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.
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. 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 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°).
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