Engineering

SPICAV-SOIR

2005: Compact high-resolution spectrometer, developed for ESA’s Venus Express spacecraft, that operates in the IR wavelength range.

SOIR (Solar Occultation in the InfraRed) is a compact high-resolution spectrometer developed for ESA’s Venus Express (VEX) spacecraft. The mission was launched on November 9th 2005. The mission was declared complete on December 16th 2014.

SPICAV-SOIR is a spectrometer aiming at the measurement of the atmospheric composition of Venus. It operates in the IR wavelength range between 2.2 µm and 4.3 µm and uses an echelle grating, in combination with an Infra-Red (IR) Acousto-Optical Tunable Filter (AOTF) for order sorting and an actively cooled HgCdTe detector.

Launch: November 9, 2005

​Mechanical integration of SPICAV-SOIRMechanical integration of SPICAV-SOIR

Mission concept

For the ESA Venus EXpress (VEX) mission to planet Venus a compact IR spectrometer that has low mass, high throughput, rather high resolution (10 times better than the Planetary Fourier Spectrometer on board Mars-Express), a rather large free spectral range (20 cm-1), and a spectral sampling interval and an instrument line profile of 0.1 resp. 0.2 cm-1, were envisaged. These requirements can be fulfilled by combining a highly dispersive element operated at high diffraction orders (a coarse-spaced echelle grating) with a diffraction order sorting device.

Order filtering can be achieved by inserting an in-line acousto-optic device, known as an Acousto-Optical Tunable Filter (AOTF) that may be considered an electronically tunable constant-bandwidth band pass filter.

Venus-Express spacecraft with SPICAV-SOIRVenus-Express spacecraft with SPICAV-SOIR

The working principle of the echelle–AOTF combination has been demonstrated in two implementations:

  • one covering the wavelength range from 1 to 1.65 µm with a resolving power of 30,000, manufactured, assembled, and tested by a joint French–Russian team [Service d’Aéronomie and the Russian Space Research Institute (IKI)], and
  • another that was intended as a precursor for the space-borne IR spectrometer, built by the BIRA-IASB Engineering service and its industrial partner OIP Sensor Systems (OIP), Oudenaarde, Belgium.

The latter is an echelle-based spectrometer with AOTF order sorting based on the same basic principles but specifically designed for the 2.2 to 4.3 µm range in the IR. This high-resolution spectrometer, called Solar Occultation in the IR (SOIR), was accepted as part of the scientific payload for the VEX mission of ESA.

This spectrometer fits within the scientific mission themes of the ESA VEX mission, such as the study of the dynamics, structure, composition, and chemistry of the Venusian atmosphere, and is highly appropriate for the investigation of the altitude range from 110 to 60 km. The application in the Venusian atmosphere of the solar occultation technique, for which this instrument is particularly designed, is limited because of the existence between altitudes from 45 to 70 km of thick cloud layers composed of fine droplets of sulfuric acid.

However, above these cloud layers the sounding of the atmosphere and the measurement of the vertical abundance profiles of atmospheric constituents is perfectly feasible up to high altitudes.

SPICAV-SOIR flight spare modelSPICAV-SOIR flight spare model

One of the key scientific themes of the VEX mission is the understanding of the past and present water balance in the Venusian atmosphere. The study of the water escape from Venus by means of the measurement of HDO and H2O profiles is therefore of utmost importance. Absorption lines of HDO and H2O of the dark side of Venus have been observed from Earth in the interval 2.34 to 2.43 µm with a resolving power of 0.23 cm-1, which is comparable to the resolving power offered by the instrument described. These results suggest pronounced deuterium enrichment in the atmosphere of Venus, expressed by an abundance ratio D/H that is approximately 120 times larger than the telluric ratio. The measurement of the HDO escape profile from 60 to 80–90 km altitude by SOIR could confirm or refute this ground based observation.

The first opportunity to design and manufacture an echelle-based high-resolution spectrometer with AOTF order sorting for the 2.2 to 4.3 µm range in the IR appeared when the ESA VEX mission was announced. The payload on board the Venus Express spacecraft comprises mainly upgraded versions of instruments already flown on the Mars EXpress (MEX) and Rosetta platforms, with some new instruments added. The SOIR instrument is to be considered a new extension on top of the existing SPICAM-L experiment flown on MEX. The combination of both is called SPICAV.

In accordance with the scientific objectives, the SOIR design was tuned for solar occultation observations of the Venusian atmosphere in the 2 to 5 µm band, a requirement that was relaxed to the wavelength range of 2.325 to 4.25 µm because of limitations imposed by the optics and the detector.

A second requirement was to achieve a spectral resolution that was approximately 10 times better than could be reached before for equal size and mass. An Instrument Line Profile (ILP), with a FWHM, of 0.2 cm-1 and a sampling interval of 0.1 cm-1 per pixel over the whole spectral range, were therefore put forward. This allows deriving the density profile of already known atmospheric constituents at higher altitudes than feasible before. It also allows the detection of yet unknown atmospheric constituents.

Electronic testing of SPICAV-SOIRElectronic testing of SPICAV-SOIR

The application of the Sun as a bright light source for the solar occultation technique determined the Field Of View (FOV) of the instrument. It constrains which rays emitted from the solar disk and refracted in the Venusian atmosphere will finally enter the spectrometer and reach the detector. Because, at Venus, the Sun has an apparent diameter of 44 arcmin, the FOV in the spatial direction was limited to 30 arcmin to prevent solar limb darkening effects. This choice is also compatible with observing the Sun (apparent diameter, 32 arcmin) during ground-based tests.

For the spectral direction, perpendicular to the spatial direction, a FOV of 2 arcmin was considered to allow for good height resolution during the inversion of the Venus occultation data. Hence, a rectangular field of view of 2 by 30 arcmin was defined. The attitude of the VEX spacecraft is controlled during an occultation observation such that the bore sight of the SOIR instrument is pointing to the Sun and that the long side of the rectangular FOV remains parallel to the planet’s limb.

Aerobreaking was done at the end of the life of the spacecraft so that minimal science would be lost if something went wrong. In 2014 the aero-braking phase was successfully executed, followed by the continuation of the scientific measurements that lasted until the end of November 2014 before the satellite ran out of propellant. This was one of ESA’s first attempts at aerobreaking. Without propellant, however, it was no longer possible to control the attitude and orient VEX towards Earth to maintain communications. It was also impossible to raise the altitude further, meaning that the spacecraft naturally sunk deeper into the atmosphere and crashed on the Venusian surface.

The SOIR instrument has performed over 750 solar occultation measurements over the course of eight years. The whole mission is considered a huge success and the SOIR instrument provided extremely valuable data to the scientific community.

The end of mission operations for Venus Express was declared on the 18th of December 2014, followed by a six month rundown phase for finalization of operational and archiving activities.

The Engineering service at BIRA-IASB was responsible for planning of all operational aspects related to the SOIR instrument.
Electronic testing of SPICAV-SOIR Electronic testing of SPICAV-SOIR.

 

Publications

  • Trompet, L.; Mahieux, A.; Ristic, B.; Robert, S.; Wilquet, V.; Thomas, I.R.; Vandaele, A.C.; Bertaux, J.-L. (2016), “Improved algorithm for the transmittance estimation of spectra obtained with SOIR/Venus Express”, Applied Optics, Vol. 55, Issue 32, 9275-9281, DOI: 10.1364/AO.55.009275
  • Vandaele, A.C.; Mahieux, A.; Chamberlain, S.; Ristic, B.; Robert, S.; Thomas, I.R.; Trompet, L.; Wilquet, V.; Bertaux, J.L. (2016), “Carbon monoxide observed in Venus' atmosphere with SOIR/Vex”, Icarus, Vol. 272, 48-59, DOI: 10.1016/j.icarus.2016.02.025
  • Vandaele, A.C.; Chamberlain, S.; Mahieux, A.; Ristic, B.; Robert, S.; Thomas, I.; Trompet, L.; Wilquet, V.; Belyaev, D.; Fedorova, A.; Korablev, O.; Bertaux, J.L. (2016), “Contribution from SOIR/VEX to the updated Venus International Reference Atmosphere (VIRA)”, Advances in Space Research, Vol. 57, 443-458, DOI: 10.1016/j.asr.2015.08.012

  • Vandaele, A.C.; Mahieux, A.; Chamberlain, S.; Ristic, B.; Robert, S.; Thomas, I.R.; Trompet, L.; Wilquet, V.; Bertaux, J.L. (2015), “A compilation of all CO observations performed by SOIR during the Venus Express mission”, EPSC Abstracts, Vol. 10, EPSC2015-106
  • Vandaele, A.C.; Mahieux, A.; Robert, S.; Berkenbosch, S.; Clairquin, R.; Drummond, R.; Letocart, V.; Neefs, E.; Ristic, B.; Wilquet, V.; Colomer, F.; Belyaev, D.; Bertaux, J.-L. (2013), “Improved calibration of SOIR/Venus Express spectra”, Optics Express, Vol. 21, Issue 18, 21148-21161, DOI: 10.1364/OE.21.021148
  • Drummond, R.; Vandaele, A.-C.; Daerden, F.; Fussen, D.; Mahieux, A.; Neary, L.; Neefs, E.; Robert, S.; Willame, Y.; Wilquet, V. (2011), “Studying methane and other trace species in the Mars atmosphere using a SOIR instrument”, Planetary and Space Science, Vol. 59, Issue 2-3, 292-298, DOI: 10.1016/j.pss.2010.05.009

  • Mahieux, A.; Vandaele, A.C.; Neefs, E.; Robert, S.; Wilquet, V.; Drummond, R.; Federova, A.; Bertaux, J.L. (2010), “Densities and temperatures in the Venus mesosphere and lower thermosphere retrieved from SOIR on board Venus Express: Retrieval technique”, Journal of Geophysical Research E: Planets, Vol. 115, Issue 12, E12014, DOI: 10.1029/2010JE003589
  • Drummond, R.; Vandaele, A.C.; Neefs, E.; Mahieux, A.; Wilquet, V.; Montmessin, F. (2008), “Detecting Trace Species on Mars with a SOIR Instrument”, Third International Workshop on The Mars Atmosphere: Modeling and Observations, November 10-13, 2008, Williamsburg, VA, USA, 9044
  • Mahieux, A.; Berkenbosch, S.; Clairquin, R.; Fussen, D.; Mateshvili, N.; Neefs, E.; Nevejans, D.; Ristic, B.; Vandaele, A.C.; Wilquet, V.; Belyaev, D.; Fedorova, A.; Korablev, O.; Villard, E.; Montmessin, F.; Bertaux, J.-L. (2008), “In-flight performance and calibration of SPICAV SOIR onboard Venus Express”, Applied Optics, Vol. 47, Issue 13, 2252-2265, DOI: 10.1364/AO.47.002252

  • Korablev, O.; Fedorova, A.; Bertaux, J.-L.; Stepanov, A.V.; Kiselev, A.; Kalinnikov, Yu.K.; Titov, A.Yu.; Montmessin, F.; Dubois, J.P.; Villard, E.; Sarago, V.; Belyaev, D.; Reberac, A.; Neefs, E. (2012), “SPICAV IR acousto-optic spectrometer experiment on Venus Express”, Planetary and Space Science, Vol. 65, Issue 1, 38-57, DOI: 10.1016/j.pss.2012.01.002
  • Marcq, E.; Belyaev, D.; Montmessin, F.; Fedorova, A.; Bertaux, J.-L.; Vandaele, A.C.; Neefs, E. (2011), “An investigation of the SO2 content of the venusian mesosphere using SPICAV-UV in nadir mode”, Icarus, Vol. 211, Issue 1, 58-69, DOI: 10.1016/j.icarus.2010.08.021
  • Bertaux, J.-L.; Nevejans, D.; Korablev, O.; Villard, E.; Quémerais, E.; Neefs, E.; Montmessin, F.; Leblanc, F.; Dubois, J.P.; Dimarellis, E.; Hauchecorne, A.; Lefèvre, F.; Rannou, P.; Chaufray, J.Y.; Cabane, M.; Cernogora, G.; Souchon, G.; Semelin, F.; Reberac, A.; Van Ransbeeck, E.; Berkenbosch, S.; Clairquin, R.; Muller, C.; Forget, F.; Hourdin, F.; Talagrand, O.; Rodin, A.; Fedorova, A.; Stepanov, A.; Vinogradov, I.; Kiselev, A.; Kalinnikov, Yu.; Durry, G.; Sandel, B.; Stern, A.; Gérard, J.C. (2007), “SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere”, Planetary and Space Science, Vol. 55, Issue 12, 1673-1700, DOI: 10.1016/j.pss.2007.01.016


​Venus Express/SOIR project on the Planetary atmospheres group website