The Mission Summary
An obvious technological improvement to the current Gaia mission is to also go into the non-thermal Near-InfraRed (NIR) with a wavelength cutoff in the K-band allowing the new mission to probe deeper through the Galactic dust to observe the structure and kinematics of the star-forming regions in the disc, the spiral arms and the bulge region, to give model independent distances and proper motions in these obscured parts of the sky. Additionally, having two 5 or 10-year Gaia-like missions separated by 20 years would give 10–20 times better proper motions for a few billion common stars and also improved parallax determinations with new observations. After the publication of the final Gaia catalogue the positions of stars will be accurately known at the chosen reference epoch. However, this accurate positional information and the accuracy of the link to the VLBI reference frame will slowly degrade due to the small uncertainties in the proper motions of the stars. Hence, it is very desirable to repeat the measurements of Gaia after about 20 years to maintain the positional accuracy of the stars and the visible reference frame. Gaia is already one of the most transformational missions ever as measured in terms of scientific output.
The accuracy of the new mission should be at least that of Gaia using tried and trusted instrumentation, techniques, and lessons learned from Gaia to unveil a wealth of new and more accurate information about our Galaxy. The science cases have been roughly divided into three Sections 1) NIR science cases; 2) improved proper motion science cases and 3) reference frame science cases. A space-based mission avoids the limitations caused by the turbulent atmosphere and the use of Earth rotation parameters and models of nutation and precession. All-sky space-based astrometry leads to a global solution and provides a rigid sphere for a celestial reference frame that cannot be accurately obtained with any other method.
A brief history of proposals
Hobbs et al. (2016) proposed to the European Space Agency (ESA) a new all-sky NIR astrometry mission, called GaiaNIR. Such a NIR space observatory is however not possible today: it requires new types of Time Delay Integration (TDI) NIR detectors to scan the entire sky and to measure global absolute parallaxes and developing such TDI-NIR detectors is a significant challenge. Twenty six proposals were received and three were selected for further study - including NIR global astrometry. In late 2017 ESA conducted a Concurrent Design Facility (CDF) study of our proposal and the results were published in early 2018.
McArthur et al. (2019) and Hobbs et al. (2019) submitted White Papers to the US decadal survey (ASTRO 2020) outlining the science cases and a possible US-European collaboration. The required technology is also being pursued elsewhere, for example, the Australian National University is developing NIR astronomical detector technology with TDI capabilities.
Also in 2019 Hobbs et al. (2019) submitted A science case white paper to ESA's Voyage 2050 call for new mission proposals. Voyage 2050 finally set sail in June 2021 when ESA selected a number of future science mission themes. Our proposal was highlighted as both a potential future large-class mission or as a future medium-class mission with international cooperation. Efforts are now underway to understand and identify suitable detector technologies and to prepare for writing mission proposals. Ideally we would like to see our mission launching around 2045 which is slightly later than originally proposed - this just reflects the great success of Gaia and its 5 year mission extension.
The measurement concept for obtaining global astrometric measurements only possible from space is well-defined and well demonstrated. Indeed, the concept is identical to that of the currently flying Gaia mission where the nominal mission lifetime is 5–6 years. However, for Gaia, the community has already proposed to continue observations for a total of 10 years if fuel consumption and the hardware on-board continue to operate as expected. Such a mission will effectively double the number of measurements giving an accuracy improvement by a factor of √2 in the positions and parallaxes but a factor of 2√2 in the proper motions which also benefit from a doubling of the measurement baseline. However, in addition to just doubling the number of measurements, if we also add a gap between missions, a simple calculation shows that the combination of two 5-year missions (labelled with subscript N for GaiaNIR and G for Gaia), assuming a positional and a proper motion accuracy of 25 μas (yr−1) with a separation of 20 years will give:
which is at least a factor of 10 better in both proper motion components. If we then assume two 10-year missions one gets an extra factor of 2 improvement in the individual positions from each mission giving an overall improvement by a factor of 20 for both proper motion components compared to the initial values of 25 μas yr−1. If a new mission follows we would get these improvements in proper motions for a few billion stars. Parallaxes will also improve mainly due to the additional measurements. but also because the proper motions are much more accurate. The parallaxes can then be better determined (an indirect improvement) which has already been demonstrated for Gaia where Hipparcos/Tycho-2 data were combined with Gaia’s to form the first data release. Residual systematic errors will be present in both Gaia and the new mission, but they should be uncorrelated and it may be possible to use a joint solution of both missions to partially reduce these errors. The lessons learned from Gaia will be invaluable and improvements in the data processing and instrument modelling can be built on already well developed concepts. To accomplish this goal we need to build a new all-sky astrometry mission to fly around 2045.
Small JASMINE (Japan). ISAS/JAXA have selected small-JASMINE for their M-3 mission with a current scheduled launch in the mid-2020s, to do relative (to Gaia) astrometry in the NIR, but only focusing on the small region within ~100 pc from the Galactic Centre and relatively bright (Hw< 15 mag) stars. This mission could also act as pathfinder for our all sky proposal and the data can be combined with Gaia to provide improved proper motions in important small regions of the Galactic centre and disk.
ESA's Euclid mission. Euclid is an ESA mission to map the geometry of the Universe and better understand the mysterious dark matter and dark energy, which make up most of the energy budget of the cosmos. The mission will investigate the distance-redshift relationship and the evolution of cosmic structures by measuring shapes and redshifts of galaxies and clusters of galaxies out to redshifts ~2, or equivalently to a look-back time of 10 billion years. In this way, Euclid will cover the entire period over which dark energy played a significant role in accelerating the expansion of the Universe. Euclid was not designed for astrometry and points mainly out of the Galactic plane, nevertheless, it is a superb instrument and will provide precise measurements of foreground stars in the halo regions of our galaxy that can be combined with astrometric data to improve proper motions in this regions.
The Vera C. Rubin Observatory, previously referred to as the Large Synoptic Survey Telescope (LSST), is an astronomical observatory currently under construction in Chile. Its main task will be an astronomical survey, the Legacy Survey of Space and Time (LSST). The Rubin Observatory has a wide-field reflecting telescope with an 8.4-meter primary mirror that will photograph the entire available sky every few nights. The astrometry obtained from this observatory is less accurate at the bright end down to around 20th magnitude but importantly this observatory goes much deeper than Gaia the new NIR mission and will thus be very complementary.