The Gaia mission (1609.04153v1)

Abstract: Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page at http://www.cosmos.esa.int/gaia.

• The paper presents a comprehensive analysis showing how Gaia’s precise astrometry maps positions and motions of over 1 billion stars.
• The paper details the use of continuous sky scanning and twin telescopes with a 106-CCD instrument to capture astrometric, photometric, and spectroscopic data.
• The paper demonstrates how iterative data processing and onboard error monitoring techniques ensure high-precision measurements critical for understanding galactic evolution.

An Overview of the Gaia Mission

The Gaia mission, spearheaded by the European Space Agency (ESA), represents an ambitious endeavor to chart the positions, distances, and velocities of approximately 1000 million celestial objects. This comprehensive exercise aims to elucidate the structure, formation, and evolution of our Milky Way galaxy. Gaia's approach is distinguished by its reliance on global astrometry through a technique of continuous sky scanning, which builds upon the methodology first tested by its predecessor, Hipparcos.

Technical Framework and Instrumentation

The Gaia spacecraft hosts an impressive payload comprised of two telescopes that project their images onto a shared focal plane furnished with 106 CCDs. These components facilitate an array of measurements across astrometric, photometric, and spectroscopic dimensions. Gaia's astrometric capacity is rooted in its ability to determine one-dimensional angular separations between stars, thus providing highly accurate parallax and positional data.

Critical to Gaia's astrometric accuracy is its ability to maintain or measure the stability of its basic angle, which is the cornerstone for converting transit-time differences into wide-angle positions on the celestial sphere. Anomalies such as basic angle variations and micro-clanks, representing mechanical shifts, challenge the mission's accuracy. The on-board Basic Angle Monitor (BAM) helps mitigate potential errors by tracking such variations with exceptional precision.

Mission and Data Processing

Launched in December 2013, Gaia orbits the second Lagrange point (L2) and employs a scanning law that optimizes sky coverage by precessing its spin axis. This ensures even distribution of observations across the celestial sphere. However, crowding in dense star fields poses challenges for data integrity, a situation managed by prioritizing brighter objects and employing sophisticated onboard data handling algorithms that balance storage limits with scientific priority.

Gaia's data processing framework, managed by the Data Processing and Analysis Consortium (DPAC), involves both daily and cyclic operations. The daily pipeline handles spacecraft telemetry, payload health checks, and preliminary data calibration. The cyclic pipeline, on the other hand, iterates over the entire data set to refine source parameters and calibrations iteratively, enhancing precision in astrometric, photometric, and spectroscopic data.

Scientific Achievements and Expected Outcomes

The primary scientific goal of Gaia is to improve our understanding of the Milky Way's structure and history. With astrometric accuracies predicted to reach micro-arcsecond levels for bright stars, Gaia stands to revolutionize fields such as stellar dynamics and galactic archaeology. In addition to producing an unprecedented spatial map of our galaxy, Gaia offers insights into the properties of stars through its photometric and spectroscopic data, contributing to stellar classification and the paper of dynamic processes.

Given its extensive cataloging and enhanced stellar parameterization, Gaia is also positioned to contribute to other areas of astrophysics, including the distance measurements of variable stars and the distribution of dark matter in the galaxy. The mission's results have far-reaching implications for the calibration of the cosmic distance scale and the broader understanding of galaxy formation and evolution.

In summary, the Gaia mission is a monumental leap in astrometry and a linchpin in the future of space observation. Its comprehensive stellar survey is expected not only to enrich our understanding of the Milky Way but also to establish a foundation for comparative studies of distant galaxies and contribute to the fundamental cataloging of celestial objects across the cosmos.