SpaDeX: India’s Space Docking Mission Explained

SpaDeX, short for the Space Docking Experiment, is an Indian Space Research Organisation (ISRO) mission that demonstrated the rendezvous and docking of two small spacecraft in orbit, and its successful docking in January 2025 made India the fourth country, after the United States, Russia and China, to achieve in-space docking. The mission was launched on 30 December 2024 aboard a Polar Satellite Launch Vehicle (PSLV) from Sriharikota.

SpaDeX used two spacecraft of about 220 kilograms each, called the Chaser (SDX01) and the Target (SDX02), to test the technologies needed to bring two orbiting objects together, connect them, transfer power and separate again. Docking is a basic capability for any space station, for missions that assemble large spacecraft from parts and for missions that return samples from other worlds. This explainer describes what SpaDeX is, how the docking worked, what the satellites carried and why the mission matters for India’s future space programme.

Key Takeaways

• SpaDeX is an ISRO technology demonstration mission that was launched on 30 December 2024 on a PSLV rocket and achieved the docking of two spacecraft in orbit in January 2025.
• India became the fourth country, after the United States, Russia and China, to demonstrate in-space docking through the SpaDeX mission.
• The SpaDeX mission used two small satellites of about 220 kilograms each, the Chaser (SDX01) and the Target (SDX02), which approached and joined each other in low Earth orbit.
• SpaDeX tested rendezvous, docking, power transfer between docked spacecraft, control of the combined stack and undocking.
• Docking technology is essential for the planned Bharatiya Antariksh Station, for the Chandrayaan-4 lunar sample return mission and for future crewed missions.
• The PSLV also carried a set of experiments on its fourth stage, the PSLV Orbital Experimental Module (POEM), which included a plant-growth test in orbit.

What is the SpaDeX mission?

SpaDeX is a low-cost technology demonstration mission of ISRO that uses two small satellites to prove the capability of automated rendezvous, docking and undocking in space. It was approved by the Government of India as a stepping stone to larger human spaceflight and space-station goals.

The mission’s purpose was technical and practical. ISRO needed to show that it could manoeuvre one spacecraft towards another in orbit, align the two precisely, join them securely and operate the combined unit before separating them. The mission used small satellites launched on a single PSLV, which kept the cost modest compared with the large crewed and station projects that depend on the same technology.

The spacecraft were built and tested by ISRO centres, with the launch and mission operations managed from Sriharikota and the ISRO Telemetry, Tracking and Command Network (ISTRAC) in Bengaluru. The mission was part of a series of steps that also includes the Gaganyaan human spaceflight programme and the Bharatiya Antariksh Station plan.

What is space docking and why is it difficult?

Space docking is the process of bringing two spacecraft together in orbit and joining them mechanically so that they form one structure, and it is difficult because both spacecraft are travelling at about 7.5 kilometres per second relative to the Earth, so the relative motion between them must be controlled with extreme precision. Even tiny errors in speed or alignment can cause a failed connection or a collision.

It helps to separate the terms:

• Rendezvous: Bringing two spacecraft to the same orbit and close together, with small relative distance and speed.
• Docking: Joining the two spacecraft with a mechanical interface so they can share structural loads and, if needed, power, data or fluids.
• Berthing: A related method in which a robotic arm grabs a spacecraft and attaches it to a port, which differs from the free-flying approach used in docking.
• Undocking: Releasing the connection and separating the vehicles safely.

Orbital mechanics makes rendezvous counterintuitive. A spacecraft that fires its engine to go faster rises to a higher orbit and then moves more slowly, so approaching a target needs careful planning of burns and waiting periods. Engineers therefore use a series of manoeuvres, with gradual reduction of distance and relative speed, and sensors that measure range and direction at each stage.

How did the SpaDeX docking work?

The SpaDeX docking worked by placing the two satellites in nearly the same orbit with a small initial separation, letting the Chaser close the gap in stages from far range to a few metres, and then performing a controlled capture and latching of the two docking mechanisms. The Chaser carried out the manoeuvres, while the Target served as the passive partner.

Launch and initial separation

After launch on 30 December 2024, the PSLV placed the Chaser and Target in a circular orbit at about 470 kilometres. The two satellites were separated with a small relative velocity so that they drifted apart before the mission began the approach phase. The initial separation was in the order of tens of kilometres.

Far-range and close-range approach

The Chaser used relative navigation to approach the Target in steps, with holding points at shrinking distances. The approach passed through far-range rendezvous at several kilometres, then closer ranges of a few hundred metres, and eventually a final approach of a few metres. The mission team performed trial approaches at short distances before the final docking, and the first planned attempt in early January 2025 was postponed more than once as engineers refined the approach.

Capture and docking

At very short range, the Chaser’s docking mechanism contacted the Target’s mechanism, capturing it and then pulling the two spacecraft into a rigid connection. The docking was achieved in January 2025, and ISRO confirmed that the spacecraft were joined and under unified control.

Post-docking operations and undocking

After docking, ISRO demonstrated the transfer of electrical power between the two spacecraft and the control of the combined object as a single unit. The satellites then undocked a few weeks later and continued to operate their payloads. The undocking showed that the system could separate on command, an important step for multi-module space stations and visiting vehicles.

What technologies did SpaDeX demonstrate?

SpaDeX demonstrated an indigenous docking mechanism, relative navigation using satellite navigation and onboard sensors, autonomous rendezvous control, inter-satellite communication, power transfer between docked spacecraft and combined-stack control. These are the building blocks of every docking-based space system.

• Docking mechanism: ISRO developed its own mechanism, designed to be androgynous, meaning that the same type of interface can mate with a copy of itself. India has filed for patent protection of the design.
• Sensors for rendezvous: The spacecraft carried laser-based and optical sensors for measuring range and relative position at different distances, including close proximity.
• Relative navigation: Software computed the relative position and velocity of the two satellites, using signals from satellite navigation systems and inter-satellite information.
• Inter-satellite link: A communication link between the spacecraft allowed them to share data and coordinate the approach.
• Power transfer: The docked pair tested the transfer of electrical power between them, which is the basis for shared power in docked systems.
• Combined control: After docking, a single spacecraft controlled the attitude of the joined stack.

The mission also helped validate ISRO’s ground tools, mission planning methods and safety constraints. Because the spacecraft were small, they could be used for repeated approaches with limited risk.

What were the Chaser and Target satellites?

The Chaser (SDX01) was the active spacecraft that carried out the approach and capture, and the Target (SDX02) was the passive spacecraft that waited to be reached; each weighed about 220 kilograms. The two had matching docking interfaces but different roles.

Both spacecraft carried payloads that continued to work after the docking tests. These included imaging and radiation-monitoring instruments, which extended the use of the satellites beyond the technology demonstration.

What was POEM and what else did the PSLV carry?

The PSLV Orbital Experimental Module (POEM) is the spent fourth stage of the PSLV, which ISRO converts into a small orbital platform for experiments after the main payloads are released. On the SpaDeX flight, the platform carried a set of experiments from ISRO, universities and start-ups.

Among the experiments was a compact research module designed to study the germination of seeds in microgravity. Seeds of cowpea were sprouted in orbit, which is relevant to long-duration crewed missions where growing food may be needed. The platform also carried other technology experiments, including tests of robotic systems and of new materials and sensors. The use of POEM follows an earlier approach where otherwise discarded rocket stages provide low-cost access to space for research.

Which countries have achieved space docking?

The countries that have achieved space docking are the United States, the Soviet Union (and later Russia), China and, since January 2025, India. The first docking of two spacecraft was achieved by the United States in 1966, and each subsequent country built its own capabilities.

India’s demonstration is distinct in that it used an indigenous docking mechanism and sensors, which means that India does not depend on foreign docking hardware for its missions. Each of the earlier nations used its own system, and international standards for docking interfaces have been developed for stations with partners such as the International Space Station.

Why does SpaDeX matter for the Bharatiya Antariksh Station?

SpaDeX matters for the Bharatiya Antariksh Station (BAS), India’s planned modular space station, because the station is to be assembled from separate modules launched individually and joined in orbit, and this requires proven docking technology. ISRO has stated a goal of establishing the first module of the station in the late 2020s and a full station in the following decade.

A space station has several requirements that rest on docking:

• Assembly: Modules launched on different rockets must be joined with accurate alignment.
• Crew vehicles: Crewed spacecraft must dock with the station to deliver astronauts and then return them.
• Cargo vehicles: Uncrewed resupply vehicles must dock to bring supplies and remove waste.
• Safety: Reliable undocking is needed for emergencies and for relocation of vehicles.

Without autonomous rendezvous and docking, a station would have to be launched in a single piece, which limits its size to the capacity of one rocket. SpaDeX therefore reduces a major technical risk for the station programme.

How does SpaDeX support Chandrayaan-4 and human spaceflight?

SpaDeX supports Chandrayaan-4 and human spaceflight by validating the docking and rendezvous techniques needed for a lunar sample return mission and for crewed missions that connect with orbital platforms. Chandrayaan-4 is planned to bring samples from the Moon back to Earth.

Chandrayaan-4

The Chandrayaan-4 mission is designed with several modules, including a lander, an ascent module to lift lunar samples from the surface and a transfer module that carries the samples back. The modules need to meet and dock in lunar orbit so that the sample container can be transferred to the vehicle that returns to Earth. The mission design, as announced, involves launching the elements separately and assembling them in space. Such a plan depends on rendezvous and docking, which SpaDeX has demonstrated at a smaller scale in Earth orbit.

Gaganyaan and later crewed missions

The Gaganyaan programme aims to send Indian astronauts to low Earth orbit on an Indian vehicle and return them safely. Initial flights do not require docking, but later steps such as visiting a space station do. The technology demonstrated by SpaDeX is a prerequisite for those steps. India has also stated long-term ambitions of landing an Indian astronaut on the Moon by 2040, which would call for complex in-orbit operations.

Other applications

Docking has wider uses, including in-orbit refuelling, satellite servicing, life extension of satellites, construction of large structures and removal of debris. These possibilities also relate to commercial and defence uses of space.

What are the limits of SpaDeX and what comes next?

SpaDeX is a small-scale technology demonstration and does not by itself provide a space station or crewed docking capability; its limits are the small spacecraft size, the uncrewed nature of the test and the need for further validation with larger vehicles. ISRO will have to extend the technology to larger modules and to vehicles with crew.

Several steps are expected to follow:

• Additional experiments: ISRO may carry out further docking tests, including with different relative approach conditions and larger systems.
• Station modules: The design and launch of the first module of the Bharatiya Antariksh Station, with docking ports compatible with crewed and cargo vehicles.
• Crewed missions: Gaganyaan flights that begin with uncrewed tests and then proceed to crewed missions.
• Lunar missions: Chandrayaan-4 and later missions that use multi-module architectures.
• Industry involvement: Indian industry and start-ups are expected to take a larger role in building and operating space systems.

Schedules for these programmes can change, and readers should check official ISRO announcements for current timelines. The significance of SpaDeX lies in showing that India has now demonstrated the core skills for orbital assembly.

Bottom Line

SpaDeX showed that India can bring two spacecraft together in orbit, dock them, transfer power and separate them again, a capability that only the United States, Russia and China had demonstrated before. That capability underpins India’s plans for the Bharatiya Antariksh Station, Chandrayaan-4 and future crewed missions. Information on this page was last reviewed on 7 October 2026.

Frequently Asked Questions

What is SpaDeX?

SpaDeX, or the Space Docking Experiment, is an ISRO mission that demonstrated the rendezvous and docking of two small spacecraft in orbit. It was launched on a PSLV on 30 December 2024, and the docking was achieved in January 2025.

Which country was the fourth to achieve space docking?

India was the fourth country to demonstrate in-space docking, after the United States, Russia and China. The achievement came with the successful docking of the SpaDeX Chaser and Target satellites in January 2025.

What are the Chaser and Target in SpaDeX?

The Chaser (SDX01) and the Target (SDX02) are the two small satellites of the SpaDeX mission, each weighing about 220 kilograms. The Chaser actively approached and captured the Target, which acted as the passive partner.

Why is space docking important for India?

Space docking is important for India because it is needed to assemble the Bharatiya Antariksh Station from modules, to support crewed missions and to carry out the Chandrayaan-4 sample return plan. It also opens possibilities such as satellite servicing and in-orbit refuelling.

What else did the SpaDeX launch carry?

The PSLV carried the POEM platform, which hosted a number of experiments on its spent fourth stage. These included a plant-growth experiment in which cowpea seeds were sprouted in microgravity.

How is docking different from berthing?

Docking is the joining of two free-flying spacecraft using a mechanical interface after the active vehicle steers itself into position. Berthing is a method in which a robotic arm grabs one spacecraft and attaches it to a port, so the arm, rather than the vehicle, makes the final connection.

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