For rocket fans, JPL's open house is the right space Steve Wicke is "just big into space." The Westminster man took four months off his warehouse job last year to visit every NASA site in the United States. On Saturday, he joined an estimated 20,000 people who swarmed the Jet Propulsion Laboratory's leafy campus for its annual open house weekend. Buses and SUVs clogged Oak Grove Drive near the La Cañada Flintridge boundary with Pasadena and filled JPL parking lots to disgorge passengers of all ages, who descended on the exhibits and activities as if they were new amusement park rides. Among the most popular sites were the "Mars tents," where visitors could see exact replicas of the robotic explorers that roam the Red Planet's surface. The lines were long to get into a Harrison Ford-narrated movie billed as a "spectacular journey through the solar system and beyond." Others checked out how spacecraft and instruments are assembled in JPL's largest "clean" room, or posed for a 3-D stereo image of themselves on the surface of Mars. Wicke, 59, was happy to take in "Eyes on the Earth," an interactive computer demonstration of how satellites constantly feed data to scientists studying changes in the planet's climate. "I try to come here every year," said Wicke, wearing a gray NASA T-shirt as he watched how scientists track Earth's "vital signs" by measuring substances such as chlorine monoxide, which correlates to damage to the protective ozone layer. "I'm not making this up," Kevin J. Hussey, a JPL manager, told the audience as he showed them evidence of the ice melting in Greenland as the climate warms up. We just collect the data and report it. You can go on our website and see for yourself. Out on the mall, Stephanie Chu waited while her son, Juno, 11, and daughter, Annabelle, 9, checked out replicas of Mars explorers. "I'm interested in how these rovers work," Juno said, eyeing a small contraption that moved around nimbly. This one is 'Spirit,' and it was the first one to climb hills. It's awesome! Having arrived at JPL on a field trip from Southeast High School in South Gate, Daisy Campos, 16, Lourdes de Santiago, 17, and Natalie Lopez, 15, said their favorite exhibits were the ones about climate change and weather. "It's all pretty interesting," Lourdes said. Nelson Brown, an aerospace engineer from Lancaster who brought his son, Nathan, 4, found that the crowds had grown considerably since his previous JPL open house visit a few years ago. "We're skipping a lot of things that have long lines," Brown said as Nathan peered at the Mars explorer replicas as the robots strutted their stuff. For some visitors, the open house was like coming home. Bob Elliott, who worked as a technical writer at JPL before retiring eight years ago, said he and his wife, Evie, both had fond memories of the facility and admired the hard work and volunteerism of its employees for making the event possible. "It's really wonderful for the kids" and may inspire some youngsters to become scientists or engineers someday, said Elliott, who lives in Granada Hills. His wife jokingly said: "If you want to meet a nerd, this is the place to come." But she hastened to add she meant that "in a good way, of course." The open house - this year's theme is "Great Journeys" - continues on Sunday from 9 a.m. to 4 p.m. There is no charge for admission or parking.