Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a pretty good government conspiracy thriller about a faked mission to Mars. (It helped to feed the crazy conspiracy theory that the moon landing was faked.)
NASA is about to send a manned mission to Mars named Capricorn One, but they discover that the life support system is defective. To avoid bad publicity from delaying or cancelling the mission, and to keep their funding, they decide to lauch the ship with no astronauts. They bring the astronauts to the desert and film the fake Mars landing on a movie set, broadcasting the images to the public as if they were watching a real Mars landing. The real ship is brought back to Earth but it is destroyed on re-entry, forcing NASA to report that the astronauts all died. They confine the astronauts in an isolated location so the fake mission will not be discovered. The astronauts realize they will never be allowed their freedom so they escape and fly away in a stolen plane. But the plane didn't have any fuel so it crash-lands, stranding them in the desert. They split up and walk in three different directions, pursued by helicopters, and with almost no survival gear.
One of the austronauts, played by James Brolin, ducks into a cave to hide from the helicopters. When he gets to the back of the cave he hears a rattling sound and sees a rattlesnake nearby that is coming towards him. He wraps a strip of material around his hand and uses it to distract the snake while he picks up a rock in the other hand and smashes its head, killing it. Then he takes his broken signal mirror and cuts open the snake's belly and starts eating it raw. We can see by his hesitation and by the look on his face that he is not a fan of raw snake sushi. But he's been in the desert for a while without food or water so it makes sense he would eat it. In fact, unlike most snake scenes in movies, the entire snake scene makes sense. It's possible he would encounter a rattlesnake in a cave, and it makes sense that he kills it with a rock since it was endangering his life and a source of food. And it's likely he would eat the snake because he's thirsty and hungry. Making the snake strike at his hand when it was wrapped only in thin cotton was a really dumb idea, but desperate people do dumb things. He should have dangled the loose material down from his hand to distract the snake and avoid getting bitten. We're supposed to believe that the snake struck at the hand and missed, but if you look closely you can see that the snake really did strike the hand, but it was a fake hand. If the scene was not fake, the astronaut would have probably taken enough venom to kill or disable him.
The live rattlesnake we see on screen is different from the rattlesnake he bashes with a rock and picks up. The live snake is probably a live Eastern Diamond-backed Rattlesnake, but the snake he kills appears to be a Western Rattlesnake that was already dead. When the dead snake is cut open, it appears they used a real dead snake. According to a comment on IMDB, the snake was stuffed with raw fish for Brolin to eat.