Rosetta Rendezvous Mission with Comet 67P/Churyumov-Gerasimenko
Rosetta is a deep space mission of ESA , it is the first mission designed to both orbit and land on a comet. ESA selected the mission in Nov. 1993 as the third cornerstone mission in its long-term science program, called 'Horizon 2000'. The goal of the Probe is to rendezvous with the Comet 67P/Churyumov-Gerasimenko and map its surface in fine detail. It will also land a package of instruments (the Philae Lander) to study some of the most primitive, unprocessed material in the solar system. 1)
Comets are among the most beautiful and least understood nomads of the night sky. To date, half a dozen of these most heavenly of heavenly bodies have been visited by spacecraft in an attempt to unlock their secrets. All these missions have had one thing in common: the high-speed flyby. Like two ships passing in the night (or one ship and one icy dirtball), they screamed past each other at hyper velocity — providing valuable insight, but fleeting glimpses, into the life of a comet. That is, until Rosetta.
Launched in March 2004 and expected to reach the comet by 2014, Rosetta will be the first mission to revolve around the comet's nucleus and deliver a lander to its surface. The spacecraft will be located at a distance of 600 million km or 4AU (astronomical units) from the sun upon the comet.
Table 1: Some background on the Rosetta mission 2)
Legend to Figure 1: The Rosetta Stone is a granodiorite stele inscribed with a decree issued at Memphis in 196 BC on behalf of King Ptolemy V. The decree is given in three languages: Egyptian hieroglyphs (top), Demotic (middle), and ancient Greek (bottom). Champollion used the Greek to decipher the hieroglyphs (a full decipherment was published in 1824). The Rosetta Stone provided the key to the modern understanding of Egyptian hieroglyphs. The Rosetta Stone has a size of 114.4 cm x 72.3 cm x 27.93 cm. 3)
The measurements goals of the Rosetta mission are: 4)
- 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.
Rosetta is truly an international enterprise, involving more than 50 industrial contractors from 14 European countries and the United States. The prime spacecraft contractor is Airbus Defence and Space (formerly EADS Astrium, GmbH, Friedrichshafen, Germany), responsible for building the spacecraft. Other contributions were provided by Airbus Defence and Space UK (spacecraft platform), and Airbus Defence and Space France (spacecraft avionics) and Alenia Spazio (assembly, integration and verification) are major subcontractors.
Rosetta comprises a large orbiter, which was designed to operate for a decade, and a lander. Each of these components carries a large array of scientific instruments that will perform the most extensive study of a comet to date. The orbiter will revolve around the comet from 1km distance examining the nucleus and environment of the comet.
The Rosetta orbiter is a platform of size 2.8 m x 2.1 m x 2 m (an aluminum box) with a payload support module, which houses the 11 scientific instruments, mounted on the top and a bus support module, housing the subsystems, on the base. Two sets of solar panels with 14 m length having an area of 64m2 extend from the side of the spacecraft and a 2.2 m diameter steerable, high-gain antenna dish sticks out from the front. The lander is attached to the back of the orbiter. The two solar panel wings rotate at ±180° to capture the maximum sunlight. 5) 6)
The spacecraft is built around a vertical thrust tube, whose diameter corresponds to the 1.194 m Ariane-5 interface. This tube contains two large, equally sized propellant tanks (each of 1106 liter), the upper one containing fuel, and the lower one containing the (heavier) oxidizer. The Orbiter also carries 24 thrusters for trajectory and attitude control. Each of these thrusters pushes the spacecraft with a force of 10 Newton, equivalent to that experienced by someone holding a bag of 10 apples. Over half the launch mass of the entire spacecraft, about 1670 kg, is made up of propellant.
Figure 2: Artist's rendition of the deployed Rosetta spacecraft (image credit: ESA)
The spacecraft is three-axis stabilized. Attitude is maintained by four reaction wheels as well as using two star trackers, sun sensors, navigation cameras, and three laser gyro packages. Power is supplied by the solar arrays. The solar cells employed are 200 µm Si solar cells of LLIT (Low Intensity, Low Temperature) type sized 37.75 mm in width and 61.95 mm in length (Figure 3). The cover glass is 100 µm thick ceria doped micro-sheet designated curb mount glass (CMG). The cover-glass covers the solar cell completely. The solar arrays will provide 395 W at 5.25 AU and 850 W at 3.4 AU, when comet operations begin. Power is stored in four 10 Ah NiCd batteries which supply the 28 V bus power.
The Rosetta deep space mission of ESA represents a rather special case of solar cell technology use. At its destination in 2014, the spacecraft is at a distance of about 675 million km from Earth, corresponding to 4.5 AU, a distance almost as far out as Jupiter , where sunlight levels are only 4% of those on Earth. In LEO (Low Earth Orbit), Rosetta is the most powerful spacecraft that ESA ever built, at 12 kW of installed solar power generation provided by two solar arrays, covered with hundreds of thousands of specially developed non-reflective silicon cells. But at the deep space distance of Comet 67P/Churyumov-Gerasimenko, the total solar power available is only ~400 W. - Rosetta is the first deep space mission ever to rely entirely on solar power generation beyond the main asteroid belt (with sunlight levels of only 3-4% as those in LEO) - without the use of RTG (Radioisotope Thermoelectric Generator) technology (as is being done by all other deep space satellites).
Illumination reduces with distance from the Sun by what is called the inverse square law – if one goes twice as far away only a quarter the solar intensity is available, at three times the distance, only one ninth of the intensity is available. This means that temperatures experienced by the spacecraft also fall with distance, though in principle this is good news for solar cell designers, since cell efficiency increases as the temperature goes down. 7) 8) 9) 10)
In practice however, in low solar intensities with temperatures dropping below –100°C, standard solar arrays show worse-than-expected performance due to unpredictable degradation of individual cells. To overcome this problem, the LLIT (Low Intensity Low Temperature) specific solar cell technology was developed. The resulting single-junction silicon cells are flying on ESA's Rosetta comet chaser, which is venturing three times further from the Sun than Earth.
Legend to Figure 4: The image was taken in 2002 showing Rosetta being checked in ESA/ESTEC in Noordwijk, The Netherlands. One hinged wing is supported on a rig to allow it to unfurl safely in Earth gravity instead of weightlessness. Each wing is made up of five hinged panels, the steerable pair of wings together stretches 32 m tip-to-tip from the box-shaped spacecraft. 11)
At Rosetta's encounter with the Comet, the spacecraft is experiencing only 11% of Earth-level solar illumination — but still better than the 4% when it was furthest from the Sun. But instead of going nuclear, Rosetta runs solely on LILT (Low-Intensity, Low-Temperature) silicon solar cells, a new European technology devised for this mission, optimized for deep-space conditions. - The same is true of Rosetta's Philae lander, whose batteries are designed to be recharged by the LILT cells covering its body.
RF communications: Communications is maintained via the high-gain antenna, a fixed 0.8 m medium-gain antenna, and two omnidirectional low gain antennas. Rosetta utilizes an S-band telecommand uplink and S- and X-band telemetry and science-data downlinks, with data transmission rates from 5 to 20 kbit/s. The communication equipment includes a 28 W RF X-band TWTA (Traveling Wave Tube Amplifier) and a dual 5 WRF S/X band transponder. Onboard heaters keep the instrumentation from freezing during the period the spacecraft is far from the sun.
The Rosetta spacecraft had to be designed for a high level of reliability as the main scientific mission is starting more than 10 years after launch, and also for a high level of availability during the early years of the mission cruise, which also contains many key technical and scientifically valuable events including three Earth swingbys, one Mars swingby, two asteroid flybys, several deep space trajectory correction maneuvers, and regularly scheduled onboard system and payload checkouts.
These outstanding efforts will assure that Rosetta will contribute significantly to answer open questions in solar system research such as: How pristine are comets? How does cometary activity work? Are the craters on comets from impacts or from other processes? What is the internal structure of cometary nuclei? How does cometary material look like and what is it made of? Are there internal heat sources that trigger normal activity and outbursts? What are the main physical and chemical processes in the coma? How does solar wind – comet interaction change at the different activity levels from 3 AU to perihelion? Are comets candidates that delivered prebiotic molecules and water to Earth?
Comets remain the poorest understood solar system objects. The future measurements of Rosetta orbiting around a comet for several months and delivering a lander to the surface will open a whole new field of research. And Rosetta will provide a much better understanding of comets and solar system formation, much as the Rosetta Stone did in our understanding of the Egyptian culture (Ref. 4).
NAVCAM (Navigation Camera): The Rosetta spacecraft uses a single camera with a 5º field of view and 12 bit 1024 x 1024 pixel resolution, allowing for visual tracking on each of the spacecraft approaches to the asteroids and finally to the comet.
Figure 5: Exploded view of major spacecraft components (image credit: ESA, AOES Medialab)
Table 2: Overview of some spacecraft parameters
Thermal louvers: Throughout its mission, the Rosetta spacecraft is exposed to extreme cold and hot temperatures. In the early and late stages of its prolonged expedition, the spacecraft will sweep across the inner Solar System, where sunlight is plentiful. However, in order to rendezvous with Comet 67P/Churyumov-Gerasimenko, Rosetta will have to probe beyond the asteroid belt, more than 5 times the Earth's distance from the Sun. In those frigid regions, the solar energy levels are only 4% of the those that we enjoy on our balmy planet. 12)
Since it is not feasible to wrap a spacecraft in multiple layers of warm clothing for periods of deep freeze, then strip these away when sunbathing is the order of the day, the ESA team has been obliged to come up with alternative ways of regulating temperature.
Designers have provided Rosetta with louvers — high-tech venetian blinds which control the spacecraft's heat loss. Lovingly polished by hand, these assemblies of thin metal blades must be handled like precious antiques, since any scratching, contamination or fingerprints will degrade their heat reflecting qualities.
The principle behind the louvers is quite simple. When Rosetta is cruising around the inner Solar System and basking in the warmth of the Sun, surface temperatures may soar to 130°C, and even internal equipment can reach 50°C. At such times, it is vital to stop the spacecraft from overheating, so the louvers are left fully open, allowing as much heat as possible to escape into space from Rosetta's radiators.
However, during its prolonged deep space exploration and comet rendezvous phases, when temperatures plummet to -150°C, heat conservation is the order of the day. Since the spacecraft's limited internal power supply - equivalent to the output from three ordinary light bulbs - then becomes the main source of warmth, it is essential to trap as much heat as possible. This means completely closing the louvers in order to prevent any heat from escaping.
Some 14 of these louver panels cover an area of 2.25 m2 on the Rosetta spacecraft, placed over its radiators across the side and back of the spacecraft. The louvers open and close on a fully passive basis, requiring no power to operate. Instead they work on a ‘bimetallic' thermostat principle. The blades are moved by coiled springs made up in this case of a trio of different metals that expand and contract at differing rates, precisely tailored to rotate as required. Designed by Spain's Sener company, the louvers were extensively tested by ESA's Mechanical Systems Laboratory in advance of Rosetta's 2004 launch. 13)
Figure 6: Photo of a louver panel (image credit: ESA–A. Le Floc'h)
Figure 7: Photo of the Rosetta spacecraft with thermal blankets, released on January 19, 2004, ready for testing in the Large Space Simulator, at ESA/ESTEC (image credit: ESA) 14)
Legend to Figure 7: Temperature control was a major headache for the designers of the Rosetta spacecraft. Near the Sun, overheating has to be prevented by using radiators to dissipate surplus heat into space. In the outer Solar System, the hardware and scientific instruments must be kept warm (especially when in hibernation) to ensure their survival.
Figure 8: Rosetta and Philae — the lander is attached to the orbiter during integration of the spacecraft (image credit: ESA)
Legend to Figure 9: Rosetta is the first spacecraft to journey beyond the main asteroid belt and rely solely on solar cells for power generation. The new solar-cell technology used on its two giant solar panels allow it to operate over 800 million km from the Sun, where light levels are only 4% of those on Earth.
Launch: The Rosetta spacecraft was launched on March 2, 2004 on Ariane 5G+ vehicle from Kourou, French Guinea.
Orbit and mission event overview:
The original target of the Rosetta mission was comet 46P/Wirtanen. A failure of an Ariane-5 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 (Ref. 4).
Rosetta could not head straight for the comet. Instead it began a series of looping orbits around the Sun that brought it back for three Earth fly-bys and one Mars fly-by. Each time, the spacecraft changed its velocity and trajectory as it extracted energy from the gravitational field of Earth or Mars. During these planetary fly-bys, the science teams checked out their instruments and, in some cases, took the opportunity to carry out science observations coordinated with other ESA spacecraft such as Mars Express, Envisat and Cluster. 15)
During the 10 year trek across our solar system, Rosetta will travel five times the distance Sun-Earth, and will pass through the asteroid belt into deep space beyond 5 AU solar distance before it reaches its destination, the periodic comet 67P/Churyumov-Gerasimenko. On its way to 67P/Churyumov-Gerasimenko the spacecraft will employ four planetary gravity assist maneuvers (Earth-Mars-Earth-Earth) to acquire sufficient energy to reach the comet (Figure 10). Each of the fly-bys required months of intense preparation. In particular the fly-by of Mars in February 2007 was a critical operation: the new mission trajectory to 67P/Churyumov-Gerasimenko required that Rosetta fly past Mars at just 250 km from the surface, and spend 24 minutes in its shadow.
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. The spacecraft will enter a hibernation phase in July of 2011. In January 2014 Rosetta will come out of hibernation and begin a series of rendezvous maneuvers for comet 67P/Churyumov-Gerasimenko in May 2014.
The rendezvous maneuver 2 at~4.5 AU from the Sun will lower the spacecraft velocity relative to that of the comet to about 25 m/s and put it into the near comet drift phase, starting May 22, 2014 until the distance is about 10,000 km from the comet (Figure 3). It will be performed on the basis of a ground-based determination of the orbit from dedicated astrometric observations, before the comet is detected by the on-board cameras. The final point of the near-comet drift phase, the CAP (Comet Acquisition Point), is reached at a Sun distance of less than 4 AU. As soon as the spacecraft with a maximum relative velocity of about 1 m/s. The time and direction of the Rosetta-Philae separation will be chosen such that the landing package arrives with minimum vertical and horizontal velocities relative to the local (rotating) surface. After delivery of the lander on November 10, 2014 at a solar distance of 3 AU, the spacecraft will be injected into an orbit which is optimized for receiving the data transmitted from the lander and to relay them to the Earth. To adjust the payload operations sequences, the lander can be commanded via the orbiter.
Table 3: Milestones of the Rosetta mission
Figure 11: Schematic showing the spacecraft maneuvers close to comet 67P/Churyumov-Gerasimenko (image credit: ESA)
Sensor complement: (OSIRIS, ALICE, VIRTIS, MIRO, ROSINA, COSIMA, MIDAS, CONSERT, GIADA,RPC, RSI)
The Orbiter's scientific sensor complement includes 11 experiments and a small Lander, which will conduct its own scientific investigations. Scientific consortia from institutes across Europe and the United States have provided these state-of-the-art instruments.
The instruments on the Rosetta Orbiter will examine every aspect of the small cosmic iceberg. Wide and Narrow Angle Cameras will image the comet's nucleus to determine their volume, shape, bulk density and surface properties. Three spectrometers operating at different wavelengths will analyze the gases in the near-nucleus region, measure the comet's production rates of water and carbon monoxide/dioxide, and map the temperature and composition of the nucleus (Ref. 4).
Our knowledge of the nucleus should be revolutionized by the CONSERT experiment, which will probe the comet's interior by transmitting and receiving radio waves that are reflected and scattered as they pass through the nucleus. 16)
Four more instruments will examine the comet's dust and gas environment, measuring the composition and physical characteristics of the particles, e.g. population, size, mass, shape and velocity. The comet's plasma environment and interaction with the electrically charged particles of the solar wind will be studied by the Rosetta Plasma Consortium and the Radio Science Investigation.
Figure 12: The Rosetta spacecraft and its scientific payload (image credit: ESA) 17)
OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System)
Multi-color imaging with a WAC (Wide Angle Camera) and a NAC (Narrow Angle Camera) to obtain high-resolution images of the comet's nucleus. PI: Holger Sierks, MPS (Max Planck Institute for Solar System Research), Katlenburg-Lindau, Germany.
The objective of the OSIRIS assembly is to observe the cometary rotation, and to study the physical and chemical processes that occur in, on, and near the cometary nucleus. It also maps the cometary morphology, which will help Rosetta's lander (Philae) to find a suitable spot for setting down in the comet's surface. -The strength of OSIRIS is the coverage of the whole nucleus and its immediate environment with excellent spatial and temporal resolution and the spectral sensitivity across the whole reflected solar continuum up to the onset of thermal emission. This provides a context for the interpretation of the results from Philae. 18) 19) 20)
The OSIRIS cameras were provided by a consortium of 9 institutes from 5 European countries and from ESA, under the leadership of the MPS . The participating institutes of the consortium are: MPS (Katlenburg-Lindau, Germany), LAM (Laboratoire d'Astrophysique de Marseille), Marseille, France; UPD (University of Padova), Padova, Italy; IAA (Instituto de Astrofísica de Andalucía ), Granada, Spain; University of Uppsala (Sweden); ESA/ESTEC, Noordwijk, The Netherlands; UPM (Universidad Politécnica de Madrid), Madrid, Spain; INTA (National Institute for Aerospace Technology),Madrid, Spain; IDA (Institute of Computer and Network Engineering at the TU Braunschweig),Braunschweig, Germany.
Figure 13: Photo of the OSIRIS cameras (image credit: MPS)
After the launch of Rosetta (March 2, 2004), OSIRIS was activated on several occasions before the arrival to the main target, comet 67P/Churyumov-Gerasimenko. It was commissioned in seven slots between March 2004 and June 2005, and it performed several important scientific observations:
• A monitoring campaign of comet 9P/Tempel 1 around the Deep Impact event on 4 July 2005
• The fly-by of asteroid 2867 Steins on 5 September 2008
• Two Earth swing-bys in Nov. 2007 and Nov. 2009
• The observation of the remnant of a collision between two main-belt asteroids in February 2010 - The flyby of asteroid 21 Lutetia on 10 July 2010
• Early observation of the comet from more than 1AU distance in March 2011.
Figure 14: Specification of the OSIRIS assembly
Science objectives: The OSIRIS science objectives for the comet nucleus, the gases and dust produced by the comet and for the asteroid flybys are:
Figure 15: Nucleus objectives of OSIRIS
Table 4: Dust objectives of OSIRIS
Table 5: Gas objectives of OSIRIS
Table 6: Asteroid flyby objectives of OSIRIS
Table 7: Mars and Martian satellites flyby objectives of OSIRIS
Table 8: Earth/Moon system flyby objectives of OSIRIS
ALICE (Ultraviolet Imaging Spectrometer)
Analyses gases in the coma and tail and measures the comet's production rates of water and carbon monoxide/dioxide. ALICE is a NASA instrument providing UV spectroscopy in the band 70-205 nm. PI: Alan Stern, SwRI (Southwest Research Institute), Boulder, CO, USA. The ALICE UV spectrometer will analyze gases in the coma and the tail, and it measures the comet's production rates of water and carbon monoxide or dioxide. It will also provide information on the surface composition of the nucleus. 21) 22)
Instrument: Light enters the Alice telescope through a 40 x 40 mm entrance aperture and is collected and focused by an off-axis paraboloidal primary mirror onto the approximately 0.1° x 6° spectrograph entrance slit. After passing through the entrance slit, the light falls onto the toroidal holographic grating of a Rowland Circle style imaging spectrograph, where it is dispersed onto a microchannel plate detector. The 2D (1024 x 32 pixel) format MCP detector uses dual, side-by-side, solar-blind photocathodes of potassium bromide (KBr) and cesium iodide (CsI). The predicted spectral resolving power (λ/Δλ) of Alice is in the range of 105 - 330 for an extended source that fills the instantaneous FOV (Field of View) defined by the size of the entrance slit. 23)
Table 9: Summary of ALICE characteristics 24)
VIRTIS (Visible and Infrared Thermal Imaging Spectrometer)
An imaging spectrometer that combines three data channels in one instrument. Two of the data channels are designed to perform spectral mapping. The third channel is devoted to spectroscopy. Maps and studies the nature of the solids and the temperature on the surface of the nucleus. Also identifies comet gases, characterizes the physical conditions of the coma and helps to identify the best landing sites. Provides VIS and IR mapping (spectroscopy) in the region 0.25-5 µm. PI: Angioletta Coradini, IAS-CNR, Rome, Italy. 25)
Instrument: The optical subsystems are housed inside a common structure - the cold box - cooled to 130 K by a radiative surface supported on a truss having low thermal conductivity. On the pallet supporting the truss, two sets of electronics and two cryogenic coolers for the detectors are mounted. The cold box is rigidly mounted on the pallet but thermally isolated from it. The pallet and cold box together form the optics module, which is mounted inside the spacecraft arranged so that the observing axes of the optical subsystems are normal to the nadir (comet) pointing wall of the spacecraft. The electronics module, containing the digital electronics and power supply, is mounted separately. 26)
The mapping channel optical system is a Shafer telescope matched through a slit to an Offner grating spectrometer. The Shafer telescope consists of five aluminum mirrors mounted on an aluminum optical bench. The primary mirror is a scanning mirror driven by a torque motor. The Offner spectrometer consists of a relay mirror and a spherical convex diffraction grating, both made of glass.
The mapping channel utilizes a silicon charge coupled device (CCD) to detect wavelengths from 0.25 µm to 1 µm and a mercury cadmium telluride (HgCdTe) infrared focal plane array (IRFPA) to detect from 0.95 µm to 5 µm. The IRFPA is cooled to 70 K by a Stirling cycle cooler. The cold tip of the cooler is connected to the IRFPA by copper thermal straps. The CCD is operated at 155 K and is mounted directly on the spectrometer.
The high resolution channel is an echelle spectrometer. The incident light is collected by an off-axis parabolic mirror and then collimated by another off-axis parabola before entering a cross-dispersion prism. After exiting the prism, the light is diffracted by a flat reflection grating, which disperses the light in a direction perpendicular to the prism dispersion. The low groove density grating is the echelle element of the spectrometer and achieves very high spectral resolution by separating orders seven through sixteen across a two-dimensional detector array.
The high-resolution channel employs a HgCdTe IRFPA to perform detection from 2 to 5 μm. The detector is cooled to 70 K by a Stirling cycle cooler.
Table 10: Summary of VIRTIS characteristics
• Aug. 1, 2014: ESA's Rosetta spacecraft has made its first temperature measurements of its target comet, finding that it is too hot to be covered in ice and must instead have a dark, dusty crust. The observations were made by the VIRTIS instrument between 13 and 21 July, when Rosetta closed in from 14 000 km to the comet to just over 5000 km. 27)
• On 14 July, 2014, the entire surface of the comet occupied one of VIRTIS's pixels, allowing the scientists to estimate the mean temperature of the nucleus – around 205 K. While this may seem rather cold, it is somehow warmer than the scientists expected, providing the scientists with some first clues on the composition and the physical properties of the surface of the nucleus.
• July 8, 2014: First measurements of VIRTIS on board Rosetta have been probing the surface temperature on the nucleus of comet 67P/C-G. 28)
MIRO (Microwave Instrument for the Rosetta Orbiter)
The MIRO investigation addresses the nature of the cometary nucleus, outgassing from the nucleus and development of the coma as strongly interrelated aspects of cometary physics. During the flybys of the asteroids and, the MIRO instrument will measure the near surface temperature of these asteroids and search for outgassing activity in an effort to understand better the relationship between comets and asteroids.
PI: Sam Gulkis of NASA/JPL, Pasadena, CA. The MIRO investigation was conceived and designed functionally by the investigation team consisting of 19 scientists from 6 different institutions, and the MIRO project office, located at NASA/JPL.
Figure 16: Illustration of the MIRO instrument (image credit: MIRO Team)
The five objectives of the MIRO instrument are to: 29)
1) Characterize the abundances of major volatile species and key isotope ratios in the nucleus ices.
The MIRO instrument will measure absolute abundances of key volatile species - H2O, CO, CH3OH, and NH3 - and quantify fundamental isotope ratios - 17O,16O and 18O, 16O - in a region within several kilometers from the surface of the nucleus, nearly independent of orbiter to nucleus distance.
Water and carbon monoxide are chosen for observation because they are believed to be the primary ices driving cometary activity. Methanol is a common organic molecule, chosen because it is a convenient probe of gas excitation temperature by virtue of its many transitions. Knowledge of ammonia abundance has important implications for the excitation state of nitrogen in the solar nebula. By providing measurements of isotopic species abundances with extremely high mass discrimination, the MIRO experiment can use isotope ratios as a discriminator of cometary origins. The MIRO investigation will combine measurements of the variation of outgassing rates with heliocentric distance with models of gas votalization and transport in the nucleus to quantify the intrinsic abundances of volatiles within the nucleus.
2) Study the processes controlling outgassing in the surface layer of the nucleus.
The MIRO experiment will measure surface outgassing rates for H2O, CO, and other volatile species, as well as nucleus subsurface temperatures to study key processes controlling the outgassing of the comet nucleus.
The MIRO experiment will measure surface outgassing rates for H2O, CO, and other volatile species, as well as nucleus subsurface temperatures to study key processes controlling the outgassing of the comet nucleus.
3) Study the processes controlling the development of the inner coma. -MIRO will measure density, temperature, and kinematic velocity in the transition region close to the surface of the nucleus.
Measurements of gas density, temperature, and flow field in the coma near the surface of the nucleus will be used to test models of the important radiative and dynamical processes in the inner coma, and thus improve our understanding of the causes of observed gas and dust structures. The high spectral resolution and sensitivity will provide a unique capability to observe Doppler-broadened spectral lines at very low temperatures.
4) Globally characterize the nucleus subsurface to depths of a few centimeters or more.
The MIRO instrument will map the nucleus and determine the subsurface temperature distribution to depths of several centimeters or more. Morphological features on scales as small as 5 m will be identified and correlated with regions of outgassing.
The combination of global outgassing and temperature observations from MIRO and in situ measurements from the Rosetta lander will provide important insights into the origins of outgassing regions and of the thermal inertia of subsurface materials in the nucleus.
5) Search for low levels of gas in the asteroid environment.
The MIRO instrument will search for low levels of gas in the vicinity of asteroids and measure subsurface temperature to provide information on the presence of water ice, and on near surface thermal characteristics and the presence or absence of a regolith.
MIRO is configured both as a continuum radiometer and a very high spectral resolution line receiver. Center-band operating frequencies are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). The spatial resolution of the instrument, operating in the submillimeter band, is approximately 5 m at a distance of 2 km from the nucleus. The MIRO spectrometer is tuned to measure four volatile species - H2O, CO, CH3OH, and NH3 and the isotopes of water —H217O and H218O. These four species have all been measured to be present in comets. The spectral resolution is sufficient to observe individual, thermally broadened, line shapes at all temperatures down to 10 K or less. The MIRO experiment will use these species as probes of the physical conditions within the nucleus and coma. The basic quantities measured by MIRO are surface temperature, and gas abundance, velocity, and temperature of each species, along with their spatial and temporal variability. This information will be used to infer coma structure and outgassing processes, including the nature of the nucleus/coma interface. 30) 31)
Instrument: MIRO consists of an assembly of two heterodyne radiometers:
- Millimeterwave receiver (at 190 GHz , ~ 1.6 mm)
- Submillimeterwave receiver (at 562 GHz, 0.5 mm).
The mm and sub-mm radiometers are configured with a broadband continuum detector for the determination of the brightened temperature of the comet nucleus and the target asteroids. The sub-mm receiver is configured as a very high resolution spectrometer for the observation of the eight molecular transitions. The instrument is constituted of four separate physical modules, interconnected by a harness. The sensor unit is mounted on the spacecraft payload plane using the baseplate as the interface. The telescope boresight direction is aligned with the Rosetta payload line of sight. The optical bench is mounted on the undersite of the baseplate, under the telescope and inside the spacecraft. The mm and sub-mm wave receiver front ends, the calibration mechanism and the quasi-optics for coupling the telescope to the receivers are installed on the optical bench. The Sensor Backend Electronic Unit contains the intermediate frequency processor, the PLL (Phase Locked Loop) and the frequency sources. Due to his power consumption, it mounted next to a louvred radiator internal to the spacecraft. The Electronic Unit contains the CTS (Chirp Transform Spectrometer), including instrument computer and power conditioning circuits. The USO (Ultra Stable Oscillator) is self contained and thermally controlled. Those items are presented in Figure 17.
Table 11: Key parameters of MIRO
MIRO post launch results: The MIRO instrument appears to be fully functional at the end of commissioning. The measured key parameters appear to be better than expected.
Table 12: Key parameters were measured using the Earth as target, they are summarized in this table
ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis)
Comets are believed to be the most pristine bodies in the solar system. They were created 4600 million years ago far away from the sun and have remained for most of the time of their existence far outside of Pluto's orbit. They are small enough to have experienced almost no internal heating. They therefore present a reservoir of well-preserved material from the time of the Solar System's creation. They offer clues to the origin of the Solar System's material and to the processes that led from the solar nebula to the formation of planets. In contrast to meteorites (the other primitive material available for investigations), comets have retained the volatile part of the solar nebula. Several interesting questions on the history of the Solar System materials can therefore be answered only by studying comets. In particular, the composition of the volatile material — the main goal of the ROSINA instrument.
ROSINA is the main mass spectrometer on the orbiter of ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko. It consists of two mass spectrometers for neutrals and ions. ROSINA's primary objective is to determine the basic properties of the gas in the comet's atmosphere and ionosphere such as composition, temperature and velocity. 32) 33) 34) 35)
The ROSINA research team comes from many institutions: Physical Institute, University of Bern, Bern Switzerland; BIRA (Belgian Institute for Space Aeronomy), Brussels, Belgium; CESR (Centre d'Etude Spatiale des Rayonnements), Toulouse, France; IPSL (L'Institut Pierre Simon Laplace), Saint Maur, France; Lockheed Martin Advanced Technology Center, Palo Alto, CA, USA; MPS (Max Plack Institute for Solar System Research), Katlenburg-Lindau, Germany; Technische Universität Braunschweig, Germany; University of Michigan, Ann Arbor, MI, USA; SwRI (Southwest Research Institute), San Antonio, TX, USA; University of Giessen, Giessen, Germany. PI: Kathrin Altwegg,Hans Balsiger (honorary PI), University of Bern, Switzerland.
Science objectives: ROSINA's main goal is to determine the elemental, isotopic, and molecular composition of the comet's atmosphere and ionosphere. In addition, the scientists are interested in the temperature and the bulk velocity of the gas as well as the reactions of the gas and ions with the dust emitted by the comet. These results may render important implications for questions regarding the origin of comets, the relation between cometary and interstellar material, and the origin and evolution of the Solar System.
To accomplish these demanding objectives, ROSINA has unprecedented capabilities:
• A wide mass range from 1 amu (atomic mass unit) to more than 300 amu. This makes it possible to detect light atoms such as hydrogen as well as heavy organic molecules.
• A high mass resolution of more than 3000 m/Δm. This means that ROSINA is, for example, able to resolve CO from N2 and 13C from 12CH.
• A wide dynamic range of 1010
• High sensitivity (more than 10-5 A/mbar) to accommodate large differences in ion and neutral gas concentrations and severe changes in the ion and gas flux as the comet's activity develops between aphelion and perihelion.
Instrument: ROSINA consists of two mass spectrometers for neutrals and primary ions with complementary capabilities and a pressure sensor. The total mass of the ROSINA assembly is 36 kg with a power consumption of 53 W max.
• The DFMS (Double Focusing Magnetic mass Spectrometer) has a mass range from 1 amu to 150 amu and a mass resolution of 3000 at 1 percent peak height. It is optimized for very high mass resolution and a large dynamic range.
• The RTOF (Reflectron Time Of Flight) mass spectrometer with a mass range from 1 amu to more than 300 amu and a high sensitivity.
• The two pressure gages, COPS (COmet Pressure Sensor), provide density and velocity measurements of the cometary gas.
Figure 18: Photo of the ROSINA DFMS (Double Focusing Mass Spectrometer), Image credit: University of Bern
Figure 19: Photos of the ROSINA RTOF instruments (image credit: MPS)
Figure 20: Photo of the COPS devices (image credit: University of Bern)
COSIMA (Cometary Secondary Ion Mass Analyzer)
The instrument will analyze the characteristics of dust grains emitted by the comet, including their composition and whether they are organic or inorganic. COSIMA is a dust mass spectrometer (SIMS, m/µm ~2000). COSIMA was built by a consortium led by the MPS (Max Planck Institute for Solar System Research), Katlenburg-Lindau, Germany - in collaboration with Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, CNRS, Université d'Orléans, France; Institut d'Astrophysique Spatiale, CNRS, Université Paris Sud, Orsay, France; FMI (Finnish Meteorological Institute), Helsinki, Finland; Universität Wuppertal, Wuppertal, Germany; von Hoerner und Sulger GmbH, Schwetzingen, Germany; Universität der Bundeswehr, Neubiberg, Germany; Institut für Physik, Forschungszentrum Seibersdorf , Seibersdorf, Austria; Institut für Weltraumforschung, Österreichische Akademie der Wissenschaften, (Graz, Austria; PI: Martin Hilchenbach, (formerly Jochen Kissel), MPS. 36) 37)
COSIMA is a secondary ion mass spectrometer equipped with a dust collector, a primary ion gun, and an optical microscope for target characterization. Dust from the near comet environment is collected on a target. The target is then moved under a microscope where the positions of any dust particles are determined. The cometary dust particles are then bombarded with pulses of indium ions from the primary ion gun. The resulting secondary ions are extracted into the time-of-flight mass spectrometer.
Science objectives: COSIMA will perform in-situ measurements on individual dust particles emitted by the target comet and collected by COSIMA dust collector subsystem. From the resulting data it will be possible to determine:
• The elemental composition of solid cometary particles to characterize comets in the framework of the solar system chemistry
• The isotopic composition of key elements in solid cometary particles such as H, C, Mg, Ca, Ti in order to establish boundary conditions for models of the origin and evolution of comets and thereby of the solar system
• The chemical states of the elements
• Variations of the chemical and isotopic composition between individual particulate components
• Changes in composition that occur as functions of time ("short-term variations") and orbital position
• The variability of the composition of different comets by comparing the results to those obtained previously from comet Halley
• The presence of an organic component that is not associated with a rocky phase
• The molecular composition of the organic phase of the solid cometary particles
• The molecular composition of the inorganic phase of the solid cometary particles
• The chemical state of the organic matter characterized by its saturation degree oxidation state and bond types.
Instrument: The core of the COSIMA instrument is a TOF (Time of Flight) secondary SIMS (Ion Mass Spectrometer) equipped with a dust collector, a primary ion gun, and an optical microscope (COSISCOPE) for target characterization. Once one of the targets on the target wheel has been exposed to cometary dust it is moved in front of the microscope and imaged under shallow angle illumination provided by light emitting diodes. On-board image evaluation detects the presence and location of dust particles with diameters exceeding a few µm and calculates their position relative to the target reference point. Once the presence of features of interest is established, the target is moved in front of the mass spectrometer. Three nanosecond duration pulses of indium-115 with an energy of 10 keV and about 10 µm in diameter from the primary ion gun hit the selected feature. Secondary ions from the cometary matter are extracted by the SIL (Secondary Ion extraction Lens) into the TOF section. After passing deflection plates for beam steering the ions travel through a field free section. Next they pass a two stage reflector, return through the drift section to the ion detector. Its main element is a single stage microsphere plate, where the ions are detected. The arrival time of each ion is measured with an accuracy of about 2 ns.
Precision in the timing of the primary ion pulses, the correct selection of the dimensions and the voltages of the mass spectrometer and the accurate measurement of the secondary ion flight time are needed to obtain high mass resolution in the COSIMA instrument. A mass resolution of 2000 is achieved for ions having a flight time of 16 µs, which occurs for ion masses of above 28 Daltons (atomic mass units).
Figure 21: Photo of the COSIMA flight model (image credit: MPS)
Operations: Since launch and commissioning, the instrument health has been monitored in checkouts every 6 months. The instrument proved to be in good health during the checkouts performed so far. Operations have included mechanism tests, instrument calibration, ion source maintenance, and interference check between COSIMA and other instruments. The instrument will be re-commissioned in April and July 2014, just before the science phase of Rosetta at comet 67P/Churyumov-Gerasimenko.
• Sept. 8, 2014: The COSIMA team presented an image of the first dust grains collected by the COSIMA instrument when Rosetta was at a distance of less than 100 km from the nucleus of comet 67P/Churyumov-Gerasimenko. On Aug. 11, the first of COSIMA's 24 target plates were exposed to space. On Aug. 24, when the COSIMA team took a look at the image of the plate, they saw a number of large dust grains from the comet on a target that had been pristine when examined one week before. A first examination of the plate indicates that the largest two grains are about 50 µm and 70 µm in width, comparable to the width of a human hair. 38) 39)
• August 8, 2014: Now that comet 67P/Churyumov-Gerasimenko is within reach, Rosetta's mass spectrometer COSIMA, is beginning to reach for cometary dust. 40)
• In early April 2014, the project uploaded new software, and after switching the COSIMA instrument off and then back on, the previous tests proved successful and COSIMA was up and running with a fresh memory. On their way to or from the spacecraft, these data travel about 40 minutes through the inner Solar System. 41)
MIDAS (Micro-Imaging Dust Analysis System)
MIDAS is designed to collect and image dust particles collected in the vicinity around the comet. The measurement principle is based on atomic force microscopy. This technique allows for true three-dimensional imaging at nanometer-scale resolution. The instrument is built to investigate the smallest grain size fraction released from the comet's surface; it will also investigate the structural complexity of grain cluster up to a few µm. More than 60 collector facets, which can be individually exposed into the dust stream, and a total of 16 imaging sensors, guarantee continuous observations throughout the mission lifetime. 42)
Scientific objectives: Dust particles emitted from comet nuclei form a major source of information for the understanding of primitive matter in our Solar System. It represents remnant material from the early times of the formation comets, asteroids and planets some 4.5 billion years ago. The prime scientific objective of the MIDAS experiment is to image the micro-topography and micro-textural units of cometary dust particles; this provides important information about the characteristics and nature of these particles, for example, about the composition of their primary building blocks. In addition, sub-features on clean crystal surfaces provide insight into either the growth conditions (twinning, screw dislocations) and/or storage environment conditions (dissolution marks).
Following the mapping of single particles with a resolution in the few nanometer range, many statistical parameters describe the cometary environment. This comprises the statistical evaluation of the collected particles according to size, volume and shape, but also temporal and spatial variations of the particle flux can be deduced.
In summary, MIDAS will meet the scientific objectives that have been established for this instrument when the following information can be obtained during the rendezvous with the comet:
• 3D images of single particles with a resolution better than 10 nm
• Search for "very small particles" (10 nm)
• Search for evidence of euhedral (well-formed, sharp-faced) crystals
• Possible detection of ferro-magnetic minerals
• Size distribution of particles
• Variation of particle fluxes on time scales between hours and days.
The instrument was developed by an international collaborative team led by IWF (Institut für Weltraumforschung - Space Research Institute), Graz, Austria; ESA/ESTEC, Noordwijk, The Netherlands; Physics Department of the University of Kassel, Germany; Institute of Applied Systems Technology of Joanneum Research, Graz, Austria; Austrian Research Centers Seibersdorf (now Ruag Space, Vienna); Vienna University of Technology, Vienna, Austria. PI: Mark Bentley, IWF, Graz, Austria. 43) 44)
Instrument: Dust grains in the size range from 4 µm down to 4 nm will be imaged in three dimensions by means of atomic force microscopy (AFM). AFM makes use of tiny physical forces (van der Waals, interatomic, magnetic, etc.) that act on a sensor in closest distance to a surface.
The sensor is a 600 µm long cantilever arm with an extremely sharp 7 µm long tip mounted underneath. The sensor is controlled by a piezoelectric mechanical system that scans above the surface and senses its topography. The dust particles enter the instrument via a funnel penetrating the spacecraft's hull and hit the collector surface. Sixty-four of these targets (coated silicon facets) are mounted on the perimeter of the dust collector wheel. The facet exposed to the dust stream is rotated and presented to the microscope, which approaches the surface automatically and starts the scanning (imaging) process.
Figure 22: Photo of the MIDAS instrument, inside view (left) and flight model (right), image credit: IWF
Table 13: Performance parameters of the MIDAS instrument
In the test sequence of the MIDAS instrument, which ran over five contact passes from the ground station to the spacecraft, the following results were obtained:
- Complete electronics checkout
- Cover opening by a pyrolytic device
- Unlocking of all clamp mechanisms
- Movement of linear stage and approach mechanism out of launch position
- Verification of all motors and mechanisms
- Verification of all 16 sensors by resonance search
- Verification of the scanner by imaging calibration surfaces
- Initial characterization of mechanical noise environment.
The overall result of the MIDAS commissioning phase and subsequent instrument checkout demonstrated full functionality and performance of the instrument up to the moment the spacecraft (and payload) was put into hibernation in June 2011.
CONSERT (Comet Nucleus Sounding Experiment by Radiowave Transmission)
CONSERT is a time domain transponder that operates between one module that will land on the comet surface and another that will orbit the comet. A radio signal is transmitted from the orbiting component of the instrument and passes through the comet nucleus to the component on the comet surface. The signal is received on the lander, where some data is extracted, and then immediately re-transmitted back to the orbiter, where the main experiment data collection occurs. The variations in phase and amplitude that occur as the radio waves pass through different parts of the cometary nucleus will be used to perform tomography of the nucleus and determine the dielectric properties of the nuclear material. 45) 46) 47)
The overall science objective of the CONSERT investigation is to gather information about the geometrical structure and electrical properties of the deep interior of the comet nucleus. Inferences about the composition of the interior of the comet will then be made from the measured electrical properties. The main scientific objectives are:
• To measure the mean dielectric properties and, through modelling, to set constraints on the cometary composition (like material and porosity)
• To detect large-scale embedded structures (several tens of meters), and stratifications
• To detect small scale irregularities within the comet.
CONSERT probes the comet's interior by studying radio waves that are reflected and scattered by the nucleus. The CONCERT instrument provides radio sounding and nucleus tomography. The instrument's mass is limited to 3 kg. PI: Wlodek Kofman, LPG (Laboratoire de Planetologie de Grenoble), CNRS/UJF, Grenoble, France.
Instrument: CONSERT works as a time domain transponder. The indirect and apparently complicated transponding procedure reduces the required accuracy of the clocks on the Orbiter and Lander, and makes it possible to stay within the constraints on mass and power consumption imposed on the space experiment. The CONSERT experiment on the orbiter and on the lander both consist of a transmit/receive antenna and a transmitter and receiver contained in a common box.
A 90 MHz radio signal, phase modulated with pseudo-randomly encoded data is transmitted from the orbiter towards the comet. The transmission lasts about 25 µs. The signal propagates through the comet nucleus and is received on the lander. The transmission cycle is repeated every 200 ms. The received signal is digitised and accumulated in the lander in order to increase the signal to noise ratio. Once the accumulation is finished, the signal is compressed to obtain a time/space resolution corresponding to 100 ns which corresponds to about 20 m in the comet. After the signal processing on the lander, which determines the position of the strongest path, the lander transmits the same pseudo-random code with a delay corresponding to that of the strongest path. The transmission cycle again lasts about 25 microseconds. The signal propagates back to the orbiter along virtually the same path, since the orbiter does not travel far during the measurement cycle. The signal is received on the orbiter, accumulated and stored in the memory in order to be sent to Earth. A complete measurement cycle lasts about 1 s.
GIADA (Grain Impact Analyzer and Dust Accumulator)
GIADA will measure the number, mass, momentum and velocity distribution of dust grains in the near-comet environment. GIADA will analyze both grains that travel directly from the nucleus to the spacecraft and those that arrive from other directions having had their ejection momentum altered by solar radiation pressure. 48) 49) 50)
GIADA is a contamination monitor providing dust velocity and impact momentum measurement. PI: Luigi Colangelo, INAF, Naples, Italy.
The primary scientific objectives of GIADA are:
• Dust flux measurement for "direct" and "reflected" grains: Two populations of cometary grains exist: "direct" (coming directly from the nucleus) and "reflected" grains (coming from the Sun direction, under the action of the solar radiation pressure). The two populations undergo very dissimilar dynamic evolution in the coma and have different times of ejection from the nucleus. In the case of Rosetta, "direct" and "reflected" grains can be collected simultaneously. The relative amount will depend on the probe position along its orbit. GIADA will be able to monitor grain fluxes coming from different directions and will allow, for the first time, discrimination between the two dust populations. This task is fundamental to the determination of the original dust size distribution. In turn, this information is required to define the dust mass loss rate.
• Analysis of the dust velocity distribution : The dust ejection velocity depends both on the grain size and on time. Moreover, grains with a given size have a wide dust velocity distribution. GIADA will allow the measurement of scalar velocity and momentum for grains coming from the nucleus direction so as to give mass and impact velocity of each analyzed "direct" grain. From this information it will be possible to derive grain mass and ejection velocity from the nucleus surface. For the first time we will obtain:
- the size dependence of the dust ejection velocity
- the relation between most probable dust velocity and dust mass
- the velocity distribution for each dust mass
- the link between velocity dispersion and dust mass.
• Study of dust evolution in the coma : Once ejected from the nucleus, grains may change their physical properties due to several processes, including, for example, fragmentation. These modifications may alter the grain size distribution. The size distribution of grains collected by GIADA in the nucleus direction should not be affected by the dust velocity dispersion. Thus, changes in the dust distribution at different nucleus distances can be linked directly to actual variations in the dust size distribution and correlation can be found with dust fragmentation and/or with emission from active areas on the nucleus.
• Correlation of dust changes with nucleus evolution and emission anisotropy : The dust environment characteristics depend on the comet-Sun distance and on the time evolution of the nucleus. The continuous monitoring by GIADA of dust flux and dynamic properties will offer the best opportunity to characterize the time evolution of the dust environment as a function of heliocentric distance. Nucleus imaging will allow us to link observed changes to the nucleus evolution and to its spin state.
• Determination of dust to gas ratio : One of the crucial parameters characterizing the comet nucleus is the dust to gas ratio. Dust flux monitoring by GIADA is needed to estimate the dust to gas ratio. This will be possible in combination with results of other experiments.
• Other objectives : The data provided by GIADA about dust fluxes and grain dynamic properties are very important for the correct interpretation of images of the coma and nucleus and mass spectrometer data. GIADA will help in the selection of the surface science package landing site. The characterization of dust emitting areas, and possibly of the dust population of different active areas, will be necessary for the site selection process to achieve a proper balance between safety and scientific interest.
GIADA will play an important role for the health and the safety of various experiments and the spacecraft itself, as it will be able to provide information about dust flux in several directions. Optical surfaces of experiments and other devices pointing to the nucleus will be polluted by the dust flux. GIADA data will allow the prediction of deposition rates and informed decision making for mission planning and operations. Data from GIADA will be the only resource to predict and allow control of the performance degradation of critical devices such as passive radiators and solar panels.
Instrument: The instrument comprises three modules: GIADA 1 measures momentum, scalar velocity and mass of single grains entering the instrument by the GDS (Grain Detection System) and the IS (Impact Sensor), placed in cascade. The GIADA 2 module contains the MBS (Micro Balances System); it controls the acquisition of data from the sensors and the operation of the other subsystems. It also provides the power supply for the whole experiment. The GIADA 3 module measures the cumulative dust flux and fluence from different directions by means of five microbalances. One microbalance points towards the nucleus, while the other four cover the widest possible solid angle. 51)
In the GDS, four laser diodes with their foreoptics are used to form a thin (3 mm) light curtain (100 cm2). For each grain passing through it, the scattered/reflected light is detected by two series of four detectors (photodiodes) placed at 90º with respect to the sources. In front of each photodiode a Winston cone is placed to achieve a uniform sensitivity in the detection area.
The IS is a thin (0.5 mm) aluminum square diaphragm (sensitive area 100 cm2) equipped with five piezoelectric sensors, placed below the corners and its center. When a grain impacts the sensing plate, flexural waves are generated on the plate, and are detected by the piezoelectric crystals. The maximum displacement of these systems is directly proportional to the impulse imparted, and the displacement of the crystal produces a proportional potential. Through calibration, a known impulse may be equated with a specific charge produced on the electrodes of the PZT crystals. The detected signal is proportional to the momentum of the incident grain through the factor (1+e), where e is the coefficient of restitution.
When a grain enters GIADA 1, the GDS gives a first estimate of the grain speed and starts a time counter that is stopped when the IS detects the grain impact and the momentum is measured. In this way, for each entering grain, speed, TOF (Time of Flight), momentum and, therefore, mass are measured.
The microbalances in GIADA 3 each consist of two quartz crystals oscillating at a frequency of about 15 MHz, one acting as a sensor, the other as a reference. The measured physical quantity is the beat frequency between the two crystals. The resonance frequency of the sensor quartz oscillator, exposed to the dust environment, changes due to the variation of its mass as a result of material accretion, while the reference crystal is not exposed to the dust flux. Thus, the output signal is proportional to the mass deposited on the sensor and dust flux and fluence are measured in time. The use of a reference crystal ensures extremely small dependence on temperature and power supply fluctuations and, thus, high sensitivity.
Figure 23: Two views of the GIADA instrument (image credit: INAF)
RPC (Rosetta Plasma Consortium)
Six sensors measure the physical properties of the nucleus; examine the structure of the inner coma;monitor cometary activity; and study the comet's interaction with the solar wind. The RPC assembly consists of:
- Langmuir Probe (LAP). PI: Anders Eriksson (formerly Rolf Boström), IRF Uppsala, Sweden
- Ion and Electron Sensor (IES). PI: Jim Burch, SRI, San Antonio, TX, USA
- Flux Gate Magnetometer (MAG). PI: Karl-Heinz Glassmeier, IGEP, Braunschweig, Germany
- Ion Composition Analyzer (ICA). PI: Rickard Lundin, IRF, Kiruna, Sweden
- Mutual Impedance Probe (MIP). PI: Jean-Gabriele Trotignon, LPCE/CNRS, Orleans, France
- Plasma Interface Unit (PIU). PI: Chris Carr, Imperial College, London, UK.
• The physical properties of the cometary nucleus and its surface. Emphasis will be given to determination of the electrical properties of the crust, its remnant magnetization, surface charging and surface modification due to solar wind interaction, and early detection of cometary activity.
• The inner coma structure, dynamics, and aeronomy. Charged particle observation will allow a detailed examination of the aeronomic processes in the coupled dust-neutral gas-plasma environment of the inner coma, its thermodynamics, and structure such as the inner shocks.
• The development of cometary activity, and the micro- and macroscopic structure of the solar-wind interaction region as well as the formation and development of the cometary tail.
In order to realize these investigations extensive in-situ monitoring of the plasma electrons and ions, their composition, distribution, temperature, density, flow velocity, and the magnetic field will be necessary. These measurements will improve the understanding of the coupling processes of cometary dust, gas, and plasma as well as its interaction with the solar wind. The plasma and fields measurements thus provide complementary information to that of other Rosetta instruments for a deeper understanding of the overall physics and chemistry of an active comet.
The flybys of asteroid Steins and asteroid Lutetia have provided an opportunity to study in detail the physics of the solar wind - asteroid interaction. RPC has excellent capabilities for the investigation of this interaction. It has also been possible to study the magnetic and electric conductivity properties of the asteroids.
ICA (Ion Composition Analyzer): ICA measures the three-dimensional velocity distribution and mass distribution of positive ions. The mass resolution is sufficient to differentiate between the major particle species such as protons, helium, oxygen, molecular ions, and heavy ion clusters (dusty plasma). The ICA comprises an electrostatic arrival angle filter, a hemispherical electrostatic analyzer employed as an energy filter, and a magnetic deflection momentum filter. Particles are detected using a large micro channel plate and a two-dimensional anode array.
IES (Ion and Electron Sensor): The IES will simultaneously measure the flux of electrons and ions in the plasma surrounding the comet over an energy range from around one electron volt, which approaches the limits of detectability, up to 22 keV. IES consists of two electrostatic analyzers, one for electrons and one for ions, which share a common entrance aperture. The charged particle optics for IES employs a toroidal top-hat geometry along with electrostatic angle deflectors to achieve an electrostatically scanned field of view of 90º x 360º. 54)
LAP (Langmuir Probe): The LAP instrument will measure the density, temperature and flow velocity of the cometary plasma. It comprises two spherical sensors mounted at the tip of deployable booms, with the sensors capable of being swept in potential to measure the current-voltage characteristic of the intervening plasma, which provides information on the electron number density and temperature. The probes can be held at a fixed bias potential to measure plasma density fluctuations and by a time-of-flight analysis of the signals from the two probes the plasma flow velocity can be determined. 55)
MAG (Flux Gate Magnetometer): The MAG will measure the magnetic field in the region where the solar wind plasma interacts with the comet. It consists of two triaxial fluxgate magnetometer sensors mounted on a 1.5 m deployable boom that points away from the comet nucleus. One sensor is mounted near the outboard tip of the boom and one is mounted part way along the boom. The use of two sensors allows the effects of the spacecraft's own magnetic field to be minimized. MAG will also study any magnetic field possessed by the comet nucleus, in cooperation with the ROMAP magnetometer experiment on the Rosetta lander.
MIP (Mutual Impedance Probe): MIP will derive the electron gas density, temperature, and drift velocity in the inner coma of the comet by measuring the frequency response of the coupling impedance between two dipoles. MIP will also investigate the spectral distribution of natural waves in the 7 kHz to 3.5 MHz frequency range and monitor the dust and gas activity of the nucleus.
PIU (Plasma Interface Unit): PIU acts as an interface between the five instruments that make up RPC and the Rosetta spacecraft by providing a single path for the transmission of scientific and housekeeping data to the ground and for the receipt and processing of commands sent from the ground. The PIU also takes power from the spacecraft and converts, conditions and manages it for the RPC instruments. PIU also performs on-board data processing for the MAG sensor unit, which has no data processing capability of its own. 56)
RSI (Radio Science Investigation)
RSI makes use of the communication system that the Rosetta spacecraft uses to communicate with the ground stations on Earth. Either one-way or two-way radio links can be used for the investigations. In the one-way case, a signal generated by an ultra-stable oscillator on the spacecraft is received on Earth for analysis. In the two way case, a signal transmitted from the ground station is transmitted back to Earth by the spacecraft. In either case, the downlink may be performed in either X-band or both X-band and S-band. — PI: Martin Pätzold, University of Cologne, Germany. 57)
The goal of RSI is to investigate the nondispersive frequency shifts (classical Doppler) and dispersive frequency shifts (due to the ionized propagation medium), the signal power and the polarization of the radio carrier waves. Variations in these parameters will yield information on the motion of the spacecraft, the perturbing forces acting on the spacecraft and the propagation medium.
Science objectives: Doppler data provide time-resolved measurements of the spacecraft motion and the plasma state and thus may be used for physical investigation of the nucleus and the inner coma of the comet. In particular, the following scientific objectives may be addressed by an analysis of dual-frequency one-way or two-way radiometric tracking data, together with information provided by other Rosetta experiments, for example the remote imaging system (OSIRIS):
• Gravity Field and Dynamics
- Cometary mass and bulk density
- Cometary gravity field coefficients
- Cometary moments of inertia and spin state
- Cometary orbit, light shift, thermal properties of the nucleus
- Asteroid mass and bulk density
• Cometary Nucleus
- Size and shape (from spacecraft occultation observations)
- Internal structure (from nucleus sounding)
- Internal structure (from nucleus sounding)
- Rotation, precession and nutation rates (from bistatic radar)
• Cometary Coma
- Distribution of mm - dm size particles (from coma sounding)
- Plasma content of the inner coma (from coma sounding)
- Gas and dust mass flux (from non-gravitational perturbations of the spacecraft)
• Solar Corona Science
- Electron content of the inner corona, solar wind acceleration, search for coronal mass ejections, turbulence.
Instrument: The two-way radio link is established by transmitting an uplink radio signal either at S-band or X-band to the spacecraft. The received uplink carrier frequency is transponded to downlinks at X-band and S-band upon multiplying by the constant transponder ratios 240/221 and 880/221, respectively, in order that the ratio of the two downlinks is 880/240 = 11/3. This radio mode takes advantage of the superior frequency stability inherent to the hydrogen maser in the ground station on Earth. This mode is used for all RSI gravity science applications, routine tracking observations when in orbit during the escort phase, and for the sounding of the solar corona.
The one-way radio link is used only during an occultation of the spacecraft by the nucleus as seen from Earth. This enables radio sounding of the immediate vicinity of the nucleus and perhaps even the nucleus itself, should the solid cometary body prove to be penetrable by microwaves. These one-way occultation experiments require an USO (Ultra-Stable Oscillator) added to the radio subsystem. The prime purpose of the USO is to serve as a phase-coherent frequency reference for the simultaneous one-way downlink transmissions at S-band and X-band. The required stability (Allan variance) of the USO is about Δf/f ~10-13 at 10-1000 seconds integration time. The one-way radio link can be transmitted either while receiving a non-coherent uplink or without any uplink contact at all.
Ground segment: Ground stations include antennas, associated equipment and operating systems in the tracking complexes of Perth (ESA, 35 m), Australia, and the DSN (Deep Space Network) of NASA, (34 m) in California, Spain and Australia. A tracking pass consists of typically eight to ten hours of visibility. Measurements of the spacecraft range and carrier Doppler shift can be obtained whenever the spacecraft is visible. In the two-way mode the ground station transmits an uplink radio signal at S-band (if the spacecraft receiver operates at S-band) or at X-band and receives the dual-frequency simultaneous downlink at X-band and S-band. The information about signal amplitude, received frequency and polarization is extracted and stored as a function of ground receive time.
SREM (Standard Radiation Environment Monitor)
In addition to these scientific experiments the orbiter is also equipped with a SREM device to monitor the high energetic, ionizing particle environment aboard the spacecraft. The objective of SREM is to provide a continuous, almost uninterrupted measurement of the high energetic particles encountered by Rosetta and provide this information for mission analysis purposes.
Philae (Rosetta Lander)
The Rosetta lander Philae can be considered a scientific spacecraft of its own that is carried and delivered by the Rosetta orbiter to the comet. Upon proposal by various scientists, lead by Helmut Rosenbauer from the MPS (Max Planck Institute for Solar System Research) in Katlenburg-Lindau, the 10 scientific instruments and the various spacecraft subsystems are provided by a consortium of spaceflight agencies and research institutes from 6 European countries and by ESA. The Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. DLR played a major role in building the lander and runs the LCC (Lander Control Center) at DLR Cologne, which is preparing for and overseeing the difficult task of landing on the comet, a feat never before accomplished. 58) 59) 60) 61) 62) 63)
The goal of Philae's mission is to land successfully on the surface of a comet, and transmit data from the surface about the comet's composition. The scientific goals of the mission focus on "elemental, isotopic, molecular and mineralogical composition of the cometary material, the characterization of physical properties of the surface and subsurface material, the large-scale structure and the magnetic and plasma environment of the nucleus."
The SONC (Science Operations and Navigation Center) is located at CNES in Toulouse, France. Both centers are directly connected to the RMOC (Rosetta Mission Operations Center) at ESOC, Darmstadt. The Rosetta science operations planning is performed at the RSGS (Rosetta Science Ground Segment) at ESAC, near Madrid. - The responsibility for the Lander delivery lies at ESA. However, close cooperation between the partners is envisaged, to reach the challenging task of the first successful landing on a comet. 64)
Figure 24: Artist's impression of the 100 kg Philae lander (image credit: ESA, DLR)
The Philae lander is designed to touch down on the comet's surface after being deployed from the main spacecraft body and "falling" from a height of 25 km at about 1 m/s towards the comet along a ballistic trajectory. Upon contact, it will deploy two harpoons to anchor itself to the surface, and the legs are designed to dampen the initial impact to avoid bouncing, because the comet's escape velocity is only around 0.5 m/s.
Communications with Earth will use the orbiter spacecraft as a relay station to reduce the electrical power needed. The mission duration on the surface is planned to be at least one week, but an extended mission lasting months is possible.
Rosetta-Philae RF link: The transceiver is a full duplex S-band transmission set for digital data developed specifically for space applications. The conception made by Syrlinks was done with drastic objectives for mass and power consumption. For this, the use of commercial parts was decided leading to a low cost product widely used afterwards on the Myriade platform family. The transceiver is composed of a transmitter, a receiver and a reception filter for dual antenna use (Figure 25). The filter protects the receiver from out-of-band signals, particularly from the transmitter. The two functions (receiver and transmitter) are fully independent and can be activated separately. Technical details are given in the Table 14 and an illustration in Figure 26. 65) 66)
There are two transceivers on both sides of the RF link. The redundancy is activated with RF switches on orbiter side (1 Tx/1 Rx active) and with diplexer on lander side (1 Tx/2 Rx active). The choice of implementing identical RF chains for transmission and reception on the orbiter and the lander has given great advantages, such as cutting procurement costs and simplifying qualification, integration and testing.
With 1W RF output power and 1 dBi gain (@ 60º) patch antennas, link establishment is possible for distances up to 150 km.
The lander telecommunication system answers to a request-to-send protocol from the orbiter at any time. This handshake protocol, which implies full duplex equipment and which was specifically designed for Rosetta mission ensures a desired quality of transmission even when the relative geometry and visibility between the orbiter and the lander is not favorable.
In the housekeeping telemetry available at orbiter side, one parameter is particularly interesting to get information beyond its intrinsic value: the RSSI (Received Signal Strength Indicator). From this raw telemetry value, it is possible to extract the received power level on orbiter side, which can be then processed.
Lander bus: The main structure of the lander is made from carbon fiber, shaped into a plate maintaining mechanical stability, a platform for the science instruments, and a hexagonal "sandwich" to connect all the parts. The main body rests on a tripod landing gear, with ice screws and sensors integrated in the feet. All instruments and the drill are depicted in their deployed configuration. The open face of Philae with instruments exposed to the cometary environment is colloquially termed "balcony".
The total mass is about 100 kg. Its "hood" is covered with solar cells for power generation.
Battery assembly: The lander energy storage is based on two types of sources: primary batteries for short term activities (1000 Wh), secondary batteries for long term activities(140 Wh). The assembly includes the associated electronics.
Legend to Figure 27: The figure shows the overall structure, the subsystems and the experiment compartment of the Lander. Some instruments are not visible in the drawing: specifically, the instruments in charge of analyzing the samples distributed by the SD2 (CIVA, COSAC, PTOLEMY), and the down-looking camera (ROLIS).
Figure 28: Side view schematics of the inner structure of the lander compartment showing the location of COSAC and PTOLEMY systems, the CONSERT antennas, the SESAME dust sensor and various CIVA cameras (image credit: Philae Team)
The general tasks of Philae are to get a first in situ analysis of primitive material from the early solar system and to study the structure of a cometary nucleus which reflects growth processes in the early solar system and to provide ground truth for Rosetta Orbiter instruments. The scientific objectives of the Lander are:
• determining the composition (elemental, isotopic, mineralogical and molecular) of the cometary surface material
• measuring the physical properties (thermal, electrical, mechanical) of the cometary surface material
• describing the large-scale structure (panoramic imaging, particles and magnetic field, and internal heterogeneity)
• monitoring the cometary activity (day/night cycle, changing distance to the Sun, outbursts).
Legend of Figure 29: The ejection maneuver takes place at an altitude on the order of 1 km only; the Lander eject velocity partly cancels Rosetta's orbital velocity, such that Philae moves on an comet-surface crossing ellipse, stabilized by a flywheel and the optional use of a cold-gas thrusters (in z direction). After touchdown on the moving comet surface, the cold-gas system is activated to provide a hold-down thrust until the harpoons have safely anchored the Lander.
Philae sensor complement:
The Rosetta Lander carries a further nine experiments, as well as a drilling system to take samples of subsurface material. The Lander instruments are designed to study in situ for the first time the composition and structure of the surface and subsurface material on the nucleus. The science payload of the lander consists of ten instruments with a mass of ~27 kg, making up nearly one-third of the mass of the lander. 67)
Table 15: The payload of the Rosetta lander Philae lead investigators: Jean-Pierre Bibring and Hermann Boehnhard
For the collection of samples and the deployment of instruments it is important to note that the Lander can be rotated around its z (vertical) axis by 360º defining a "working circle" around the Lander body axis. Thus, arbitrary locations can be accessed by the sampling drill (SD2), the down-looking camera (ROLIS), and the APXS sensor; the MUPUS-PEN and the SESAME-PP electrodes are attached to the latter two instruments. Also the stereo camera pair of CIVA will be able to image a full panoramic of 360º using the Lander rotation capability.
Note that the landing gear also provides a tilting capability. This capability had to be drastically reduced in range (to ±5º) after the change of target comet in 2003 to ensure a safe landing.
The science operations of Philae are divided into various phases. During the 10 year cruise, check-ups, calibrations, software and command up-loads are scheduled as well as occasional observation campaigns for CIVA-P (flybys and ROMAP (flybys, solar wind, comet tail crossings).
After arrival at the comet, global mapping by the Orbiter instruments and the selection of a landing site, the Separation–Descent–Landing phase begins. Immediately before release from the Orbiter, thermal preparation and battery charging are foreseen. Immediately after the eject, the Landing Gear is unfolded, thereby releasing the CONSERT antennas. Then, the ROMAP boom is deployed. A telemetry contact to the Orbiter will be established a few minutes after release until well after landing. During descent (30–60 min) to the comet's surface, scientific measurements (images by ROLIS, magnetic field measurements by ROMAP-MAG, dust impact by SESAME-DIM and -CASSE, calibrations of SESAME-CASSE and MUPUS-TM) will be performed to monitor the cometary environment between the Orbiter and the surface of the nucleus, to observe the nucleus while approaching, to characterize remotely the landing site and to document the touchdown event of the Lander at the surface. ROLIS descent images will be taken until touchdown and MUPUS-ANC measurements during the actual anchoring.
During the "first science sequence" of approximately five days, Philae will be operated mainly on primary batteries, thus minimizing sensitivity to landing geometry (solar irradiance of the cells). In the first 60 hours following the touch-down, all instruments will work in their baseline mode at least once at full completion of their relevant science goals. In particular a full panorama of the landing site will be taken by CIVA-P immediately after landing and cometary samples will be acquired by SD2, both from the surface and from the maximum depth reachable with drill (i.e., about 0.2 m); these samples will then be processed by the relevant instruments (COSAC, Ptolemy, CIVA-M). MUPUS-PEN and APXS will be deployed and thermal conductivity, thermal diffusivity, strength measurements be made by MUPUS and the first X-ray and alpha spectra will be recorded by APXS. CONSERT will sound the nucleus over at least one full Orbiter orbit relative to the Lander. ROMAP will observe the daily variation of magnetic field and the plasma properties. All three parts of SESAME will perform measurements (PP only after MUPUS-PEN and/or APXS have been deployed). The Lander resources should enable at least a partial redo of this sequence over the following 60 hours, if partial failure (e.g., in data transmission) had happened. If performed successfully, the first sequence will secure a "minimum science success" of the Lander mission. 68)
During the "long-term science mission" (up to three months until r = 2 AU is reached) all instruments will be operated mostly sequentially, powered by the solar cells and buffered by the secondary (rechargeable) batteries. The Lander has enough flexibility to allow—by rotation around its body axis—the optimized orientation of the solar cells with respect to the local time, to drill several boreholes, and to measure physical properties all around the landing site.
The data volume to be uplinked to Earth is 235 Mbit during descent and the first five days, and 65 Mbit during each subsequent 60 hour period. However, depending on actual telemetry coverage and Orbiter requirements, a significantly larger data volume is expected.
With the current best estimate of the comet environment, about 52–65 hours of primary mission operations are feasible (incl. a 30% system margin). Primary power during the first science sequence is 15–20 W; the solar cells generate 10 W during the day at 3 AU.
The long-term operations then rely entirely on the solar generator; the end of life will be determined either by overheating (the thermal system is designed for a range of 2–3 AU) or by insufficient power if the solar cell degradation (mainly by dust deposition) becomes too severe.
Figure 30: Artist's impression (not to scale) of the Rosetta orbiter deploying the Philae lander to comet 67P/Churyumov–Gerasimenko (image credit: ESA, C. Carreau, ATG medialab) 69)