were flown for the first time on Apollo 13; the other experiments had been flown before. To power the ALSEP, the SNAP-27 radioisotope thermoelectric generator Apollo 13 (April 11–17, 1970) was the seventh crewed mission in the Apollo space program and would have been the third Moon landing. The craft was launched from Kennedy Space Center on April 11, 1970, but the landing was aborted after an oxygen tank in the service module (SM) exploded two days into the mission, disabling its electrical and life-support system. The crew, supported by backup systems To… The Apollo 13 prime crew undertook over 1,000 hours of mission-specific training, more than five hours for every hour of the mission's ten-day planned duration. Each member of the prime crew spent over 400 hours in simulators of the CM and (for Lovell and Haise) of the LM at KSC and at Houston, some of which involved the flight controllers at Mission Control. Flight controllers participated in many simulations of problems with the spacecraft in flight, which taught them how to react in an emergency. Specialized simulators at other locations were also used by the crew members. The astronauts of Apollo 11 had minimal time for geology training, with only six months between crew assignment and launch; higher priorities took much of their time. Apollo 12 saw more such training, including practice in the field, using a CAPCOM and a simulated backroom of scientists, to whom the astronauts had to describe what they saw. Scientist-astronaut Harrison Schmitt saw that there was limited enthusiasm for geology field trips. Believing an inspirational teacher was needed, Schmitt arranged for Lovell and Haise to meet his old professor, Caltech's Lee Silver. The two astronauts, and backups Young and Duke, went on a field trip with Silver at their own time and expense. At the end of their week together, Lovell made Silver their geology mentor, who would be extensively involved in the geology planning for Apollo 13. Farouk El-Baz oversaw the training of Mattingly and his backup, Swigert, which involved describing and photographing simulated lunar landmarks from airplanes. El-Baz had all three prime crew astronauts describe geologic features they saw during their flights between Houston and KSC; Mattingly's enthusiasm caused other astronauts, such as Apollo 14's CMP, Roosa, to seek out El-Baz as a teacher. Concerned about how close Apollo 11's LM, Eagle, had come to running out of propellant during its lunar descent, mission planners decided that beginning with Apollo 13, the CSM would bring the LM to the low orbit from which the landing attempt would commence. This was a change from Apollo 11 and 12, on which the LM made the burn to bring it to the lower orbit. The change was part of an effort to increase the amount of hover time available to the astronauts as the missions headed into rougher terrain. The plan was to devote the first of the two four-hour lunar surface extravehicular activities (EVAs) to setting up the Apollo Lunar Surface Experiments Package (ALSEP) group of scientific instruments; during the second, Lovell and Haise would investigate Cone crater, near the planned landing site. The two astronauts wore their spacesuits for some 20 walk-throughs of EVA procedures, including sample gathering and use of tools and other equipment. They flew in the "Vomit Comet" in simulated microgravity or lunar gravity, including practice in donning and doffing spacesuits. To prepare for the descent to the Moon's surface, Lovell flew the Lunar Landing Training Vehicle (LLTV) after receiving helicopter training. Despite the crashes of one LLTV and To power the ALSEP, the SNAP-27 radioisotope thermoelectric generator (RTG) was flown. Developed by the U.S. Atomic Energy Commission, SNAP-27 was first flown on Apollo 12. The fuel capsule contained about 3.79 kilograms (8.36 lb) of plutonium oxide. The cask placed around the capsule for transport to the Moon was built with heat shields of graphite and of beryllium, and with structural parts of titanium and of Inconel materials. Thus, it was built to withstand the heat of reentry into the Earth's atmosphere rather than pollute the air with plutonium in the event of an aborted mission. A United States flag was also taken, to be erected on the Moon's surface. For Apollo 11 and 12, the flag had been placed in a heat-resistant tube on the front landing leg; it was moved for Apollo 13 to the Modularized Equipment Stowage Assembly (MESA) in the LM descent stage. The structure to fly the flag on the airless Moon was improved from Apollo 12's. For the first time, red stripes were placed on the helmet, arms and legs of the commander's A7L spacesuit. This was done as, after Apollo 11, those reviewing the images taken had trouble distinguishing Armstrong from Aldrin, but the change was approved too late for Apollo 12. New drink bags attached inside the helmets, and that were to be sipped from as the astronauts walked on the Moon, were demonstrated by Haise during Apollo 13's final television broadcast before the accident. Apollo 13's primary mission objectives were to: "Perform selenological inspection, survey, and sampling of materials in a preselected region of the Fra Mauro Formation. Deploy and activate an Apollo Lunar Surface Experiments Package. Develop man's capability to work in the lunar environment. Obtain photographs of candidate exploration sites." The astronauts were also to accomplish other photographic objectives, including of the Gegenschein from lunar orbit, and of the Moon itself on the journey back to Earth. Some of this photography was to be performed by Swigert as Lovell and Haise walked on the Moon. Swigert was also to take photographs of the Lagrangian points of the Earth-Moon system. Apollo 13 had twelve cameras on board, including those for television and moving pictures. The crew was also to downlink bistatic radar observations of the Moon. None of these were attempted because of the accident.