According to a summary report by NASA, the explosion of the number 2 oxygen tank on Apollo 13 was traced back to a small component: a thermostat switch designed for 28 volts that remained in a circuit operating at 65 volts. The spacecraft was approximately 200,000 miles from Earth, or about 322,000 km, which is roughly eight times the Earth’s equatorial circumference. The damage had already occurred long before the launch, on the ground. What ultimately caused the tank to fail was not the 5 cm drop it suffered during a handling operation or the spark from a fan—both of which were well documented in the investigation—but rather a thermostat that was never upgraded to match the new voltage level. This thermostat was meant to control the tank heater by opening the circuit when the tank reached a specific temperature. The chain of events leading to the explosion stretched from 1965 to April 14, 1970. In 1965, the allowable voltage for the oxygen tank heaters was increased from 28 to 65 volts, which matched the electrical supply at the Kennedy Space Center during ground operations. Most components were updated to handle the higher voltage, but the thermostatic switches on the heaters were overlooked. These switches were designed to open the circuit and turn off the heater when the tank reached about 80 °F (27 °C). Inside the tank, oxygen was kept at very low temperatures, typically between -300 and -100 °F (-184 to -73 °C), and the heaters were usually used only briefly to maintain pressure and circulate the oxygen. As a result, the thermostat was never upgraded to handle the increased voltage. The number 2 tank of Apollo 13 had originally been installed for Apollo 10. It was removed during a modification and fell 2 inches (5 cm) during extraction, slightly shaking an internal filling tube. Another tank replaced it on Apollo 10 after an external inspection, but the internal damage went unnoticed and the tank was later installed on Apollo 13. During ground tests, this tank showed anomalies and did not drain properly, possibly due to the damaged tube. On Earth, tanks were emptied by forcing in gaseous oxygen to expel the liquid, a process not needed in space. The teams then decided to use the heater to boil off the excess, which required 8 hours of power at 65 volts. This put excessive strain on the thermostat, which was only rated for 28 volts, likely soldering it in the closed position. The heater then heated the tank without limit, reaching over 1,000 °F (540 °C), which damaged the Teflon insulation of the wires connected to the fans. Approximately 56 hours after launch, around 03:06 UT on April 14, the tank fans were turned on for a cryogenic stir that homogenizes the oxygen and allows sensors to accurately measure the remaining quantity. This maneuver was standard for every Apollo mission. However, the exposed wires of the fans short-circuited. The Teflon insulation, weakened by the heat from the ground test, caught fire. The fire spread along the electrical conduit on the side of the tank, which weakened and gave way under the nominal pressure of about 1,000 psi. The number 2 tank exploded, tearing away the panel of bay 4 on the service module and rupturing a line or damaging a valve on the number 1 tank, which quickly lost its oxygen. All reserves disappeared in about 3 hours, along with the loss of water, electricity, and the propulsion system. Apollo 13 was to be the third attempt at landing on the Moon, but the mission was abandoned after the tank rupture. The three-man crew was safely splashed down in the Pacific near the Samoas on April 17, 1970, six days after launch. NASA described the incident as an oversight during a design change, where a single undersized component was not updated when the voltage increased from 28 to 65 volts. This kind of error is not unusual, which makes it particularly concerning. After the crew's return, tests confirmed that the thermostat failed when the heaters were powered at 65 volts, the voltage used by Kennedy technicians during filling and draining cycles. The 1965 modification was intended to pressurize the tanks more quickly using ground power supply.