There are several reasons why we need a satellite to measure parallaxes:
The first and most obvious is that observing from space allows us to avoid the disturbing effects of the atmosphere, which limit the accuracies at which those measurements can be obtained.
The second is the possibility to see the whole sky with the same instrument and under the same observational conditions. Homogeneous all-sky surveys can only be obtained from space.
Most importantly, measuring parallax from space allows the observations to be done with a telescope with two apertures (see The scan). This allows the direct comparison of stars for which the parallax displacements are in different directions. In the small fields observed from the ground, or even with Hubble Space Telescope, this can never be the case, and leads to calibration uncertainties. The two apertures of both, Hipparcos and Gaia, allow for the direct determination of absolute parallaxes, and provide the only way to do so on a large scale.
Next: The detectors
Page last updated: 23 December 2013