1998 WB₂

This asteroid was discovered by LINEAR on 1998 Nov. 18, and was announced in MPEC 1998-W21 two days later. The object was on an approach trajectory to the Earth (minimum distance 0.0168 AU on 1998 Dec. 5) on 1998 Nov. 28 when photometric observations were made by R. Whiteley. These observations were carried out using the UH 2.2 meter telescope on Mauna Kea.

Observations of 1998 WB₂ were carried out as part of an ongoing program to identify the spectral characteristics of the near-Earth asteroid population (Whiteley and Tholen 1999). The photometric measurements presented here were taken in the ECAS photometric system (Zellner et al. 1985). The primary purpose of the measurements is taxonomic identification, and not rotational information. However, in order to remove the effects of the asteroid's lightcurve from the determination of its color indices it is necessary to repeatedly measure the brightness of the asteroid through one of the filters. In this case, there are nine observations made through the v filter, and two points in each of the u, b, p, x and w filters. The time distribution of the measurements is shown in Fig. 5 (see below for assumed color indices). Errors on all data points are about 0.02 mag. This error estimate includes a systematic error due to the color term calibration being impossible to perform without formal color indices.

Obviously, the scatter of points in Fig. 5 is much greater than the errors and represents a real variation. To search for a period solution, we had to assume some color indices. We started with color indices typical for small S-type asteroids as measured in Whiteley and Tholen's survey: u-v=0.35, b-v = 0.15, v-w=0.15, v-x=0.10 and v-p=0.10. All but one of these values gave a good fit. The only exception was v-p for which a value of 0.15 has been found to provide an acceptable fit and was adopted. Although the small number of points does not allow us to make a firm statistical conclusion, values differing by more than 0.05-0.10 mag from the assumed/adopted color indices gave an unsatisfactory fit. Although somewhat uncertain, the best-fit color indicesare highly suggestive of an S-type classification.

The period  min has been found in the data. (Its error includes also the uncertainty of the color indices.) The composite lightcurve is shown in Fig. 6. It has a peak-to-peak amplitude of 0.6 mag, and is dominated by the second harmonic. There is also significant signal in the first harmonic, which is somewhat unusual at a phase angle of , but not unheard of. The mean, absolute magnitude has been derived from the zeroth-order coefficient of the best fit Fourier series, extrapolated to zero solar phase angle assuming .

The smoothing of the lightcurve due to the 60-sec integration time is small; maximum deviations are about 0.01 mag, negligible in comparison with other errors. The longer integration time of the two u points must have a somewhat greater effect. Considering the placement of the points in the lightcurve, they must contain shifts up or down by about 0.03 mag. These shifts are comparable to the photometric errors of the data points,and are smaller than the uncertainty in the adopted u-v. The effect is therefore not important and we have not made any correction for it in thecomposite lightcurve.

We conclude that the lightcurve of 1998 WB₂ is due to the object's fast rotation. The adopted color indices suggest an S-type taxonomic classification. Taking a geometric albedo of 0.17 (typical for S asteroids), and the measured absolute magnitude, we estimate a mean diameter of the object of 120 meters. The shape is somewhate longated; an equatorial axis ratio has been estimated from the lightcurve amplitude.