Second STS-120 spacewalk finds damage to station power system

William Atkins
Monday, 29 October 2007 19:14

Science - Space

Regularly scheduled extravehiclular activity (EVA) tasks proceeded nominally however astronaut Daniel Tani found possible metal shavings on one of the station’s Solar Array Rotating Joints, indicating an improperly moving joint.        

The Solar Array Rotating Joint (SARJ) is a large joint assembly that allows the station’s solar panels to rotate with the use of bearing assemblies and a servo control system. With such motion, the two SARJs are always facing the Sun in order to generate electricity from sunlight.

One SARJ is located between the P3 and P4 truss segments (“P” is for port side, right side, of the station) and the other is located between the S3 and S4 truss segments (“S” is for starboard side, left side). The joints rotate continuously to keep the solar array panels on the truss segments always oriented towards the Sun. Each SARJ is about 10 feet (3 meters) in diameter and weight around 2,500 pounds (1,100 kilograms) on Earth.

Tani commented, within the New York Times article “Space Station Has Power System Damage,” upon seeing the shavings, “It’s quite clear. There’s metal-to-metal scraping, or something, and it’s widespread.” [subscription required: http://www.nytimes.com/2007/10/28/science/space/28cnd-shuttle.html?em&ex=1193803200&en=a94eed39cac9fa6b&ei=5087%0A]

The spacewalking astronauts, Tani and Scott Parazynski, were asked to investigate a possible problem with the SARJ when a NASA flight controller noticed abnormal readings to the SARJ motor as it ran over the past two months.

NASA managers with the mission are unsure why this is occurring to the SARJ that is positioned on the right side of the station, which was installed only four months ago. The SARJ for the left side of the space station is working properly.

NASA mission managers are discussing their options with the troublesome SARJ. In the meantime, the grinding joint will be fixed in position so that further grinding does not occur. Images taken by the spacefaring astronauts are being looked at by NASA engineers and, once samples of the flakes are returned to Earth, they will be studied in greater detail.

If the problem is major in scope, it could have an impact on future missions. NASA officials hope this will not be the case. The next two missions, in December 2007 and April 2008, will attach European and Japanese scientific laboratories, respectively, to the recently installed Harmony module. NASA already has backup systems (such as motors and rotors) installed that may be able to take over for the faulty motor. However, NASA managers will not make a decision until all options are considered and the best decision made.

On a more positive note, the space shuttle Discovery has been given the ok to return to Earth and re-enter the Earth’s atmosphere at the end of its mission. Its thermal tiling system look fine, with no damage incurred during its launch from ice or foam coming off of the external tank.