Why did EOS-09 satellite mission fail? ISRO reveals initial observation

After taking off at the scheduled time of 5.59 am on Sunday, the 101st launch of the Indian Space Research Organisation (ISRO) failed mid-flight during its third stage, just minutes after the launch.
ISRO's otherwise reliable PSLV C61 rocket launched from Satish Dhawan Space Centre in Sriharikota with EOS 09, an earth observation satellite, onboard. The mission was to carry the Earth observation satellite into a Sun Synchronous Polar Orbit (SSPO). However, the launch suffered a technical glitch during the third stage of PSLV's flight and failed.
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'On 18th May 2025, the 101st launch was attempted, PSLV-C61 performance was normal till the 2nd stage. Due to an observation in the 3rd stage, the mission could not be accomplished,' said the space agency.
According to ISRO's Chairman V Narayanan, the Polar Satellite Launch Vehicle is a 4-stage rocket, and the first two stages were normal. However, during the third stage, there was a 'fall in the chamber pressure of the motor case' which caused the mission failure.
Also read: EOS-09 mission: What ISRO said on the failed satellite launch
"Today, we targeted the 101st launch from Sriharikota, the PSLVC61 EOS-09 mission. The PSLV is a four-stage vehicle, and up to the second stage, the performance was normal. The third stage motor started perfectly, but during the functioning of the third stage, we are seeing an observation," Narayanan said.
"...and the motor pressure--there was a fall in the chamber pressure of the motor case, and the mission could not be accomplished. We are studying the entire performance, we shall come back at the earliest," he added, according to a PTI report.
EOS-09 is a repeat satellite of EOS-04, designed with the mission objective to ensure remote sensing data for the user community engaged in operational applications and to improve the frequency of observation, according to ISRO.
The mission was ISRO's 101 launch. Had the mission been successful, the Orbit Change Thrusters (OCT) would have been used to reduce the altitude of the PS4 stage, followed by passivation to limit its orbital life after the deployment of EOS-09.
The mission carried a Synthetic Aperture Radar (SAR) payload capable of providing images for various earth observation applications under all-weather conditions.

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