And, yeah, I wish we were really going to Saturn by 2020, but, given that "Tomorrow is Yesterday" took place in the sixties, I figure 2020 was about about as far in the future as I could push the Saturn mission without making Shaun Christopher a retiree!
The old Star Trek Chronology actually put Shaun's mission in 2009, but, alas, that ship has sailed.
Why can't that still have happened in '09 in the Trek universe, even if it didn't in ours?
Personally, I don't see the need to keep revising the Trek timeline of the Late 20th/Early 21st centuries to fit what is going on in our timeline. It's always been pretty clear that they are two very different places. Space Seed already shows that spacecraft technology in the 1990's of the Trek-verse was way ahead of anything we've got in our timelline. Their NASA also seems to have launched a lot more unmanned probes than we did (their Voyager 6 vs. our Voyager 2).