“Chandrayaan’s first sensor failed much earlier”

BANGALORE: Even as the failure of Chandrayaan’s ‘star sensor’ continues to make news, top officials at the Indian Space Research Organisation (ISRO) have revealed that the April 26 snag, announced on Friday, was the second sensor failure — the first one having occurred “much earlier.”

The lunar spacecraft had on board two star sensors, with one as back-up, to determine the orientation or “attitude” while in orbit. Although the ISRO maintained that the “spacecraft started malfunctioning on April 26,” necessitating a switch-over to a contingency gyroscope, it appears that the malfunction had taken place earlier.

At a press conference on Friday, ISRO chief Madhavan Nair said the star sensor failure was due to “excessive radiation from the sun.” It was detected on May 16. As the sensor could not be recovered at this stage, the remainder of the two-year mission would be completed using a gyroscope, an electro-mechanical device that was used in Indian Remote Sensing satellites.

Gyroscopes, however, needed regular intervention to stabilise their orientation, and the ISRO’s ground stations had begun weekly attitude corrections, the official said. With the failure of the two star sensors, the number of technical glitches Chandrayaan has encountered in its eight-month lunar orbit stands at three — the third being the failure of a Bus Management Unit, which has been replaced with a back-up unit.

The public relations office at the ISRO did not confirm that the April 26 sensor snag was the second of its kind.

Thermal heating
The Rs.400-crore satellite encountered problems of thermal heating also. In one instance in January, the temperature within the spacecraft rose to 80 degrees Celsius, according to another ISRO official. The optimal temperature for electronic packages and payloads is zero to 40 degrees.

Chandrayaan was launched on October 22 carrying 11 payloads (scientific experiments), including the moon impact probe that crash-landed on a designated location near the moon’s South Pole in November.

Five payloads were developed by international space agencies, including the National Aeronautics and Space Administration (NASA) and the European Space Agency. link...