P. doi:10.1002/asna.18740840902. 
    2. ↑ 2.0 2.1 2.2 2.3 135 Hertha at the JPL Small-Body Database
    3. ↑ 3.0 3.1 3.2 Carry, B. (December 2012), "Density of asteroids", Planetary and Space Science 73: 98-118, arXiv:1203.4336, Bibcode:2012P&SS...73...98C, doi:10.1016/j.pss.2012.03.009.  See Table 1.
    4. ↑ 4.0 4.1 4.2 Torppa, J.; et al. (August 2003). "Shapes and rotational properties of thirty asteroids from photometric data". Icarus 164 (2): 346–383. Bibcode:2003Icar..164..346T. doi:10.1016/S0019-1035(03)00146-5. 
    5. ↑ 5.0 5.1 5.2 Dotto, E.; et al. (October 1992). "M-type Asteroids: Rotational properties of 16 Objects". Astronomy and Astrophysics Supplement Series 95 (2): 195–211. Bibcode:1992A&AS...95..195D. 
    6. ↑ Cellino, A.; Vincenzo, Z. (October 1993). "Asteroid 'clans': Super-families or multiple events?". Celestial Mechanics and Dynamical Astronomy 57 (1–2): 34–37. Bibcode:1993CeMDA..57...37C. doi:10.1007/BF00692459. 
    7. ↑ Cellino, A.; et al. (August 2001). "The Puzzling Case of the Nysa–Polana Family". Icarus 152 (2): 225–237. Bibcode:2001Icar..152..225C. doi:10.1006/icar.2001.6634. 
    8. ↑ Shepard, M.K.; et al. (September 2006). "More Results from a Long-Term Radar Survey of M-Class Asteroids". Bulletin of the American Astronomical Society 38: 626. Bibcode:2006DPS....38.7101S. 
    9. ↑ Sampson, W.T.; Shufeldt, R. W. (1884). "Observations of (135) Hertha made at the Naval Observatory Washington with 9.6 inch equatorial". Astronomische Nachrichten 107 (20): 323. Bibcode:1884AN....107..323S. doi:10.1002/asna.18841072006. 
    10. ↑ Hill, G.W. (March 1904). "Variability of (135) Hertha". Astronomical Journal 24 (557): 42. Bibcode:1904AJ.....24...42H. doi:10.1086/103543. 
    11. ↑ Harris, A.W.; et al. (January 1992). "Asteroid lightcurve observations from 1981". Icarus 95 (1): 115–147. Bibcode:1992Icar...95..115H. doi:10.1016/0019-1035(92)90195-D. 
    12. ↑ 12.0 12.1 Rivkin, A.S.; et al. (June 2000). "The Nature of M-Class Asteroids from 3-μm Observations". Icarus 145 (2): 351–368. Bibcode:2000Icar..145..351R. doi:10.1006/icar.2000.6354. 
    13. ↑ Rivkin, A.S.; et al. (March 2002). "Hydrated Minerals on Asteroids: The Astronomical Record" (PDF). Technical Report, Massachusetts Institute of Technology. 

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