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.
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
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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