I did some quick experimentation. The spiders have 85 HP and do a respectable amount of damage. The ship I sent in to explore had 579 shields, and the spider took it down to about 2/3rds. They took me down to 1/2 health on foot pretty fast, but I wasn't really trying to evade them.
They'd put up a fair fight for anyone on foot. If you got jumped and didn't have your gun in your hotbar already, I'd put my money on the spider. With your gun out, I'd say 90% of the time the Player would win.
The station I explored had 2 spiders.
Edit: ****, make that 3 spiders. The third one got me while I was forum-ing.
Edit 2: There was a fourth spider, but my radar was definitely clear after the 3rd. Tentatively, I'd say they spawn.