We report on the development of a miniature ion trap mass spectrometer to be deployed on European Space Agency's Rosetta Mission [Rosetta Comet Rendezvous Mission, ESA publication SC1(93)7 (1993)] to analyze the comet P46/Wirtanen. The mission is due to be launched in 2003 and following a long journey through space the comet rendezvous will take place in 2011 at a distance of approximately 465 million miles from earth. The instrument has been developed on a stringent space budget of < 75 g in weight, 1 W maximum power consumption, and resolution down to <1 amu. Silicon field emitters have been successfully used as the electron source for producing ions by impact ionization. Mass spectra have been obtained for a number of,oases at pressures in the region of 10(-4) Torr.