1 Introduction

Nereid is the most eccentric satellite of the known Solar System (e= 0.75). This satellite of Neptune was discovered by Kuiper (1949) and from 1949 until now, there are 79 ground based published observations of this satellite. Its magnitude is about 19.5 and therefore there are not many Nereid's astrometric positions and those are not uniformly distributed in time. From 1949 to 1969 there were 47 photographic observations made by Van Biesbroeck and Roemer (Rose 1974 and Van Biesbroeck et al. 1976). The majority of this observations (35) are distributed uniformly between 1949-1955 and the remaining (12) in the interval 1967-1969. In 1977-78 two positions were taken at MacDonald Observatory by Shelus & Mulholland (Veillet 1982). From 1981 to 1987 there are 15 positions observed by Veillet (1988), 3 by Landgraf (1988) and 8 by the authors (Veiga et al. 1996). All these positions were obtained with photographic plates. In 1987, four CCD positions were obtained by Shaefer & Shaefer (1988). During the Voyager 2 encounter with Neptune 83 unpublished positions of Nereid where determined (Jacobson et al. 1991). In this paper we present 229 new CCD astrometric positions of this satellite obtained in 23 nights in the period 1993-1998.

Many computations of Nereid's orbit have been taken. The orbital parameters were calculated by Rose (1974) using a Keplerian orbit. In 1975, Mignard (1975) studied the orbital motion of a satellite with large eccentricity considering the perturbation of the Sun and applied these results to Nereid. Later, Mignard (1981) applied his theory to Van Biesbroeck observations and gave the expressions to calculate the mean elliptical elements for any given time. In Veillet (1982) and Veillet & Bois (1988) new determination was made of the initial elliptic elements for the Mignard's theory and Vieira Martins (1989) added the perturbations of Triton to the Mignard expressions. Jacobson (1990) used all the published observations to calculate the barycentric state vector of Nereid referred to the earth mean equator and equinox of 1950.0 at Julian ephemeris date 2447080.5. Finally, Jacobson et al. (1991) made a new determination of the state vector using additionally the 2 ground observations of Pascu and 83 positions given by the Voyager spacecraft. In this paper we made a new determination of the state vector for Nereid considering the previous ground base observations put together with our positions.

The large eccentricity and the small number of observations of Nereid make the determination of the periapsis very difficult. This could be a domineering source of errors in its orbit determination. To study this problem we placed many of our CCD observations near the periapsis. In particular, some 199 observations of Nereid was taken at this region. Using these positions and all others published, we fitted a new orbit for Nereid through a numerical integration.