The Legacy

The era of all-sky space astrometry began with the Hipparcos mission in 1989 and provided the first very accurate catalogue of apparent magnitudes, positions, parallaxes and proper motions of 120 000 bright stars at the milliarcsec (or milliarcsec per year) accuracy level. Hipparcos has now been superseded by the results of the Gaia mission. The third Gaia data release contained astrometric data for ~1.8 billion sources with tens of microarcsec (or microarcsec per year) accuracy in a vast volume of the Milky Way and future data releases will further improve on this. Gaia has now completed its nominal 5-year mission (July 2019), but continues its operations for an extended period of 5 years through to mid 2024. Its final catalogue to be released by 2030, will provide astrometry for ∼2 billion sources, with astrometric precisions reaching 10 microarcsec. 

The Vision

All-sky visible and Near-InfraRed (NIR) astrometry with a wavelength cutoff in the K-band is not just focused on a single or small number of key science cases. Instead, it is extremely broad, answering key science questions in nearly every branch of astronomy while also providing a dense and accurate visible-NIR reference frame needed for future astronomy facilities.

For almost 2 billion common stars the combination of two all-sky space observatories would provide an astrometric foundation for all branches of astronomy – from the Solar System and stellar systems, including exoplanet systems, to compact galaxies, quasars, neutron stars, binaries and dark matter (DM) substructures. The addition of NIR will result in up to 8 billion newly measured stars in some of the most obscured parts of our Galaxy, and crucially reveal the very heart of the Galactic bulge region.

Rather than improving on the accuracy to answer specific science questions, a greater overall science return can be achieved by going deeper than Gaia and by expanding the wavelength range to the NIR. An obvious question to ask is – can a more accurate all-sky mission than Gaia be done? Clearly the answer is yes, if we can build a space telescope with a larger aperture (D) but in practice it is very difficult to do this without greatly inflating the cost of the mission. Gaia was designed very well and only just fitted in the available launchers so improving the telescope’s angular resolution (i.e. minimum angular separation) R → λ/D at a fixed wavelength λ is very costly. Other mission proposals have tried to avoid this by employing long focal lengths and advanced metrology systems for ultra-accurate narrow field proposals, like SIM, NEAT and Theia, but these missions were focused on answering important specific science cases and did not aim to do a broad all-sky astrometric survey. Nevertheless, the metrology systems explored may find application in improving a future all-sky mission.

A new all-sky NIR astrometric mission will expand and improve on the science cases of Gaia using basic astrometry. Key topics are focused on what dark matter is and how is it distributed, how the Milky Way was formed and how has it been impacted by mergers and collisions? How do stars form and how does stellar feedback affect star formation; what are the properties of stars, particularly those shrouded in dust, and small Solar System bodies; how are they distributed and what is their motion? How many co-planar systems like ours (with Earth-sized and giant planets) are there and what fraction have planets with long period orbits?