That background is kind of like Aliens and Dead Space. The rules allow you to create crew members on a ship that is exploring a huge and dangerous universe.
Core mechanicsThe main dice used are d10s are the main mechanism is a d% check as per so many Runequest-inspired games.
The other elements such as saves and advantage and disadvantage will probably be familiar to most people through Dungeons and Dragons though.
SkillsThe skills are maybe the most distinctive thing about the system as they are setup in a tree with three tiers where higher-tier skills have prerequisites.
Higher-tier skills cost more to buy and offer a bigger percentage towards checks (which seems like a sensible way of dealing with percentile whiff).
While a bit involved (and with the naturally debatable choices about whether given skills sit in the right tier and have the right prerequisites) I can see that this might be an acceptable balance of crunch with simplicity of play.
StarshipsLike skills the rules for Starships are both relatively short while not being simple. You choose the things you want your ship to have and then things like engines and so on are calculated on the basis of your choices.
The entire ship spend then goes towards building the stats for the ship.
It is reminiscent of Black Book Traveller but again has a number of improvements over that system while still benefiting from a calculator. Mainly that it focuses on the outcome you want and then backfills the costs and the calculations.
ArtMothership is hands-down the best illustrated game of 2018. That's because its art is integral to the content and used not just to illustrate but to illuminate the meaning of the text. This is not merely beautiful drawings that sit alongside the text, here form and function merge and become one. It's beautiful and utilitarian and is setting a new benchmark for me.
OverallMothership seems a dense but neat system for playing combat and exploration orientated sci-fi games. However as a set of rules it doesn't really have a lot to say about horror. It is one of those games which hopes that throwing out a simulationist approach will result in the game it wants. Ultimately there's nothing to explain why horror will happen on the protagonists will get involved in it.
I like Mothership a lot but its going to take a mix of the right idea and the right group of players to make it work.