Climate on Earth is determined by the Earth Radiation Budget (ERB), which quantifies the incoming and outgoing radiative energy fluxes at the top-of-atmosphere (TOA). The ERB can be monitored from space by non-scanning wide field-of-view radiometers (WFOV), or by scanning narrow field-of-view radiometers. Recently, WFOV radiometers have gained renewed interest as illustrated by the development of the RAVAN and SIMBA 3U CubeSats. RAVAN uses a Vertically Aligned Carbon Nanotubes (VACNT) coating, while the SIMBA CubeSat uses a novel cavity-based geometry with a Black Velvet coating. Both VACNT and Black Velvet are diffuse coatings, but when applied to flat sensors, the VACNT coating has a significantly lower reflectivity in comparison to classic diffuse or specular black coating materials. When used on a cavity radiometer, it is currently unclear if a VACNT coating would improve the measurement accuracy compared to other diffuse coatings, such as Black Velvet. In this paper, we therefore investigate the potential benefits of using the VACNT coating as an alternative for Black Velvet, in our in-house developed radiometer. Our analysis includes the evaluation of the influence of the cavity geometry as well as the coating absorption factor. The comparison of the VACNT with the Black Velvet coating is based on the absorption factor of the cavity that is determined using radiation view factor calculations. Scattering and stray-light analyses are carried out using commercially available ray-tracing software (ASAP R, Breault Research). We evaluated whether the coating or the geometry is the main contributing factor to the performance of the radiometer. As a conclusion, we observed that for cavity-type radiometers, the difference in the cavity absorption factor between Black Velvet and VACNT becomes negligible, favoring the use of Black Velvet, since Black Velvet has a long space heritage and appears more user friendly from a fabrication point of view, as it can be deposited in an easier and more reproducible manner on the radiometer cavity walls, including non at surfaces.
The development of small space-based platforms for nulling interferometric observations could be the pathfinder of a new era in exoplanetology. While planetary transit and radial velocity are the most productive ways to detect exoplanets, such techniques are indirect detections. For deeper characterization of exoplanets, direct detection techniques should be developed. By injecting direct light coming from exoplanets into spectrometers, we could study their chemical composition, search for biosignatures, and possibly infer the presence of life.
The low number of photons to be gathered from the planets, high contrast with the star and small angular resolution are the major difficulties for a direct detection. However, nulling interferometry seems to be a solution to tackle these challenges. By combining the light of two or more telescopes, we would considerably increase the angular resolution, and thus could potentially lead to the detection of Earth-size rocky exoplanets around Solar-type stars. Moreover, with a π- phase shift between the two interferometer arms, the starlight is reduced which allows the detection of much fainter objects around the star. In this paper it will be presented the development of a new mission based on nulling interferometry and dedicated to the Alpha Centauri system. As our nearest stellar system, it is a prime target to investigate for the research of new worlds. Monte-Carlo simulations about potential exoplanet yield of such an interferometer will be described, for different assumptions such as the detection wavelength and telescope size. Single-mode fibers and integrated optics will also be investigated for this mission. This could lead to low-cost type missions with a high potential of scientific return.
Proxima b is our nearest potentially rocky exoplanet and represents a formidable opportunity for exoplanet science and possibly astrobiology. With an angular separation of only 35 mas (or 0.05 AU) from its host star, Proxima b is however hardly observable with current imaging telescopes and future space-based coronagraphs. One way to separate the photons of the planet from those of its host star is to use an interferometer that can easily resolve such spatial scales. In addition, its proximity to Earth and its favorable contrast ratio compared with its host M dwarf (approximately 10-5 at 10 microns) makes it an ideal target for a space-based nulling interferometer with relatively small apertures. In this paper, we present the motivation for observing this planet in the mid-infrared (5-20 microns) and the corresponding technological challenges. Then, we describe the concept of a space-based infrared interferometer with relatively small (<1m in diameter) apertures that can measure key details of Proxima b, such as its size, temperature, climate structure, as well as the presence of important atmospheric molecules such as H2O, CO2, O3, and CH4. Finally, we illustrate the concept by showing realistic observations using synthetic spectra of Proxima b computed with coupled climate chemistry models.