ISRO's Space Docking Roadmap: SPADEX-2, SPADEX-3 and the Path to Chandrayaan-4
In a parliamentary question, the government revealed that ISRO is studying two follow-on orbital docking experiments — SPADEX-2 and SPADEX-3 — building on the success of the original SpaDeX mission completed in early 2025.
SPADEX-2 is designed to replicate docking in a highly elliptical orbit, serving as a direct precursor technology demonstrator for the Chandrayaan-4 lunar sample return mission, which requires docking in the elliptical lunar orbit.
SPADEX-3 will demonstrate docking in a circular orbit, replicating the conditions required for assembling and operating the Bharatiya Antariksh Station (BAS), India's planned space station.
The original SpaDeX mission — launched aboard PSLV-C60 on 30 December 2024 — made India only the fourth country in the world to demonstrate in-space docking using indigenous technology.
The two SPADEX spacecraft (SDX01 Chaser and SDX02 Target), each weighing 220 kg, were placed in a 475 km circular orbit at 55° inclination and successfully docked using an indigenous Androgynous Docking Mechanism.
Space Docking Technology: Why It Matters
Space docking is the process of joining two independently orbiting spacecraft in space. It is a foundational capability for human spaceflight, in-space refuelling, satellite servicing, and multi-module space station assembly. Without docking, large missions — such as moon landings requiring fuel transfer, or orbital assembly of heavy structures — are impossible with a single rocket launch.
Key Details
- Only four nations have demonstrated autonomous in-space docking: the USA (Gemini 8, 1966), USSR/Russia (Cosmos 186/188, 1967), China (Tiangong-1/Shenzhou-8, 2011), and India (SpaDeX, 2025).
- India's docking mechanism (450 mm diameter) is smaller than the International Docking System Standard (IDSS) used globally (800 mm) — reflecting an indigenous, lightweight design philosophy.
- The docking approach velocity was approximately 10 mm/second, assisted by Laser Range Finders and Corner Cube Retro Reflectors.
- Docking enables power transfer between spacecraft, a capability ISRO tested and validated during SpaDeX.
SPADEX-2 and SPADEX-3 extend this validated technology to mission-specific orbital conditions — elliptical orbit (moon missions) and circular orbit (space station) — making them risk-reduction exercises for India's flagship future missions.
Chandrayaan-4: India's Lunar Sample Return Mission
Chandrayaan-4 is ISRO's planned mission to land on the Moon, collect surface samples, and return them to Earth. Unlike Chandrayaan-3 (which landed but did not return samples), Chandrayaan-4 involves assembling multiple spacecraft components in lunar orbit — a process that requires docking in the Moon's highly elliptical orbit environment.
SPADEX-2's simulation of highly elliptical orbit docking is directly calibrated to the conditions Chandrayaan-4 will face, making it a mandatory technological milestone before that mission can proceed.
Bharatiya Antariksh Station (BAS): India's Space Station
The Bharatiya Antariksh Station (BAS), also called Bharatiya Space Station, is India's planned modular orbital space station, targeted for commissioning by 2035. Like the International Space Station (ISS), it will be assembled in segments launched on separate rockets, requiring repeated docking operations in circular low Earth orbit.
SPADEX-3 validates docking in the precise orbital regime where BAS will operate, providing flight heritage data before human-rated docking systems are developed for the station.
- PSLV-C60 launched SpaDeX on 30 December 2024 from Satish Dhawan Space Centre, Sriharikota.
- SpaDeX spacecraft mass: 220 kg each (SDX01 Chaser + SDX02 Target).
- Orbital parameters: 475 km circular orbit, 55° inclination.
- Countries with indigenous docking capability: USA, Russia, China, India (4th).
- India's Strategic Petroleum Reserve (Phase I): 5.33 MMT across Visakhapatnam, Mangaluru, and Padur.
- Chandrayaan-3 landing date: 23 August 2023; landing site: Shiv Shakti Point (lunar south pole region).
- BAS target commissioning year: 2035.
- Gaganyaan: India's first human spaceflight mission, crewed phase targeted for 2026-27.