This is a free game you can download from the NASA website, or from Steam. It's basically a first person game in which you can maintain a base on the surface of the moon. It uses realistic physics and generally shows the whole process of lunar habitability in as realistic a manner as feasible in a computer game.
It's a sim-lite, really. The interface is nice and straight-forward, and the computer basically tells you what you have to do next.
The goal - in the first scenario at least - is to repair damaged components on the lunar surface. You need to use a variety of tools and technologies to do this in as short a timeframe as possible.
It's all very interesting, and the science nut in me loves this to bits. It will not appeal to someone who's expecting it to be a shooter. There are no aliens, no zombies... no bad guys.
The difficulty is in the design of the game, really. It's intended to be played cooperatively. There's not a lot of time to complete the requirements of each scenario - lets face it, when you've got an oxygen leak, you can't really wait around - so you do need to have a few people working alongside you to figure out the problems, weld things together, and drag stuff around.
Unfortunately, when attempting to get into a multi-player game I've found that most people want to just drive the buggy around quickly and crack bad jokes about Tiger Woods while I'm trying to nail two bits of hab-module motherboard together.
Playing as a singleton is fine, but it takes away too much of the challenge, removing time limits and other restrictions altogether, giving you a sandbox. A mix of the two would be better... if you've only got one player, increase the time limit a bit... that's all.
In short, this is a fun little game if you're the sort of person who prefers making things over shooting things, especially if you've got a couple of like-minded friends who are happy to log in and play together.
This is precisely the sort of computer game that would go down spectacularly well in a 5th form science class, I suspect.
"Are you not entertained? Are you not entertained? Is this not why you are here?"