having been working with various astronomical data for years now there are a lot of good sources out there. some of my favourites:<p><a href="https://pds.nasa.gov/" rel="nofollow">https://pds.nasa.gov/</a> - planetary data service has a lot of data collected from space missions if you are willing to poke around, use some funky tools and maybe even port awful non-standard C or fortran code from the 70s to read files :P<p><a href="http://vizier.u-strasbg.fr/viz-bin/VizieR" rel="nofollow">http://vizier.u-strasbg.fr/viz-bin/VizieR</a> - a huge collection of catalogues of stars, galaxies and other things that can be searched and browsed.<p><a href="http://ssd.jpl.nasa.gov/?horizons" rel="nofollow">http://ssd.jpl.nasa.gov/?horizons</a> - a great source of planetary, cometary and other ephemerides, with e-mail and telnet interfaces that are surprisingly convenient.<p><a href="http://pds-geosciences.wustl.edu/missions/venera/" rel="nofollow">http://pds-geosciences.wustl.edu/missions/venera/</a> - a small selection of data from the russian venera program - a bit of a special case - but i've not found much of this data available (online at least - offline is something else :P)<p>its a bit of a shame that the fortran friendly file format still seems to the the most popular way to store this kind of data... but its easy to parse thankfully :)