Space instrumentation

Abstract:

The modern world is increasingly computerised and, as more functionality is shifted to the edges of systems in bids to reduce latency or energy consumption, the Internet of Things (IoT) increasingly includes safety-critical systems. A typical such system is 'functionally dense,' with a time-shared processing element (PE) carrying out many tasks; we believe that they should instead be functionally sparse, with many small PEs dedicated to single tasks. We present case studies of this design approach and introduce FUNK, our effort to build the tools that will support the development of hard real-time, functionally safe systems that follow this approach.

Abstract:

ESA’s Ariel space telescope is due to launch in 2029 and aims to measure broadband (0.5 – 7.8 µm) spectra from ∼1000 known transiting exoplanets. For optimum stability the payload uses an all aluminium telescope with an extremely narrow (30 ′′) field of view. Prior to integration with the spacecraft, the Ariel payload will undergo a TVAC campaign to check that the telescope and instruments are capable of achieving the mission’s scientific goals. During these tests the payload will be illuminated by a quarter aperture test beam generated by a highly stable and adaptable cryogenic optical system – the payload optical ground support equipment (OGSE). This OGSE is required to output a 275 mm x 183 mm elliptical collimated beam with an RMS wavefront error (WFE) of < 214 nm to enable diffraction limited imaging in Ariel’s instruments. Furthermore this must be achieved after the collimation and beam expansion optics are cooled to an operating temperature of 70 K. In this proceeding, the optical design, specifications and simulations of the OGSE cryogenic optical system are presented. This system is designed to be athermal between room temperature and 70 K while also producing a low WFE output by using a combination of glass and aluminium mirrors. To prevent stressing and deforming the optics during cooldown, a selection of low distortion and minimally constraining mounts have been designed and simulated. The alignment and verification philosophy of the optical system is also presented. Emphasis is placed on precision mechanical alignment with only a minimal number of adjustable degrees of freedom required to focus and optimise the system. Following alignment, the end-to-end optical quality of the OGSE is verified in a single-pass configuration with a Shack-Hartmann based metrology system.

Abstract:

This paper presents developments in the thermal design and testing of the optical ground support equipment (OGSE) used to characterise the optical performance of the Ariel space telescope at payload level. As part of ESA’s Cosmic Vision programme, Ariel will carry out transit spectroscopy on approximately 1,000 exoplanets over four years of operation. To simulate a flight-like thermal environment during payload level testing, the payload is enclosed in the cryogenic test rig which provides a 35K radiative environment. The payload is illuminated by the OGSE, which comprises an illumination module that illuminates the payload and a periscope module that steers the test beam into the payload line of sight. To enable verification of the dark current requirement in the payload’s detectors (< 1 e− pix−1 s−1), both the periscope and illumination module must be kept at 70 K. The illumination module has a two-stage thermal design consisting of a thermally isolated optical bench and a radiative shroud constructed from low-emissivity MIRO-27 sheeting. The optical bench and radiative shield are cooled using a pair of cryocoolers connected to flexible, high-conductivity copper straps. To de-risk the thermal design, a thermal breadboard of the illumination module has been constructed. Here, we present the thermal designs of the periscope and illumination module as well as the results of the thermal breadboarding activities. In particular, the cryocooler performance was found to be consistent with the manufacturer’s specifications. The thermal performance of the flexible copper straps was characterised, and the influence of thermal interface materials on interface conductance was quantified, with 40 μm graphite foil providing approximately twice the interface conductance of direct metal-to-metal contact for heaters operating at approximately 100 ◦C. The dominant thermal loads acting on the illumination module shroud were identified, and the effective emissivity of a MIRO-27-based shroud was determined to be 3.5 %. Furthermore, the periscope thermal design was experimentally verified at its operational temperature of approximately 70 K, and several critical aspects of the periscope thermal design were de-risked.

Abstract:

In this proceeding, we present the verification and test methods used on the Ariel Optical Ground Support Equipment (OGSE). Ariel is an ESA medium-class mission that will use the transit spectroscopy method to observe the atmospheres of ~1000 exoplanets over a nominal four-year mission. Ariel is a 1 m class cryogenic (∼ 40 K) space telescope that will be placed in a halo orbit around the Earth-Sun L2 point. To detect atmospheric molecular absorption features, the payload contains spectroscopic and photometric instruments that span 0.5–7.8 μm. To achieve Ariel’s science goals, the payload requires detailed calibration and performance verification. The payload-level engineering model calibration of the Ariel payload will take place in 2027 in a 5-m vacuum chamber at the Rutherford Appleton Laboratory’s Space Instruments Test Facility. The payload will be enclosed in a Cryogenic Test Rig (CTR) to provide a space-like (~35 K) thermal environment. During cryogenic vacuum testing, the payload will be illuminated by the OGSE. To test the payload on the ground, the OGSE needs to meet very stringent performance requirements. Notable examples include: the 214 nm RMS wavefront flatness at a 70 K operational temperature, 24 ppm detector photometric stability on hour timescales, radiometric calibration with 5% relative accuracy, and a flat fielding capability with a uniformity better than 0.5%. In addition to the performance requirements, the OGSE system features cryogenic mechanisms, a custom cryostat, and a vacuum chamber coupling system; the functionality of each must be verified. Finally, since the OGSE shares a common vacuum environment with the payload, cleanliness is therefore a key requirement on the project. In this proceeding, we will present the integration and verification plan for the OGSE system, showing how the different technical requirements will be verified.

Abstract:

Abstract We investigate the diurnal temperature variations of Europa’s surface using brightness temperatures taken by Galileo’s Photopolarimeter-Radiometer (PPR) instrument. Diurnal curves created from these data are compared to those predicted by a thermophysical model to determine what thermal inertias and Bond albedos can fit the data within a reduced chi-squared cutoff of $\chi _{\textrm {red}}^2\le 1.0$. This analysis is used to extensively quantify the uncertainty for the first time of the two thermophysical parameters derived from PPR. We map the albedo and thermal inertia for 33% and 24% of Europa’s surface area, respectively. We find a range of 0.375-0.75 for albedo and 20-110 J m−2 K−1 s−1/2 for thermal inertia. Our uncertainty analysis indicates well-constrained estimates for albedo, while upper limits for thermal inertia remain poorly constrained. When averaged across nine geological areas, albedo varies as expected: lower values are obtained for darker regions in visible wavelengths. Thermal inertia appears to vary independently from geological boundaries. The surface was also divided based on electron bombardment energy flux and non-ice composition fraction. Thermal inertia behaves opposite of what is expected: lower thermal inertias are located in regions of high-energy electron bombardment. This possibly indicates a mechanism competing with electron-induced sintering is present that is lowering the surface conductivity. No endogenic anomalies were detected that could not be explained by passive emission alone. Nevertheless, these results aid in preparing for future thermal measurements from Europa Clipper and Juice by improving the surface coverage of Europa’s passive thermal properties and providing uncertainties of their values.