Doomsday Machine--the one released in 1972.
SPOILER ALERT (if anyone cares):
Actually filmed in 1967, most of it. They ran out of money and put everything on hold. When they scraped together a few bucks they decided to finish it--but without any of the original actors. The 1967 info makes sense because when they looked through the viewfinder, the earth was a pink version of the old Universal logo. I thought, Wait a minute--this was filmed after the moon landing! But it actually wasn't.
I'm not making this up. It out-Woods Ed Wood using his wife's chiro as Lugosi's character in
Plan 9 after Lugosi's death.
The movie has 3 sets: part 1 on earth with all the standard pre-launch stuff and additional spy stuff to justify turning the ship into a 3 Adams plus 3 Eves re-seeding *cough* attempt. Part 2 in a totally impractical ship that's supposedly going to take 2 years to land on Venus and colonize it. (Go with me here.) Part 3, filmed in 1972, has the ship running out of fuel, so they hook up with one of the old Soviet experimental ships. 2 of them make the spacewalk, so all we see is a super-dark interior set with 2 people encased in space suits. the voices are suitably muffled--how convenient. They sort of get the old ship working and try to get the others to come over so they can land on Venus. Then they lose contact with the others. Then--A booming voice fills their ears! It's the hive-mind of the super-evolved, super-ancient inhabitants of Venus. They've killed the others because humans are ebil and a threat to all other species. They're going to send the 2 remaining humans out beyond the edge of the galaxy because "something extraordinary awaits them there." (Since they've killed the other humans, this plot point makes no sense. I wonder if it was a sop to the audience so they didn't leave the theater all depressed because humanity was extinct.) Then the little spaceship zooms out into space. Roll credits.
It's free on YouTube. If you have 90 minutes and want a laugh, check it out. IMO a better version of this kind of movie was
The Angry Red Planet. (Not on YouTube, alas.)