The craft was launched successfully toward the Moon, but two days after launch a faulty oxygen tank exploded, and the Service Module became damaged, causing a loss of oxygen and electrical power. There was a very large chance that the astronauts would die before they could return to Earth. They were very short of oxygen. Oxygen is not just used to breathe; on the Apollo spacecraft it was used in a device called a Fuel cell to generate electricity. So they conserved their remaining air by turning off almost all their electrical equipment, for example heaters. It became very cold in the spacecraft.