The case for an Astrometric Mission Extension of Euclid. Extending Gaia by 6 magnitudes with Euclid covering one-third of the sky

Abstract

The nominal duration of Euclid's main mission is six years, but current best estimates indicate that the observatory has sufficient propellant to operate for up to ~14 years in total. In this work, we advocate dedicating six of these ~8 additional years to repeating the main survey, covering approximately one-third of the sky. This repetition would not only improve the sampling, signal-to-noise, quality, and depth of the survey, but -- most importantly -- would provide a six-year time baseline between two epochs if executed in the same sequence. The availability of multiple epochs would enable the derivation of proper motions for stars as faint as V~27, i.e., more than five magnitudes fainter than those measured by the Gaia mission. Although it may seem early to propose such a mission extension, in this work we quantitatively illustrate its immense scientific potential. We therefore intend to initiate the technical and scientific discussions early to ensure optimal planning. The here proposed extension would employ only the VIS channel -- owing to its superior astrometric capability and depth -- while simultaneously using NISP in slitless-spectroscopy mode to enhance the signal-to-noise ratio of first-epoch spectra that would also benefit of proper motions to identify and reject objects within the local Universe.