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 Hipparcos has now been superseded by the results of the Gaia mission.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 second Gaia data release contained astrometric data for almost 1.7 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 just completed its nominal 5-year mission (July 2019), but is expected to continue in operations for an extended period of an additional 5 years through to mid 2024. Its final catalogue to be released \(\sim \) 2027, will provide astrometry for \(\sim \) 2 billion sources, with astrometric precisions reaching 10 microarcsec. Why is accurate astrometry so important? The answer is that it provides fundamental data which underpin much of modern observational astronomy as will be detailed in this White Paper. 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.

全天空空间天文测量的时代始于1989年的Hipparcos任务，并提供了第一个非常精确的目录，以毫安秒（或每年毫毫秒）的精度水平显示了12万颗明亮恒星的视星等，位置，视差和适当运动。Hipparcos现在已被盖亚（Gaia）任务的结果所取代。盖亚的第二次数据发布包含大量银河系中近17亿个源的天文数据，精度为数十微弧度（或每年微弧度），未来的数据发布将对此进行进一步的改进。盖亚（Gaia）刚刚完成了其名义上的5年任务（2019年7月），但预计将继续运营另外5年，直到2024年中。其最终目录将发布\（\ sim \）2027年，将提供天体测量\（\ SIM \） 2种十亿源，与天体测量精度达到10 microarcsec。为什么精确的天文测量法如此重要？答案是，它提供了基础数据，这些基础数据将在本白皮书中详细介绍，它们是现代现代观测天文学的基础。K波段波长截止的全天可见和近红外（NIR）天文测量法不仅关注单个或少数关键科学案例。取而代之的是，它极为广泛，几乎回答了天文学的每个分支领域中的关键科学问题，同时还提供了未来天文学设施所需的密集且准确的可见近红外参考系。