The European Space Agency launched the Rosetta space probe {wiki} in 2004 to study the comet 67P/Churyumov-Gerasimenko. It should reach the comet by 2014. Rosetta has just passed Mars, and sent back some beautiful photographs.
In the lead image above, an orange (red), green and near-UV colour filter composite image of Mars is shown; the UV channel (the blue color) has been enhanced. The enhanced UV signal clearly shows the presence of the cloud system covering most of the Martian disk.
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If you included medical/science terms it's over a million for the count in English. But current usage has us at 250k, not counting derivatives and historical words. If you include the full lexicon it's somewhere above 750k.