The mission

MAJIS is the acronym for Moons And Jupiter Imaging Spectrometer. It is one of the science instruments of the ESA L-Class mission JUICE (Jupiter ICy Moons Explorer).  It will be launched in 2022 from Kourou on an Ariane 5 rocket and will travel 7.5 years through the Solar System before arriving at Jupiter in 2031. JUICE (Figure 1) will perform detailed observations of the Jovian System, including Jupiter and the Galilean satellites.

Figure 1. Schematic view of the JUICE satellite and its scientific payloads (ESA credit).


The MAJIS spectrometer combines two different spectral channels: the VIS-NIR channel from 0.5 to 2.35 µm and the IR channel from 2.25 to 5.54 µm, with spectral resolutions of 3.66 nm and 6.51 nm, respectively. The MAJIS detectors consists of a Teledyne H1RG HgCdTe array of 1024 x 1024 pixels. MAJIS is a French instrument under the scientific and technical responsibility of the Institut d'Astrophysique Spatiale (IAS, Orsay), the supervision of the Centre National d'Etudes Spatiales (CNES) and the Agenzia Spaziale Italiana (ASI).

Science objectives

Thanks to both the VIS-NIR and IR spectrometers, the main science objectives of the MAJIS spectrometer are to obtain detailed information about Jupiter and the Galilean satellites. More specifically, MAJIS will be involved in the study of:

  • the composition and physical properties of the icy moon surfaces of Ganymede, Europa and Callisto (ices, salts, minerals, organic compounds),
  • the composition, structure, and spatial and temporal variability of the exospheres of Ganymede, Europa and Callisto,
  • the composition, structure, dynamics and evolution of Jupiter's atmosphere from the troposphere to the stratosphere,
  • the composition and physical properties of Io, small moons, rings and dust in the Jupiter system.

BIRA-IASB objectives

The MAJIS detectors must be radiometrically characterized before being integrated on the spacecraft to ensure a proper data processing of the signals that will be collected by the instrument during the mission. In this way, it will be possible to assure the performances of the detectors within the in-flight operation, by reproducing high-vacuum and cryogenic conditions in a safe controlled environment. A Memorandum Of Understanding (MOU) was signed in 2015 between Belgium and France for MAJIS. Following this, the Royal Belgian Institute for Space Aeronomy (BIRA-IASB) and the Royal Observatory of Belgium (ROB) became responsible for the complete characterization of the spare (SM) and flight models (FM) of the MAJIS VIS-NIR detectors. The funding was supported by BELSPO, ESA PRODEX Office and ESA JUICE mission. It is worth to mention that the project was also supervised by the Belgian User Support and Operations Center (B.USOC) as the Product and Quality Assurance (PA/QA) of the measurement campaigns. Different properties were analyzed in order to properly characterize the response of the MAJIS VIS-NIR detectors. A summary of the measurements performed is available in Figure 2.

Figure 2Summary of the characterization measurements for the MAJIS VIS-NIR detectors.

Characterization facility

The facility to characterize the MAJIS VIS-NIR detectors was developed at BIRA-IASB in 2019, and fully validated in early 2020. This facility is located in an ISO-5 (class 100) certified cleanroom, which is composed of a large vacuum chamber coupled to a cryo-cooling system, an optical equipment external to the vacuum chamber, and a high-level security system (Figure 3). A second ISO-7 area is available to prepare the equipment to be integrated into the cleanroom.

Figure 3. General view of the VIS-NIR facility (left). MAJIS VIS-NIR detector integrated in the vacuum chamber, ready for a cooling-cycle (right).

The facility built at BIRA-IASB is a complete system able to:

  • Control the FPU in safe conditions inside a Vacuum Chamber (VC) by means of the Electrical Ground Support Equipment (EGSE, provided by IAS) and a security unit,
  • Explore the operating temperature range (116 K – 160 K) under vacuum,
  • Fully characterize the MAJIS VIS_NIR detectors under key configurations driven by the Optical Ground Support Equipment (OGSE), developed by BIRA-IASB.

A dedicated detector mount was developed for the MAJIS VIS-NIR project to properly thermalize the detector but also to provide options to characterize in dark and light conditions thanks to the different movable plate positions (Figure 4).

Figure 4Schematic of the detector mount and its associated components.

The opto-mechanical facility developed to characterize the MAJIS VIS-NIR detectors is constituted by three main blocks (Figure 5):

  • Light entrance: a 1000 W QTH lamp produces a stable radiance with a continuum including the MAJIS VIS-NIR spectral range (400 nm – 2600 nm). Moreover, by combining different neutral density filters, up to 30 different illumination levels can be provided to the detector. For latency measurements, an electronic shutter is especially included in this block.
  • Spectrometer: It consists in a double monochromator and its electronics for phase-sensitive or unmodulated detection. The spectrometer includes its own internal filter wheel for second order rejection. The monochromatic beam obtained thanks to the illumination of the entrance slit by the tungsten lamp is directed to the third block by an optical fiber.
  • Detection: This block is constituted by the integrating sphere and the vacuum chamber. Absolute radiometry, for quantum efficiency measurements, was performed using calibrated Silicon (Si) and Lead Sulfide (PbS) photodiodes connected to the IS. Thanks to measurement equation and validated parameters, it was possible to characterize the absolute flux and homogeneity illumination on the detector surface.
OptoMechanicalEquip min
Figure 5. Opto-mechanical equipment of the MAJIS VIS-NIR facility: 1) light entrance, 2) spectrometer, and 3) detection blocks.

Moreover, the optical system disposes of three distinct configurations that have been conceived to perform different measurement conditions (temperature, illumination, exposure time, beam uniformity, …), and therefore achieve the complete characterization of the MAJIS VIS-NIR detectors. The three optical configurations available for the MAJIS VIS-NIR characterization are:

  1. Dark conditions.
  2. Light conditions with beam uniformity.
  3. Light conditions with 11° of beam convergence that reproduces the convergence of the light rays that are focalized on the detector inside the MAJIS VIS-NIR spectrometer, which also includes the use of a Linear Variable Filter (LVF).

Characterization campaigns and results

FM and SM characterization campaigns were performed in July/September 2020 and May 2021, respectively. The first results demonstrated a nominal characterization of the MAJIS VIS-NIR detectors. The whole MAJIS project has been a success and a great achievement for the BIRA-IASB team. Some results of the FM campaign are available here below.

Figure 6Example of acquisitions under monochromatic illumination to verify the alignment of the LVF with respect to the detector: a) 1020 nm to visualize two fiducial markers, b) 1765 nm to visualize the three fiducial markers.
Linearity DIT2
Figure 7Results from linearity measurements VS integration time performed during the characterization campaign without LVF (detector at 125 K): a) Image acquisition in slow mode, b) Image acquisition in fast mode.


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