EOS-09 mission: Why failure of Isro’s PSLV launch is so rare

The Indian Space Research Organisation (Isro) suffered a setback on Sunday (May 18) after its 63rd PSLV mission to place the EOS-9 surveillance satellite in orbit failed.

The Earth observation satellite, launched using the Polar Satellite Launch Vehicle (PSLV), encountered a failure due to a pressure problem in the third stage of the rocket.

Isro Chairman V Narayanan said that the PSLV is made up of four stages and that the first two stages performed as expected.

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This failure is considered rare, as the PSLV has been a reliable launch vehicle for India’s space missions. Since its first flight, the rocket has failed only twice - the first time in 1993 and again in 2017.

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But why do PSLV launch failures happen so rarely?

In this explainer, we break down why PSLV launch failures are rare, what caused the earlier failures, and what the Isro chief said about this latest attempt.

Before diving into that, here’s a brief look at what PSLVs are.

What is PSLV?

Satellites used for navigation, mapping, and other tasks are sent into Space using launch vehicles or rockets, such as the Polar Satellite Launch Vehicle (PSLV).

These rockets have strong engines that produce the energy needed to lift heavy payloads like satellites into orbit, pushing against the pull of Earth’s gravity.

The PSLV can carry more than one payload at a time. To do this, it uses special structures called multi-payload adaptors inside the top section of the rocket, known as the payload fairing, according to Isro.

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Notably, PSLV earned the title ‘the workhorse of Isro’ by reliably placing a wide range of satellites into low Earth orbit, as per the space agency.

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Why PSLV launch failures are rare

Since the PSLV’s first launch in 1993, it has rarely failed. Only a few missions have faced problems over the years.

When failures have occurred, they were mostly caused by issues with how different stages of the rocket separate or by problems in the rocket’s engine system. These could include trouble in controlling the rocket’s direction or underperformance during a stage.

In September 1993, the PSLV-D1 was launched from the Satish Dhawan Space Centre in Sriharikota to carry the IRS-1E remote sensing satellite into Low Earth Orbit (an altitude of 2,000 km or less).

But the mission did not succeed. A fault in the rocket’s onboard software caused it to miss orbit and fall into the ocean.

The next failure came in 2017, when the rocket’s payload fairing failed to detach. This left the satellite stuck inside the rocket. Isro called the mission unsuccessful after the final stage ended, marking the first failure since 1993.

In the case of PSLV-C61, failures like this may be caused by several reasons – from problems in the propulsion stages, mistakes during separation of the rocket stages, or faults in how the rocket is guided and controlled.

What happened this time?

This was Isro’s 101st launch from Sriharikota.

The mission aimed to place an Earth observation satellite into a sun-synchronous polar orbit at an altitude of around 597 km. In this type of orbit, the satellite would pass over the same location at the same local time each day.

#WATCH | Indian Space Research Organisation (ISRO) launches PSLV-C61, which carries the EOS-09 (Earth Observation Satellite-09) into a SSPO orbit, from Sriharikota, Andhra Pradesh.

EOS-09 is a repeat satellite of EOS-04, designed with the mission objective to ensure remote… pic.twitter.com/4HVMZzXhP0

— ANI (@ANI) May 18, 2025

If the mission had gone as planned, Orbit Change Thrusters (OCT) would have been fired to lower the altitude of the PS4 stage after the EOS-09 satellite was deployed. The rocket stage would then have been passivated to shorten its time in orbit.

The satellite carried a Synthetic Aperture Radar (SAR) payload, designed to capture Earth images in all weather conditions for a variety of observation uses.

EOS-09 is a follow-up to EOS-04 and is intended to supply remote sensing data to users involved in operational tasks. It also aims to increase the frequency of such observations, according to Isro.

What led to the failure then?

Isro Chairman V Narayanan explained that the issue occurred during the third stage of the launch. He said there was a “fall in the chamber pressure of the motor case,” which led to the failure.

VIDEO | Explaining the failure of PSLV-C61 vehicle launch mission, ISRO chairman V Narayanan says, "Today we attempted a launch of PSLV-C61 vehicle, the vehicle is a four stage vehicle, first two stages were performed as expected, during the third stage, it's a solid motor… pic.twitter.com/z9QzhgRQEH

— Press Trust of India (@PTI_News) May 18, 2025

“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,” he 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, as reported by PTI.

The space agency has set up a failure analysis committee to carefully study the flight and telemetry data to find out what went wrong and to put in place steps to prevent it from happening again.

With inputs from agencies