I. INTRODUCTION
The JUpiter ICy moons Explorer (JUICE) mission, launched on April 14, 2023, is the first large mission within the Cosmic Vision Program 2015–2025 of the European Space Agency.1,2 The orbiter will first orbit Jupiter to perform detailed investigations of Jupiter and its system in all their inter-relations during ∼3.5 years. A second orbit insertion maneuver around Ganymede is scheduled in late 2034 to study this complex moon as a planetary body during a 130 days long orbital phase. Investigations of Europa and Callisto, during flybys, will make it possible to compare the characteristics of the three icy Galilean moons, addressing key scientific questions about their potential habitability. Among the set of ten scientific instruments, the Moons And Jupiter Imaging Spectrometer (MAJIS) is the imaging spectrometer that will provide hyperspectral images in the 0.5–5.56 μm range in 1016 nominal spectral channels at various spatial resolutions from tens of m to km scale during the closest approaches of satellites up to 100–150 km for Jupiter during the perijove. An extensive calibration campaign was performed before integrating MAJIS aboard the JUICE spacecraft. In addition, in-flight calibration acquisitions were acquired a few weeks after the launch. In this introduction, we present the context of the special collection of six papers dedicated to the calibration of the MAJIS instrument that provides key calibration results and a summary of the statement of compliance of the performances with the high-level instrument requirements.
II. OVERVIEW OF CALIBRATION ACTIVITIES
After the successful launch on April 14, 2023, followed by a 8.25 year long cruise and insertion into Jupiter’s orbit in July 2031, the JUpiter ICy moons Explorer (JUICE) spacecraft will start its scientific nominal mission by a Jovian tour of a ∼3.5 year duration. During this phase, it will focus on characterizing Jupiter’s atmosphere (circulation, meteorology, chemistry, and structure from the cloud tops up to the thermosphere) and magnetosphere (three-dimensional properties of the magnetodisc and coupling processes), and their interaction with the Galilean satellites. Specific observations of the three ocean-bearing worlds, Ganymede, Europa, and Callisto, will be also performed during tens of flybys to characterize these moons as planetary bodies and potential “habitats” for the two first.1 The study of the diversity of the satellite system will be enhanced by additional remotely sensed information on Io, the smaller moons, and the rings. During the second phase of the mission, Ganymede has been selected for enhanced investigation during a nine-month orbital phase starting late 2034. Thanks to the 11 JUICE scientific instruments, Ganymede will be investigated as an unprecedented target for deciphering the nature, evolution, and potential habitability of icy worlds, while enabling the study of its unique magnetic and plasma interactions with the surrounding Jovian environment. JUICE science objectives and an overview of the S/C and mission operations are described in detail in a series of articles published in the JUICE Space Science Review collection.3
The visible and infrared hyperspectral imager Moons And Jupiter Imaging Spectrometer (MAJIS) is one of the four remote sensing instruments on the JUICE payload. This set of instruments will perform remote sensing observations from UV to millimeter wavelengths to characterize the geology, the surface composition, and the exospheres of satellites as well as the Jovian atmosphere at high spatial and spectral resolutions.
The scientific objectives and the main parameters of MAJIS are described in detail in the study by Poulet et al.4 In the following, we briefly present the instrument and invite the reader to carefully read the study by Poulet et al.4 for additional information on the technical and scientific requirements of the instrument. The spectral range is covered by two channels: the VISNIR (VISible and Near-InfraRed) channel (0.5–2.35 μm) and the IR (InfraRed) channel (2.25–5.56 μm). MAJIS is a cryogenic instrument requiring the optics and the VISNIR focal plane detector to work at a temperature below 140 K and the IR detector below 95 K. This is achieved with a passive cooling scheme based on two dedicated radiators, one cooling the whole optical head (OH) and the other one directly connected to the IR detector by a thermal strap.4 H1RG detectors from Teledyne have been selected for the two focal plane arrays but having two different wavelength cutoffs. These detectors are 1024 × 1024 pixels in size with a pitch of 18 μm. The effective area for photon collection is constituted by 800 lines of 1016 pixels. A binning by 2 in the spatial direction is implemented, yielding an IFOV (Instantaneous Field Of View) of 150 μrad (corresponding to a pixel size of 36 μm) and a FOV of 60 mrad (full slit). The requirements nominal operating mode implements a binning by 2 in the spectral direction (508 spectral samples). In addition, dedicated radiation shielding on the most sensitive optical elements (including reflective and antireflection coatings) and electronics devices has been added so as to comply with the on the total ionizing dose (TID) and the total non-ionizing dose (TNID).
The calibration of the MAJIS instrument is critical for a reliable scientific interpretation of images and spectra returned from the Jovian system. This task required an in-depth characterization, calibration, and verification of instrumental performances, from single subsystems up to integrated system levels during assembly and commissioning according to the flowchart shown in Fig. 1:
Step 1. Sub-system level for sensitive units is to be integrated. The outcomes of these tests were described in the internal review documents and reports only, and they are not addressed in this collection.
Step 2. On-ground calibration on the fully integrated instrument is performed prior to its delivery for integration on the spacecraft (S/C). This step is the main purpose of this collection.
Step 3. S/C calibrations: measurements and tests are performed on the instrument when integrated on the S/C, prior to its launch; these activities were dedicated to thermal performances and EMI/EMC tests and are, therefore, not addressed in the collection.
Step 4. In-flight calibrations, both during commissioning (and later on, during the cruise and science operations). The analysis of these first measurements acquired after launch showed that there were minor evolutions compared to calibration, which are also presented in this collection.
MAJIS calibration flowchart. The special collection covers the outcome of the activities in green, red, and black rectangles. IAS, Institut d’Astrophysique Spatiale; LDO, Leonardo; ICU, Internal calibration unit; OH, Optical head; ME, Main electronics; FPU, Focal plane unit; FPA, Focal plane array.
MAJIS calibration flowchart. The special collection covers the outcome of the activities in green, red, and black rectangles. IAS, Institut d’Astrophysique Spatiale; LDO, Leonardo; ICU, Internal calibration unit; OH, Optical head; ME, Main electronics; FPU, Focal plane unit; FPA, Focal plane array.
During step No. 2, the MAJIS experiment has undergone two major on-ground test campaigns in order to characterize the instrument spectral, spatial, and radiometric responses as a function of various parameters that might evolve while MAJIS will be operating. The measurements were performed in a representative subset of operative modes. Key measurements were repeated at different temperatures of OH and FPA’s covering the expected operating range during science acquisitions for checking possible drifts in the instrumental response (spectral response, including overlapping between the two channels and radiometric response). Due to planning constraints at the instrument level, some measurements performed at the FPU and OH levels were not fully reproduced at this instrument level. As a secondary goal, the on-ground calibration provided data that helped validate operational procedures, functional performances of the ME (including the data handling, telemetry flow, data compressor, and commanding and operative modes) and further scientific reduction algorithms.
The first flight measurements (step No. 4) using the internal calibration unit were acquired shortly after launch during the commissioning phase with the following objectives:
To provide post-launch calibration verification with the aim to check in space the validity of the results obtained during ground calibration and take proper corrective actions, if needed.
To monitor the instrumental performance and detect any changes, particularly those associated with possible degradation by contamination, radiations, and aging of the components.
Given the complexity of the on-ground calibration activities, the large amount of data (including both ground and flight measurements) acquired with versatile setups and the complex data analyses, it was out of scope to address all these items in one paper. Therefore, a series of papers was initiated to detail the calibration activities, their analysis, and the retrieval of the key calibration parameters once the instrument was fully integrated (Table I).
List of the papers of this collection and their objectives.
First author . | Title . | Objectives . |
---|---|---|
Vincendon et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer): I. On-ground setup description and characterisation | Describe the calibration objectives, the major on-ground calibration setup, and the validation of its performances |
Filacchione et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). II. Spatial calibration | Present the (on-ground and in-flight) measurements dedicated to the spatial calibration, describe the analysis of data, and retrieve the spatial key parameters |
Haffoud et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). III. Spectral calibration | Present the (on-ground and in-flight) measurements dedicated to the spectral calibration, describe the analysis of data, and retrieve the spectral key parameters |
Langevin et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). IV. Radiometric calibration | Present the (on-ground and in-flight) measurements dedicated to the radiometric calibration, describe the analysis of data and retrieve the radiometric key parameters |
Rodriguez et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). V. Validation with mineral samples and reference materials | Present the on-ground measurements of mineral samples and calibration reference materials, describe the analysis of data, and compare the retrieved performances with spectral, spatial, and radiometric key parameters |
Stefani et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). VI. The Inflight Calibration Unit (ICU) | Describe the main characteristics of the internal calibration unit dedicated to the in-flight monitoring of the health and calibration of the instrument |
First author . | Title . | Objectives . |
---|---|---|
Vincendon et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer): I. On-ground setup description and characterisation | Describe the calibration objectives, the major on-ground calibration setup, and the validation of its performances |
Filacchione et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). II. Spatial calibration | Present the (on-ground and in-flight) measurements dedicated to the spatial calibration, describe the analysis of data, and retrieve the spatial key parameters |
Haffoud et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). III. Spectral calibration | Present the (on-ground and in-flight) measurements dedicated to the spectral calibration, describe the analysis of data, and retrieve the spectral key parameters |
Langevin et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). IV. Radiometric calibration | Present the (on-ground and in-flight) measurements dedicated to the radiometric calibration, describe the analysis of data and retrieve the radiometric key parameters |
Rodriguez et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). V. Validation with mineral samples and reference materials | Present the on-ground measurements of mineral samples and calibration reference materials, describe the analysis of data, and compare the retrieved performances with spectral, spatial, and radiometric key parameters |
Stefani et al. | Calibration of MAJIS (Moons And Jupiter Imaging Spectrometer). VI. The Inflight Calibration Unit (ICU) | Describe the main characteristics of the internal calibration unit dedicated to the in-flight monitoring of the health and calibration of the instrument |
III. CALIBRATION METRICS AND STATEMENT OF COMPLIANCE TO THE SPECIFICATIONS
The MAJIS instrument was designed to fulfill the main scientific performance requirements identified in the Science Requirement Document of the JUICE mission.5 These requirements were back propagated to the instrument performances and key calibration parameters. We refer to the study by Poulet et al.4 for the traceability between science objectives, design requirements, and metrics. The tables report the metrics and the statement of compliance for the major instrumental parameters regarding the spectral (Table II), spatial (Table III), and radiometric performances (Table IV).
Statement of compliance of the spectral parameters to specifications at nominal temperatures (∼130 K). Green box, compliant (achieved values are in line with the required values) when applicable. The deviation with respect to specification is in bold. The values of the parameters are reported when useful.
![]() |
![]() |
Nominal sampling is obtained by averaging the detector physical pixels by 2 in the spectral direction.
Due to optical aberrations in the OH, a spectral shift (smile effect) is present over the entire field of view (FOV), leading to a variable curvature of the slit image across the spectral range.
Same as presented in Table II but for the spatial performances. The deviation with respect to specification is in bold.
![]() |
![]() |
No required value because it can be mitigated by telecommand.
Keystone is an optical distortion introducing a pixel to pixel magnification, which changes with the wavelength. The net effect is a spectral tilt of the spectrum along the focal plane.
A short recap of the justification of the main performances is presented in the following. To distinguish spatially and spectrally surface material species that have different implications (e.g., exogenic vs endogenic processes) for the satellite compositions or to deconvolve gaseous atmospheric signatures that trace different atmospheric Jovian phenomena, adequate spectral range, spectral and spatial resolutions, and samplings are necessary. This requires a sufficiently large spectral range (0.5–5.55 μm), a high-density spectral sampling (≤3.65 and 6.5 nm for the VISNIR and IR channel, respectively), and a narrow FWHM (full-width at half-maximum) of the instrument response in the spectral direction (≤5.5 and 10 nm for the VISNIR and IR channel, respectively). The instrument was also designed to have an angular FWHM ≤225 μrad (corresponding to a nominal spatial FWHM ≤54 μm across the detector), thus providing the capability to resolve small-scale deposits (<75 × 75 m2) on the surface of the icy moons. This value corresponds to 1.5 times the nominal IFOV (Instrument Field of View) of 150 μrad (or 36 μm considering the nominal pixel size on the detectors). Finally, as for all previously flown imaging spectrometers, the science return of MAJIS in terms of SNR (signal to noise ratio) is critically dependent on the reliability of its radiometric calibration, which makes it possible to determine the radiance in W/μm/m2/str for a given pixel and wavelength from the DN level of the corresponding data element. The radiometric performances [non-linear behavior and operability of the detectors, conversion efficiency (e-/DN), readout noise, dark as a function of the temperature, instrument transfer function, and stray light] and related metrics have been retrieved from two steps: detector characterization and then radiometric calibration of the integrated MAJIS instrument at various operating temperatures (from 110 to 150 K for the optical head and from 75 to 105 K for the IR detector).
We refer to the various papers listed in Table I for detailed description and results concerning the calibration measurements, analyses, and retrieved metrics. Tables II–IV that present the outcome of these papers demonstrate that MAJIS meets, and most often exceeds, the requirements except for radiometric performances toward the short wavelengths side of the VISNIR channel that are impacted by a stray light issue experienced on ground (however, it should be noted that relevant spectral signatures can still be identified). To disentangle the combined effect of the setup (non-solar sources) and the instrument, the stray light behavior will be further studied in flight by using Solar System bodies as sources, in particular, the Moon, observed as much as possible extended in the MAJIS FOV.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Y. Langevin: conceptualization (equal); data curation (supporting); funding acquisition (supporting); methodology (equal); project administration (equal); writing – original draft (supporting). G. Piccioni: conceptualization (equal); data curation (supporting); funding acquisition (supporting); methodology (equal); project administration (equal); writing – original draft (supporting). F. Poulet: conceptualization (equal); data curation (equal); funding acquisition (equal); methodology (supporting); project administration (equal); writing – original draft.
DATA AVAILABILITY
Data sharing is not applicable to this article as no new data were created or analyzed in this study.