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.
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.
The Hindu, July 19
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