MIRS: an imaging spectrometer for the MMX mission

MIRS (MMX InfraRed Spectrometer) is an imaging spectrometer onboard of MMX (Martian Moon eXploration) mission. MMX is a JAXA sample return mission that will be launched in September 2024 to Martian system, to bring back to Earth sample from Phobos, to observe in detail Phobos and Deimos and to monitor Mars’s atmosphere with observations of dust storm, clouds, and distributions of total amount of water vapor. The main objectives of the mission are to understand the origin of Martian moons, to constrain the processes for planetary formation and to understand the evolutionary processes of the Martian system. MIRS is a push-broom imaging spectrometer working in the range from 0.9 to 3.6 micron.


INTRODUCTION
First of all, the exploration begins with the discover of Phobos and Deimos in 1877 and the mystery over their formation is big ; there are two possibilities : a giant impact between Mars and an asteroid ( Figure 1) OR a capture of asteroids ( Figure 2). The composition of surface materials on the martian moons [1] reveales that both have a low mass and a low density (relative to the Earth's moon) (Figure 3). That's why the MMX Mission will be launched to analyse the surface of Phobos especially and will come back on Earth with samples of Phobos ground. The spacecraft is landing for several hours to collect a sample of at least 10g using a corer that can gather material from a minimum of 2cm below the moon's surface.  The different instruments on board are listed below :

Push-broom Spectrometer
MIRS is a spectrometer that uses the well-known push-broom acquisition principle ( Figure 4). A single detector acquisition (2D matrix) provides the image of a strip in one direction (spatial), and the spectrum of each point of the strip in the second direction (spectral). The second spatial dimension results from the motion of MIRS Line of Sight in the along-track direction either thanks to the spacecraft speed or by actuation of MIRS scanner.
The start of each image acquisition has to be adjusted so that: -All images are as contiguous as possible: no overlaps, no holes.
-Integration time is sufficient to guarantee the required SNR, and short enough not to degrade the spatial resolution.
We define the subsatellite point as the projection of MMX -Z axis on the celestial body of interest. Subsatellite ground speed varies and might not be adapted to MIRS acquisition principle. For Phobos observation on low altitude trajectories, ground speed can be too high to ensure sufficient integration time between 2 images. However, for Phobos observation during descent phases, ground speed is nearly null. For Mars observation, ground speed is low enough to cover large zones.
In order to adjust ground speed of MIRS Line of Sight projection on celestial body of interest to these situations, MIRS is equipped with an along-track scanner with +/-20° optical amplitude. This scanner can also be used to perform observations of zones with specific phase angle and local solar time by commanding a constant along-track bias. Meanwhile, the cross-track latitudes can be reached using the spacecraft maneuvers.

Instrument concept
MIRS is an infrared imaging spectrometer ( Figure 5). Based on scientific objectives, the MIRS technical requirements have been derived as presented in section 2.3. The spectral dispersion is obtained with a low-density groove grating working in order 1. The telescope is composed of two free form mirrors focusing the scene target on the entrance slit of the spectrometer. The collimator is also composed of two free form mirrors projecting the entrance slit at infinity. The grating is located at the pupil of the instrument. Then two dioptric objectives are used. The first one, composed of 3 lenses, projects the spectral image on a filter that sorts the grating orders. The second one, composed of 5 lenses, projects this spectral image on the detector but also images the pupil on a cold stop in order to limit the background flux due to the thermal emission of the spectrometer. The detector and the cold stop are encapsulated in a cryostat and cooled down to 110 K (120K for the cold stop).
A shutter is placed in the slit plane in order to close the spectrometer cavity after the telescope and acquire background images that can be subtracted to science data.
A cover is positioned at the entrance of the instrument to limit dust pollution when landing on Phobos.
-An Optical Box (OBOX) containing all the optical components: mechanisms (scanner, shutter, dust cover), a telescope, a spectrometer, a calibration source, and a detector package with its dedicated proximity electronic. MIRS OBOX is mounted on a dedicated bracket outside MMX eXploration module, with a nadir pointing. An open-close mechanism called dust cover is implemented on the MIRS OBOX entrance to protect the spectrometer from dust contamination.
The optical alignment is mechanically ensured with a stiff foot called Reference foot and a flexible one, i.e. a bipod called Axis foot. The third foot is mounted with regard to the Detector package center of mass.
The EBOX is the electronic box of the MIRS-F spectrometer. Its mass is about 1.2 kg. This unit is thermally coupled to the spacecraft descent module and is black anodized on its external surfaces.
MIRS data shall be composed of: • Science data is 2D raw data (Phobos, Deimos, Mars).
• Calibration data is 2D images of dark (shutter activated) and 2D images with the calibration source.
MIRS has a data interface with the Mission Data Processor (MDP) which also has a data interface with the MMX spacecraft Management Unit (SMU) and the Mission Data Recorder (MDR). All telemetry data will be first transferred into the MDR via MDP without any compression. Telemetry data will be reloaded from MDR to MDP for the compression process (CCSDS122.0-B-1 standard).

Requirements
The requirements are the following : MIRS-PE-006 SNR: ≥ 100 in [2.7 -3.2] µm in less than 2 sec integration, for 30° solar incidence, at 1.5 au, with Lambertian albedo at 30° phase angle MIRS-PE-007 Modulation Transfer Function shall be higher than 8% at Nyquist frequency in the spatial direction MIRS-PE-008 MIRS boresight orientation with respect to MMX spacecraft shall be known with an accuracy better than +/-1.4 mrad.

MIRS-PE-009
MIRS boresight relative orientation between successive acquisitions of an observation sequence shall be known with an accuracy better than +/-0.17 mrad.
Note: an observation sequence is: -a complete observation run when the scanner is not used (e.g. Phobos global mapping), -the observation phase corresponding to a scan when the scanner is continuously rotating during images acquisition (e.g. Mars, Phobos landing)

The Afocal part
We have four Aluminium reflective optics (achromatic), made of 2 parts : -The telescope : Schwarschild-like design, with 2 freeform mirrors. It makes an image of the scene (FOV = +/-1.65) in a focal plane, where the slit is placed. It works at F# = 4. -The collimator : Schwarschild-like design , with 2 freeform mirrors too. It collimates the light towards the grating and conjugates the entrance pupil onto the grating.

Detector overview
The detector is provided by the LYNRED company. This is a Neptune SMW detector & a RICOR cryocooler. Here are some characteristics :  Table 3 Figure 6 -the afocal part

Mechanical architecture
The mechanical architecture is called "OBOX" for Optical Box. This is just an overview to illustrate :