some systems. The problem appeared to be in the ammonia pump module that circulates the ammonia cooling fluid. Several subsystems, including two of the four CMGs, were shut down. The failed ammonia pump was returned to Earth during STS-135 to undergo root cause failure analysis. Planned operations on the ISS were interrupted through a series of EVAs to address the cooling system issue. A first EVA on Saturday, 7 August 2010, to replace the failed pump module, was not fully completed due to an ammonia leak in one of four quick-disconnects. A second EVA on Wednesday, 11 August, successfully removed the failed pump module. A third EVA was required to restore Loop A to normal functionality. 2011 – Near collision with space debris On 28 June 2011, an unidentified piece of space debris was predicted to pass near the space station with a 1 in 360 chance of collision. The object flew by at a relative velocity of and a distance of only from the station. Warning of the potential collision came less than 15 hours before closest approach leaving insufficient time to plan an avoidance maneuver, so the six-person crew boarded the Soyuz capsules and closed the hatches on the station and Soyuz to prepare to undock in the event of an impact. Four minutes after the moment of closest approach, mission controllers gave the all-clear signal meaning that the danger had passed and the crew could return to work. This was the second time that crew had needed to take such precautions on-board the space station. 2011–2012 – Failure of Main Bus Switching Unit #1 and replacement EVA The four Main Bus Switching Units (MBSUs, located in the S0 truss), control the routing of power from the four solar array wings to the rest of the ISS. In late 2011 MBSU-1, while still routing power correctly, ceased responding to commands or sending data confirming its health, and was scheduled to be swapped out at the next available EVA. In each MBSU, two power channels feed 160V DC from the arrays to two DC-to-DC power converters (DDCUs) that supply the 124V power used in the station. A spare MBSU was already on board, but the Aug 30 2012 EVA failed to be completed when a bolt being tightened to finish installation of the spare unit jammed before electrical connection was secured. The loss of MBSU-1 limited the station to