I hope you don't mind me jumping into this discussion after you already did so much work (props to you!). If you do, feel free to disregard my opinion altogether (actually, feel free to do it regardless. It's all that it is - an opinion).
I think length-wise you're great. It's definitely not too long in my opinion, and I also think overall it's a good blurb, but let me break it down into more helpful parts.
I think that's a great start. I don't know if I'd go with maybe 'desperate measures' instead, but regardless it's unique and had me immediately interested.
This was the first "bump" I felt in my writing, and I think it comes down so 'one struggling military'. It's such a wide definition that it makes your story sound like more of an idea than a story (and I sneaked a peek, it certainly doesn't look like you're at the idea level). I'd suggest using the proper name for said space military. Even if the potential readers don't know it yet, they will soon, and I think using something like 'X space division' would make it read much more smoothly while also being more interesting and engaging.
This might be an entirely me thing, but the temp of the entire paragraph would sound better to me if this was one sentence. So for example 'Losing means losing the little respect they have left, along with a few lives.' Again, might totally be just me, but I'm already here so I'm sharing.
Now, the second paragraph is the one that I think was the biggest "hurdle" to me in keeping my attention (which is notoriously bad, so keep in mind that's the pov you're getting here). I honestly also think that's the hardest part about blurbs - how much do we include in them and how much we leave for the readers to find out?
I think you can do without the last sentence ('most of the crew are fresh out of highschool...'). And as for the one before it, I'd maybe incorporate it in what comes before, maybe something like this:
A roll of dice and a lack of options led to the selection of the Celestion-5’s crew, a collection of inexperienced astronauts with personalities ranging from space-loving dreamers to cynics only in it for the money.
I feel like that keeps it more in the 'gives you a taste while making you want more' territory and also a fun, readable text rather than a description.
As for the last paragraph:
It's great. I wouldn't change a thing. I feel like you're finishing strong and making us want to open the book and start reading (also adding another problem on top of... you know, everything that can already happen when you're traveling to Neptune). So great work here!
Again, I think it's a good blurb. I think you start and finish strong, with only a few bump in the middle that can easily be smoothed out (should you choose to do so). And with your artwork looking fun and unique (because let's face it, many of us absolutely judge books by their cover), I'm sure people would be very much inclined to read it.
If you have any questions please don't hesitate to hit me up (here or through a dm) Good luck with your novel and a happy new year!