From my initial read-through and by drawing from sources outside of the core book, yes, IMO, solo play should run pretty well.
The only "missing mechanic" for any RPG to be "solo playable" is some method for the solo player to be surprised by the outcome themselves.
Without that, you are simply plotting a novel, rather than playing.
Most RPGs do not include solo play mechanics, but there are a few examples of "how to solo play" out there which can usually be adapted easily.
Traveller, the early sci-fi RPG still going strong 40 years later, has been a breeding ground of solo play advice.
For a sci-fi RPG, there tends to be 3 "big" areas where solo play framework helps:
1. How do you drive the characters in the adventuring party on the adventure / or in combat?
2. How do you handle space trading?
3. How do you handle space combat?
All three of those can apply to Free League ALIEN RPG as well.
So that we have something specific to hang our hats on:
https://www.freelancetraveller.com/feat ... alone.html
In this, you will a checklist of "actions" that the solo player works through, regarding a space trading group getting into adventures along the way.
That should give you an idea of "how" to solo play with an RPG if you never have before.
One thing I really like about solo play is that it is a fantastic way to "test" adventures before running them for players.
Take Chariot of the Gods with the provided characters, and run through the entire mission yourself as if you were the players.
Then roll up a group of players from the core book, and do the same for Hope's Last Day.
As you can see the Freelance Traveller solo notes cover a number of contingencies, but not everything.
You can simply fill in the missing bits and re-run...
Or you can check out other Solo game rules across the net and available for purchase and build from there.
Is the time worth it?
I think so - solo playing also helps you flesh out your game map, as well as provide re-usable material like NPCs, situations, etc that can be run with a player group later.
It's also let's you "prep" an adventure to figure out where the PCs might throw curve balls at you - so you can pre-set some response which may not have been included in the adventure.
One of the authors of the Free League ALIEN RPG is a guy named Paul Elliott.
Paul writes for other RPGs as well, and one of his books, SOLO, is a set of Solo Play rules and advice for Cepheus Engine RPG.
It is an expanded version of an earlier book he put together called STAR TRADER for Traveller, and Solo includes and expands on the Star Trader material.
The Solo supplement for Cepheus has a "field-tested" solo play checklist, and a decent mechanic for how to resolve party adventures in a way that can surprise you rather than being totally pre-ordained.
So, if the online notes above aren't enough to get you going with a Free League ALIEN solo game, consider looking at Solo as well.
Or maybe Free League can get Paul to adapt his Solo book specifically to ALIEN and we all get the benefit?
We live, as we dream -- alone. ~ Joseph Conrad