What is Astrometry?

Astrometry has two main scientific objectives:

1. to provide a non-rotating stellar reference frame to which the motions of objects in the Solar System and stars in the Galaxy may be referred, and which can be used as a reference framework for relating optical observations to those in other regions of the electromagnetic spectrum.
2. to provide, through the use of such a framework, basic observational data for the studies of stellar properties (luminosities, mass, etc.), the spatial distribution of stars within the Galaxy, and their motions.

Fundamental though the stellar astrometric parameters (positions, parallaxes and proper motions) are, the angles involved are extremely small (typically parallaxes and annual proper motions are measured in milliarcsec), and the measurements required to quantify them must be extremely precise. Conditions experienced from ground-based astronomical observatories, such as the perturbing atmosphere, lack of all-sky visibility, and gravitational and thermal flexure of telescopes, complicate the measurements and the separability of the positional and time-dependent parameters. These factors tended to result in a limit beyond which progress proved technologically complex and observationally time-consuming.

The next step was to make astrometric measurements from space, hence by-passing many of these difficulties. The Hipparcos mission was the first space-based astrometric mission. Following its success a number of missions aiming at higher precision measurements have been proposed by various space agencies. The most ambitious of these is ESA's Gaia mission, launched in 2013.