Astrometric observations of Phobos and Deimos during the 1971 opposition of Mars^⋆

¹ Institut de Mécanique Céleste et de Calcul des Éphémérides IMCCE, Observatoire de Paris, UMR 8028 du CNRS, UPMC, Univ. Lille 1, 77 avenue Denfert-Rochereau, 75014 Paris, France
e-mail: robert@imcce.fr
² Institut Polytechnique des Sciences Avancées IPSA, 7−9 rue Maurice Grandcoing, 94200 Ivry-sur-Seine, France
³ United States Naval Observatory USNO, 3458 Massachusetts Ave NW, Washington DC 20392, USA
⁴ Royal Observatory of Belgium ROB, avenue Circulaire 3, 1180 Uccle, Belgium

Received: 12 June 2014
Accepted: 11 September 2014

Abstract

Context. Accurate positional measurements of planets and satellites are used to improve our knowledge of their dynamics and to infer the accuracy of planet and satellite ephemerides.

Aims. In the framework of the FP7 ESPaCE project, we provide the positions of Mars, Phobos, and Deimos taken with the U.S. Naval Observatory 26-inch refractor during the 1971 opposition of the planet.

Methods. These plates were measured with the digitizer of the Royal Observatory of Belgium and reduced through an optimal process that includes image, instrumental, and spherical corrections to provide the most accurate data.

Results. We compared the observed positions of the planet Mars and its satellites with the theoretical positions from INPOP10 and DE430 planetary ephemerides, and from NOE and MAR097 satellite ephemerides. The rms residuals in RA and Dec of one position is less than 60 mas, or about 20 km at Mars. This accuracy is comparable to the most recent CCD observations. Moreover, it shows that astrometric data derived from photographic plates can compete with those of old spacecraft (Mariner 9, Viking 1 and 2).