This idea has been archived
idea not further considered in this campaign as recommended by the evaluation board.
Dec 22, 2020
All authors
José A. Caballero
Abstract
Stellar radii, one of the most important parameters of stars, are determined indirectly from luminosities and effective temperatures through the Stefan-Boltzmann law, or measured directly by analysis of eclipsing binaries, the use of expensive interferometry from the ground (and only for the stars with the largest angular diameters because of their closeness or large size) or by timing of occultations. During an occultation, a star is hidden by a Solar System body (the Moon, the planets and, more rarely, minor planets) that passes between it and the observer. However, occulted stars are only in a narrow strip on the sky: the ecliptic. One way to solve this problem is locating the observer on a highly elliptical Near Rectilinear Halo Orbit (NRHO) at the L2 Earth-Moon Lagrangian point. In parallel, ESA has raised several calls for instrumentation onboard the Lunar Orbital Platform-Gateway, the planned lunar-orbit space station, which just has that NRHO orbit at L2. --- I propose a very small automatic telescope for stellar occultations, attached to an external bay of the Gateway. The instrument , dubbed GATO (Gateway Automatic Telescope for Occultations) would be equipped with a single fast read-out optical EMCCD and/or a near-infrared CMOS with millimagnitude/millisecond capability (thus insensitive to Gateway vibrations), and an artificial-intelligent scheduler for maximising the observation time. After only one year of regular operation, the instrument should have increased by several orders of magnitude the number of single stars with precise radius determination. The results obtained with such an instrument, with a relatively small budget compared to dedicated space missions, may have an extraordinary impact on stellar astrophysics and complement the luminosities and effective temperatures determined by Gaia.
Stellar radius is a key astrophysics parameter, but has been directly measured only for a small number of single stars. GATO would measure stellar radii via Moon occultations from an external bay at the Lunar Orbital Platform-Gateway. On the contrary to what can be done from the ground, numerous stars outside the ecliptic would be observable by GATO because of the peculiar Gateway orbit (NRHO).
The GATO instrument shall be relatively simple, small, compact, robust and of low mass. The most expensive high-level mission requirement is by far the transport to Gateway and its installation in an external bay (probably with a robotic arm). The mission cost below does NOT include launch and installation in Gateway, but only hardware (optomechanics, electronics) and human resources.
The GATO mission architecture shall include only: optics (baseline: <10 cm refractor camera), mechanics (movable mount, baffle), electronics (detector, power, temperature control), control system (pointing/tracking/guiding, scheduler, pipeline, data flow).
This is a brand new idea, except for the fact that stellar occultations by the Moon have been performed from the ground since at least the sixth century (Aldebaran was observed in year 509). There has not been related space missions or studies in the past: usually, astrophysics space missions do not point near the Moon. Since this is a fresh idea, there has not been time yet for quantifying the increase in number of potentially occulted stars by the Moon from Gateway with respect to from the Earth, determining orbit-dependent occultation times, preparing a preliminary target catalogue or defining high-level scientific requirements (minimum detector readout speed, field of view, pixel and PSF size, broadband filter and system throughput, dynamical range, scheduler efficiency). A number of coauthors (both engineers and astronomers) would be added to the project after GATO is selected for a feasibility study.
All data shall be publicly available to the whole astrophysics community in monthly releases.
There are no time restrictions except for that the Lunar Orbital Platform-Gateway should be ready to accept small instruments on an external bay with rather continuous Moon visibility. GATO should go through all space mission stages (conceptual, preliminary, final design; manufacture, assembly, integration, verification; launch; installation on Gateway; commissioning; start of operations), but with the Gateway restrictions. GATO should be operative at least for 2+3 years (depending mostly on the space needs in Gateway external bays).
| Name | Added on | |
|---|---|---|
 | GATOpitch.pdf | Oct 2, 2020 |
Centro de Astrobiología (CSIC-INTA)
José A. Caballero is an astrophysicist and staff researcher at the Centro de Astrobiología (CSIC-INTA) in ESAC, near Madrid. He is an expert in exoplanets, brown dwarfs, stars and instrumentation for studying them. He was the project manager and currently is the instrument astronomer of CARMENES, one of the most successful instruments for exoplanet radial-velocity surveys built to date. He has worked in Madrid (CAB, Universidad Complutense de Madrid), the Canaries (Instituto de Astrofísica de Canarias, Isaac Newton Group of Telescopes) and Germany (Max-Planck-Institut für Astronomie, Landessternwarte Königstuhl Universität Heidelberg). He has published >130 refereed papers in Q1 journals, including Nature and Science, with more than 4700 citations, has been a member of the most important telescope time allocation committees (ESA/NASA, ESO, IAC), has received several awards and is a recognised communicator in his country, Spain.
caballero@cab.inta-csic.es