By the time Mission Control issued the wake-up call to the crew for flight day two, the spacecraft was about away from the Earth, traveling at about . As it was not due to arrive in lunar orbit until flight day four, flight days two and three were largely preparatory days, consisting of spacecraft maintenance and scientific research. On day two, the crew performed an electrophoresis experiment, also performed on Apollo 14, in which they attempted to prove the higher purity of particle migrations in the zero-gravity environment. The remainder of day two included a two-second mid-course correction burn performed by the CSM's service propulsion system engine to tweak the spacecraft's trajectory. Later in the day, the astronauts entered the lunar module for the second time in the mission to further inspect the landing craft's systems. The crew reported they had observed additional paint peeling from a portion of the LM's outer aluminum skin. Despite this, the crew discovered that the spacecraft's systems were performing nominally. Following the LM inspection, the crew reviewed checklists and procedures for the following days in anticipation of their arrival and the Lunar Orbit Insertion burn. Command Module Pilot Mattingly reported a "gimbal lock" warning light, indicating the craft was not reporting an attitude. Mattingly alleviated this by realigning the guidance system using the Sun and Moon. At the end of day two, Apollo 16 was about away from Earth.