Astronomy

What are the Objectives of the Rosetta Mission



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The Rosetta mission was decided as part of the European Space Agency’s (ESA) Horizon 2000 programme as a planetary cornerstone mission. Approved in November 1993 it builds upon the success of its predecessor mission GIOTTO which provided data from flybys of two comets 1P/Halley and 26P/Grigg-Skjellerup. Rosetta draws its name from the Rosetta stone, the key in unravelling the mysteries of the civilization of ancient Egypt. It is hoped that the Rosetta spacecraft will do the same for the origin of the solar system by studying the origin of comets. It is believed that the composition of comets is close to that of the proto-solar nebula from which comets and planets in the solar system were formed, therefore results obtained could help in not only understanding the formation and evolution of comets but also of the solar system itself. Previous missions have shed some light on the misunderstood subject of cometary science and Rosetta helps to build on these successes[1]

The measurement goals of Rosetta as described by Schwem and Schulz include:

Global characterisation of the nucleus, determination of dynamic properties, surface morphology and composition Determination of chemical, mineralogical and isotopic compositions of volatiles and refractories in a cometary nucleus Determination of the physical properties and interrelation of volatiles and refractories in a cometary nucleus Study of the development of cometary activity and the processes in the surface layer of the nucleus and inner coma (dust/gas interaction)[2] 

To achieve these goals Rosetta will implement a much wider range of instruments than previous cometary missions. Although originally planned as a sample return, it has since been revised. The mission now contains an orbiter; the Rosetta probe, and a lander; Philae. Scientific instrumentation is split into two sections, 12 different instruments are present on the orbiter for measuring and examining the cometary nucleus whilst orbiting it and 10 instruments within the surface science package on the Philae lander. 

The original mission target was comet 46P/Wirtanen in 2011 but due to concerns about the reliability of the Ariane 5 launcher following a catastrophic failure in December 2002 the launch was postponed and a new mission plan devised. The new choice of comet, 67P/Churyumov-Gerasimenko has significant consequences for the Philae lander. The nucleus of the new target is estimated to be far larger than that of the previous target (Roughly 4km instead of 1.2km) resulting in an increased landing risk due to higher projected impact velocities. Previous landing velocities were optimised for a .5m/s decent with an upper limit of 1m/s, since the alternative mission plans came into force rigorous testing and technical adjustments have allowed a new upper limit of 1.5m/s to counter increased impact velocities.[3] 

Rosetta was eventually launched on March 2nd 2004 due to arrive at its new target the periodic comet 67P/Churyumov-Gerasimenko in 2014. Discovered in 1969 it is a short periodic comet believed to be of the Jupiter comet family with a perihelion distance of 1.29 AU[4]

During the years prior to its rendezvous, Rosetta will employ four gravity assist manoeuvres to gain enough energy to reach the comet, in between which it will fly by two main belt asteroids, 2867 Steins and 21 Lutetia. The asteroid flybys were carefully selected from the observations of potential candidates. 21 Lutetia was deemed interesting as it was the only one which would allow mass determination and density estimation by radio science experiments. Its primitive composition allows it to fit in neatly with the scientific objectives of investigating the early solar system. Secondly, 2867 Steins was chosen as it has relatively unusual spectral properties and could be possibly classified as an E-type asteroid, a rare type of asteroid with a surface most consistent with pyroxene[5]

After Rosetta has completed all its gravity assist manoeuvres and has come out of hibernation, ‘rendezvous manoeuvre 2’ will be performed on May 22, 2014 at roughly 4.5 AU from the sun which should put Rosetta into a relative velocity of 25m/s of that of the comet, this will continue until it is about 10,000km away. At this point ground based astrometric observations will be used to guide the spacecraft until the onboard cameras can be used to resolve the comet and dictate further movements. The final stage will be reached at a distance from the sun of less than 4AU, from which point the solar arrays will be able to provide sufficient power to allow the craft to acquire adequate images from the on board navigational camera. After around 30 days the relative velocity should have decreased to 1.5m/s with a distance of roughly 300 comet radii. From this distance point landmarks and radiometric measurements can be used to guide the spacecraft closer and the orbit insertion process can begin. Global mapping of the comet surface will then take place with orbit radii of 5-25 cometary nuclei distance before reducing the distance to around 1 radii for close observation, at which point all the orbiters instruments should be collecting data[6]. 

With observations completed the landing site for the Philae lander will be determined based on the collected data, the orbiter will then eject the lander from a low altitude eccentric nearest the pericentre of the landing site at a maximum relative velocity of 1m/s in such a direction as to minimise vertical and horizontal velocities to the local surface. The orbiter will move back into a position to optimise data reception from the Philae lander so as to transmit it back to Earth. After this the orbiter will spend most of its time in a close orbit to the comet in order to analyse gas, dust and plasma in the inner coma from the onset until peak activity closer to the sun. 

The lander itself is a rectangular box of roughly 8 cubic metres. It has a High Gain Antenna (HGA) on one side with a diameter of 2.2m and two degrees of freedom allowing it to point independently of the spacecraft body. Its solar panels span over 32m and can rotate +270 degrees and -90 degrees driven by the Solar Array Drive Mechanism (SADM).

The majority of instruments are orientated such that they can point more or less anywhere in the sky whilst the HGA can continue to point to Earth. This is however limited by some thermodynamical constraints, the Philae lander and some of the instruments such as the UV spectrometer ALICE cannot be pointed within 11 degrees of the sun without risking damage and the thrusters which are thermally decoupled from the craft cannot be exposed to the sun at all when closer than 1.8 AU. Another constraint is communication; the HGA uses X and S band radio waves and typically transmit to the ESA deep space antenna in New Norcia, Australia. The typical bitrate for science is between 41-22kbps with pass durations of 8-10 hours during the comet phase. Power is typically provided by the solar panels which are optimal between 3.2-1.4AU of the sun, too far and there is not enough power, too close and power has to be reduced to avoid overheating. All these aforementioned factors conspire for the need of detailed timelining and priority setting in instrumentation operations[7] 

As previously mentioned the orbiters instrumental payload contains 12 experiments:

ALICE: A UV imaging spectrometer whose scientific objectives are to ‘Search for and determine the evolved rare gas content of the nucleus to provide information on the temperature of formation and thermal history of the comet since formation’[8].

CONSERT: A comet nucleus sounding experiment the purpose of which is to determine from the measurement of the propagation delay the mean dielectric properties of the cometary nucleus[9]

COSIMA: A secondary ion mass analyser to determine the characteristics of dust grains emitted by the comet, including composition and whether they are organic or inorganic.

GIADA: A grain impact analyser and dust accumulator to give a broader view of the dust grains, i.e. measuring the number, mass, momentum and velocity distribution of dust grains

MIDAS: A micro-imaging dust analysis system will study the dust environment around the comet and provide information on particle population, size, volume and shape

OSIRIS: An optical, spectroscopic and IR imaging system, equipped with both a wide angle and narrow angle camera to obtain high resolution images.

ROSINA: A spectrometer for ion and neutral species to determine the composition of the comets atmosphere and ionosphere

RPC: Five instruments designed to examine how the comet is affected by its approach toward the Sun in particular with the solar wind, it will also examine the physical properties of the nucleus.

RSI: Radio science investigator will use Doppler shifts to measure the mass density and gravity of the cometary nucleus

VITRIS: Visible and IR mapping spectrometer used to study the surface of the nucleus, but can also identify gases and physical conditions to help pick a suitable landing area
Along with these scientific experiments the orbiter also contains SREM a radiation environment monitor, designed to measure the ionising particle levels the spacecraft encounters. 

The lander Philae contains 10 scientific instruments to allow it to land on and subsequently analyse the comet. ROLIS the descent camera and CIVA, the panoramic camera system will be used to image and control descent they will continue imaging along with the other instruments, which after touchdown will be powered by an on-board battery for 5 days. SESAME, the acoustic sounding probe will monitor the surface and dust impacts from cometary activity. CONSERT has a part on the orbiter but also contains some equipment on the landing allowing both large scale and localised results. ROMAP a magnetometer will be used to explore the magnetic and plasma interactions within the landing site and its interaction with the solar wind. Surface and sub-surface measurements will be taken by the APXS spectrometer and MUPUS the multipurpose sensor; these will be assisted by the drill and sampling unit SD-2 and by the mass spectrometers and gas chromatographs PTOLEMY/COSAC which will determine isotopic/molecular composition and chirality after being imaged by the IR microscope on CIVA. Results from these experiments will be relayed via the orbiter to earth. Once the primary Batteries run out, a secondary solar rechargeable battery takes over and a long-term exploration program is adopted for as long as the lander survives which is expected to be a few months[10]

The Rosetta Mission will provide a unique insight into cometary science and the implications it has on solar system evolution. Most strikingly the Philae lander will be the first controlled touchdown of a robotic lander on a cometary nucleus providing the first images from a cometary surface and in situ analysis of cometary composition.


References:


[1] The Rosetta Mission: Flying Towards the Origin of the Solar System, Karl-Heinz Glassmeier, Hermann Boehnhardt, Detlef Koschny, Ekkehard Kuhrt and Ingo Richter, 2007, Space Sci Rev Vol. 28, Iss. 1-4, p1-21

[2] Rosetta goes to Comet Wirtanen, 1999, G. Schwehm and R.Schulz, Space Sci. Rev Vol. 90 Iss. 1-2, p313-319

[3] Rosetta Lander – Philae: Implications of an alternative mission, S. Ulamec, S. Espinasse, B. Feuerbacher, M. Hilchenbach, D. Moura, H. Rosenbauer, H Scheuerle, R. Willnecker, Acta Astronautica 58 (2006) p435-441

[4] The Rosetta Mission: Flying Towards the Origin of the Solar System, Karl-Heinz Glassmeier, Hermann Boehnhardt, Detlef Koschny, Ekkehard Kuhrt and Ingo Richter, 2007, Space Sci Rev Vol. 28, Iss. 1-4, p1-21

[5] Asteroid target selection for the new Rosetta Mission baseline: 21 Lutetia and 2867 Steins, M.A. Barucci, M. Fulchignoni, S. Fornasier, E. Dotto, P. Vernazza, M. Birlan, J. Carvao, F. Merlin, C. Barbieri, I. Belskaya, Astron. Astrophys. 430, p33-317

[6] The Rosetta Mission: Flying Towards the Origin of the Solar System, Karl-Heinz Glassmeier, Hermann Boehnhardt, Detlef Koschny, Ekkehard Kuhrt and Ingo Richter, 2007, Space Sci Rev Vol. 28, Iss. 1-4, p1-21

[7] Scientific Planning and Commanding of the Rosetta Payload, 2007, D. Koschy, V. Dhiri, K. Wirth, J. Zender, R. Solaz, R. Hoofs, R. Laureijs, T. M. Ho, B. Davidsson and G. Schwehm, Space Sci. Rev Vol. 90, Iss 1-2, P167-188

[8] Alice: The Rosetta UltraViolet Imaging Spectrograph, 2007, S.A. Stern, D.C. Slater, J. Scherrer, J. Stone, M. Vertseef, M. F. A’hearn, J. L. Bertaux, P.D. Feldman, M.C. Festou, Joel Wm. Parker and O. H. W. Siegmund, Space Sci. Rev Vol. 90, Iss. 1-2 P507-527

[9] The Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT): A short description of the instrument and of the commissioning stages, 2007, W. Kofman, A. Herique, J. P. Goutail, T. Hagfors, I.P. Williams, E. Nielsen, J. P. Barriot, Y. Barbin, C. Elachi, P. Edenhofer, A. C Levasseur-Regourd, D. Plettemeier, G. Picardi, R. Seu and V. Svedhem, Space Sci. Rev Vol 90 Iss 1-2 P413-432

[10] The Rosetta Mission: Flying Towards the Origin of the Solar System, Karl-Heinz Glassmeier, Hermann Boehnhardt, Detlef Koschny, Ekkehard Kuhrt and Ingo Richter, 2007, Space Sci Rev Vol. 28, Iss. 1-4, p1-21

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