Adv. Space Re.a. Vol. 1I, No. 12, pp. (12)193-(12)196, 1991 0273-11T'H91 $0.00 + .50 Printed in Great Britain. All fightsrmervexl. Copyright© 1991 COSPAR PHOTOELECTRIC AND CCD PHOTOMETRY OF 951 GASPRA C. Blanco,* M. Di Martino,** W. Ferreri,** M. Gonano,*** S. Mottola*** and G. Neukum*** * lstituto di Astronomia, Universitd di C atania, L95125 C atania, Italy **Osservatorio Astronomico di Torino, 1-10025 Pino Torinese, Italy ***DLR German Aerospace Research Establishment, D-8031 Oberpfaffenhofen, Germany ABSTRACT The asteroid 951 Gaspra is a fly-by target for the Galileo space mission. This encounter, planned for October 1991, will represent the first possibility to show an asteroid in close-up. In preparation to this close encounter an earth-based international observing campaign started in 1988 /1,2/, with the goal to provide a substantial data base on the photometric and rotational properties of this object. This will allow on one side to support and optimize the planning of the fly-by and, on the other side, to verify the accuracy of current models for the determination of shapes, surface textures, spin axis orientation and composition. In this paper we present the composite lightcurve of 951 Gaspra obtained from photoelectric and CCD observations, carried out during the 1990 apparition at the observatories of Asiagn, Catania and Loiano (Italy). We have determined the sidereal rotational period, the prograde sense of rotation, the H and G parameters and the B-V and V-R color indices. OBSERVATIONS The observations of this S-type asteroid span from January, 26 to March, 21 1990. In TABLE 1 are listedthe observers, the detectors, the observed bands and the telescopes used. A description of the DLR CCD Camera is given in the Mottola et a/. /3/ paper. TABLE 1, Observation Circumstances. Date Observer Telescope Detector Band 26/27/28 Jan 1990 21/22/23Feb 1990 21Mar 1990 Blanco, Ferreri 91-cm Catania EMI 9789 Q.A. B,V Gonano 182-cm Asiago DLR CCD B,V,R Gonano 152-cm Loiano DLR CCD V The aspect data are given in TABLE 2. The magnitudes have been reduced at 1 AU distance from the Earth and from the Sun and the times have been corrected for the light travel path. A composite of the V-light curves of 951 Gaspra obtained during three nights in February is shown in Figure 1. The light curves present a two maxima and two minima profile. The principal min- imum (ml) magnitude differs from the one of the secondary (m2) of about 0.08 mag. An exact estimation of the maximum light curve amplitude is possible for the observations of February 1990, resulting in a value of 0.46 mag. at a phase angle of 11 deg. This value, compared with the amplitude of 0.8 mag. obtained at 25 deg. phase angle by W.Z. Wisniewski /4/ shows an increase of the light curve amplitude with increasing phase angle, in agreement with what has been detected before for several other asteroids and with laboratory simulations/5/. (12)193