MIST-A onboard EMA: observation planning and expected performance

The Emirates Mission to the Asteroid Belt (EMA), set for launch in March 2028, is a multi-target mission, aimed at investigating primitive, water-rich asteroids [1]. EMA will tour the main belt, performing flybys of six asteroids from 2030 to 2033, before the rendezvous with (269) Justitia in 2035. (269) Justitia is one of the two reddest objects in the asteroid belt, recently classified as a Z type [2], making it similar to red-colored trans-Neptunian objects [3] and suggesting a possible origin in the giant planet zone. The flyby targets (10253 Westerwald, 623 Chimaera, 13294 Rockox, 88055, 23871, and 59980) of EMA encompass different spectral types, with most of them likely being primitive objects and/or members of collisional families.

The EMA scientific payload includes a collection of remote sensing instruments, among them the Mid-wave Infrared Spectrometer for Target-Asteroids (MIST-A; [4]). MIST-A is a slit hyperspectral imager operating in the wavelength range 2-5 µm, with a spectral sampling <10 nm. This wavelength interval is optimal to investigate the surface composition of EMA targets, as it covers the spectral signatures of a variety of compounds relevant for understanding primitive bodies, including organics (3.3-3.5 µm) [5], carbonates (3.4-4.0 µm) [6, 7], ammonium salts (2.2 µm, 3.1-3.3 µm [8, 9]) hydrated silicates (~2.7 µm) [6, 10, 11], and water ice (~3 µm) [12].

MIST-A has an Instantaneous Field of View (IFOV) of 238 µrad, corresponding to a spatial sampling of 24 m from 100 km distance. Thanks to a scanning mirror, it can observe a composite FOV of 3.52°X12° and control boresight pointing stability. The current configuration of MIST-A on the EMA spacecraft (MBR Explorer) aligns the spectrometer slit parallel to spacecraft velocity direction. According to this, the observation scene is built thanks to the movement of the scanning mirror, operating in the across-slit direction, as a collection of hyperspectral images of the target through the slit (so-called lines).

In this contribution we present the observation strategy and envisaged performance of MIST-A during the EMA mission, in terms of expected mapping coverage, data volume, and observation SNR at Justitia and during the flybys.

Observations at Justitia: the prime science observations at Justitia will be carried out during the Primary Science Orbit (PSO) phase (~14 weeks). During this phase, according to the current simulated scenarios, MBR will be on nearly polar circular orbits, with a period of ~12-21 h and orbital radius of 89-96 km (MIST-A spatial sampling of 15-16 m/pix), depending on the gravitational field, and β angle ~30°-45°. MIST-A will operate on the day-side arc of the orbit, with a nominal repetition of 2 s (time step between the acquisition of consecutive lines, including a nominal exposure time of 0.8 s and 1.2 s for the detector readout), smaller than the expected dwell-time of ~4-7 s, thus minimizing pixel smearing. A full scan of the scene width (covering the maximum 12° across-slit field) would require ~870 contiguous lines, taking approximately 29 minutes. Depending on the orbital period, the scan extent (i.e., duration) and cadence can be adjusted to avoid gaps and ensure minimal overlap between consecutive scans along the same track.

Our coverage simulations show that, depending on the orbital scenario and mission operation constraints, MIST-A will be able to map 50% of Justitia's surface in 4-6 weeks, with near-complete coverage (>90%) achievable during the PSO.

Simulations including the MIST-A radiometric model and spectrophotometric modeling of Justitia's reflected and thermal radiance indicate that MIST-A can achieve SNR>100 over most of the investigated spectral range from the equator up to mid-latitudes with the nominal 0.8 s exposure time.

Strategies for further optimization of coverage, redundancy, data volume, and SNR will also be discussed.

Flybys: During the flybys, MIST-A will acquire scientific data primarily in a temporal window of ±2 hr around the Closest Approach (CA), during which the instrument boresight will be pointed towards the photometric center of the asteroid thanks to the spacecraft Tracking and Autonomous Landing Onboard Navigation System (TALONS). The extent of the scan will be adjusted during the flyby to match the expected apparent size of the target and to accommodate pointing uncertainties.

Our simulations for the different targets indicate that at minimum phase angle (typically within a few minutes from the closest approach) MIST-A will achieve average SNRs between a few to several hundred for wavelengths below 3.5 µm, and >50 (up to a few hundred) at longer wavelengths, with nominal exposure time of up to 0.8 s (repetition of 2 s).

Further optimization of the exposure and repetition times is under study for each flyby to maximize the SNR over the entire science phase, considering the reflectance properties of the different targets and the varying observation geometry and distance, while avoiding saturation and minimizing pixel smearing. By adapting the exposure time, it should be possible to extend observations throughout the entire flyby science windows (CA-10 hr; CA+2 hr), including the acquisition of subpixel images of the targets.

Acknowledgements: MIST-A science, calibrations, operations, and data processing responsibilities are in INAF/IAPS, Rome. The MIST-A program is managed by the Italian Space Agency (ASI) as leading funding agency. Leonardo (Campi Bisenzio, Florence) is the primary industrial contractor (ASI contract n. 2025-33-I.0). Funding for the Emirates Mission to Explore the Asteroid Belt is provided by the United Arab Emirates Space Agency, in collaboration with its knowledge partner, the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder.

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