Here’s the ones I’ve played over the past couple of years.
Mothership is survival-horror in space. Very easy system. Old school in its approach. Highly lethal for characters. The Player’s Survival Guide is available for free (though it’s the Zero-Edition version, with First Edition dropping later this year after being Kickstarted). The published materials are solid with nice variety. Also a lot of third party support for the game.
As others have mentioned, the Alien RPG is a good game. It has some very evocative elements, and the Stress/Panic mechanics are very cool. I think it likely needs a bit of work for a longer campaign, especially if you wanted to do like a colony or space-station type campaign. I imagine they’ll put out a campaign guide for that style of campaign at some point. Absent that, the game is really strong for short mini-campaigns, or “cinematic play” as they call it.
The Heart RPG is a blend of different types of horror… survival horror, body horror, a dash of cosmic horror… combined with dungeon delving. Built on the Resistance System, the game casts the player-characters as delvers into the Heart, a living, twisted tear in reality that changes to suit those who enter it. It’s a living dungeon that tries to give people what they want, but its understanding of people is poor, so what it gives people is always some weird twisted take on their needs or desires. The game is very much about the toll this lifestyle takes on those who choose it… the toll on their body, mind, and soul. It’s a great game, with a very unique setting that allows for all manner of locations, situations, and characters.