One of ESA's first major successes in space science was the encounter of its Giotto spacecraft with comet Halley in March 1986. This flyby mission provided a brief snapshot of the comet nucleus at a relative speed of 68 km/s. Four other spacecraft- two each from the Soviet Union (VeGa 1 & 2) and Japan (Sakigake and Suisei)- encountered Halley at greater distances. NASA had already diverted ISEE-3 (one of its solar-terrestrial exploration spacecraft) to visit comet Giacobini-Zinner, renaming it ICE (International Cometary Explorer) in the process. Giotto went on to a second comet, Grigg-Skjellerup, in 1992. Following these 'pathfinder' missions and the years of data analysis and interpretation since, we are now ready for the next stage of cometary exploration. The impact of comet Shoemaker-Levy 9 on Jupiter in 1994 and the recent spectacular apparitions of comets Hyukatake and Hale-Bopp have raised interest in comets further. These three comets have been better observed than any others, due to the combination of ground- and space-based observations and the improvements in the technology and coverage of both techniques.
In May 1985 ESA's Solar System Working Group recommended that one of the cornerstone missions of the Horizon 2000 science programme should be a comet nucleus sample return mission. A joint ESA / NASA Science Definition Team was formed by the end of 1985 to define the scientific objectives of Rosetta, as the mission was named. Planning began in earnest after the Halley encounter, which provided an important 'first look' at the type of body Rosetta was due to visit. Rosetta was initially conceived as a comet nucleus sample return with NASA as a partner agency. In 1992, however, financial and programmatic difficulties within NASA (related to its own ill-fated CRAF (Comet Rendezvous and Asteroid Flyby) mission proposal) made it necessary for ESA to examine solutions that could be carried out with European technology alone. In 1993 it was decided that a sample return would be too ambitious for the resources available. Since then the aim has been to send the laboratory to the comet, rather than bring a sample back to Earth for analysis.
The Rosetta spacecraft is due to be launched in January 2003 on an Ariane 5, following an interplanetary trajectory via Mars, Earth (twice) and two asteroids. It will arrive in 2011 at Wirtanen, the target comet, which has an orbital period of 5.45 years and may only be a couple of kilometres in diameter. The rendezvous will occur at a heliocentric distance of just over 3 AU, at which point the spacecraft will attempt to orbit the comet nucleus- not an easy task when its mass will still be unknown! The comet may already be showing signs of activity- gas drag on the orbiter will be another challenge for ground controllers. For the first few weeks the orbiter will perform an initial phase of reconnaissance measurements necessary for the selection of a site for the lander, a 75 kg package provided by a German-led international consortium. It is hoped that the lander will operate on the nucleus surface for at least six months, while the orbiter is intended to stay with the comet until after its perihelion at 1.08 AU from the Sun in 2013.
Only recently has it become clear what form the lander and its payload will take. The original sample-return design (1991) involved using a cruise stage based on an RTG-powered NASA Mariner Mark-II bus (similar to Cassini and CRAF) carrying a European lander stage and an Earth Return Capsule (ERC). Following launch by a Titan IV / Centaur and either a Venus-Earth or Venus-Earth-Earth gravity assist trajectory, Rosetta would arrive at the target comet. Several possible comets and trajectories were under consideration, each scenario having a different launch mass and mission timeline. After comet rendezvous the whole combination would descend to the nucleus surface for sampling and in situ measurements. The cruise stage / ERC section would then detach itself and set off for Earth. On arrival the ERC would be ejected and perform a re-entry and splashdown. Figure 1 shows the spacecraft configuration as it would have appeared during on-comet operations.
Figure 1. Spacecraft configuration of the original sample return concept (ESA, 1991).
Following the re-orientation of the mission a completely new scenario was adopted. A solar-powered orbiter spacecraft (based on a communications satellite bus) would carry one or more surface packages to the comet. These would be ejected and descend to the surface, leaving the 'mother ship' to continue remote measurements from cometary orbit until perihelion. The nature of the packages as well as their number was left open; the intention being that they would be provided by experiment teams as instruments, rather than by ESA itself. CRAF-style penetrators as well as various landers were considered for mission analysis purposes, however. Figures 2-4 show three alternative concepts from ESA's 1993 System Definition Study, focussing on alternative approaches for cushioning the lander's impact on the nucleus surface.
Figure 2. Lander concept with passive impact damping (ESA, 1993).
Figure 3. Lander concept with active impact damping (ESA, 1993).
Figure 4. Lander concept with combined active and passive impact damping (ESA, 1993).
In 1994 two consortia emerged, each proposing landers rather than penetrators. At this stage ESA had decided that two 45 kg surface packages could be accommodated on the orbiter. One concept, RoLand (Rosetta Lander) was proposed by a German-led consortium, while the other, Champollion, was proposed by a joint CNES / NASA team. Figure 5 shows the initial concept for RoLand, while figure 6 shows how Champollion might appear on the comet. By autumn 1995 RoLand had become pentagonal in shape- figure 7 shows a demonstration model of this design.
Figure 5. Cut-away view of the initial concept for the RoLand lander (RoLand Consortium, autumn 1994).
Figure 6. Concept for the Champollion lander, early 1995 (image from the former web site at NASA JPL). The lander touches down on six crushable legs and deploys three anchoring spikes to secure itself to the surface.
Figure 7. The RoLand demonstration model, autumn 1995 (photo: DLR). By this stage the main body had become pentagonal in shape, incorporating a balcony area on the baseplate. The demonstration model primary structure was constructed using representative lightweight composite material.
In the summer of 1996 the original two-lander concept was dropped when NASA pulled out of the Champollion partnership. The French half of the team then joined RoLand to provide a single, larger lander based on the RoLand concept. This is currently called simply the "Rosetta Lander", though this will no doubt change to something more poetic before launch. Figure 8 shows the lander as it would appear on the nucleus surface with its landing gear deployed. On the right a suite of externally mounted experiments can be seen. Figure 9 shows an ESA Visulab image of the orbiter design from 1996.
Figure 8. The Rosetta Lander with its three legs deployed (image: Rosetta Lander consortium). The main body of the lander is approximately 850 x 850 x 660 (height) mm. Most of the instruments are either lowered down through the lander baseplate or mounted on the external "balcony" area.
Figure 9: The Rosetta orbiter- a 3-axis stabilised spacecraft based on a communications satellite bus. The lander will be mounted on one side of the orbiter before deployment to the surface (image: ESA Visulab, 1996).
The physical state of the target nucleus will remain largely unknown until Rosetta arrives- even its size and rotation state are a matter of debate, though current best guesses from recent observations put the radius at about 700 m and the rotation period about 6 hours. Although the mass of the nucleus is unknown, comet nuclei are considered to be very porous, with densities only about half that of water. All the uncertainties regarding the physical state of the nucleus put severe demands on the lander's system engineers to ensure safe landing over a broad range of surface conditions. In addition the lander must be able to cope with low temperatures (about -140°C) at 3 AU but increasingly warm conditions as the comet approaches the Sun.
Sending a probe to the surface of a comet can hardly be considered a 'landing' when the surface gravity may be as low as 1/500000th of that on Earth, with an escape velocity of only 17 cm/s. Gas drag from sublimating ices may well threaten to eject a lander from the surface, while the rebound on landing may exceed the escape velocity. Activities such as lowering an instrument to the surface or drilling for samples also cause significant reaction forces. For these reasons an anchoring harpoon will be fired into the surface on touchdown, as soon as two of the three feet are in contact with the surface.
What will the Rosetta Lander do on the surface of comet Wirtanen? A mass of 24.3 kg is available for the science payload. This has been divided between several instrument teams, each of which is an international collaboration between European institutes. The aim of the payload, summarised in the table, is to analyse the comet's chemical, isotopic and mineralogical composition and physical state. Some of the measurements will also provide 'ground truth' data for experiments on the orbiter.
|APXS||Alpha-Proton-X-ray Spectrometer (elemental composition)|
|COSAC||Evolved gas analyser (elemental, isotopic, chemical and mineralogical composition)|
|MODULUS||Evolved gas analyser (elemental, isotopic, chemical and mineralogical composition)|
|ÇIVA||Visible and infrared camera system|
|MUPUS||Sensors for thermal and mechanical properties; surface density|
|SESAME||Acoustic sounding, electrical permittivity and detection of back-falling dust particles|
|ROMAP||Magnetometer and plasma package|
|CONSERT||Radio sounding of the nucleus|
The UK is heavily involved in the Rosetta Lander. Prof. Colin Pillinger of the Open University will be providing the MODULUS instrument, in partnership with the Rutherford Appleton Laboratory. That part of the MUPUS experiment which will measure the surface density profile at the landing site is due to be provided jointly by the University of Kent and the Mullard Space Science Laboratory. The University of Kent is also involved in the sensor for detecting back-falling dust particles, part of the SESAME experiment. UK institutes are also involved in the plasma package and dust flux analyser experiments on the Rosetta orbiter.
Work on Rosetta and the lander is now proceeding quickly in many institutes and companies around Europe. The mission should prove to be a worthy cornerstone to ESA's science programme. NASA also has its plans for cometary exploration, of course. The Discovery mission Stardust, due to be launched in February 1999, will fly by a comet and collect samples of dust for return to Earth. The Champollion concept is being adapted for a possible New Millennium mission called Deep Space 4, which would land on a comet, perform experiments and return a small sample to Earth (an artist's impression of this scenario from late 1996 is shown in Figure 10). Rosetta, however, will still be a distinctive mission. The strong payloads on both the orbiter and the lander, coupled with the extended period over which measurements will be made, will ensure the mission's unique importance in cometary science.
Figure 10: Concept for a NASA New Millennium mission Deep Space 4, based on the Champollion design. The picture shows the Earth return capsule being ejected from the lander. The orbiter is shown in the background (image: NASA JPL).
Rosetta Lander: roland.mpae.gwdg.de
New Millennium Program: nmp.jpl.nasa.gov
Andrew J Ball, 1997.
Published in CapCom, the newsletter of the Midlands Spaceflight Society, Vol. 8, No. 2, November 1997.