Instrument Design Updated of the Venus Emissivity Mapper (VEM) for VERITAS and Envision

The Venus Emissivity Mapper (VEM) is a groundbreaking instrument designed to transform our understanding of Venus. Developed for NASA’s VERITAS and ESA’s EnVision orbiters, this advanced push-broom multispectral imager is tailored to reveal the planet’s surface. VEM features a 14-band filter assembly, an InGaAs detector with thermoelectric cooling, a turn-window mechanism, and a dual-stage baffle system to ensure performance in Venus's harsh environment. Weighing just 6 kilograms, it will achieve over 70% global coverage, enabling unprecedented surface mapping. VEM promises to unveil Venus's secrets and deepen our understanding of Earth’s enigmatic sister planet.

 

Optics: The VEM/VenSpec-M Optics (VEMO), developed by CNES and LESIA, uses a telecentric 3-lens system for precision imaging. Its entrance lens focuses the Venusian scenery onto a filter assembly, while relay optics with <1 magnification transfer the image to a 16.4 mm focal length detector. With a 46.4° field of view, it achieves a 207 km swath width at a 250 km orbit. The filter assembly (0.86–1.18 µm), developed by Bertin Winlight and CILAS, maps the image across 14 filter stripes, enabling surface emission measurements and atmospheric corrections.

Detector: The VEM detector features an InGaAs short-wave infrared sensor with an integrated thermoelectric cooler, eliminating the need for a cryocooler. Qualified for space, it undergoes rigorous environmental testing. The detector covers 400–1700 nm (optimized to 790–1510 nm), with 640 x 512 pixels at a 20 µm pitch, operating from -40°C to +70°C (baseline 0°C). Its quantum efficiency peaks at ~85%, aligning with the VEM spectral range. System-level analyses show signal-to-noise ratios around 100, even at the mission's end, with a margin exceeding 100%. While a radiation test campaign is complete, managing increased dark current from proton irradiation remains a challenge.

Testing and Analysis: A thermal cycling test was conducted as part of the evaluation campaign on two flight-representative sealed detectors following the MIL-STD-883 standard (method 1010, condition A). The temperature range was adjusted to -43°C to -40°C and +85°C to +88°C, staying within the detector’s non-operative limits. The test involved 100 cycles with a 125°C total range achieved in 16 minutes, resulting in a gradient of approximately 8°C per minute. A follow up of the structural analysis has been performed showing that the loads at the VEMO interface are higher than initially expected, resulting in increased loads at the detector level. The engineering team will explore potential solutions, including updates to the mechanical design to mitigate the loads. 

Keywords: VERITAS, VEM, EnVision, VenSpec-M, Venus, IR, N-IR, SWIR, InGaAs, Imaging

Acknowledgments: CNES/LESIA for its contribution on the optics development.