SpaDeX (Space Docking Experiment) is ISRO’s technology demonstration mission in which two small satellites, SDX01 (Chaser) and SDX02 (Target), launched together on PSLV-C60 on 30 December 2024, performed autonomous rendezvous, docking and undocking in low Earth orbit. With the successful docking on 16 January 2025, India became the fourth country – after the United States, Russia and China – to master in-space docking, a capability needed for the Bharatiya Antariksh Station, Chandrayaan-4 and future crewed missions.
Context
- ISRO docked the two SpaDeX satellites on the morning of 16 January 2025, after attempts planned for 7 and 9 January were postponed and a trial approach to 3 metres was made on 12 January.
- Power transfer between the docked satellites was demonstrated, the pair was controlled as a single spacecraft, and undocking was completed on 13 March 2025; a second docking round was carried out in April 2025 to build confidence for future missions.
Mission Objectives
- Primary: demonstrate the technology for rendezvous, docking and undocking of two small spacecraft in a low-Earth circular orbit.
- Secondary: demonstrate electric power transfer between docked spacecraft, composite spacecraft control, and payload operations after undocking during the two-year mission life.
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Mission Design
- Two satellites of about 220 kg each, injected by PSLV-C60 into a 470 km circular orbit at 55 degrees inclination, with a small relative velocity so that they drifted 10-20 km apart.
- The Chaser then closed in through hold points at 5 km, 1.5 km, 500 m, 225 m, 15 m and 3 m before capture, at approach speeds of about 10 mm per second.
- Bharatiya Docking System (BDS): an androgynous, peripheral docking mechanism – identical on both satellites – with two motors, compared with 24 in the International Docking System Standard; capture, retraction and rigidisation are followed by latching and power transfer.
- Sensors and navigation: laser range finder, rendezvous sensor, proximity and docking sensor, and a satellite-to-satellite link with differential GNSS for relative position.
- Payloads: a high-resolution camera on SDX01; a miniature multispectral payload and a radiation monitor on SDX02, used after undocking for Earth observation and radiation studies.
- POEM-4: the spent fourth stage of PSLV-C60 carried 24 experiments from ISRO centres, start-ups and academic institutions.
Why Docking Is Hard
- The satellites orbit at about 28,800 km per hour; the relative speed at contact must be brought down to a few centimetres per second with precise attitude control and synchronised timing.
Why It Matters for India
- Bharatiya Antariksh Station: modules will be launched separately and docked in orbit from 2028 onwards.
- Chandrayaan-4: the lunar sample-return mission needs docking in lunar and Earth orbit.
- Gaganyaan and beyond: crew transfer, refuelling, satellite servicing and in-orbit assembly all depend on docking.
- India joins the United States, Russia and China; the technology also opens satellite-servicing markets.
Important Facts for Examsmost asked
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- Launch: 30 December 2024, PSLV-C60, Satish Dhawan Space Centre, Sriharikota; two satellites SDX01 (Chaser) and SDX02 (Target), 220 kg each, 470 km orbit.
- Docking: 16 January 2025 – India the fourth country; undocking: 13 March 2025.
- Docking mechanism: Bharatiya Docking System; approach speed about 10 mm/s.
- Fourth stage: POEM-4 with 24 payloads.
FAQs
What is in-space docking?
The process by which two orbiting spacecraft align, approach and mechanically join into one unit so that power, data, crew or cargo can move between them.
Which country docked spacecraft first?
The United States, with Gemini 8 docking to an Agena target vehicle in 1966; the Soviet Union achieved the first automated docking in 1967.
Why is SpaDeX linked to Chandrayaan-4?
Chandrayaan-4 will bring lunar samples back to Earth using multiple modules that must dock in orbit, a manoeuvre SpaDeX has now demonstrated.
