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I.INTRODUCTIONThe thermal properties of small bodies in the Solar System extensively contribute to the knowledge of their global physical properties and dynamical evolution. Their determination allows us to constrain the surface properties (roughness, presence of regolith), the internal structure through the thermal inertia, and to quantify the Yarkovsky effect that controls the non-gravitational evolution of the orbit. The latter is particularly important for the prediction of the orbit of NEAs (Near Earth Asteroids), such as Apophis that could impact the Earth, hence for the design and development of future space missions to change the trajectory of those bodies. The thermal properties are best determined by the space-resolved mapping and spectroscopy of the surface of the body. This requires a specific and dedicated instrument, implemented on a spacecraft for a flyby, or better a rendezvous mission. We are realizing the feasibility study of a thermal spectro-imager for future space missions to small bodies in the Solar System. With the recent progress of uncooled micro-bolometers (silicon detectors) technology and its space qualification under progress at the Centre National d’Études Spatiales (CNES, France), we aim to use such an infrared (9-14 μm) detector as a baseline for our thermal spectro-imager. We have defined four objectives to be achieved in order to reach our goal:
In this paper, we show our progress on the first two objectives of this work. II.EXPERIMENTOur experiment to perform uncalibrated and calibrated radiometric images is composed of:
Fig. 1.The IRXCORE640 module: a 640x480 micro-bolometer array (front) and its associated electronics (back).  III.RESULTSA. Uncalibrated imagesTo complete our first objective, we took a series of uncalibrated images (Fig. 4) while heating the black body. Thus, we were able to characterize the response of the detector for different integration times, gains, and detector operating temperatures. We first established that for an integration time between 9 μs and 36 μs, and an operating temperature of the detector between 20°C and 30°C, the variations of the response of the detector are negligible (<0.3 %). We also tested the response of the detector for gain values ranging from 1.0 to 4.5. As expected, larger gain values lead to a larger dynamic but a smaller measurable temperature range. Finally, we tested the stability of the signal of the detector over time (Fig. 5). It takes approximately three hours for the detector to reach a temperature equilibrium with its surroundings (the room temperature is 20°C) after it is switched on. After those three hours, the signal remains stable, with for example variations lower than 0.2 % over a 16 minutes period. The long stabilization time is likely due to the initial heating of the read out electronics placed close to the detector. B. Calibrated radiometric imagesTo complete this objective, we took a series of calibrated images (Fig. 4) while heating the black body and monitoring its temperature from 10°C to 70°C. We measured ten responses curves which can be seen in Fig. 6. These curves were all plotted using measurements done in the same conditions, though not on the same date, and they all show good repeatability. Fig. 6.Absolute response of the detector measured using a black body as our temperature controlled scene, with a temperature ranging from 10°C to 70°C.  According to the theory of microbolometers, we expect the response curves to be proportional to T4, where T is the temperature of the scene. So, we fitted the ten responses with a fourth degree polynomial equation, and computed the mean deviations and maximum deviations (Tab. 1). Assuming the temperature indicated by the black body to be exact as a first approximation, we converted the counts into Celsius degrees. Residuals are lower than 0,4% over the temperature range 10-70°C, for a mean value of 0,1%, corresponding to 0,1°C. Since we aim for absolute measurements of the temperature with an accuracy of about 1K, this looks a promising result. Mean deviation!: Tab. 1.Mean deviations and maximum deviations of the residuals between fitted and measured response curves of the detector.
IV.CONCLUSION AND PERSPECTIVESOur first results to demonstrate the possibility to use an uncooled microbolometer for a thermal imager for space missions to small bodies look promising. However several points need to be addressed before we can design the instrument. In particular, during the next months, we will extend the temperature range of the scene to lower temperatures (up to -10°C), we will optimize the calibration (absolute accuracy and number of sources), and we will investigate the feasibility of spectroscopic measurements. ACKNOWLEDGMENTSThis work is supported by the Centre National d’Etudes Spatiales (CNES) and Thales Alenia Space (TAS). REFERENCESJ.L. Tissot,
“IR detection with uncooled sensors,”
Infrared Physics & Technology, 46
(1-2), 147
–153
(2004). https://doi.org/10.1016/j.infrared.2004.03.018 Google Scholar
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