Planetary and Space Science 55 (2007) 1398–1413 The BepiColombo Laser Altimeter (BELA): Concept and baseline design N. Thomas a,Ã , T. Spohn b , J.-P. Barriot c , W. Benz a , G. Beutler d , U. Christensen e , V. Dehant f , C. Fallnich g , D. Giardini h , O. Groussin i , K. Gunderson a , E. Hauber b , M. Hilchenbach e , L. Iess j , P. Lamy k , L.-M. Lara l , P. Lognonne´ m , J.J. Lopez-Moreno l , H. Michaelis b , J. Oberst b , D. Resendes n , J.-L. Reynaud k , R. Rodrigo l , S. Sasaki o , K. Seiferlin a , M. Wieczorek m , J. Whitby a a Physikalisches Institut, Sidlerstr. 5, University of Bern, CH-3012 Bern, Switzerland b Deutsches Zentrum fu ¨ r Luft- und Raumfahrt-Institut fu ¨ r Planetenforschung, Rutherfordstr. 2, D-12489 Berlin, Germany c Bureau Gravimetrique International, Avenue Edouard Belin 18, F-31055 Toulouse, France d Astronomy Institute, Sidlerstr. 5, University of Bern, CH-3012 Bern, Switzerland e Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Str. 2, 37189 Katlenburg-Lindau, Germany f Royal Observatory of Belgium, 3 avenue Circulaire, B-1180 Brussels, Belgium g Physikalische-Technische Bundesanstalt (PTB), Bundesallee 100, D-38116 Braunschweig, Germany h Institute of Geophysics/Swiss Seismological Service, Swiss Federal Institute of Technology, (ETHZ), Honggerberg, CH-3093 Zurich, Switzerland i Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA j Dipartimento di Ingegneria Aerospaziale e Astronautica, Universita’ degli Studi di Roma ‘‘La Sapienza’’, Via Eudossiana 18, I-00184 Rome, Italy k Laboratoire d’Astrophysique de Marseille, Site Peiresc, Traverse du Siphon-Les Trois Lucs, 13376 Marseille Cedex 12, France l Instituto de Astrofisica de Andalucia, C/ Camino Bajo de Hue´tor 24, 18008 Granada, Spain m De´partement de Ge´ophysique Spatiale et Plane´taire/UMR7096-CNRS, Institut de Physique du Globe de Paris, 4 Avenue de Neptune, F-94100 Saint Maur des Fosse´s Cedex, France n Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal o Planetary Sciences Group, Geological Institute, University of Tokyo, Hongo, Tokyo 113-0033, Japan Received 26 July 2006; received in revised form 21 February 2007; accepted 2 March 2007 Available online 23 March 2007 Abstract The BepiColombo Laser Altimeter (BELA) has been selected for flight on board the European Space Agency’s BepiColombo Mercury Planetary Orbiter (MPO). The experiment is intended to be Europe’s first planetary laser altimeter system. Although the proposed system has similarities to the Mercury Laser Altimeter (MLA) currently flying on board NASA’s MESSENGER mission to Mercury, the specific orbit and construction of the MPO force the use of novel concepts for BELA. Furthermore, the base-lined range-finding approach is novel. In this paper, we describe the BELA system and show preliminary results from some prototype testing. r 2007 Elsevier Ltd. All rights reserved. Keywords: Mercury; BepiColombo; Laser altimetry; Instrument design 1. Laser altimetry Laser altimetry is a powerful remote sensing technique although the measurement concept is simple (Hofton et al., 2000). The time-of-flight of a pulse of light from a laser to a reflecting surface and back is measured (Fig. 1). The range from the laser to the illuminated spot on the surface is related to the laser pulse time-of-flight by z ¼ cDT 2 , (1) where c ¼ 299,792,458 m/s (Cohen and Taylor, 1997). Hence, a 1 ns delay is E15 cm range. This travel time, ARTICLE IN PRESS www.elsevier.com/locate/pss 0032-0633/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2007.03.003 Ã Corresponding author. Tel.: +41 316 314406; fax: +41 316 314405. E-mail address: nicolas.thomas@space.unibe.ch (N. Thomas).