When I first watched ALIEN, the first Alien movie I saw, I did have some knowledge on the biology of the Alien. I always assumed that the reason the Nostromo takes some damage when it lands on LV-426 is because it landed on top of an Alien, and the Alien's acid blood was causing some short circuting of the Nostromo.
Look carefully when the Nostromo lands, right before the exploding begins. You can see on the right side of the screen, one of the Nostromo's legs appears to land on something that looks kind of like an Alien. It appears to crush whatever this lump on the ground is. Either that, or the weight causes this object to sink into the ground... but something definitely happens there. It looks like some kind of creature, and whatever it is appears to be dead.
Whatever this object is, it definitely is NOT the Xenomorph we are familiar with. But that would make sense, because it is likely the chestburster which burst from the SPACE JOCKEY. So this Xenomorph has taken on DNA from the Space Jockey. It would make sense that this Xenomorph is dead, because it had been there by itself for hundreds or thousands of years while the space jockey was fossilizing, so it likely starved to death.
I do think this explains why parts of the Nostromo exploded and caught on fire when it landed. ACID BLOOD. I've never quite understood why this happened to the Nostromo when it landed, and this explains it. It does look like it lands on top of a weird object, possibly a dead alien. Check it out.
It may sound coincidental that the Nostromo landed on top of the same alien that burst from the Space Jockey, but it might not be the same alien. For all we know there were other Space Jockeys on the derelict which were impregnated.