in a nutshell, the earlier space missions were to break the barrier of what we didnt know about space in general. sputnik, Apollo, Gemini, etc. they were all the 'first to be' if you will. space walks, landing on the moon, proving the u.s. could be the Russians to the moon, etc. nowadays, its more of branching off to what we already know. but, mainly to put satellites in orbit, getting better pictures of dopplar radar, mars, clusters, further images of space, hence the hubble. but again, the difference of today as opposed to then is less "pushing the envelope" and more of enhancing and branching off into tangets of what we already pushed prior!