European Space Agency

International Rosetta Mission: the comet rendezvous mission

G. Schwehm

Rosetta homepage

The main objective of the Rosetta mission, which was approved in November 1993 as the Planetary Cornerstone mission of ESA's Horizon 2000 long-term programme, is a rendezvous with Comet Wirtanen. A subsidiary aim is to have two close encounters with asteroids en route to the comet. As in situ investigation of the cometary nucleus is regarded as of the upmost scientific interest, the mission will carry a Lander to the nucleus and deploy it on the surface. The mission is a collaborative one, with NASA providing Orbiter payload elements and ground-support by the Deep Space Network (DSN).

As far as possible, the mission will satisfy the objectives of the comet nucleus sample return mission (as the mission was originally called) and will concentrate on the in situ investigation of cometary matter and the structure of the nucleus, with the added potential of studying the evolution of the cometary processes as a function of heliocentric distance.

Studying cometary material represents a major challenge, owing to the very characteristics that make it a unique repository of information about the formation of the solar system, namely its high content of volatiles and organic material. Two solutions to the problem of obtaining unaltered material can be considered: returning a sample of a cometary nucleus to Earth (the original Rosetta concept) or staying close to the comet and performing comprehensive in situ analyses of material from the surface or the coma. The latter approach, which results in a less complex and cheaper mission, guarantees by design minimal perturbations of the cometary material, at low temperatures and in a microgravity environment. It also provides the opportunity for observing the onset and development of cometary activity at close range, which results in the spectacular displays that have captured the imagination of mankind over the centuries.

A fundamental question that has to be addressed by the mission is: to what extent can the materials accessible to analyses be considered as representative of the bulk material constituting the comet and of the early nebular condensates that constituted the cometesimal 4.57 109 years ago? This representativity issue has to be addressed by first determining the global characteristics of the nucleus (mass, density, state of rotation), which can provide clues concerning vertical gradients, and hence the relationship between the outer layers and underlying material.

Rosetta Orbiter Payload
Table 3.1.5/1: Rosetta Orbiter Payload

The dust and gas activity observed around comets, as well as its rapid response to insolation, guarantees the presence of volatiles at or very close to the surface in active areas. Analysing material from these areas will therefore provide information on both the volatile and the refractory constituents of a cometary nucleus. The selection of a proper site for surface science investigations should be relatively straightforward, given the extensive remote sensing observation phase and the advanced Rosetta Orbiter instrumentation, which covers a broad range of wavelengths. The surface science site can be monitored during surface activities, as well as during a large fraction of the activity cycle, which should bring to light clues concerning the compositional heterogeneity of active regions.

The dust-emission processes are induced by very low density gas outflows and should preserve the fragile texture of cometary grains. These grains can be collected at low velocities (a few tens of metres per second) by the spacecraft after short travel times (of the order of minutes), which will minimise alterations induced by the interaction with solar radiation. Similarly, gas analysed in jets or very close to the surface should yield information on the volatile content of cometary material in each source region.

Based on these considerations, the prime scientific objective of the mission is to study the origin of comets, the relationship between cometary and interstellar material and its implications with regard to the origin of the solar system. The measurements to be made in support of this objective are:

The AO for Rosetta Orbiter Investigations and Inter- disciplinary Scientists was issued by ESA on 1 March 1995. After a Peer Review Process, the Science Pro-gramme Committee (SPC) in February 1996 endorsed the Rosetta Orbiter payload for a 1-year Science Verification Phase, two Surface Science Packages (Champollion to be provided by NASA/JPL/CNES and Roland by a European consortium led by Max Planck Institute and DLR from Germany) and five Interdisciplinary Scientists.

In November 1996, the French SPC delegation announced that they would not be able to fund DFA. Following Italy's commitment to support the Italian DFA Co-Investigators to an extent that they would be able to lead the investigation, the PI responsibility was transferred.

The 1-year Science Verification Phase will lead to clear definition of interfaces, identify critical areas where more development work has to be done, and provide a demonstration of the feasibility of some of the novel techniques for which flight hardware will be implemented for the first time. In addition, the member state authorities have to provide a final commitment on the funding levels of the scientists' contributions from the respective member states. After successful completion of their Experiment Conceptual Design Reviews in the spring of 1997, all investigations will have to be reconfirmed by the SPC in June 1997.

In September 1996 NASA, for budgeting and programmatic reasons, announced their withdrawal from the Champollion Lander. This resulted in a complete reappraisal of the Roland Lander and its role in fulfilling the Rosetta scientific objectives. CNES and the Roland team are actively pursuing the merging of the science of both Landers into a joint European Lander based on the Rola

The Lander payload will focus on in situ measurements of the composition of the nucleus material. These measurements are aimed at determining the elemental, molecular, mineralogical and isotopic composition of the comet's surface and subsurface materials. Highest priority is given to the elemental and molecular determination, as it is believed that some mineralogical and isotopic measurements can be carried out adequately from the Orbiter. In addition, the characterisation of near-surface strength, density, texture, porosity, ice phases and thermal properties was regarded as a very important objective for the Lander. Texture characterisation will include microscopic studies of individual grains.

Tentative Rosetta Lander Payload
Table 3.1.5/2: Tentative Rosetta Lander Payload

Five Interdisciplinary Scientists have been nominated for an initial period of 3 years to support the implementation of the mission:

The baseline mission scenario foresees a launch by Ariane 5 in January 2003. One gravity-assist manoeuvre at Mars and two at Earth will provide the orbital energy for the spacecraft to rendezvous with Comet Wirtanen in August 2011 (Fig. 3.1.5). After the comet approach and an extensive mapping phase, science operations will start at 3.25 AU heliocentric distance. Shortly thereafter, the Lander will be deployed. The Orbiter will follow the nucleus very closely through perihelion at distances of the order of 2-20 nucleus radii.

Rosetta mission scenario
Figure 3.1.5: Rosetta mission scenario. (1) spacecraft leaves Earth; (2) Mars swingby for energy gain; (3) first Earth swingby for energy gain; (4) flyby of asteroid Mimistrobell; (5) second Earth swingby; (6) flyby of asteroid Rodari or Siwa; (7) rendezvous with the comet.

The spacecraft will be operated by ESOC. For all critical mission phases, the Rosetta Science Operations Centre (RSOC) will be collocated with the Rosetta Mission Operations Centre (RMOC) at ESOC.

An extensive ground-based observation campaign of Comet Wirtanen is underway. The comet was recovered in June 1995 and since March 1996 it has been monitored on a regular basis. The new observational data indicate a small (radius ~650 m) nucleus, already showing an early onset of activity, beyond 3 AU. The next perihelion passage will be on 27 March 1997 (see also

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Published August 1997.