As the spacecraft was on its way to the Moon, at a distance of 321,860 kilometers (199,990 mi) from Earth, the number two oxygen tank, one of two tanks contained in the Service Module (SM), exploded. Mission Control had requested that the crew stir the oxygen tanks, a task required to prevent the oxygen ‘slush’ from stratifying. Damaged Teflon-insulated electrical wires, powering the stirrer motor, caught fire when power was applied. The fire heated the surrounding oxygen, increasing the pressure inside the tank above its nominal 1,000 PSI (7 MPa) limit causing the tank to explode. The true cause of the explosion was unknown at the time; one conjecture was that a meteoroid had impacted the SM or even the LM.
This explosion damaged other parts of the Service Module, including critical damage to the number one oxygen tank. The Command/Service Module (CSM) relied on the oxygen tanks to generate electricity, so as a result of the explosion, very little power was available for the spacecraft. The Command Module (CM) contained batteries for use during re-entry, after the SM was jettisoned, but these would only last about ten hours. Because this power needed to be saved for re-entry, the crew survived by using the Lunar Module (LM), still attached to the CSM, as a "lifeboat". The LM "lifeboat" procedure had actually been created during a training simulation (in the simulator) not long before the flight of Apollo 13.[7](Lovell and Kluger 83-87)
The damage to the CSM meant that the Moon-landing at the Fra Mauro Highlands had to be abandoned. To return the crew as quickly and safely as possible, only a single pass around the Moon was made in what is called a free return trajectory, which uses the Moon's gravity to effectively "slingshot" the spacecraft back to Earth. To enter this trajectory, a significant course-correction was required; this would normally have been a simple procedure, using the SM propulsion engine. However, the flight controllers did not know the extent of the damage that the service module had suffered, and did not want to risk firing the main engine. Instead, the course correction was performed by firing the LM's descent engine, an option selected after extensive discussion among the engineers on the ground. The initial maneuver to change to a free return trajectory was made within hours of the accident. After passage around the Moon, the descent engine was fired again for a PC+2 burn (PeriCynthion + 2 hours) in order to accelerate the spacecraft's return to Earth. Only one more descent engine burn was required later, for a minor course correction.
Considerable ingenuity under extreme pressure was required from both the crew and the flight controllers to work out how to jury rig the craft for the crew's safe return, with much of the world watching the developing drama on television (although due to the severe electrical power limitations following the explosion, no live TV broadcasts were made from the craft for the remainder of the mission; network commentators had to use models and animated footage to illustrate their coverage).
Interior of the Lunar Module, showing the "mailbox" built to adapt the Command Module's Lithium Hydroxide canisters to fit the LM's environmental systems.One of the major stumbling blocks was that the LM "lifeboat" was only equipped to sustain two people for two days, but it was now required to sustain three people for four days. One of the most critical problems was that the lithium hydroxide canisters available for the LM's carbon dioxide scrubbers would not last for all four days. Although the CM had an adequate supply of replacement canisters, they were the wrong shape to fit the LM's receptacle; an adapter had to be fabricated from materials in the spacecraft. This adapter would come to be refered to as the "mailbox" by the astronauts.
As the re-entry to Earth's atmosphere approached, NASA took the unusual step of jettisoning the Service Module before the Lunar Module, so pictures of the SM could be taken for later analysis. When the crew saw the damaged service module, they reported that the access panel covering the oxygen tanks and fuel cells, which extended the entire length of the Service Module's body, had been blown off.
There was some fear that the extensive water condensation in the CM, due to reduced temperatures during the return leg, might have seriously damaged the electronics of the Command Module, which would only become apparent upon activation. But the equipment worked perfectly when activated, at least partly due to the extensive design modifications made to the CM after the fire aboard Apollo 1.
Apollo 13's successful splashdown
Command module being loaded onto deck of the USS Iwo JimaThe crew returned unharmed to Earth, although Haise had a urinary tract infection, resulting from the scarcity of potable water on the damaged ship and the difficulty of disposing of urine, and had to be treated in an infirmary.
The crew was instructed to store urine and other waste products on board instead of dumping it into space to avoid disturbing the trajectory which might have required an additional course correction.
Although the explosion forced the mission to be aborted, the crew was fortunate that it occurred on the first leg of the mission, when they had a maximum of supplies, equipment, and power to use in the emergency. If the explosion had occurred while in orbit around the Moon, or on the return leg after the LM had been jettisoned, the crew would have had a significantly smaller chance of survival.
Paradoxically, the crew's lives may have been saved by another failure in the oxygen tanks. At around 46 hours and 40 minutes into the mission, the oxygen tank 2 quantity gauge went "off-scale high" (reading over 100%) and stayed there. As a result of this failure, and to assist in determining the cause, the crew was asked to perform cryo tank stirs more often than planned: in the original mission plan, the stir which blew out the tank would have occurred after the lunar landing.
2007-02-28 19:57:31
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answer #2
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answered by Suzanne D 2
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