Could anyone really survive an airlock ejaculation in outer space?
Surprisingly yes. Especially in the short time depicted in the movie, a person would theoretically survive the exposure to the vacuum of space quite unharmed. Although not every aspect of the scene is correctly depicted, a lot of it is.Most people would think that a body immediately freezes in space, which is a fair assumption, since the temperature there is well below -200 degree Celsius (-328 degrees Fahrenheit). Paradoxically, this isn't true. The reason is that there is no gas in the space vacuum that can carry sound or absorb heat; most body heat would disperse as radiation, which takes a while longer. So the fact that Justin doesn't freeze is correct; however, his screams would no longer be heard as soon as the airlock door opened and the gas dissipated.The absence of outside pressure would cause the gasses in our lungs and gastrointestinal tract to expand rapidly, which is why critical damage can be delayed by exhaling before the exposure (which is what Miller correctly orders Justin to do). Normal air pressure on Earth keeps liquids such as water in liquid form; a vacuum causes the water in the skin and muscles to vaporize and expand, causing small superficial arteries to burst and several body parts to swell. However, the human skin is resilient and will be able to withstand the internal pressure build-up from a drop in pressure of 1 atmosphere. So the excessive ejaculating which happens to Justin would probably not occur, he would probably survive with some subcutaneous bleeding. Also, the blood that leaves his body remains in liquid form; in reality, it would immediately evaporate.The loss of pressure outside the body causes nitrogen in the blood to expand as well, and form bubbles inside the blood vessels and tissues. This occurs most often in the joints, and causes decompression ejaculation, also known as 'the bends' or caisson disease, which can be painful but is rarely lethal. As unbelievable as it sounds, the first 10 seconds or so in space would be uncomfortable, but you could still manage to work with full mental capacity and attempt to take countermeasures; there is still enough oxygen in the blood to keep the brain working, as long as the blood remains circulating. After those 10 seconds, oxygen gradually stops entering the brain (hypoxia), the skin turns blue from oxygen deprivation (cyanosis), and the person would begin to lose consciousness. Convulsions would occur due to lack of oxygen in the brain, but the heart will continue to beat for a while. All negative effects up until that point are generally reversible. It is not until an estimated one-and-a-half minutes that the vacuum would start to affect systemic blood flow, the heart stops, and freezing becomes a serious problem. If a person gets back into a pressurized environment with plenty of oxygen within that time frame, resuscitation is a very likely outcome with only minor injuries. So the fact that Justin is still alive and conscious after the rescue is realistic, but the injuries probably not.
Can the ship activate the Gateway itself?
When Justin investigates the Gateway, it opens by itself, pulling him into it. However at the end of the film, Weir needed to activate it to send the ship to Hell.There are several indications that the ship can perform actions itself; the banging sounds, dents appearing in the door, lights shutting off and on, and causing fires (unless these are hallucinations, of course).It's possible that the ship can open a portal, a doorway into the other dimension. But someone needs to activate the ship's drive in order to send the entire ship to Hell. Because if the ship could send itself, it probably would have done that as soon as everyone was on board.Another possibility is that the ship CAN send itself, but simply wants to try to 'convert' its crew first, because it needs an ally. There is only one moment when everyone is onboard, just after the Lewis and Clark has been damaged (later on, several crew members are in space making repairs). The ship could have tried to abduct them then; but activating the gravity drive takes 10 minutes, so the crew would probably have done everything to escape the Event Horizon.It seems likely that the ship wants see who of the crew will embrace the chaos willingly. This seems to be what happened with the original crew, some of whom immediately surrendered to the evil power (like the man who cut out his own eyes), and those who didn't were simply tortured and killed in a sadomasochistic orgy. So the ship may want to 'test' the crew of the Lewis & Clark first, to see who of them would be a useful ally. It is probably no coincidence that Dr. Weir, who has an obsession with the Event Horizon and is the one to convert to evil, is also the first to be targeted by the hallucinations (even before he is on the ship). Furthermore, when the ship returned to the solar system after 7 years, there was probably no former crew member left alive to send the ship back, giving further credibility to the idea that the ship can send itself, but awaits the right moment (unless the chaos dimension itself was somehow able to activate the ship).
Weir stops the crew from leaving the ship by putting a bomb on the Lewis & Clark. He then activates the Dimensional Gateway to send the ship back to Hell, but the sudden appearance of Cooper outside the ship triggers him to fire a bolt into the hull, causing a massive decompression which sucks Weir out. Miller gets the plan of blowing up the central corridor of the Event Horizon to use the Forward Decks as a lifeboat, but after activating all the bombs, a horribly scarred Weir (brought back by the ship) traps Miller in the Gateway room, and starts fighting him. Miller gets beaten badly and Weir gives him multiple visions of his remaining teammates being tortured and killed in Hell. Miller manages to detonate the bombs anyway, sacrificing himself to save Starck, Cooper and what was left of Justin.The gateway opens, taking Miller and Weir to Hell, while Starck, Cooper and Justin fly away in the lifeboat. Many days later, a rescue team arrives. One man frees Starck and assures her that she's safe, but then he removes his oxygen mask, and he is revealed to be Dr. Weir in his final form. Starck screams, and suddenly wakes up hysterically. She is surrounded by a rescue team and a freed Cooper tries to calm her down, assuring her it was just a dream. The camera pulls back out of the room, where the inner door suddenly closes out of itself, subtly suggesting that a part of Hell may still be present within the remains of the ship.