THE ROSETTA MISSION: FLYING TOWARDS THE ORIGIN OF THE SOLAR SYSTEM KARL-HEINZ GLASSMEIER1, HERMANN BOEHNHARDT2, DETLEF KOSCHNY3, EKKEHARD KUHRT¨ 4 and INGO RICHTER5 1Institut fur¨ Geophysik und extraterrestrische Physik, Technische Universitat¨ Braunschweig, Mendelssohnstrasse 3, D-38106 Braunschweig, Germany 2Max-Planck-Institut fur¨ Sonnensystemforschung, Lindau, Germany 3Research and Scientific Support Department, ESA/ESTEC, Noordwijk, The Netherlands 4Institut fur¨ Planetenforschung, Deutsches Zentrum fur¨ Luft- und Raumfahrt, Berlin, Germany 5Technische Universitat¨ zu Braunschweig, Germany (∗Author for correspondence: E-mail: [email protected]) (Received 16 November 2006; Accepted in final form 12 December 2006) Abstract. The ROSETTA Mission, the Planetary Cornerstone Mission in the European Space Agency’s long-term programme Horizon 2000, will rendezvous in 2014 with comet 67P/Churyumov- Gerasimenko close to its aphelion and will study the physical and chemical properties of the nucleus, the evolution of the coma during the comet’s approach to the Sun, and the development of the in- teraction region of the solar wind and the comet, for more than one year until it reaches perihelion. In addition to the investigations performed by the scientific instruments on board the orbiter, the ROSETTA lander PHILAE will be deployed onto the surface of the nucleus. On its way to comet 67P/Churyumov-Gerasimenko, ROSETTA will fly by and study the two asteroids 2867 Steins and 21 Lutetia. Keywords: ROSETTA, solar system, comets, asteroids 1. Introduction The International ROSETTA Mission, approved in November 1993 by the Science Programme Committee of the European Space Agency (ESA), is the planetary cornerstone mission in ESA’s long-term programme Horizon 2000 (Bonnet, 1985). The ROSETTA mission is a logical follow-up mission of ESA’s very successful first mission, GIOTTO, to comet 1P/Halley (Reinhard, 1986). The prime scientific objectives of the ROSETTA mission are to investigate the origin of our solar system by studying the origin of comets (e.g. Schwehm and Schulz, 1999). Originally planned as a comet nucleus sample return mission (e.g. Huber and Schwehm, 1991), it consists of two mission elements, the ROSETTA orbiter and the ROSETTA lander PHILAE. The ROSETTA mission is a cooperative project between ESA, various European national space agencies, and NASA, and is comprised of twenty-five experiments, making it unprecedented in scale. The mission is named after a plate of volcanic basalt currently in the British Museum in London, called the Rosetta Space Science Reviews (2006) DOI: 10.1007/s11214-006-9140-8 C Springer 2007 K.-H. GLASSMEIER ET AL. Stone. This plate was the key to unravel the civilization of ancient Egypt. Just as the Rosetta Stone provided the key to an ancient civilization, the scientific instruments onboard the ROSETTA spacecraft are designed to unlock the mysteries of the oldest building blocks of our solar system, the comets. Why a cometary mission as a planetary cornerstone? Comets are widely con- sidered to contain the least processed material in our solar system since their con- densation from the proto-solar nebula. Most likely even pre-solar grains have been preserved in these bodies. The physical and compositional properties of comets may therefore be a key to their formation and evolution, hence to the formation of the solar system. Direct evidence on cometary material, in particular on the composition of cometary volatiles, is however extremely difficult to obtain. Un- til the first spacecraft missions to comet 21P/Giacobini-Zinner, 1P/Halley, and 26P/Grigg-Skjellerup only ground-based spectroscopic evidence about the com- positional character of comets could be used to hypothesize on the properties of comets. These early ground-based observations led to the popular view of a comet as being a “dirty snowball” (Whipple, 1950, 1951). In-situ measurements of the named cometary missions and their successors, the Deep-Space 1 mission to comet 19P/Borrelly (Boice et al., 2002), the Stardust mis- sion to 81P/Wild2 (Tsou et al., 2004), and the Deep Impact mission to 9P/Tempel1 (Sunshine et al., 2006; Belton et al., 2006), led to a major revision of our current un- derstanding of cometary nuclei. Major results of previous missions are: Cometary activity is rather localized at the nucleus surface with only minor parts being active and the activity being highly constant in short time scales. However, sometimes outbursts with dramatic increase of activity but unclear genesis occur. The domi- nant component is not water ice and a more proper characterization of the nucleus would be as an “icy dirtball”. Organic material, that is CHON particles, have been observed, and some isotopic ratios and most elemental abundance differ from solar ones. Their surfaces can be evolved with very different landforms visible; impact structures have been observed. The Deep Impact results even suggest a layered structure with different physical characteristics, which led to the development of a layered pile model (Belton et al., 2006). Furthermore, some minerals must have been processed in hot (T about 2000 K) environment near the Sun or other stars. All the present results indicate that cometary nuclei are unique, have their own history, and are not entirely pristine. Many physicochemical processes, like sublimation, (photo)chemical reactions, and interactions with the solar wind and the high energy radiation in space alter the material originally present in the nucleus. The species observable from Earth and even in situ during flyby missions are consequently not representing the immediate molecular composition of the nucleus from formation times, although the currently available information already demonstrates the low level of evolution of cometary material. To retrieve information about the composition of a cometary nucleus and the processes altering it, the nucleus and the near-nucleus environment of a comet needs to be monitored in situ by very sensitive analytical investigations, THE ROSETTA MISSION including also the ionized gas or plasma environment of the comet whose evolution during perihelion approach is not fully understood (e.g. Neugebauer, 1990; Ip, 2004; Hansen et al., this issue). The measurement goals of the ROSETTA mission therefore include (Schwehm and Schulz, 1999): – a global characterization of the nucleus, – the determination of its dynamic properties, – the surface morphology and composition, – the determination of chemical, mineralogical and isotopic compositions of volatiles and refractories in the cometary nucleus, – the determination of the physical properties and interrelation of volatiles and refractories in the cometary nucleus, – studies of the development of cometary activity and the processes in the surface layer of the nucleus and inner coma, that is dust/gas interaction, – studies of the evolution of the interaction region of the solar wind and the outgassing comet during perihelion approach. To achieve these goals ROSETTA will study in detail the properties of the nucleus, the physicochemical evolution of the cometary coma, and the plasma en- vironment of comet 67P/Churyumov-Gerasimenko from onset of activity at large solar distances (possibly around 3 AU) through perihelion at 1.2 AU. Table I pro- vides an overview of the different mission phases. ROSETTA’s payload comprises 12 scientific instruments and instrument groups on board the orbiter (see Table II) and 10 on the Surface Science Package, the Rosetta lander PHILAE (see Table III), details of which are described in the respective instrument papers of the special ROSETTA issue introduced here. ROSETTA combines two strategies for characterizing the properties of a cometary nucleus. On one hand, the comet’s evolution along the orbit with de- creasing heliocentric distance will be investigated with the orbiter instruments by monitoring the physical and chemical properties of the nucleus and in situ analysis of the near-nucleus environment. On the other hand, the ROSETTA lander PHILAE will provide ground truth by analyzing the nucleus material directly. 2. Mission Overview The original target of the ROSETTA mission was comet 46P/Wirtanen. A fail- ure of an Ariane rocket in December 2002 forced ESA to postpone the initially scheduled January 2003 launch and to re-target ROSETTA, now heading for comet 67P/Churyumov-Gerasimenko. ROSETTA was finally launched by an Ariane-5 G+ launch vehicle from the Guyana Space Center in Kourou, French Guyana, on March 2, 2004 ( Figure 1). During a circuitous ten-year trek across our solar system, ROSETTA will travel the distance Sun-Earth five times, and will pass through the K.-H. GLASSMEIER ET AL. TABLE I Milestones of the ROSETTA mission. Mission event Nominal date Launch March 2, 2004 First Earth Gravity Assist March 4, 2005 Mars Gravity Assist February 25, 2007 Second Earth Gravity Assist November 13, 2007 2867 Steins Flyby September 5, 2008 Third Earth Gravity Assist November 13, 2009 21 Lutetia Flyby July 10, 2010 Rendezvous Manoeuvre 1 January 23, 2011 Start of Hibernation July, 2011 Hibernation Wake Up January, 2014 Rendezvous Manoeuvre 2 May 22, 2014 Between 4.5 and 4.0 AU Start of Near-Nucleus August 22, 2014 Operations at 3.25 AU PHILAE Delivery November 10, 2014 Start of Comet Escort November 16, 2014 Perihelion Passage August, 2015 End of Nominal Mission December 31, 2015 asteroid belt into deep space beyond 5 AU solar distance before it reaches its des- tination, the periodic comet 67P/Churyumov-Gerasimenko. On its way to 67P/Churyumov-Gerasimenko the spacecraft will employ four planetary gravity assist manoeuvres (Earth-Mars-Earth-Earth) to acquire sufficient energy to reach the comet (Figure 2). In between the last two Earth swingbys ROSETTA will fly by the main belt asteroid 2867 Steins at a distance of 1700 km and at a relative velocity of 9 km/s on September 5, 2008. After the third Earth swingby ROSETTA will enter the main asteroid belt again and fly by the main belt asteroid 21 Lutetia at a distance of 3000 km and a speed of 15 km/s on July 10, 2010.