On the ambiguity of rotational periods of asteroids: The peculiar case of 52 Europa

Recent results support a possible ambiguity for several adopted values of the spin rates. An analysis of the present data set on the rotational properties is made in order to select a number of asteroids, for which new observations should be performed to check the reliability of their adopted parameters. The most significant example is presented and it concerns the very large asteroid 52 Europa. Its lightcurve presented two maxima and two minima during 1976 opposition leading to a period of 11^h.26. In 1983 the same number of extrema appeared in 5^h.63 only. The amplitude was small in both oppositions (～0^m.08 and ～0^{m.10, respectively)} and suggests the possibility of surface irregularities which dominate the light variation. Other hypotheses can be made but generally the period determination of asteroids is more difficult than previously believed.