The Indian Space Research Organisation (ISRO) has launched a space docking experiment — a critical mission for India’s upcoming human spaceflight campaign. The mission will demonstrate the technology to rendezvous, dock, and undock in orbit. The successful completion of the mission will be a key step towards India’s ambitions for future missions to the lunar surface and the construction of an orbiting space station. It would also make India the fourth country to develop space docking technology.
Meanwhile, exactly ten years after the first launch of a test crew module, assembly has begun on the human-rated launch vehicle that will one day launch crew to orbit as part of India’s Gaganyaan program.
ISRO’s Space Docking Experiment mission, or SpaDeX, successfully lifted off from the First Launch Pad at the Satish Dhawan Space Center in Sriharikota, India, on Monday, Dec. 30, at 16:30 UTC. A Polar Satellite Launch Vehicle (PSLV) rocket carried a pair of satellites for this experiment, each massing 220 kg, to a circular low-Earth orbit at 470 km, inclined 55 degrees.
PSLV-60 vehicle is transported to the pad in advance of the SpaDeX mission. (Credit: ISRO)
The PSLV is India’s third-generation medium-lift launch vehicle and the first to adopt liquid stages. Due to the low mass of the payloads, this mission utilizes the PSLV-CA (or Core Alone) configuration. This variant omits the six strap-on boosters used on the standard PSLV-G and extended PSLV-XL versions and usually flies with less propellant in its upper stage.
The vehicle’s fourth stage utilizes storable liquid engines that can be reignited several times in orbit to release its payloads. This mission also carried the PSLV Orbital Experimental Module (POEM) platform, enabling experiments to be conducted, drawing its energy from a lithium-ion battery and solar panels around the fourth stage’s tank. This platform was last used on Jan. 1, 2024, on the XPoSat mission.
SpaDeX docking demonstration
The SpaDeX experiment will be conducted around 10 days following launch when the two satellites, the SDX01 “Chaser” and the SDX02 “Target,” will be released with a small relative velocity between them. The pair will drift apart for around a day until they are separated by a distance of around 10 to 15 km. Once achieved, Target will eliminate the velocity difference between itself and Chaser using its propulsion system. The pair will then enter a health check period while orbiting around 20 km apart in what is referred to as a “Far Rendezvous.” Chaser will then begin incrementally moving closer to Target, first to five kilometers and then to 1.5 km, gradually decreasing the distance between them to three meters.
Diagram of the SpaDeX spacecraft (left) and the pair docked (right). (Credit: ISRO)
Approaching each other at around 10 mm per second, the pair will eventually perform a controlled docking using four rendezvous and docking sensors. These include laser range finders and corner cube retro reflectors to determine relative range. At the same time, laser diodes facilitate the Proximity and Docking Sensor (PDS) from 30 m away to the final approach. A Mechanism Entry Sensor (MES) detects contact in the last eight centimeters, and the event will also be captured using an onboard video camera.
The androgynous system, identical on both spacecraft, is similar to the International Docking System Standard (IDSS) used on the International Space Station and its visiting spacecraft. With one degree of freedom, it is smaller than the 800 mm IDSS at 450 mm and has only two motors compared to the 24 on the IDSS hexapod system.
Secondary objectives for the mission include demonstrating electric power transfer between the two docked craft — a capability that will be particularly relevant for future robotics missions. Composite spacecraft control, where one craft controls the attitude of the other while docked, will also be demonstrated. The pair will then finally demonstrate an undocking maneuver and perform payload operations, after which they are expected to remain in operation for up to two years.
The SpaDeX craft during assembly and testing. (Credit: ISRO)
Developed by ISRO’s UR Rao Satellite Centre (URSC) in Bangalore, the two satellites will use a differential Global Navigation Satellite System (GNSS) for positioning. The onboard Novel Relative Orbit Determination and Propagation (RODP) processor determines Chaser and Target’s relative position and velocity. This technology will pave the way for lunar missions where spacecraft cannot use GNSS, which would rely on Earth-based receivers for autonomous docking or undocking. The planned Chandrayaan-4 mission to the Moon will require multiple docking maneuvers of this kind. For the SpaDeX mission, however, the ground stations within the ISRO Telemetry, Tracking, and Command Network (ISTRAC) will be controlling the satellites while in orbit.
Additional payloads
A total of 24 additional payloads accompanied the primary mission as part of the aforementioned POEM-4 experimental module. POEM acts as a satellite bus with power generated by solar panels, providing all necessary subsystems for the experimental payloads onboard. These include the Micro Orbital Infrastructure Technology Demonstration (MOI-TD) by TakeMe2Space, described as India’s first satellite lab in space, offering a platform for researchers and students to conduct experiments in microgravity.
MOI-TD tech demonstration of an AI-lab in space undergoing final testing. (Credit: TakeMe2Space)
The company is building affordable satellite infrastructure hosting AI-powered computer and storage services in low-Earth orbit (LEO). TakeMe2Space previously collaborated with ISRO to create the Radiation Shielding Experiment Module (RSEM), which launched on the XPoSat mission in January. The mission employed its innovative radiation-shielding coating in various thicknesses and saw the coating exposed to multiple C-class and X-class solar flares while in orbit. The coating demonstrated a 10x reduction across the range in Total Ionisation Dosage (TID) and will benefit satellite builders using non-radiation-hardened electronics in their circuits.
The SpaDeX mission will test a new radiation monitoring system mounted in the SDX02 Target craft. The system estimates electron and proton radiation, which will inform the development of radiation monitoring instruments on future missions and protect astronauts and equipment.
POEM platform with Varuna in position (left) and a close-up of Varuna (right). (Credit: Piersight)
PierSight’s Varuna will demonstrate synthetic aperture radar (SAR) in a cubesat form factor. Mounted to the POEM platform for this demonstration, Varuna will test the electronics, study the deployable antenna, and test the pre-driver stage of the solid-state power amplifier onboard. The mission will prove the flightworthiness of the subsystems and reach its highest level of technical readiness. The company aims to build a constellation that would monitor oceans with a 30-minute revisit time, providing information on oil spills and illegal fishing, amongst other maritime operations and security usages.
India’s Gaganyaan program
The demonstration of successful docking and undocking during the SpaDeX mission will validate key technology critical for India’s human spaceflight program. These missions will use the Gaganyaan crewed spacecraft, whose name translates to “celestial vehicle.” The capsule will host three crew members for missions lasting up to seven days and will return to Earth with a splashdown under a parachute after a mission. Once intended to be launched in 2021 on the new Human Launch Vehicle Mark 3 (HLVM3) rocket, revised estimates have the maiden launch and orbital test flight for this vehicle as no earlier than 2025.
Assembly begins of the S200 motor (left) for the HLVM3 vehicle (right). (Credit: ISRO)
The HLVM3 is the human-rated version of ISRO’s Geosynchronous Satellite Launch Vehicle (GSLV) Mark III rocket. The three-stage LVM3 has been active for a decade and has previously lofted the Chandrayaan-2 Chandrayaan-3 lunar lander missions and the GSAT and OneWeb satellites. The 53 m tall vehicle can carry 10,000 kg to LEO, and the HLVM3 evolves the design by adding a Crew Module and Crew Escape System. Assembly of the first human-rated HLVM3 vehicle began this month at the Satish Dhawan Space Center with the stacking of the nozzle end segment of the S200 solid rocket motors.
The program will first conduct three uncrewed launches before carrying the first crewed mission in 2026 or 2027. The first crewed mission is expected to launch three crew members into a 400 km orbit for three days before returning to Earth with a splashdown in the Indian Ocean. The first uncrewed test flight of the capsule is currently set for no earlier than February 2025. The crew module is being integrated at the Vikram Sarabhai Space Centre (VSSC), while the C32 cryogenic and L110 liquid stages stand ready for integration at the launch complex. URSC is currently integrating the Service Module and will later integrate and test the Orbital Module.
BAS Space Station and beyond
The Chandrayaan-4 lunar sample return mission will require two separate launches of the LVM3 vehicle as it carries five modules across two separate missions. The Descender module, which will collect and containerize lunar regolith samples, will launch together with the Ascender module in one stack. The Propulsion, Transfer, and Reentry Modules will follow in a separate stack and include the technology required to receive and return the lunar samples to Earth. The mission is expected to be completed by late 2027 and includes docking and undocking in lunar orbit.
Starting no earlier than 2028, additional missions will begin to construct India’s first space station, the Bharatiya Antariksha Station (BAS). The first module for the station, BAS-1, will provide cargo vehicle docking and propellant transfer capabilities. The current timeline has additional modules progressively added to the station, with short-duration crewed missions possibly starting around 2031 and ultimately longer-duration missions and science experiments by around 2035. By this point, the station will mass around 56 tonnes, which is 75% the mass of China’s Tiangong space station.
Render of the BAS in orbit. (Credit: ISRO)
Measuring approximately 30 m in length and 25 m in width, the BAS will be a quarter of the International Space Station’s (ISS) volume in this first phase. The station will have two docking ports, an internal robotic arm, and an external robotic arm to support maintenance and inspection. The BAS will orbit at around 450 km, inclined 51.6 degrees — around 30 km higher than the ISS and in a similar inclination. A continuous presence, with resupply missions every six months, is anticipated as the station moves into its second expansion phase in 2035.
The Indian Cabinet approved the budgets for several space programs in September 2024, securing funding for Chandrayaan-4, the BAS-1 module, and eight supporting Gaganyaan missions. ISRO will endeavor to complete four precursor missions by 2026, and the remaining four, including BAS technical demonstrations and validation, will be completed by the end of December 2028. The Cabinet also approved the Shukrayaan Venus orbiter mission and the development of the Next-Generation Launch Vehicle (NGLV) rocket. The three-stage NGLV is also referred to as “Soorya” by ISRO.
From left to right: PSLV, LVM3, HLVM3, and the NGLV single, reusable, and side booster configurations. (Credit: ISRO)
At over twice the height of the LVM3, this 92 m tall vehicle will have a five-meter diameter core and is intended to be partially reusable. It will support BAS missions with the capacity to carry up to 30,000 kg to LEO or 12,000 kg to a geostationary transfer orbit with the addition of solid motors. The vehicle will be propelled using clustered engines that burn liquid methane and oxygen. The LM470 core first stage is planned to support propulsive landings and is accompanied by two additional cores that act as side boosters in the most powerful “NGLV Heavy” variant. A human-rated NGLV-H is also proposed as part of India’s plans to put Indian boots on the Moon by 2040.
The Shukrayaan mission is scheduled for the March 2028 launch opportunity and will study the Venusian surface, subsurface, and atmosphere. The mission will investigate the underlying causes of the planet’s evolution away from a more habitable environment. The spacecraft will spend 112 days travelling to Venus, arriving mid-July 2028.
Vapor cone seen as PSLV-C59 ascends during the PROBA-3 mission in December 2024.(Credit: ISRO)
On Dec. 4, ISRO and the European Space Agency (ESA) signed a Technical Implementing Plan (TIP) to provide ground-tracking support for the Gaganyaan missions. The two agencies have a longstanding collaboration, and ISRO recently launched the PROBA-3 mission in collaboration with ESA in early December. In signing the TIP agreement, ESA will help ensure continuity in the flow of communications and data with orbital operations. This agreement followed the signing of an Implementation Agreement between ISRO and the Australian Space Agency in late November. This agreement ensures collaboration on crew module recovery for Gaganyaan missions, including contingency search and rescue plans in the event of an abort during the ascent phase near Australian waters.
(Lead image: PSLV-60 arrives at the First Launch Pad at the Satish Dhawan Space Centre for the SpaDeX mission. Credit: ISRO)
