ESA Science & Technology - Scientific overview

Rosetta's Scientific Instruments

Instruments

Composition analysis

The Rosetta Lander

	Figure 3. The Rosetta Lander

Lander Project Managers
S. Ulamec, DLR, Köln Porz-Wahn, Germany
D. Moura, CNES,
Toulouse, France
R. Mugnuolo, ASI, Matera, Italy

Lead Scientists
H.R. Rosenbauer, MPAe Lindau
J.P. Bibring, IAS, Orsay

Rosetta Lander Payload

The Lander science will focus on the in situ study of the composition and structure of the nucleus material.

Measurement goals include the determination of the elemental, molecular, mineralogical, and isotopic composition of the cometary surface and subsurface material. Highest priority is given to the elemental and molecular determinations as it is believed that some mineralogical and isotopic measurements can be carried out adequately by orbiter science investigations. In addition properties like near-surface strength, density, texture, porosity, ice phases and thermal properties will be derived. Texture characterisation will include microscopic studies of individual grains.

CONSERT, with hardware on both the lander and orbiter, will try to reveal the coarse structure of the nucleus through radio sounding.

Instruments

Interdisciplinary Science (IDS)

The Interdisciplinary Scientists are:

• M. Fulchignoni, Paris-Meudon Observatory, France
• M. Fulle, Osservatorio Astronomico, Trieste, Italy
• E. Grün, MPI Kernphysik, Heidelberg, Germany
• R. Schulz, ESA/SSD, Noordwijk, The Netherlands
• P. Weissman, JPL, Pasadena, USA

Comet 46 P/Wirtanen

Figure 4. Comet 46P/Wirtanen

Comet 46P/Wirtanen imaged with the MPG/ESO 2.2 m Telescope in September 1996. The dust coma and the already developed dust tail are visible.

Comet 46 P/Wirtanen was discovered on 15 January 1948 at Lick Observatory by Carl A. Wirtanen. Two close approaches to Jupiter in 1972 (0.28 AU) and 1984 (0.46 AU) changed the orbit from an initial perihelion distance of 1.63 AU and a period P= 6.71 years to a period P = 5.46 years and a perihelion distance of 1.06 AU.

The comet belongs to the Jupiter family comets, which represents a large group of the short-period comets in the Solar System. Their orbits with an aphelion around Jupiter orbit make them in principle observable along their entire orbit. Except in 1980, Comet Wirtanen was observed during all its apparitions. However, only the coordinated observation campaign in 1996/1997 made this object to one of the best monitored comets.

Assuming an albedo of .04 a radius of about 700 m was derived. For its small size the nucleus is fairly active and produces about 1 028 molecules s-1.

For the comet's daily orbital position see also
http://galileo.mpi-hd.mpg.de/~mmueller/

Classification of comets

New cometary designation system

A new cometary designation system was adopted by the IAU in August 1994, and took effect at the beginning of 1995.

The principal designations for discoveries of comets, according to the Catalogue of Cometary Orbits, consist of the year and an upper-case letter to indicate the halfmonth of discovery in that year, so:

A = Jan 1-15

B = Jan 16-31

C = Feb 1-15, ..., Y = Dec 16-31

(I is omitted to avoid confusion)

followed by a numeral showing the order of announcement of discovery in that half month.

The half months are as used for discoveries of minor planets, the difference being that minor-planet designations have a second letter (as well as possible numerals) to indicate the order of discovery announcement.

'Discovery' refers to the time when the discovery observation is actually made (and in the case of a photographic observation, for example, it is usually the time of mid-exposure), even though the comet might not actually be recognized until long afterward. Unlike that for minor planets, the cometary system is precisely the same for comets discovered before and after 1925 (Universal Time is used in either case), with the usual astronomical convention of using the Julian calendar before October 1582 and the Gregorian calendar afterward.

The new cometary designations are mainly prefixed by:

• P/ for 'short' period comet
• C/ otherwise

More rarely:

• D/ considered defunct - i.e. it would be ill-advised seriously to consider a prediction for a future return, because either it is known no longer to exist, or it has failed to show at several expected returns, or because its orbit is poorly known.
• A/ would be used to denote an object given cometary designation but deemed to be a minor planet (no examples so far).
• X/ is used for comets for which it is not possible to compute orbits - and that in some cases may never even have existed.

Why comet 46 P/Wirtanen?

When the periodicity of a comet is well-established, either because of a recovery or an identification at a second passage through perihelion, this is shown by assigning a sequential periodic-comet number, which normally appears in front of the P/ (or D/) prefix.

There is also the possibility of assigning such a number to a comet, if and when it is observed through its first aphelion passage after discovery. An attempt has been made to assign these periodic comet numbers in a historically meaningful manner (e.g. 1 P/Halley, 2 P/Encke, etc), to allow continuity between past and future returns. Thus comet Wirtanen was the 46th periodic comet to be assigned a number in this way.

Components of comets that are observed to split are indicated by -A, -B, etc after the P/ (or D/) number.

Why a new system?

The new system abandons the tradition, introduced by the Astronomische Nachrichten in 1846, of applying Roman numerals to comets in the order they were observed to pass perihelion in a particular year. In practice this proved troublesome. A subsidiary system, adopted by the Astronomische Nachrichten in 1870, which assigned lower-case letters to comets in order of announcement of discovery in a particular year, was applied too erratically, with different letters, or no letter at all, being assigned to the same comet. In reality, a substantial fraction of both Roman numeral and letter designations had come to be applied not to discoveries, but to recoveries of periodic comets on subsequent perihelion passages.

It was felt that a modern designation system should generally restrict acknowledgement of recoveries to those comets making their first predicted returns (when the error is expected to be greatest).

Finally, the distinction between a comet and a minor planet is often quite unclear, whether considered in terms of particular observations or of the general evolution of the solar system. It was therefore felt that any new cometary designation system should be more similar to that used for minor planets.

Taken from the tenth edition of the Catalogue of Cometary Orbits, by B.G. Marsden and G.V. Williams, available from the International Astronomical Union's Minor Planet Center:

Minor Planet Center
Smithsonian Astrophysical Observatory
60 Garden Street
Cambridge, MA 02138
USA

Past Cometary Missions

The beginning

Launch 12 August 1978

	Figure 5. International Cometary Explorer

International Cometary Explorer (ICE) achieved the first ever comet encounter. This NASA spacecraft was originally known as ISEE-3 (International Sun-Earth Explorer). Having completed its original mission, it was reactivated and diverted to pass through the tail of Comet Giacobini-Zinner on 11 September 1985. It also observed Halley's Comet from a distance of 28 million km (17 million miles) in March 1986.

Launch 15 and 20 December 1984

Figure 6. Vega 1

Vega-1 and Vega-2, two Russian probes, each left a lander on the surface of Venus as they flew past it on the way to investigate and photograph Halley's Comet. Vega 1 made its closest approach to the comet on 6 March at a distance of 39 000 km (24 ,235 miles). Vega 2 flew in closer to the comet nucleus at a distance of 8 030 km (4 990 miles) on 9 March 1986.

Launch 8 January 1985 and 19 August 1985

	Figure 7. Sakigake and Suisei

Sakigake and Suisei were Japan's first deep space missions. Suisei approached to within 151 000 km (93 825 miles) of Halley's Comet on 8 March 1986 to observe its interactions with the solar wind. Sakigake approached to within 7 million km (4.35 million miles) of the comet on 11 March 1986 in order to study radio and plasma waves. Launch 2 July 1985

Launch 2 July 1985

Figure 8. Picture taken by Giotto

Giotto obtained the closest pictures ever taken of a comet. This European Space Agency satellite flew past the nucleus of Comet Halley at a distance of less than 600 km (373 miles) on 13 March 1986. Images showed a black, potato-shaped object with active regions which were firing jets of gas and dust into space. Giotto then became the first spacecraft to visit two comets when it passed within 200 km (124 miles) of Comet Grigg-Skjellerup on 10 July 1992. It was placed in hibernation on 23 July 1992, and the spacecraft has since been inactive. Giotto returned to the vicinity of the Earth on 1st July 1999. The distance of its closest approach was very uncertain, the estimate being about 220 000 km (136 700 miles), just over half the Earth-Moon distance. No communication with the spacecraft took place at this time. Giotto will continue to orbit the Sun for the foreseeable future, completing six revolutions roughly every seven years.

Launch 25 October 1998

Figure 9. Deep Space 1

Deep Space 1 is the first spacecraft in NASAs New Millennium programme. Its primary mission is to test 12 new advanced technologies. Most of these technologies were validated during the first few months of flight. It then approached to within 26 km (16 miles) of asteroid 9969 Braille on 28 July 1999. The few pictures returned showed that Braille's longest side is about 2.2 kms (1.3 miles) across and its shortest side appears to be 1 km (0.6 miles). The mission has been extended to include visits to two comets -Wilson-Harrington in January 2001, and Borrelly in September 2001. During the flybys, it will take close-up pictures, measure the size and shape of their nuclei, examine the gases in the surrounding coma and study the interaction of the comet with the solar wind. More at the NASA/JPL Deep Space 1 site.

Current and Future Missions

Launch 7 February 1999

	Figure 10. Stardust

Stardust is a NASA mission that was launched from Cape Canaveral, Florida. It will travel into the cloud of ice and dust that surround the nucleus of Comet Wild-2, coming to within 150 kilometres (100 miles) of the nucleus itself on 1 January 2004. There, it will gather comet dust particles and deliver them back to Earth. En route to the comet, Stardust will attempt to capture interstellar particles that are believed to be blowing through our Solar System. The mission ends in January 2006, when the Stardust sample return capsule will return to Earth and parachute to a designated landing spot in the Utah desert. More at the NASA/JPL Stardust site.

Launch 13 August 2002

Comet Nucleus Tour (Contour) is a NASA Discovery mission to improve our understanding of comet nuclei. Encounters are planned with Comets Encke (12 November 2003), Schwassmann-Wachmann 3 (18 June 2006), and d'Arrest (16 August 2008). There is a possibility that the spacecraft will be retargeted between 2004 and 2008 to a new comet.

More at the NASA/JPL Comet Nucleus Tour (Contour) site.

Launch 6 January 2004.

DEEP IMPACT is a NASA Discovery mission to Comet Tempel 1. It will consist of two craft that will separate when the comet is reached. The main spacecraft is an instrument platform that will fly slowly by the comet and record visual images and infrared spectral mapping data of the comet. The second craft is the 500 kg "impactor", which will separate from the flyby craft and be propelled into a target site on the sunlit side of the comet on 4 July 2005. It will crash into the sunny side of the comets nucleus at 10 km per second, creating a crater which is 120 m across and 25 m deep (25 m). This will be the first time any man-made object has impacted with a comet. Cameras and other instruments on the mother craft and back on Earth will study the newly created crater and the gases which are created as the nucleus is vapourised by the impact.

The NASA/JPL Stardust site has more information about these missions

For more background information on comets from 'Views of the Solar System'

Scientific objectives

Cometary nuclei, the prime target bodies of the Rosetta mission, and - to a lesser extent - asteroids, represent the most primitive solar-system bodies. They are assumed to have kept a record of the physical and chemical processes that prevailed during the early stages of the evolution of the solar system. It is the abundance of volatile material in comets that makes them particularly important and extraordinary objects: they demonstrate that comets were formed at large heliocentric distances and have been preserved at low temperatures since their formation. The only alteration processes since that time were induced by irradiation close to the surface and, late, by the thermal wave experienced by the comet when it was transferred from the Oort cloud to a Jupiter family trajectory, a few thousand years ago. Cometary material therefore represents the closest we can get to early condensates in the solar nebula. This may also be true to a lesser extent of primative (carbonaceous) asteroids.

Figure 1. Halley's Comet. Photo: MPAE, courtesy Dr H.U. Keller.

ESA's Giotto probe returned 2333 images during the Comet Halley encounter of 13/14 March 1986. All were recorded before the closest approach of 596 km at 00:03:02 UTC on 14 March 1986; the last from a distance of 1 180 km, 15 s before closest approach. The six shown here range from 375s (no. 3416) to 55s (no. 3496) before closest approach.

Our knowledge of small solar-system bodies, the comets and asteroids, has dramatically improved over the last 20 years. The major milestones were undoubtedly the first flybys of comet P/Halley by ESA's Giotto and the Russian Vega probes in 1986; followed in 1991 by the first near encounter with a main-belt asteroid, Gaspra, by the Galileo spacecraft on its way to Jupiter. During the same period, telescopic observations from the ground or in Earth orbit have drastically expanded and diversified. They constitute the basis for understanding small bodies as a population, since we can now compare observations of a very large variety of objects, and can undertake investigations of cometary activity. Systematic observations in the visible spectrum are performed for short-period comets as well as main-belt and near-earth asteroids, and it is now possible to observe cometary nuclei very far from the Sun, both from the ground and with the Hubble Space Telescope. Furthermore, small bodies can now be studied systematically at all wavelengths from the UV (IUE) via the infrared (ISO), to microwave and radio wavelengths.

From this wealth of new information, it is becoming apparent that small solar-system bodies, asteroids and comets, constitute an almost continuous suite of progressively less evolved objects, reflecting the radial gradient in the swarm of planetesimals during the formation of the solar system. Indeed, the outermost asteroids present spectral similarities with the bare cometary nuclei observed far from the Sun. The most distant 'asteroid', Chiron, whose orbit is outside Saturn's, is often considered a giant cometary nucleus. Furthermore, short-period comets should ultimately evolve into asteroids after the depletion of their volatile components. A better understanding of the relationship between asteroids, comets and planetesimals throughout the solar nebula is an essential step in unravelling the first stages of the formation of our solar system.

Figure 2. Halley's Comet - Close up. Photo: MPAE, courtesy Dr H.U. Keller.

Cometary material has been submitted to the lowest level of processing since its condensation from the protosolar nebula. It is considered likely that presolar grains may have been preserved in comets. As such, cometary material should constitute a unique repository of information on the sources which contributed to the protosolar nebula, as well as on the large condensation processes that resulted in the formation first of planetismals, then of larger planetary bodies. While tantalising results were obtained in situ from cometary grains, and from interplanetary dust particles collected on Earth, these cannot be considered as fully representative, in particular in terms of the organic and volatile complement. Direct evidence on cometary volatiles is particularly difficult to obtain, as species observable from Earth, and even during the Halley flybys, result from physico-chemical processes such as sublimation, interaction with solar radiation and the solar wind. Information currently available on cometary material gained from in situ studies and ground-based observations demonstrates the low level of evolution of cometary material. The tremendous potential of cometary material for providing on the constituents and early evolution of the solar nebula has yet to be exploited.

The Rosetta mission has specifically been designed to achieve this important scientific goal by focusing on in situ investigations of cometary material. The surface-science package will provide information on the chemical and physical properties of a selected area. It should also be possible to determine the isotopic composition. This information will be used as ground-truth for high-resolution coverage of the nucleus by state-of-the-art remote-sensing investigations. Sophisticated in situ analyses will be performed on the dust grains and the gas flowing out from the nucleus. the physico-chemical processes that link material in the coma to volatile and refractory species in the nucleus will also be investigated. To achieve these goals, the spacecraft will remain for most of the mission within a few tens of kilometres of the nucleus, where the analysed dust and gas is likely to present minimal alterations relative to surface material, and where it can be traced back to specific active regions on the cometary nucleus. The physics of the outer coma and the interaction with the solar wind will also be studied.

Rosetta will constitute a major step towards a better understanding of the formation and composition of planetesimals in the outer solar system, as well as their evolution over the last 4.57 X 10⁹years. The global characterisation of a cometary nucleus and one or two asteroids will provide essential information about the provenance of meteorites and interplanetary dust from which we have obtained most of our present knowledge about the formation of the solar system. Last, but not least, the Rosetta mission has important and exciting astrophysical implications, as it will provide a link between the solar system and nucleosynthetic processes, the formation and evolution of molecular clouds, and their further evolution to protostars and planets.

The prime scientific objective of the Rosetta 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 to achieve this are:

• the global characterisation of the nucleus, determination of dynamic properties, surface morphology and composition;
• the determination of the chemical, mineralogical and isotopic compositions of volatiles and refractories in a cometary nucleus;
• the determination of the physical properties and interrelation of volatiles and refractories in a cometary nucleus;
• the study of the development of cometary activity and the processes in the surface layer of the nucleus and the inner coma (dust/gas interaction);
• the global characterisation of asteroids, including determination of dynamic properties, surface morphology and composition.