On Thu, 25 Oct 2018 00:43:32 -0700 (PDT), Quadibloc
wrote:
While Hipparcos has done great work in deterimining the distances
to many stars,
ideally to be sure of the accuracy of a distance, one would like
photographs of
the star's position against a background of more distant stars.
That's the
conventional way that stellar parallaxes were obtained.

Many traditional observing methods in astronomy have been abandoned.
For instance, transit circle measurements are no longer used to
determine the Earth's rotation, instead GPS satellites are used for
that.



The Earth's orbit around the Sun provides a 2 AU baseline for such
observations.
A probe orbiting a planet more distant from the Sun than the Earth
could provide
a longer baseline. Observations with a baseline from the site of
the probe to
the Earth could be made simultaneously, avoiding the need to
account for the
Sun's own proper motion.


Of course, this is an expensive way to obtain a small improvement
in accuracy,
which is likely why it isn't being considered. But given the
uncertainty in the
cosmic distance scale, it would be nice to do something...

Gaia, the successor to Hipparcos, is in operation since a few years
back. While Hipparcos was situated in LEO, Gaia is situated near the
L2 Sun-Earth Lagrangian point, therefore Gais baseline isn't much
larger than Hipparcos' baseline. Despite this, Gaia obtains much
higher precision, through a larger mirror and improved measurement
techniques.

So something is done.