Main Scientific objectives of VIRTIS
The VIRTIS instrument on board the Rosetta spacecraft is a part of the orbital payload focusing on studies of the comet nucleus and coma composition. The main scientific objectives are analyses of the ices, minerals and organics on the cometary nucleus, the thermal characteristics of the surface material, the development of cometary activity and texture of the surface material. During the observation phase of the early studies the investigation of the comet by the instrument will contribute to the selection of the landing place for the Rosetta Lander. As fare as VIRTIS enables observations with high spatial resolution (VIRTIS-M) as well as with high spectral resolution (VIRTIS-H) the surface material can be determined straightforward and mapped over the cometary surface. After the setup of cometary activity VIRTIS continue its observation by studies of the cometary coma. The aim of these studies is determination of parent and daughter molecules in the cometary coma. During the cruise phase VIRTIS will map the surface mineralogy of asteroids in flight by scenarios.
With these studies the observations of VIRTIS will contribute to answer the major questions of evolution of the solar system, the formation of comets and their contributions to the origin of life.
The Instrument
VIRTIS, the V isible and I nfrared T hermal I maging S pectrometer for the ESA/Rosetta and Venus Express missions, is a scientific instrument devoted to multispectral analysis of a cometary nucleus and its environment. Rosetta will observe the Comet Churyumov-Gerasimenko in 2014, 10 years after launch in February 2004.
The VIRTIS-M channel is an imaging spectrometer ( M apper optical subsystem: -M) from 0.25 to 5µm at medium spectral resolution, and the VIRTIS-H channel ( H igh resolution optical subsystem:-H) for high spectral resolution from 2 to 5µm. The optics is passively cooled down to 130K and the detector is actively cooled down to 70K.
The instrument is composed of four separate hardware modules, the Optics Module ( OM ), two Proximity Electronics Modules (PEMs) and the Main Electronics Module (ME). The Optics Module contains the three infrared and visible detectors, the active coolers for cooling down the IR detectors, covers for protecting the optical entrance including emergency cover actuators, a scan mirror for expanding the field of view in a scan mode, shutters, calibration devices, decontamination and annealing heaters. Two separate Proximity Electronic Modules (PEM) contain the electronics either for the –M or –H channel data read-out and device control with interfaces to the Optics and Main Electronics modules.
German contribution for VIRTIS
DLR provides the Main Electronics and on-board software for the scientific instrument VIRTIS. It has been developed by DLR with partners Kayser Threde/Munich, EADS/Astrium GmbH/Munich, IB Ulmer/Frankfurt/Oder, Institute of Planetary Research, Laben/Italy.
The Main Electronics module provides two Data Processing Units (DPUs) including the telecommand and telemetry interfaces to the Spacecraft, the power supply and power distribution unit for the whole instrument, the cross-strapping and interface electronics between the DPUs and the –M and –H channels as well as two Cooler Control Electronics. The main or redundant DPU and its power converter operate exclusively (cold redundancy) while the –M or –H channel dedicated electronics is powered-on when the –M and/or –H channel operates.
All instrument software is installed inside the Main Electronics. No additional de-central processor units are implemented in VIRTIS. This implies a special focus on software complexity, performance and independency requirements as well as advantages of flexibility, operability and software maintenance. Identical on-board software is implemented on each DPU. If one DPU fails, VIRTIS can be operated without any performance degradation by means of redundant DPU and software.
Main functions of the software are scientific data processing as well as instrument control of the two instrument sub-systems VIRTIS-M and –H. Due to the very low Rosetta telemetry bandwidth of 5….20kbit/s reducing of the data volume and increasing the scientific data output is the main goal of data processing. Controlling of the three detector channels, two cover stepper motors and emergency actuators, two active coolers, two shutters, a scan unit, and annealing heater implies partially strong real time and safety aspects to be considered.
A safe management of the start-up procedure after power-on VIRTIS and the in-flight maintenance of the application software by means of up- and download are essential features to ensure the VIRTIS performance over the 10 years mission life cycle.
According to operation planning VIRTIS has to be driven in certain instrument modes getting different telemetry data rate or instrument power consumptions. The software has the task to detect and execute the different instrument modes and to ensure that only allowed mode transitions are possible to be commanded.
In order to ensure the “visibility” of VIRTIS performance health checking, event and error handling are necessary to detect instrument anomalies as well as inconsistent instrument commanding. The software receives, verifies and executes all telecommands, formats and transfers different type of telemetry to the spacecraft following ESA standard PSS-04-107/106. It guarantees a telecommanding and telemetry data consistency from the on-ground mission operation center up to the instrument on the spacecraft.
German science contributions
The German science contributions to the experiment include studies of the nucleus mineral composition and especially the analysis of the nature of organic material described in the following separate section (Moroz, Arnold, Wäsch). The studies of relevant organic materials led by the German team performed in cooperation with several German, Russian, Italian, Ukrainian, and French institutions. This work requires laboratory spectral measurements of relevant material. The German group provides a spectral library to the team. For the data analysis compositional models are developed being the base for the cometary nucleus composition analysis. Furthermore, the group will contribute to the asteroid surface composition analysis.
In addition several cometary models have been studied and they are discussed below in another separate section of this documentation.