The Indian Space Research Organisation (ISRO) has embarked on a groundbreaking mission with the PSLV-C60 SpaDeX (Space Debris Experiment), redefining the potential of spent rocket stages in space exploration. Traditionally discarded as debris, the fourth stage of the PSLV rocket has been ingeniously repurposed into the PS4-Orbital Experiment Module (POEM-4), transforming it into a sophisticated orbital laboratory. This ambitious platform hosts an unprecedented 24 experiments, showcasing India’s advancements in space technology, scientific research, and sustainable space utilisation. The launch is planned for the end of the year.
A critical feature of the SpaDeX mission is its integration of an innovative indigenous docking system, which represents a significant leap in India’s space technology capabilities. Docking systems are essential in space missions, enabling spacecraft to connect seamlessly in orbit for purposes such as crew transfer, modular space station assembly, satellite servicing, and resupply missions. Over the years, countries have developed sophisticated docking mechanisms tailored to their needs.
“POEM gives scientists a chance to conduct microgravity experiments in space for up to three months. Normally, the empty stage of the rocket would just become space debris after launching the main payload. Instead, POEM allows researchers to make use of it for valuable experiments. These experimental payloads help test new ideas and technologies that could be important for future space missions. The PSLV-C60 SpaDeX mission is the fourth mission in the POEM series, known as POEM-4. This mission aims to achieve specific objectives related to space exploration and technology,” remarked space expert Girish Linganna.
The PSLV-C60 SpaDeX mission includes two small spacecraft, called 'Chaser' and 'Target,' each weighing about 220 kg. The mission aims to demonstrate space docking technologies. Both spacecraft will be launched independently and simultaneously by the PSLV-C60 rocket into a circular orbit at 470 km altitude and a 55° inclination, with a local time cycle of about 66 days.
“The POEM-4 mission will carry a total of 24 payloads. Of these, 14 are from ISRO and its centers, while 10 are from various non-government entities (NGEs), including universities and startups, which were selected through IN-SPACe (Indian National Space Promotion and Authorisation Centre). This mission marks a major improvement, with the capacity of POEM increasing three times compared to the previous POEM-3 platform, which only carried eight payloads,” remarked Linganna.
Russia had pioneered docking technology with systems like the Androgynous Peripheral Attach System (APAS), first used during the Apollo-Soyuz Test Project in 1975. Later, the SSVP system, a reliable probe-and-drogue mechanism, was widely used in the Mir space station and later integrated into the International Space Station (ISS). The United States introduced the Soft Capture Mechanism (SCM) as part of NASA's International Docking System Standard (IDSS), offering compatibility with diverse spacecraft and missions, including the ISS and future lunar explorations. China’s Tiangong space station employs an advanced system derived from APAS, emphasizing autonomy and precision, while the European Space Agency’s Automated Transfer Vehicle (ATV) demonstrated sophisticated docking for resupply missions to the ISS.
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The POEM-4 mission by ISRO carries 14 cutting-edge payloads developed by its centers. These include five from Vikram Sarabhai Space Centre (VSSC), four from the Space Physics Laboratory (SPL) at VSSC, three from the ISRO Inertial Systems Unit (IISU), one collaborative payload from SPL and IISU, and one from the Indian Institute of Space Science and Technology (IIST). Some of these payloads include the 'RRM-TD: Walking Robotic Arm' that can perform tasks in space like walking on surfaces, repairing satellites, and supporting space station development. Then there is the Debris Capture Robotic Manipulator, a robot designed to capture and remove dangerous space debris, keeping space safe and clean. Besides this, there is Multi-Sensor Inertial Reference System (MIRS) a navigation tool that uses multiple sensors to ensure spacecraft move and remain stable in space. There is also a Lead Exempt Experimental System (LEXS) that tests environmentally friendly, lead-free electronics in space for future sustainable technologies.
Apart from these there are 10 payloads from Non-Governmental Entities (NGEs) and start-ups such as the Amity Plant Experimental Module in Space (APEMS) that has been developed by Amity University, Mumbai and will study how plant cells (using spinach) grow and adapt in microgravity compared to Earth. It uses LEDs for light, a gel for nutrients, and cameras to monitor growth. This experiment helps understand plant growth in space, essential for future space missions. There is also the RVSat-1 payload, developed by RV College of Engineering, Bengaluru, which will study the growth of gut bacteria in space with prebiotics. This will help understand astronaut health and supports advancements in antibiotics, waste recycling, and space exploration. There is also a Green Propulsion Thruster VYOM-2U payload, developed by Manastu Space Technologies, Mumbai and demonstrates a safer hydrogen peroxide-based thruster for satellites, offering improved performance over hydrazine with high thrust, efficiency, and long continuous firing capability. Besides this, there is this MOI-TD payload, developed by TakeMe2Space Hyderabad and demonstrates an AI lab in space, processing Earth observation data in real time, testing subsystems, and using AI to capture and analyze sharp Earth images.
India’s docking system, tested on SpaDeX, features a compact, low-impact androgynous design compatible with modular spacecraft configurations. “Advanced dual-motor actuation ensures precise alignment and secure connection at low velocities. Equipped with state-of-the-art sensors such as Laser Range Finders, Proximity Sensors, and Rendezvous Cameras, it supports real-time navigation and alignment. An Inter-Satellite Communication Link (ISL) facilitates seamless data exchange during docking operations, enhancing the system's autonomy and reliability. This docking system positions ISRO at the forefront of orbital assembly, servicing, and space station construction, laying the foundation for future missions, including the proposed Bharatiya Antariksh Station,” remarked Srimathy Kesan, founder and CEO of Space Kidz India.
The SpaDeX mission’s experimental array further underscores its significance. ISRO’s Walking Robotic Arm (RRM-TD), developed by the Inertial Systems Unit, marks India’s first venture into space-based robotics with walking capabilities. With seven degrees of freedom and advanced motion planning, this robotic manipulator is a precursor to modular space station operations. Complementing this is the Debris Capture Robotic Manipulator, which autonomously tracks and manages space debris using innovative visual tracking and motion prediction algorithms.
“The SpaDeX mission exemplifies ISRO’s ability to foster collaboration among government institutions, academic centers, and private companies. This cooperative effort not only advances multiple fields of space science but also establishes a sustainable model for future space research. By transforming a spent rocket stage into a versatile orbital laboratory, ISRO demonstrates its commitment to innovation, resource optimization, and scientific discovery,” added Kesan.
As these experiments unfold in orbit, they promise to yield valuable insights that could reshape our approach to space exploration. The success of the POEM-4 platform could pave the way for regular orbital laboratories, creating a new paradigm in sustainable space research. Through this mission, ISRO not only showcases its technical excellence but also sets a visionary path for sustainable and inclusive space exploration.