[Phalanx]

Brock wasn't an astronaut, not originally (comic history wise) anyway. He was a competing photographer/journo to Parker, another guy working at the bugle.
What happens...
We'll just call it a "suit"...whereas without a human host, it's more of a puddle of slime that can like...crawl around in liquid form.
*Suit gets on spider-man, enhances his abilities, "kinda" talks to him, suit manifests as per the users imagination/will.
*Parker points out Brock is using crap info sources, Brock is disgraced and fired
*Suit starts taking over S-M's mind, warping his sense of justice, becoming increasingly violent, even at times when Parker is asleep inside it, it also starts effecting his "Parker" life as well.
*S-M decides to ditch the suit, which is effected by sonics and loud noises in a belltower of a church (the bell = loud)
*The disgraced Brock is contemplating suicide in said church.
*Suit is thought to be destroyed, but bonds with Brock.
*Brock hates Parker + suit hates spider-man/Parker for "abandoning" it.
*Brock + suit = the venom symbiote, who is larger than parker (mainly attributed to the fact that Brock WAS a much bigger person, at least in the comic), suit also retains "spider-powers", and is seemingly stronger. Suit can also evade "spider-sense", and turn invisible.
Basically a badguy who knows a lot about spider-man and Parker.

The character has since risen and fallen in popularity up and down, initially a stalker/villain/killer, then vigilante...but goes between "almost good" and downright evil allover the place.
Has had about 8 series of comics on his own, aside from the many spider-man appearances. He was the definative SM badguy from the mid 80's through mid 90's. Brock has since made the symbiote abandon him after it's ruined all other aspects of his life...Brock is currently dying of cancer, and comatose.


Origin confusion?
Actually, where the symbiote IS from space...Jameson Junior never really had anything to do with it. That concept was bought in only to service the storyline in the 90's spider-man cartoon. Initially spider-man himself bought it back...secret wars, miniseries about a bunch of superheroes getting put on a planet with a bunch of supervillains, a whole "lets see good vs evil" thing orcestrated by a cosmic entity called the traveller. I think the watcher may have had something to do with it.
Spider-man gets his original suit trashed, and Reed Richards (Mr fantastic - the stretchy guy from fantastic 4) being all scientist-y, finds a semi-stable substance on the planets surface which he uses to fashion spider-man a new costume, which he wore on earth for some time. It's powers and personality didn't manifest until later in the series...the vaunting of his powers were something he gradually realised. Other than that it's all pretty much the same.