First test is always adjusting my sizing so the module sizing matches the sizing it would be on a piece of paper if I printed it. I can't decide if you pass this test or not. I feel like I might have to do an actual print to be sure. The font size feels like it could stand to go up by a point or two. At the same time, you didn't crowd it so tightly that it feels like an impossible wall of text. I think how I am going to fall on this will be decided by how I feel at the end of the review.
So, starting with the cover you went with the bold choice to have a cover and then not crowd it with information. What is here does tell me most of the important things. Title, designer/writer, system, level expectations, setting style, and imagery that lets me know this is taking place in space. Not bad for a cover that looks fairly open. Good job there. With the cover being so open, I decide to flip to the last page now as a glance. Yep, feels like a back cover in keeping with the front one. Excellent. At A6, this means it feels like a legitimate trade paperback of the time period. We're off to a good start.
We start on page one with an overview. I've met GMs who would be confused here, but I think most would get it. Since such individuals do exist, you were kind enough to have a colorful call-out telling us the key goals of the module. Bonus points for not looking like another bulleted list. Probably only possible due to the simple 3-goal setup.
Next is a map, but the page after isn't just the key. Instead, we diverge to the countdown clock. I feel like (if you can do it with the space) I would have felt it was more natural if the countdown clock was swapped with the map. This means I have an immediate understanding of the ongoing risk before I ever see the map. Orbital decay information and a clear graphic for the tracking of what is going on. That would cause the map to move down where the number key is so they are together. From there, I move to the information for each of the numbered rooms.
I'm not saying this layout is bad. Printed in a configuration where pages open in the correct order, 2 and 3 will face each other. Not a huge issue in that case. However, if someone decides to scale things up and single-page prints each, it means turning back and forth between two pages due to the way information was split. Something to consider.
Formatting was done so the borders of each line are aligned. Usually this looks very nice, but the very first line of the second paragraph under Orbital Decay Timer ends up having very open spacing between words. It may be worthwhile to find a way to reword this so it doesn't have such a jarring break. This happens in a few other places in the document as well.
Regarding the timer itself, I kind of love how this works. Nice changes with each drop down the ladder. Having down constantly change could be a real harrowing experience.
What I don't like is an all-or-nothing game. For a one-off that's fine, but if I wanted to integrate this into an ongoing game somehow (not sure how, but could be fun) I am going to be in a rough spot. If players fail, the universe dies. Never my preference. Basically, this adventure is absolutely limited to a one-shot game.
Two words: Space Pterodactyls. If anything at all screamed theme, this is it. I'm suddenly remembering the old movies and cartoons from the late 70s that were inspired directly from the pulp era stories. I don't think you need the stat diff for calming a second time on the same page.
Blasphemy is painful and sad. An interesting use of a monster besides the obvious 'it attacks' situation. Giving it a backstory of it's own and making it affect the room without directly affecting the characters is nice. They'll almost certainly mention her name if they find the journal though, so it sets up an event that won't trigger the moment they enter the room. Solid.
Next room has an interesting mechanic where players have a hazard that can be converted into a boon if they figure out how. Nice usage. And of course, a valuable item under the ooze. Of course at least one of them is getting coated. It's probably the one already bearing a low intelligence and wisdom anyway though.
Kraken is a rough enemy, but you give several ways to handle it. My experience is players never willingly put out a torch in ShadowDark, so having other options is nice. The passage shifting is an interesting mechanic as well. Gems should have a bit more clarity. (50gp each) vs (50gp total). Different people will read this information quite differently.
Flesh tanks are fair enough. A trade-off effect if you get in them and some elements to put a twist on a common enemy. I only just now realize I should note good job with using bold to call out key elements. When I glance back over things that's important.
Legal stuff, sure enough. Handled cleanly. Tiny print doesn't matter on this. Anyone who cares enough will pull out a magnifying glass or look at the PDF. Nice to see another familiar name involved in the project too.
Area 6 again offers a way to avoid combat if players wish. It probably costs a little extra time, but if they took a beating earlier it may be vital. Ape is another sad element that adds a lot to this adventure. Also interesting to see people can wager their souls on a game of cards. It would be more impactful if this wasn't a one-shot style setup though.
Simple enough boss fight where there's enough change to keep it interesting. Snake swarm may not be much of a deterrent to players. Depends on the group class spread. In the treasure after, the Duskblade is probably the only one I am likely to pull into another game. The rose is basically useless outside of low g and the apple is mostly interesting if used in a game with a lot of contact with angels.
Gravity generator is well explained. Numbers where I need them, details laid out clear enough. If they end up here before the final battle, they deal with a flying snake pool. I hope they break it for that alone!
It ends with the encounter table. Logical spot for it considering the overall design. Encounter 6 going off in some spots would end in strange results. Somehow contracting to push you through an entire room to reach another. Otherwise the encounters are varied enough to create interest. If you make the top 20, I would use the either the front or back page 'appendix N' notation to indicate your placement as a sales point. It's already in both spots, so you may as well make it do double duty. The one on the back is most easily modified this way, but the front cover is going to get seen more.
Honestly, it seems to be a relatively complete module that's self-contained. Nothing is hard to find when I go hunting for it and the look, feel, and concepts are all solidly within the theme. There were some smart encounters built into this and several uses of creatures to enhance the overall theme and feel of the setting. Bonus: it passes the 2 year test. I could pull this out blind and manage to run it with relatively little digging through books or confusion over text. Good job here.