Don't get me wrong, I have fond memories of
Space: 1999. The visual effects were generally pretty amazing. But even as a teen watching it in the 70s, I knew the science was attrocious and some of the acting a little dodgy. And I got sick of the "only-the-Commander-can-see-what's-going-on-so-everyone-thinks-he's-crazy" episodes, which happened even more in the 2nd series.
Well, as I said the 1976 reboot was a disaster. I have no regard for it. (Which some people seem to think is a "second series" - crazy!

)
I was using "series" more in the English sense, what we in America would call a second "season". And yes, there were A LOT of pretty big changes from the first to second season, largely unexplained.
Yes, I know. I'm English. I wouldn't have thought you meant anything else.

That's my whole point though. The second series
is a reboot. Despite what its producers might have claimed for it. They change the setting, the costumes, the music, they change the style of storytelling, they eliminate most of the characters and bring in new ones without any explanation. But worse than that, they change the philosophy, the ethos.
The modern Trek series were certainly guilty of over-relying on technobabble, moreso than the original series. But I'd still place
Star Trek (TOS) head-and-shoulders above
Space:1999 in the science department, in general. (Yes, TOS DID have some real

science moments, I'll agree. But overall...)
Well, the way I see it, most popular tv sci-fi mucks around with the laws of physics. The approaches are different. It strikes me that Star Trek's attitude is to say, it's the future and man's technology has overcome the nature of reality. So you can have matter transportation, and faster-than-light travel, and what have you, as long as you've got a machine that you say can do these things. It's just a magic wand really, but it's dressed up in technical-sounding terms. Space: 1999 on the other hand, presented the universe as a strange, scary and unknowable place - our heroes were not futuristic space adventurers, they were completely unprepared to face what was happening to them. So when things happened that defied all their (20th Century) understanding of science, sometimes they just had to roll with it.
Admitedly, I haven't seen Space:1999 in a looong time (though I'd love to watch it again), but even the main premise (the Moon blasted out of orbit by an explosion of nuclear waste, unless you accept that some "higher power" was responsible, as was hinted at later; and traveling between star systems weekly) was beyond credibility;
Funny, isn't it? Everyone always says that (i.e. travelling between star systems weekly) but it doesn't happen at all. Not first series anyway. In the 24 episodes, they actually encounter 9 star systems (and a couple of wandering rogue planets) and the series events take place over a period of at least three years. Now obviously, I accept that several months journey time between star systems is impossible. But I don't think that's what the show is about. It's a sort of modern-day myth really, an epic saga about the group of wanderers cast adrift in the wild and hostile universe. It's the journey that's important, not the means by which it takes place. And like the ancient myths, the "gods" are driving the fate of our heroes. Yes, I certainly do accept that higher powers are responsible for what's going on. It's not a hint, it's definitely stated within the series. Their odyssey is planned out for them. On the way, they restore lift to the planet Piri, advance the Atherians to their higher state of being, and return human life to its original location on Arkadia. And Arra prophesies that the Alphans' destiny is to multiply and spread throughout the universe. Now, I don't believe in "gods", but if non-corporeal beings are guiding what's going on, we don't really know what technology they've got at their disposal, only that it's impossible for us mere humans to grasp what it is. It's an application of Clarke's third law. If they're guiding the Moon's course, then it's not a stretch to me to imagine that they're using space warps of some kind to advance the Moon through the universe (and they certainly do encounter more than one space warp in the actual episodes) - nor is it a stretch to go right back and postulate that the Moon's initial breakaway was somehow engineered by fiendishly complex technology. They even say in the series that it's impossible, that they should have been destroyed.
Ah, a true believer.

As an old-school
Star Trek fan, I recognize the devotion. So we'll have to disagree on the relative placement of our favorites in the SF TV pantheon.

But I
am re-inspired to try to re-watch
Space:1999 sometime soon to re-experience the fun.
Do it! Just stay away from series 2...