Doomsday Machine

Our film opens with an American spy breaking into the Chinese Embassy. With the help of another agent on the inside, she discovers that the Chinese have built a doomsday bomb. She reports to her superiors that the bomb will go off in just over two days.
We jump to the seven-man crew of the first US Venus probe preparing to launch for it's two year trip. For some reason, the launch has to be hurried, and three of the men are ordered to be replaced by three female officers.
Then comes the launch. The crew are strapped in into La-Z-Boy recliners wearing motorcycle helmets. No, really. Recliners and motorcycle helmets. Someone sacrificed their living room furniture for this movie. They use stock footage of an Apollo take-off, but the rocket in the movie is more along the lines of a classic Destination: Moon big-finned space-ship. Then suddenly it looks like a round space station. Then it becomes a different round space station. Then it's a different finned rocket!
Anyway, it turns out that the Pentagon decided to put the women on board to preserve the American species just in case the Chinese ending up destroying the world. And before you can say, "Whoops, apocalypse!" the earth is laid waste in a montage of clips from Japanese monster movies.
The astronauts realize that the future of the human race now depends on them, so naturally they pair off and start,er, arguing. They have to dodge meteors from the remains of the Earth. Interestingly, the meteors made from the bits of earth come from in front the ship, not behind it where the Earth actually is.
It turns out that they need to drop a bunch of weight from the ship in order for it to land on Venus safely. Some people will have to sacrificed. In a remarkable coincidence, the bitch female astronaut and the bastard male astronaut luckily get sucked out of an airlock.
As they come in to land, two of the astronauts go outside to repair the ship and are believed lost. But they find an old Soviet Venus probe and manage to make their way to it.
Now things get strange, because the ending of the film was never shot in 1967, and when filming resumed in 1972, none of the actors were available. Probably they didn't want to available. Anyway, the last scene of the movie is the two astronauts who board the derelict Soviet craft. They are filmed in spacesuits with opaque visors so that you can't see that they aren't the original actors, and their dubbed voices really don't resemble the original actors' either.
Anyway, they board the Soviet ship and are trying to get it going. They spend an inordinate amount of time bumbling around silently in the dark until a disembodied voice announces to them that the American ship has been destroyed. It turns that Venus is home a race of super-powerful intergalactic busybodies who speak in stilted English: "Last of Man, listen. Of this, we will tell you." They decide that instead of allowing the last two humans alive to land, they will propel the spacecraft on a journey beyond the rim of the universe. Whatever the hell that means.
This film originally began life when filming began in 1967 as Armageddon: 1975. For reasons lost to obscurity, the film was never finished and it was shelved. In 1972, someone got hold of the film and added additional scenes, namely the teaser scene at the beginning with the secret agent, and the ridiculous ending.
Look for Casey Kasam (America's Top Ten and the voice of Shaggy from Scooby-Doo) as a Mission Control Officer, and Mike Farrell (B.J. from tv's M*A*S*H) with one line of dialogue as a reporter.