and educate visitors. Other exhibits offer a hands-on understanding of concepts related to rocketry or space travel. A bell jar demonstrates the reason for using a rocket instead of a propeller in the vacuum of space. A wind tunnel offers visitors the opportunity to manipulate a model to see how forces change with its orientation, and The Mind of Saturn exhibit demonstrates gyroscopic force (necessary for rocket navigation). An Apollo trainer offers visitors the opportunity to climb in. Some simulators on exhibit were used for astronaut training. A Project Mercury simulator shows the cramped conditions endured by the first Americans in space. A Gemini simulator shows visitors the accommodations when two people flew together to space for the first U.S. missions involving extra-vehicular activities and space rendezvous. Exhibits also cover the future of space flight. Two Orion spacecraft exhibits show the next NASA spacecraft, and a Bigelow Aerospace commercial habitat model details a space tourism effort. Bus tours The Space & Rocket Center offers bus tours of Marshall Space Flight Center. The tour offers views of all four National Historic Landmarks at the center including a stop at the landmark Redstone Test Stand, where Alan Shepard's Redstone Rocket was tested prior to launch. Another scheduled stop is the Payload Operations and Integration Center, which serves as mission control for a number of experiments. Bus tours originally started July 4, 1972, but were suspended following the September 11 attacks in 2001. Tours resumed July 20, 2012, the 43rd anniversary of the Apollo 11 Moon landing, limited to United States citizens because of security protocol at the Army installation, Redstone Arsenal, which contains Marshall Space Flight Center. Traveling exhibits In the summer of 2010, the Space and Rocket Center began hosting traveling exhibits. The first was Star Wars: Where Science Meets Imagination with other exhibits planned. The United States Space Camp hosted at the facility has provided themed camps in conjunction with the exhibits, including a Jedi Experience camp. Other traveling exhibits include: The Chronicles of Narnia: The Exhibition Traveling Exhibit CSI: The Experience Traveling Exhibit A T-Rex Named Sue and Be the Dinosaur 100 Years of Von Braun: His American Journey Mammoths and Mastodons: Titans of the Ice Age Miss Baker gravesite The U.S. Space & Rocket Center is the resting place of Miss Baker, a squirrel monkey who flew on a suborbital test flight of the PGM-19 Jupiter rocket on