1 Introduction

Ephemerides of the Solar System are generated at JPL by numerical integration of the equations of motion of the planets, the Moon and the more massive minor planets, with initial conditions determined from observations. The initial conditions are refined continuously by the addition of more recent observations, and the resultant ephemerides are issued in the series DExxx, where xxx is a sequence number. DE405 is the most recent ephemerides to be issued by JPL through their Horizons On-Line Ephemeris System http://ssd.jpl.nasa.gov/cgi-bin/eph interface at their Web site. This is a major departure from the DE2xx series in the sense that it is referred to the new International Celestial Reference System (ICRS)(Arias et al. 1995). In the ICRS the axes of the international celestial reference frame (ICRF) are fixed by the positions of $\sim$600 quasars which have absolute positions determined to better than 1mas by VLBI (Ma et al. 1998) These axes have been aligned to within $\sim$ 20 mas (Mignard & Froeschle 1998) with the previous optical frame, FK5. The major advantage of the ICRS over the previous system is that the axes of the ICRF are fixed (to within $\sim$20 $\mu$as) and there is no rotation at this level.

The orientation of the orbits of the four innermost planets and the Moon have been fixed relative to the ICRF with an accuracy approaching 1mas using VLBI observations of spacecraft and lunar laser ranging. The relative angles between these objects and their inertial motions have been determined with an accuracy of 1mas and 20mas/cy, respectively, using ranging to the Viking Lander spacecraft, radar observations, VLBI and radiometric observations of spacecraft, and lunar laser ranging (Standish 1998). No optical data were used in this linking process because their accuracy is too crude. However, for the outermost planets, Jupiter to Pluto, optical data have been used in generating the numerical integrations because there are either insufficient or no radio data.

The main strength of the optical data is the long time-series of observations: their main weakness is that the positions are referred to the FK5 frame, or its precursor, the FK4. These frames introduce regional distortions in the sky of 100mas or more, and the frames have spurious rotations with time, reaching 300mas/cy. Recently, however, the Hipparcos Catalogue has become available for use in realizing the optical frame. The axes of the Hipparcos Catalogue are aligned with the ICRF to an accuracy of 0.6mas at the epoch 1991.25 and with a time-dependent part of 25mas/cy. Modern optical positions of the planets can now be made relative to the ICRF using the Hipparcos and Tycho Catalogues (ESA, 1997).

This paper compares DE405 for the planets Jupiter to Pluto with modern optical positions, either observed with respect to the ICRF via the Hipparcos Catalogue or adjusted retrospectively to the ICRF. This provides a check on the alignment of their orbits relative to the ICRF. In some cases this check is not completely independent because some of the optical positions were used in deriving the starting conditions for the DE405 integration. The degree of circularity in the comparisons will be addressed later in the discussion of the individual planets.