ESA Science & Technology - Orbiter Instruments

Introduction

Rosetta is an interplanetary spacecraft whose main objective is to rendezvous with Comet 67P/Churyumov-Gerasimenko. In order to investigate the comet nucleus and the gas and dust ejected from the nucleus as the comet approaches the Sun, Rosetta carries a suite of eleven instruments on the comet orbiter and Philae, a lander equipped with a further ten instruments which perform surface measurements.

The orbiter instruments combine remote sensing techniques, such as cameras and radio science measurements, with direct sensing systems such as dust and particle analysers.

The instruments are provided by collaborative efforts between scientific institutes in ESA member states and the USA. Principal investigators in different European countries and America lead the nationally funded collaborations.

Instruments In Brief

Alice

Alice, an Ultraviolet Imaging Spectrometer, will characterize the composition of the nucleus and coma, and the nucleus/coma coupling of comet 67P/Churyumov-Gerasimenko. This will be accomplished through the observation of spectral features in the extreme and far ultraviolet (EUV/FUV) spectral regions from 70 to 205 nm.

Alice will make measurements of noble gas abundances in the coma, the atomic budget in the coma, and major ion abundances in the tail and in the region where solar wind particles interact with the ionosphere of the comet. Alice will determine the production rates, variability, and structure of H₂O and CO, and CO₂ gas surrounding the nucleus and the far-UV properties of solid grains in the coma.

Alice will also map the cometary nucleus in the FUV. En route to comet 67P/Churyumov-Gerasimenko, Alice has studied Mars and the Rosetta asteroid flyby targets, Steins and Lutetia.

CONSERT

CONSERT (Comet Nucleus Sounding Experiment by Radio wave Transmission) will perform tomography of the comet nucleus. CONSERT operates as a time domain transponder between one module which will land on the comet surface and another that will orbit the comet. A radio signal passes from the orbiting component of the instrument to the component on the comet surface and is then immediately transmitted back to its source. The varying propagation delay as the radio waves pass through different parts of the cometary nucleus will be used to determine the dielectric properties of the nuclear material.

COSIMA

COSIMA (Cometary Secondary Ion Mass Analyser) 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.

GIADA

GIADA (Grain Impact Analyser and Dust Accumulator) will measure the scalar velocity, size and momentum of dust particles in the coma of the comet using an optical grain detection system and a mechanical grain impact sensor. Five microbalances will measure the amount of dust collected as the spacecraft orbits the comet.

MIDAS

MIDAS (Micro-Imaging Dust Analysis System) is intended for the microtextural and statistical analysis of cometary dust particles. The instrument is based on the technique of atomic force microscopy. This technique, under the conditions prevailing at the Rosetta orbiter permits textural and other analysis of dust particles to be performed down to a spatial resolution of 4 nm.

MIRO

MIRO (Microwave Instrument for the Rosetta Orbiter) is composed of a millimetre wave mixer receiver and a submillimetre heterodyne receiver. The submillimetre wave receiver provides both broad band continuum and high resolution spectroscopic data, whereas the millimetre wave receiver provides continuum data only.

MIRO will measure the near surface temperature of the comet, allowing estimation of the thermal and electrical properties of the surface. In addition, the spectrometer portion of MIRO will allow measurements of water, carbon monoxide, ammonia, and methanol in the comet coma.

OSIRIS

OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) is a dual camera imaging system operating in the visible, near infrared and near ultraviolet wavelength ranges. OSIRIS consists of two independent camera systems sharing common electronics. The narrow angle camera is designed to produce high spatial resolution images of the nucleus of the target comet. The wide angle camera has a wide field of view and high straylight rejection to image the dust and gas directly above the surface of the nucleus of the target comet. Each camera is equipped with filter wheels to allow selection of imaging wavelengths for various purposes. The spectroscopic and wider band infrared imaging capabilities originally proposed and incorporated in the instrument name were descoped during development.

ROSINA

ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) consists of two mass spectrometers, since no one technique is able to achieve the resolution and accuracy required to fulfil the Rosetta mission goals over the range of molecular masses under analysis. In addition, two pressure gauges provide density and velocity data for the cometary gas.

The two mass analysers are:

• A double focusing magnetic mass spectrometer with a mass range of 1 - 100 amu and a mass resolution of 3000 at 1 % peak height, optimised for very high mass resolution and large dynamic range
• A reflectron type time-of-flight mass spectrometer with a mass range of 1 - 300 amu and a mass resolution better than 500 at 1 % peak height, optimised for high sensitivity over a very broad mass range

RPC

RPC (Rosetta Plasma Consortium) is a set of five instruments sharing a common electrical and data interface with the Rosetta orbiter. The RPC instruments are designed to make complementary measurements of the plasma environment around comet 67P/Churyumov-Gerasimenko.

The RPC instruments are:

• an Ion Composition Analyser (ICA)
  to measure the three-dimensional velocity distribution and mass distribution of positive ions
• an Ion and Electron Sensor (IES)
  to simultaneously measure the flux of electrons and ions in the plasma surrounding the comet
• a Langmuir Probe (LAP)
  to measure the density, temperature and flow velocity of the cometary plasma
• a Fluxgate Magnetometer (MAG)
  to measure the magnetic field in the region where the solar wind plasma interacts with the comet
• a Mutual Impedance Probe (MIP)
  to derive the electron gas density, temperature, and drift velocity in the inner coma of the comet

All five instruments are controlled by the common Plasma Interface Unit (PIU) which also provides the interface to the Rosetta spacecraft and distributes power from the spacecraft to the five RPC instruments at the required secondary voltages.

RSI

RSI (Radio Science Investigation) 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.

RSI will investigate the nondispersive frequency shifts (classical Doppler) and dispersive frequency shifts (due to the ionised 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.

VIRTIS

VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) is 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.

Alice: Ultraviolet Imaging Spectrometer

Alice, an Ultraviolet Imaging Spectrometer, will characterize the composition of the nucleus and coma and the nucleus/coma coupling of comet 67P/Churyumov-Gerasimenko. This will be accomplished through the observation of spectral features in the extreme and far ultraviolet spectral regions ranging from 70 to 205 nm.

Alice will make measurements of noble gas abundances in the coma, the atomic budget in the coma, and major ion abundances in the tail and in the region where solar wind particles interact with the ionosphere of the comet. ALICE will determine the production rates, variability, and structure of H₂O and CO, and CO₂ gas surrounding the nucleus and the far-ultraviolet properties of solid grains in the coma.

Alice will also map the cometary nucleus in the FUV. En route to comet 67P/Churyumov-Gerasimenko, Alice has studied Mars and the Rosetta asteroid flyby targets, Steins and Lutetia.

Science Objectives

The scientific objectives of the Alice investigation are to characterize the composition of the nucleus and coma, and the coma/nucleus of comet 67P/Churyumov-Gerasimenko. This will be accomplished through the observation of spectral features in the 70 - 205 nm extreme and far ultraviolet spectral region.

Ultraviolet spectroscopy is a powerful tool for studying astrophysical objects, and has been applied with dramatic success to the study of comets. Alice will provide unprecedented improvements in sensitivity and spatial resolution over previous cometary ultraviolet observations. For example, Alice will move the sensitivity threshold from the ~1 Rayleigh level achievable with the Hubble Space Telescope to the milliRayleigh level. In addition, Alice will (by virtue of its location at the comet) move the spatial exploration of nucleus ultraviolet surface properties from the present-day state-of-the-art (no data available on any comet) to complete nuclear maps at Nyquist-sampled resolutions of a few hundred meters. Stars occulted by the absorbing coma will also be observed and used to map the water molecule spatial distribution, giving hints as to the location of the production regions on the nuclear surface.

Through its remote-sensing nature, Alice will be able to:

• Obtain compositional and morphological information on the comet prior to the rendezvous, thereby providing planning observations for in situ instruments prior to entering orbit about the comet
• Map the spatial distribution of key species in the coma, and small coma dust grains, as a function of time as the comet responds to the changing solar radiation field during its approach to the Sun
• Obtain compositional and production rate measurements of nuclear jets and other inner coma features even when the Orbiter is not in the vicinity of these structures
• Obtain certain ion abundance measurements around perihelion in order to connect nucleus activity to changes in tail morphology and structure, and coupling to the solar wind

The primary scientific themes of the Alice investigation are the following:

• Determine the rare gas content of the nucleus to provide information on the temperature of formation and the thermal history of the comet since its formation. Argon and Neon will be primary targets of the Alice investigations
• Determine the production rates and spatial distributions of the key parent molecule species, H₂O, CO and CO₂, thereby allowing the nucleus/coma coupling to be directly observed and measured on many time-scales in order to study the chemical heterogeneity of the nucleus and its coupling to the coma
• Obtain an unambiguous budget of the cosmogonically most important atoms (Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur through the detection of their emissions far from the nucleus. This is required to understand their production processes and to derive the elemental composition of the volatile fraction of the nucleus. Coupled to the measurement of the major molecule abundances of the nucleus, this will give us the total contribution of the secondary parent species to the composition of the nucleus
• Study the onset of nuclear activity and nucleus output variations related to changing solar aspect and nuclear rotation with unprecedented sensitivity

Additional scientific themes Alice will address include the following:

• Spectral mapping of the complete nucleus at far-ultraviolet wavelengths to characterize the distribution of ultraviolet absorbers on the surface, in particular icy patches and organics
• Photometric properties and ice/rock ratio of small grains in the coma as an aid to understanding the size distribution of cometary grains and how they vary in time. Also, studying the grain coma to establish the relative contributions of the nucleus and coma grains to the observed gases
• Mapping the time variability of O⁺, N⁺, and possibly S⁺ and C⁺ emissions in the coma and ion tail in order to connect nuclear activity to changes in tail morphology and structure, and tail interaction/coupling to the solar wind

Instrument Description

Light enters the Alice telescope through a 40 × 40 mm entrance aperture and is collected and focused by an off-axis paraboloidal primary mirror onto the approximately 0.1° × 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 2-D (1024 × 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 field-of-view defined by the size of the entrance slit.

 
Rosetta Blog articles
 

CONSERT: Comet Nucleus Sounding Experiment by Radiowave Transmission

CONSERT (Comet Nucleus Sounding Experiment by Radio wave Transmission) 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.

Science Objectives

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 metres), and stratifications
• To detect small scale irregularities within the comet

Instrument Description

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 microseconds. The signal propagates through the comet nucleus and is received on the lander. The transmission cycle is repeated every 200 milliseconds. 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 nanoseconds, which corresponds to about 20 metres 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 second.

 
Rosetta Blog articles
 

COSIMA: Cometary Secondary Ion Mass Analyser

COSIMA (Cometary Secondary Ion Mass Analyser) 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

which in turn will allow:

• Comparison of the composition of the solid particles to the elemental and isotopic composition of the neutral and ionised atmosphere of the comet
• Gaining insight into the molecular composition of the inorganic phase of the particulate matter
• Assessment of the exobiological relevance of the cometary organic matter as possible organic precursor material
• Evaluation of the relation of the association of inorganic phases and mineral components in cometary matter to the formation of prebiotic organic molecules on the early Earth

Instrument Description

The core of the COSIMA instrument is a time-of-flight (TOF) secondary ion mass spectrometer (SIMS) 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 secondary ion extraction lens (SIL) 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).

 
Rosetta Blog articles
 

GIADA: Grain Impact Analyser and Dust Accumulator

GIADA (Grain Impact Analyser and Dust Accumulator) will measure the number, mass, momentum and velocity distribution of dust grains in the near-comet environment. Giada will analyse 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.

Science Objectives

The primary scientific objectives of GIADA (Grain Impact Analyser and Dust Accumulator) 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 analysed "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 characterise 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 characterising 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 characterisation 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 Description

The instrument comprises three modules: GIADA 1 measures momentum, scalar velocity and mass of single grains entering the instrument by the Grain Detection System (GDS) and the Impact Sensor (IS), placed in cascade. The GIADA 2 module contains the main electronics (ME); 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.

In the GDS, four laser diodes with their fore-optics 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 90 deg 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) aluminium square diaphragm (sensitive area 100 cm²) equipped with five piezoelectric sensors, placed below the corners and its centre. 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, 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.

 
Rosetta Blog articles
 

MIDAS: Micro-Imaging Dust Analysis System

MIDAS (Micro-Imaging Dust Analysis System) will study the dust environment around the comet. In particular, MIDAS will perform microtextural and statistical analysis of cometary dust particles. The instrument is based on the technique of atomic force microscopy. This technique, under the conditions expected to prevail, both at the comet and during the cruise phase, permits textural and other analysis of dust particles to be performed down to a spatial resolution of 4 nm. MIDAS also studied the dust environment of the two asteroids, Steins and Lutetia, that were flown by during the journey to comet 67P/Churyumov-Gerasimenko.

Science Objectives

During the rendezvous with the comet MIDAS will provide the following information:

• Images of single particles with a spatial resolution of 4 nm
• Statistical evaluation of the particles according to size, volume, and shape
• Size distribution of particles ranging from about 4 nm to a few µm
• Shape, volume and topographic structure of individual particles
• Temporal variation of particle fluxes
• Spatial variation of particle fluxes
• Measurements on local elastic properties if studies show that they do not affect the tip lifetime

During the cruise phase to the comet MIDAS provided:

• Characterization of the dust environment in the vicinity of the asteroids Steins and Lutetia
• Characterization of dust particles detected in the vicinity of the comet

MIDAS will deliver global images, that is complete images of the entire scan field, and images of individual dust particles. The individual images are contained in the global images, since candidate particles for detailed imaging are identified using the global image. The selected particles are then re-scanned with a much higher resolution.

Instrument Description

MIDAS is designed to analyse microdust particles collected in the interplanetary and cometary environment, irrespective of their electrical conductivity and shape, by means of atomic force microscopy. The sizes of the particles range from about 4 nm to a few µm. The dust collector includes a mechanism which controls the particle flux onto a wheel made of polished silicon. After analysis, another of the 64 facets of the wheel is exposed to the ambient dust flux. The MIDAS microscope consists of five functional parts: a one shot cover and a funnel to protect the aperture on the ground and during launch, the shutter to define the exposure time to the dust flux, the robotics system for manipulation of the dust particles, the scanner head, and the supporting electronics.

The heart of the atomic force microscope (AFM) is a very small tip which maps the surface of the particle. An AFM is capable, in principle, of imaging details down to atomic resolution. In the simplest case, the tip remains in permanent contact with the surface and follows its height variations with a control mechanism which keeps a constant force on the tip (contact mode). In a technically more complex mode, the tip scans the surface while its supporting cantilever vibrates at one of its natural resonance frequencies. There are two dynamic modes: (a) the tip does not come closer to the surface than a few tenths of a nanometre (non-contact mode) or (b) the tip hits the surface during its sinusoidal oscillation (tapping mode). In all three modes it is essential either to keep the force constant or to measure it accurately in order to derive an image of the surface.

The tip must move over the surface in a reproducible manner, which can be relatively easily achieved by piezo electric scanners in three independent directions. The combination of the tip, supporting cantilever, and piezo-electric actuators is called scanner head. Due to lifetime requirements, several tips will be employed.

The MIDAS instrument consists of one mechanical unit. The top part of the instrument unit houses the elements of the atomic force microscope and the system to collect and transport the dust samples to the head of the microscope. The dust intake system is connected to the instrument unit and protrudes through the outer spacecraft wall. The control electronics which must be near the sensor and the actuators are also accommodated in the top part of the instrument unit. The lower part of the instrument unit contains the remaining digital and analogue electronics and the interfaces to the spacecraft.

The dust intake system consists of a dust cover at the outside and a funnel. The cover is opened after launch by a pyro actuator. The path of the dust particles leads through a funnel through the spacecraft skin. The inner edge of the funnel points towards the entrance slit of the main instrument box, with some minimum clearance towards the box. The particles can enter the AFM via a slit. Beneath the slit there is a shutter which can be opened and closed in order to control the optimum exposure time of the facets on the dust collector; 64 facets, with an area of about 3.5 mm² each, are located on the circumference of the collector wheel. This surface area is defined with a diaphragm located between the slit and the facet. After exposure, the facet rotates from the position behind the slit to the analysis position.

 
Rosetta Blog articles
 

MIRO: Microwave Instrument for the Rosetta Orbiter

MIRO will investigate the nature of the cometary nucleus, outgassing from the nucleus and development of the coma. MIRO is configured both as a continuum and a very high spectral resolution line receiver. Centre-band operating frequencies are near 188 GHz (1.6 mm) and 562 GHz (0.5 mm). Spatial resolution of the instrument at 562 GHz is approximately five metres at a distance of two kilometres from the nucleus; spectral resolution is sufficient to observe individual, thermally broadened, line shapes at all temperatures down to 10 K. Four key volatile species - H₂O, CO, CH₃OH, and NH₃ and the isotopes H₂¹⁷O and H₂¹⁸O - are pre-programmed for observation. The primary retrieved products are the abundance, velocity, and temperature of each species, along with their spatial and temporal variability. This information will be used to infer the structure of the coma and its creation processes, including the nature of the nucleus/coma interface.

MIRO will sense the subsurface temperature of the comet nucleus to depths of several centimetres or more using the continuum channels at millimetre and submillimetre wavelengths. Model studies will relate these measurements to electrical and thermal properties of the nucleus and address issues connected to the sublimation of ices, ice and dust mantle thickness, and the formation of gas and dust jets. The global nature of these measurements will allow in situ lander data to be extrapolated globally, while the long duration of the mission will allow us to follow the time variability of surface temperatures and gas production. Models of the thermal emission from comets are very crude at this time since they are only loosely constrained by available data. MIRO will offer the first opportunity to gather subsurface temperature data that can be used to test thermal models. MIRO complements the IR mapping instrument on the orbiter, having a similar spatial resolution but greater depth penetration.

Science Objectives

The five objectives of the MIRO instrument are to:

• Characterise 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 kilometres 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 volatilisation and transport in the nucleus to quantify the intrinsic abundances of volatiles within the nucleus.
   

• 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 investigation will use correlated measurements of outgassing rates and nucleus thermal properties to test models of gas formation, transport, and escape from the nucleus to advance our understanding of the important processes leading to nucleus devolatilisation.
   

• 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.
   

• Globally characterise the nucleus subsurface to depths of a few centimetres or more.

  The MIRO instrument will map the nucleus and determine the subsurface temperature distribution to depths of several centimetres 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.
   

• 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.
   

Instrument Description

The MIRO experiment will acquire both high resolution molecular line spectra in absorption and emission, and broadband continuum emission data from which gas abundances, velocities, temperatures, and nucleus surface and subsurface temperatures will be derived.

The MIRO instrument is composed of a millimetre wave mixer receiver operating with a centre-band frequency of 188 GHz, and a submillimetre heterodyne receiver operating with a centre-band frequency of 562 GHz. The two receivers are fixed tuned. The submillimetre wave receiver provides both broad band continuum and high resolution spectroscopic data, whereas the millimetre wave receiver provides continuum data only.

The submillimetre wave spectroscopic frequencies allow simultaneous observations of six molecules which are known constituents of comets. The submillimetre wave lines observed include the ground-state rotational transition of water 1(10)-1(01) at 557 GHz, the corresponding lines of two oxygen isotopes of water, and the 572 GHz ground state rotational line of ammonia J(1-0). Since these lines are ground-state transitions (between the lowest rotational levels of these molecules), they are expected to be the strongest in cometary conditions. The submillimetre spectrometer can also observe the CO J(5-4) line, and three methanol lines.

The millimetre and submillimetre wavelength continuum channels will sense the subsurface temperature of the nucleus to depths of several centimetres or more. Model studies will relate these measurements to electrical and thermal properties of the nucleus and address issues connected to the sublimation of ices, ice and dust mantle thickness, and the formation of gas and dust jets.

 
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OSIRIS: Optical, Spectroscopic, and Infrared Remote Imaging System

OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) is a dual camera imaging system operating in the visible, near infrared and near ultraviolet wavelength ranges. OSIRIS consists of two independent camera systems sharing common electronics. The narrow angle camera is designed to produce high spatial resolution images of the nucleus of the target comet. The wide angle camera has a wide field of view and high straylight rejection to image the dust and gas directly above the surface of the nucleus of the target comet. Each camera is equipped with filter wheels to allow selection of imaging wavelengths for various purposes. The spectroscopic and wider band infrared imaging capabilities originally proposed and incorporated in the instrument name were descoped during development.

Science Objectives

The OSIRIS science objectives for the comet nucleus, the gases and dust produced by the comet and for the asteroid flybys are:

Instrument Description

OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) is a dual camera imaging system operating in the visible, near infrared and near ultraviolet wavelength ranges. OSIRIS consists of two independent camera systems sharing common electronics. The narrow angle camera is designed to produce high spatial resolution images of the nucleus of the target comet. The wide angle camera has a wide field of view and high straylight rejection to image the dust and gas directly above the surface of the nucleus of the target comet. Each camera is equipped with filter wheels to allow selection of imaging wavelengths for various purposes.

The OSIRIS experiment comprises the two cameras, two sensor readout electronics boxes, the OSIRIS common electronics box and a set of interconnecting harnesses.

The cameras are mounted on the outside of the spacecraft and thermally isolated from it. The sensor readout electronics boxes are located inside the spacecraft, each as close as possible to its camera to minimise harness length. The central electronics box is located elsewhere inside the spacecraft with a dedicated cooling louvre on the outside panel surface.

 
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ROSINA: Rosetta Orbiter Spectrometer for Ion and Neutral Analysis

ROSINA, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, is a combination of two mass spectrometers and a pressure sensor. The mass spectrometers will determine the composition of the comet's atmosphere and ionosphere, measure the temperature and bulk velocity of the gas and ions, and investigate reactions in which they take part. The ROSINA pressure sensor is capable of measuring both total and ram pressure, and will be used to determine the gas density and rate of radial gas flow.

Science Objectives

The primary measurement objective of the ROSINA investigation is to determine the composition of the atmospheres and ionospheres of comets, the temperature and bulk velocity of the gas and ions and the homogenous and inhomogeneous reactions of gas and ions in the dusty cometary atmosphere and ionosphere. In determining the composition of the atmospheres and ionospheres of comets, the following scientific objectives will be achieved:

• Determination of the global molecular, elemental, and isotopic composition and the physical, chemical and morphological character of the cometary nucleus
• Identification of the processes by which the dusty cometary atmosphere and ionosphere are formed and to characterize their dynamics as a function of time, heliocentric and cometocentric position
• Investigation of the origin of comets, the relationship between cometary and interstellar material and the implications for the origin of the solar system
• To investigate possible asteroid outgassing and establish what relationship, if any, exists between comets and asteroids

Instrument Description

To accomplish these scientific objectives, the ROSINA has unprecedented capabilities, including:

• Very wide mass range - from 1 amu (hydrogen) to more than 300 amu (organic molecules)
• Very high mass resolution - able to resolve CO from N₂ and ¹³C from ¹²CH
• Very wide dynamic range and high sensitivity, to accommodate large differences in ion and neutral gas concentrations and large changes in ion and gas flux as the comet approaches perihelion
• Ability to determine cometary gas and ion outflow flow velocities and temperatures

No single instrument could have the capabilities required to accomplish the ROSINA science objectives, so a three-sensor approach has been adopted. Each sensor is optimised for a part of the scientific objectives, while at the same time complementing the other sensors. The three sensors are: the Double Focusing Mass Spectrometer (DFMS), the Reflectron Time of Flight Spectrometer (RTOF), and the Comet Pressure Sensor (COPS).

Double Focusing Mass Spectrometer

The Double Focusing Mass Spectrometer (DFMS) is a high-resolution mass spectrometer (resolution m/Δm greater than 3000 at 1% peak height) with a high dynamic range and a good sensitivity.

The DFMS has two operation modes: a gas mode for analysing cometary gases and an ion mode for measuring cometary ions. Switching between the gas and ion modes only requires changing some of the potentials in the ion source and suppression of the electron emission that is used to ionise the gas. All other operations are identical for the two modes.

The three main parts of the DFMS are the ion source, the analyser and the detectors. The instrument is housed in a vacuum-tight enclosure and has been thoroughly degassed by baking and was launched under vacuum. The ion source region will be opened during the cruise phase to the comet by removing the protective cap. At the same time the analyser section vent, pointing at free space, away from the comet and the ion source will also be opened.

Reflectron Time of Flight Spectrometer

The mass analysis in the Reflectron Time-of-Flight (RTOF) sensor is performed using the time-of-flight technique. This technique allows the combination of extremely high mass resolution (m/Δm = 3000 at 50% peak height) and time resolution (theoretically limited by the extraction frequency of 10 kHz). The instantaneous recording of the whole mass range (1 to 1000 amu) is possible.

The sensor consists of four parts: the ion sources, the drift tube, the electronics and a closable cover. Two ion sources are used to produce and extract the ions into the drift path. One ion source is optimised for neutral particles, which are ionised via electron impact (gas mode); the other source is optimised for the direct measurement of cometary ions (ion mode). For redundancy reasons both sources can be operated in either ion or gas mode.

The drift tube is the time-of-flight path for the particles. The ions originating from the ion sources are extracted with a short start pulse and accelerated using a constant voltage to a fixed energy. After this acceleration process, the particles drift through the tube, with the heavier ions drifting more slowly than the lighter ions. The ions reach the microchannel plate detector (MCP) where they generate another pulse (stop pulse). The time difference between start and stop pulses is used to directly calculate the mass of the ion.

The electronics generates the various voltage supplies needed for the instrument components, measure and report housekeeping data, control the cover motor and measure the exact time-of-flight of the ion packets. In order to achieve a high mass resolution, the measurement of the time difference between the start and the stop pulse has to be very accurate. The ROSINA electronics offers a timing resolution of 550 pS.

The cover protects the ion sources and sensitive microchannel plate detectors (MCPs) from contamination during ground handling and launch and also from the gases released by the spacecraft thrusters during manoeuvres.

Comet Pressure Sensor

The Comet Pressure Sensor (COPS) instrument consists of two sensors dedicated to measurement of the neutral gas parameters around the comet, primarily the total density and the radial flow.

The total density, or pressure, is measured using a Bayard-Alpert type gauge mounted at the end of a boom. The pressure measurements will improve existing models of the inner coma and will be used to protect other Rosetta instruments. If the pressure rises too high for correct operation of the instruments, for example during close approaches to the comet, they will be shut down.

The molecular flow from the comet, referred to as the ram pressure, is measured using an extractor-type gauge situated in an equilibrium chamber, a spherical chamber whose opening is facing the comet.

 
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RPC: Rosetta Plasma Consortium

RPC, the Rosetta Plasma Consortium, is a set of five instruments sharing a common electrical and data interface with the Rosetta orbiter. The RPC instruments are designed to make complementary measurements of the plasma environment around comet 67P/Churyumov-Gerasimenko.

Science Objectives

RPC is intended to investigate the following scientific areas of interest:

• 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.

Instrument Description

RPC consists of five sensors:

• Ion Composition Analyser (ICA)
• Ion and Electron Sensor (IES)
• Langmuir Probe (LAP)
• Fluxgate Magnetometer (MAG)
• Mutual Impedance Probe (MIP),
  
  as well as a joint
   
• Plasma Interface Unit (PIU)
  
  acting as instrument control, spacecraft interface, and power management unit.

Ion Composition Analyser

The Ion Composition Analyser (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 analyser 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.

RELATED LINK

ICA at the Swedish Institute of Space Physics

Ion and Electron Sensor

The Ion and Electron Sensor (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 analysers, 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 90 × 360 degrees.

Langmuir Probe

The Langmuir Probe (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.

RELATED LINK

LAP at the Swedish Institute of Space Physics

Fluxgate Magnetometer

The Magnetometer experiment (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 metre 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 minimised.

MAG will also study any magnetic field possessed by the comet nucleus, in cooperation with the ROMAP magnetometer experiment on the Rosetta lander.

RELATED LINK

MAG at the Technical University of Braunschweig

Mutual Impedance Probe

The 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.

RELATED LINK

MIP at Laboratoire de Physique et Chimie de l'Environnement

Plasma Interface Unit

The 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.

RELATED LINK

PIU at Imperial College

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RSI: Radio Science Investigation

The 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.

RSI will investigate the nondispersive frequency shifts (classical Doppler) and dispersive frequency shifts (due to the ionised 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)
  -	Dielectric constant and roughness of the surface (from bistatic radar experiment)
  -	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 Description

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 Ultra-Stable Oscillator (USO) 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 Deep Space Network (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.

 
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VIRTIS: Visible and Infrared Thermal Imaging Spectrometer

VIRTIS is an imaging spectrometer that combines three data channels in one instrument. Two of the data channels are committed to spectral mapping and are housed in the Mapper (-M) optical subsystem. The third channel is devoted solely to spectroscopy and is housed in the High resolution (-H) optical subsystem.

Science Objectives

The primary scientific objectives of the VIRTIS during the Rosetta mission are:

• To study the cometary nucleus and its environment
• Determine the nature of the solids of the nucleus surface
• Identify the gaseous species
• Characterise the physical conditions of the coma
• Measure the temperature of the nucleus

Secondary objectives include helping with the selection of landing sites and providing support to other instruments.

Tertiary objectives include detection and characterisation activities during asteroid flybys.

Instrument Description

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.

Mapping channel

The mapping channel optical system is a Shafer telescope matched through a slit to an Offner grating spectrometer. The Shafer telescope consists of five aluminium mirrors mounted on an aluminium 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.

High resolution channel

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.

 
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