One half is canonical monster movie, a well-paced buildup to berserk cathartic terror. The other half a documentary about space travellers, a reimagination of the future. The other half a meditation on the loneliness of the long-distance trucker. The other half, a testing ground for innovative spaceship design and genetic engineering so good it should have been illegal.
Unreliable maths aside, Alien is a lot more than it ought to have been.