Compared to today's mega thrill rides, Walt Disney World's Space Mountain is a simple spin.No upside-down loops. No 3D effects synchronized to the movement of the cars. It doesn't even reach 30 mph — half the speed of Busch Garden's Kumba. Yet as Florida's oldest roller coaster turns 40 on Thursday, it has one enduring advantage: the thrill of the unknown in pitch-darkness.Strapped into a retro rocket ship, you blast through space for 2 1/2 minutes with no idea when the next drop or sharp turn is coming. You never do see the ride in full. Like the ambiguity of a Stanley Kubrick film, the less you see, the more the tension builds.It's one of the first rides that scared the pants off us as kids — and still does.One of Kristi Walker's favorite pictures is of her then-5-year-old daughter Rylee grinning ear to ear as she stands next to the height limit post, finally tall enough at 44 inches to ride Space Mountain last summer."When she got in it, she screamed the whole time and swore she'd never ride it again," said Walker, 37, a nurse from Amarillo, Texas, who visits Walt Disney World at least twice a year.But then when the family visited again a few months later, she caught sight of Space Mountain."She did it again. And then again and again and again," Walker said.Age doesn't really matter. Quarterback Tom Brady, who will lead the New England Patriots at the AFC Championship game Sunday, recently posted a "throwback Thursday" picture of himself looking terrified on Space Mountain in 2011. "Should have gone on the teacups ride with the kids," he wrote.Opened on Jan. 15, 1975, Space Mountain was launched in the Magic Kingdom's Tomorrowland with a network musical variety special that you can still see on YouTube hosted by Lucie Arnaz and Broadway legend Tommy Tune. (Because this was the '70s and we loved musical variety shows and groovy fashions).The park released 50,000 balloons on opening day, according to news reports at the time, had a 2,000-piece band and the late astronaut Gordon Cooper was on hand (Dennis Quaid played "Gordo" in The Right Stuff) because the Mercury 9 and Gemini 5 astronaut served on the Space Mountain creative team as a consultant.That's right, this ride has space cred from one of NASA's most legendary spacemen of swagger.When it debuted, Space Mountain was the first fully computer-controlled thrill ride, according to Walt Disney World. Using specific brake zones, the computer is able to gauge the weight of each car and its distance between other cars, allowing for multiple cars on one track. Most roller coasters today have similar programs.The idea for Space Mountain came from Walt Disney himself, inspired by the jet age of the early 1960s. He envisioned a ride that simulated an astronaut's journey from a shadowy futuristic space station into deep space.It took more than 10 years to develop and three years to build, so while it may have been smartly timed for the Star Wars craze that followed George Lucas' 1977 masterpiece, the design has more in common with Kubrick's 2001: A Space Odyssey, with clean white hallways and large geometric shapes.There are now five Space Mountains around the globe, one in every Disney Magic Kingdom park. An instant classic, the ride was created by legendary Disney designer John Hench, who is the artist old Walt most often turned to make his ideas a reality, which he detailed in Designing Disney about his 50-year history with the cartoon kingpin.More than just a roller coaster in the dark, Space Mountain shuts out the outside world and lets you imagine what it might be like to board a spaceship for your next vacation.It starts with the architecture.You can see that mountain from everywhere in the park, its sloping cone rising 183 feet high. It looks futuristic, but in a retro 1960s style, with strong vertical lines that draw the eye up toward the sky from its massive base.Along the "Star Tunnel" line, visitors encounter "Infinity Windows," which have changed from the original Jetsons-era vision of the future brought to you by co-sponsor RCA. In 2009, the ride got a massive reboot that made the pre-show slicker, with interactive buttons, clever jokes that remind us of airport security checks and a "Welcome, space travelers" story line.But the best part is you never see the whole ride. You move along a queue that seems to go for miles, but at no point do you see what you are in for. The long corridors grower dimmer and the roar of the unseen roller coaster and shrieks of the riders build the tension.That's because, as Hench said in 1971 as Disney World was finalizing the design, Space Mountain "evokes such ideas as the mystery of outer space, the excitement of setting out on a journey, and the thrill of the unknown."Margaret Mackintosh, 47, a software analyst from Pinellas Park, was one of those kids who first road Space Mountain back in the '70s when her family visited from New Jersey every year."I think that's what Disney is, that you have parents who were there as kids and take their kids and say, 'This is the ride I enjoyed when I was a kid and I want you to enjoy,' " Mackintosh said. "My daughter is 5 and not ready just yet, but I can't wait to take her."Times staff writer Kelly Stefani contributed to this report.