How much farther away could Hipparchos probe our galaxy relative to measurement on the Earth?
Hipparcos’ mission was to determine the positions of selected stars as accurately as possible using trigonometric parallax. The orbit of the earth around the sun causes nearby stars to appear to shift back and forth against the background of more distant stars and galaxies, and the amount of shift is proportional to the star’s distance from the earth. At a great enough distance, a star’s parallax shift becomes much smaller than the apparent diameter of the star itself and cannot be accurately measured. The upcoming Gaia probe, scheduled for launch in 2011, is expected to measure the parallax of stars tens of thousands of light-years from earth.
