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The Hipparcos mission (1989-1993) resulted in the first space-based stellarcatalogue including measurements of positions, parallaxes and annual propermotions accurate to about one milli-arcsecond. More space astrometry missionswill follow in the near future. The ultra-small Japanese mission Nano-JASMINE(launch in late 2013) will determine positions and annual proper motions withsome milli-arcsecond accuracy. In mid 2013 the next-generation ESA mission Gaiawill deliver some tens of micro-arcsecond accurate astrometric parameters.Until the final Gaia catalogue is published in early 2020 the best way ofimproving proper motion values is the combination of positions from differentmissions separated by long time intervals. Rather than comparing positions fromseparately reduced catalogues, we propose an optimal method to combine theinformation from the different data sets by making a joint astrometricsolution. This allows to obtain good results even when each data set alone isinsufficient for an accurate reduction. We demonstrate our method by combiningHipparcos and simulated Nano-JASMINE data in a joint solution. We show asignificant improvement over the conventional catalogue combination.

Combining and comparing astrometric data from different epochs: A case study with Hipparcos and Nano-JASMINE