@Chinspinner,
I have to say that your synopsis is not very clear and so it's tough to form an opinion on your story other than it's a bit incoherent.
How do you plan on keeping the reader in the dark about the setting being post-apocalyptic, and why do you want to?
A professor using a telescope salvaged from anything called "The Ark" to follow an orbiting craft, when he knows the skies should be clear of all satellites sort of tips the reader off that something catastrophic has happened to the world. Throw in a post apocalyptic cliché like a marauding band of Scavengers attacking a lone settlement and I'm not sure your subterfuge is going to be successful.
Keep the reader in the dark as to what caused the apocalypse has been done before but if you can pull off hiding the fact the world has gone through an apocalypse it could be very interesting. It won't be easy though and based on your synopsis it doesn't appear that you'll be able to do it.
It appears that you plan to have the orbiting craft play a central role in the plot, but here doesn't seem to be much a payoff when it lands. Is this how the characters learn that there was a civilization before their time? Again, if the professor has been following it and knows it can't be a satellite because they've all run down and fallen from the skies, it implies he already knows or at least should know what's happened. Is he keeping the knowledge from the other characters? If so why?
What is The Ark and how does it factor in the story? Must be more than a source for a telescope.
Are you saying the Stranger doesn't learn she missed the flight (and her role in whatever mission was involved) until after the craft lands? Amnesia?
Are the megalomaniacal aggressor terrestrial or aliens?
Needs a lot of work on the plot or you need to share greater detail with us.
I like post-apocalyptic fiction but there's nothing here that grabs my interest except your stated goal of keeping the reader in the dark.
Who are the other characters?
The dialogue in your snippet is more promising, but most of your tags are forced. Sometimes it’s the hardest part of writing dialogue and poorly done it will break the rhythm you can establish. Suggest you select a few books that you really like and focus on dialogue and specifically the tags the author uses. I prefer it when they are kept rather simple. Every section doesn't require an accompanying action and all of the tags don't require adverbs, especially if they are all trying for drama.
How does one shake one's head sadly? When you read that someone did, does it immediately conjure a specific gesture or is it being used to tell you the character is sad. If the dialogue is well written you won't need the author to tell you the guy is sad, and if it's not, the "sad shake of a head" will be jarring. "The guy is supposed to be sad?" you may think.
I don't know what the story is that you want to tell and am wondering if you do either, but stick with it and continue to share if any of this is of value to you.
Did you write a story outline before you began writing? If not, you may want to give it a try. It doesn't have to be all that detailed, just something to serve as a framework.
Good luck