Thermal validation of a balloon-borne infrared telescope (EXCITE) using test flight measurement data and TAITherm

Abstract

EXCITE (the EXoplanet Climate Infrared TElescope) is a balloon-borne telescope designed to study atmospheres around exoplanets and other worlds beyond our own solar system. Led by NASA’s Goddard Space Flight Center, EXCITE aims to collect infrared sensor data observed during circumpolar long-duration scientific balloon missions, determining atmospheric composition, temperature distribution, and how atmospheric molecules absorb and emit light via phaseresolved spectroscopy. EXCITE is designed to fly at altitudes up to 40 kilometers via a helium-filled scientific balloon, rising above 99.5% of the Earth’s atmosphere to space-like conditions. This enables exoplanetary observations with minimal deleterious atmospheric effects on infrared remote sensing. However, these sensitive infrared observations depend on the measurement of very small variations in detector readings. Reduction in the sensor noise floor is achieved by cooling the spectrometer’s detector below -210 degrees Celsius with a cryostat, an illustration of how vital thermal envelopes are for achieving mission success. To better understand thermal rejection requirements for operation of high-fidelity spectrometer instrumentation in the stratosphere, a high-fidelity 3D thermal model of the balloon-tethered gondola and payload was constructed in 2022. For this pre-flight analysis, a steady state polar orbit condition and a transient ascent scenario were simulated, and a virtual design study explored the impact of material and surface property choices. Subsequently, a successful test flight was performed on August 31, 2024 from Fort Sumner, New Mexico (USA). During this test flight, temperature data was collected on the gondola and telescope. In this work we evaluate the predictive power of the thermal model by comparing transient temperature predictions (which employ measured boundary conditions) to in-flight measurement data recorded in 2024. We present the results of this high-altitude thermal validation as well as conclusions regarding important sources of temperature discrepancy to inform on models for general balloon-borne telescope instrumentation.