and the firing of the attitude control thrusters. Communications and telemetry to Earth were lost for 1.8 seconds, until the system automatically corrected by switching the high-gain S-band antenna, used for translunar communications, from narrow-beam to wide-beam mode. The accident happened at 55:54:53 (03:08 UTC on April 14, 10:08 PM EST, April 13). Swigert reported 26 seconds later, "Okay, Houston, we've had a problem here," echoed at 55:55:42 by Lovell, "Houston, we've had a problem. We've had a Main B Bus undervolt." William Fenner was the guidance officer (GUIDO) who was the first to report a problem in the control room to Kranz. Lovell's initial thought on hearing the noise was that Haise had activated the LM's cabin-repressurization valve, which also produced a bang (Haise enjoyed doing so to startle his crewmates), but Lovell could see that Haise had no idea what had happened. Swigert initially thought that a meteoroid might have struck the LM, but he and Lovell quickly realized there was no leak. The "Main Bus B undervolt" meant that there was insufficient voltage produced by the SM's three fuel cells (fueled by hydrogen and oxygen piped from their respective tanks) to the second of the SM's two electric power distribution systems. Almost everything in the CSM required power. Although the bus momentarily returned to normal status, soon both buses A and B were short on voltage. Haise checked the status of the fuel cells and found that two of them were dead. Mission rules forbade entering lunar orbit unless all fuel cells were operational. In the minutes after the accident, there were several unusual readings, showing that tank2 was empty and tank1's pressure slowly falling, that the computer on the spacecraft had reset and that the high-gain antenna was not working. Liebergot initially missed the worrying signs from tank2 following the stir, as he was focusing on tank1, believing that its reading would be a good guide to what was present in tank2; so did controllers supporting him in the "back room". When Kranz questioned Liebergot on this, he initially responded that there might be false readings due to an instrumentation problem; he was often teased about that in the years to come. Lovell, looking out the window, reported "a gas of some sort" venting into space, making it clear that there was a serious problem. Since the fuel cells needed oxygen to operate, when Oxygen Tank1