ISRO's POEM-4 Turned a Spent PSLV Upper Stage Into a Laboratory With 24 Experiments, Sprouted Cowpea Seeds in 4 Days and Completed 1,000 Orbits Before Re-entering on 4 April 2025

The empty fourth stage of the PSLV-C60 rocket stayed in orbit for 95 days after it released ISRO’s SpaDeX satellites on 30 December 2024. It carried 24 experiments, completed 1,000 orbits on 4 March 2025, and impacted in the Indian Ocean at 02:33 UTC on 4 April 2025, ISRO says.

ISRO’s statement of 7 March 2025 says all 24 payloads “have successfully completed the intended experiments in orbit.” Fourteen came from ISRO and Department of Space centres, and ten from universities and start-ups. The stage is called POEM-4, and the SpaDeX docking test that shared its launch is told in a separate article.

A spent stage with a second job

POEM stands for PS4 Orbital Experiment Module. According to ISRO’s description, the module reuses the spent fourth stage of the PSLV, which would otherwise become space debris once the main satellites are released, so scientists can run microgravity experiments for up to three months.

Three earlier flights appear in ISRO’s list of re-entry missions: POEM on PSLV-C53 on 30 June 2022, POEM-2 on PSLV-C55 on 22 April 2023 and POEM-3 on PSLV-C58 on 1 January 2024. Between them the first three missions flew 21 payloads, ISRO says.

The PSLV-C60 mission brochure describes the platform as a stage stabilised on three axes. A flexible solar panel and a 50 Ah lithium-ion battery supply its power, and its own avionics handle navigation, guidance, control and telecommands.

From satellite release to a 350 km orbit

The brochure’s flight table puts the release of the two SpaDeX satellites about 909 seconds after liftoff, at an altitude of about 477 km. The brochure says the fourth stage was then restarted twice to bring it to a circular orbit at 350 km and change its inclination. The leftover propellant was vented in a set order, which ISRO calls passivation, to reduce the risk of the stage breaking up.

The final orbit, ISRO’s re-entry page says, was nearly circular at 350 km with an inclination of 55.2 degrees. The previous flight, POEM-3, was lowered from 650 km to the same 350 km, which ISRO’s POEM-3 note says shortens the time the stage takes to decay. That stage, left over from the XPoSat X-ray satellite launch, re-entered on 21 March 2024.

The 14 ISRO payloads

ISRO’s POEM-4 page counts five payloads from the Vikram Sarabhai Space Centre, four from its Space Physics Laboratory, three from the ISRO Inertial Systems Unit, one shared by those last two and one from the Indian Institute of Space Science and Technology. Proof-of-concept tests for future missions is how the page describes them.

Two were robotic arms. The Walking Robotic Arm, built by the Inertial Systems Unit, is a 7-degree-of-freedom arm that relocates itself between fixed points on the platform by walking end over end, in the manner of an inchworm. ISRO’s page calls it India’s first space robotic manipulator with walking capability and a precursor for robotics on the Bharatiya Antariksh Station, whose approval and cost are laid out in our piece on the station plan. The Vikram Sarabhai centre’s Debris Capture Robotic Manipulator was to capture a tethered object, using camera images to guide the arm. ISRO says that, once proven, it could capture free-floating debris and refuel spacecraft on later POEM flights.

The others tested a lead-free power converter, small gyroscopes and magnetic sensors, a laser-fired igniter for thrusters, a set of probes for electrons and ions in the ionosphere, a CubeSat radio board, and a receiver that uses NavIC, India’s regional navigation satellite system, to measure electrons in the plasmasphere near 350 km.

Cowpea seeds that sprouted in four days

The experiment with the clearest published timeline is CROPS, the Compact Research Module for Orbital Plant Studies, built by the Vikram Sarabhai centre. It is a sealed box 300 mm across and 450 mm tall, holding eight cowpea seeds in four chambers of clay pellets mixed with slow-release fertiliser. Cowpea was chosen after ground trials because it germinates quickly, ISRO says.

The platform reached 350 km and the box was switched on. About 90 minutes later, controllers on the ground opened an electric valve to let water into the soil. Carbon dioxide readings rose in the following orbits, which ISRO reads as germination.

On the fourth day ISRO’s page records seeds sprouting from their holding strips, and its images are dated 4 January 2025 for the first sprout and 5 January for two leaves. Lights ran on a 16-hour on, 8-hour off cycle, and a camera and sensors for oxygen, carbon dioxide, humidity and soil moisture reported back during ground-station passes. Two leaves on the fifth day met the objective, in ISRO’s account. Its page calls growing plants in orbit a step toward producing food in space and giving astronauts an activity on long crewed missions. India’s own crewed programme is covered in our Gaganyaan article.

The ten outside payloads

ISRO says the ten payloads from non-government groups reached the platform through IN-SPACe. Bellatrix Aerospace of Bengaluru fired its green monopropellant thruster, Rudra, for 60 seconds. In a release of 1 January 2025 the company said the burn pushed the platform around its yaw axis at 1.4 degrees a second, for a turn of 80 degrees. The company said it was the second firing of that package in space, and the PTI report quotes its co-founder, Yashas Karanam, as saying the aim was to validate the same design again and establish repeatability.

Manastu Space Technologies of Mumbai flew a 1.1 newton thruster fuelled by hydrogen peroxide, which ISRO’s page describes as aiming to be a safer alternative to hydrazine, a common but more hazardous rocket fuel. GalaxEye of Bengaluru tested electronics for processing radar images on board, with an aim, in ISRO’s words, of shrinking 400 MB of raw data to under 1.5 MB in under 10 minutes. Piersight Space of Ahmedabad tested parts of a radar small enough for a CubeSat, including a reflectarray antenna that unfolds in orbit.

Students and a university took part too. A team at RV College of Engineering, Bengaluru, set out to draw the growth curve of a gut bacterium, Bacteroides thetaiotaomicron, in orbit. Amity University, Mumbai, flew spinach callus, a lump of plant tissue grown on gel, alongside a twin experiment on the ground. SJC Institute of Technology, Karnataka, built an amateur-radio transmitter for voice, text and images, working with the ISRO satellite centre’s radio club. N Space Tech of Andhra Pradesh tested a radio link to an ISRO ground station, and MIT-WPU of Pune tested four commercial motion sensors.

TakeMe2Space of Hyderabad planned to send three machine-learning models up from the ground, run them on its satellite computer and send the results back. ISRO’s March 2025 statement says a space start-up tested the uplink and execution of an AI model, and it does not name the company. India’s wider start-up sector is covered in our NewSpace piece.

What the platform itself proved

ISRO’s March statement adds that POEM-4 validated a mission management computer built on Vikram3201, a 32-bit processor with a custom architecture developed by ISRO, and used a nano star sensor for navigation for the first time. It also used reaction wheels to hold its attitude, and cold gas thrusters that extended its orbital life by at least 45 days compared with earlier POEM flights. The statement says POEM-4 carried the most payloads of any POEM mission so far.

Coming down on a date ISRO predicted

US Space Command facilities and ISRO’s radars tracked the stage throughout, and ISRO’s debris-monitoring system, IS4OM, updated the forecast. By the time ISRO predicted re-entry for 4 April 2025, the orbit had decayed to 174 km by 165 km. The stage entered the atmosphere and impacted in the Indian Ocean at 02:33 UTC, which is 08:03 IST, as ISRO’s page records. The re-entry counts toward ISRO’s Debris Free Space Mission, the page says. In March 2024 ISRO said POEM-3 had left practically zero debris.

What ISRO has published and what it has not

ISRO’s documents disagree on one number. The POEM-4 page says POEM-3 hosted eight payloads, while ISRO’s two POEM-3 pages say nine. Both statements agree that POEM-4 set the record.

The reports on results are mostly collective. Day-by-day detail exists for CROPS and the re-entry. For the rest, ISRO’s re-entry page says “all payloads worked as expected yielding valuable science data.” The pages opened for this article do not give separate results for the walking arm, the debris-capture arm or the Amity plant tissue. The RV College team said in February 2025 that its data would be analysed and published in a research paper, and none was found.

The next phase of CROPS will run, ISRO’s page says, for 30 to 45 days, with active control of carbon dioxide, oxygen, humidity, soil moisture and temperature. The pages opened give no launch date for it.