What is Docking & Undocking of Satellites? All you need to know about it

On March 13, 2025, at approximately 09:20 Hrs, the Indian Space Research Organisation (ISRO) successfully carried out the undocking of SPADEX satellites in their first attempt. This marks another milestone in India's advancements in space technology, making the country one of the few with space docking and undocking capabilities.

 Congratulations to India on this milestone! 🇮🇳✨ #Spadex #ISRO #SpaceTech pic.twitter.com/7u158tgKSG

— ISRO (@isro) March 13, 2025

Key Details on Undocking of SPADEX Satellite

• Initial Docking: The SPADEX satellites were initially docked on January 16, 2025.
• Orbit Details: The undocking operation was conducted in a 460 km circular orbit with a 45-degree inclination.
• Satellite Status: Post-undocking, the satellites are orbiting independently, and their health remains normal.
• Ground Monitoring Stations: The operations were monitored through ground stations located in Bengaluru, Lucknow, and Mauritius.
• Future Experiments: Further experiments with the satellites are planned in the coming days to enhance capabilities.

Source:isro.gov

Importance of the Achievement

• Demonstrates rendezvous, docking, and undocking technologies in a circular orbit.
• Enhances India’s capabilities in autonomous space operations.
• Supports future missions, including India’s planned space station and Chandrayaan-4, which aims to return lunar soil and rock samples.

Congratulations to India on this milestone! 🇮🇳✨ #Spadex #ISRO #SpaceTech pic.twitter.com/7u158tgKSG

— ISRO (@isro) March 13, 2025

Understanding Docking and Undocking in Space

What is Docking?

Docking is the process where two spacecraft physically connect in space for purposes like crew transfer, cargo resupply, or spacecraft assembly.

Key Steps in Docking:

1. Rendezvous: The approaching spacecraft aligns with the target.
2. Approach: Precise manoeuvres guide the spacecraft toward the docking port.
3. Capture: Docking mechanisms (e.g., mechanical arms or magnetic latches) lock the two spacecraft together.
4. Sealing: The docking port is sealed to maintain pressure.

Examples of Docking in Space Missions:

• Apollo Lunar Missions (1969-1972): Apollo Command Module (CM) docked with the Lunar Module (LM).
• International Space Station (ISS): Regular docking by spacecraft like SpaceX Dragon and Russia’s Soyuz.
• SpaceX Crew Dragon: Uses an automated docking system to connect with the ISS.

What is Undocking?

The separation of two spacecraft that were previously connected is called undocking. Undocking refers to the controlled procedure of disengaging a spacecraft, satellite, or module from another structure, such as a space station or a companion satellite. This process follows a carefully planned sequence of steps to ensure a smooth and safe separation. For example, in the case of the SDX-01 (Chaser) and SDX-02 (Target) satellites, undocking involved their gradual detachment after being launched aboard the PSLV-C60 from the Satish Dhawan Space Centre on December 30, 2024.

 Congratulations, Team ISRO! 🇮🇳 #ISRO #Spadex #SpaceTech pic.twitter.com/2H1Ts8ATkN

— ISRO InSight (@ISROSight) March 13, 2025

Key Steps in Undocking:

1. Release: Unlocking of docking mechanisms.
2. Separation: Thrusters fire to move the spacecraft apart.
3. Move to Safe Distance: Spacecraft manoeuvres to avoid collisions.

Examples of Undocking in Space Missions:

• Apollo Missions: The Lunar Module was undocked before the moon landed and returned.
• SpaceX Crew Dragon: Undocks from ISS after completing its mission.
• Soyuz Spacecraft: Regularly undocks from ISS before returning to Earth.

SPADEX: India’s Space Docking Mission

Mission Details

• Launch Date: December 30, 2024.
• Satellites:
• SDX01 (Chaser)
• SDX02 (Target)
• Orbit: 470 km circular orbit.
• Purpose: Demonstrate India's docking and undocking technology.

How has SPADEX demonstrated Docking?

• Initial Distance: Satellites started 20 km apart.
• Manoeuvring: Slowly brought together using thruster control.
• Locking Mechanism: Satellites docked via an extended locking system.
• Power Sharing: Demonstrated power-sharing capabilities after docking.

Why is this achievement important?

History of Space Docking

Conclusion

The successful undocking of the SPADEX satellites marks a significant step for India’s space ambitions. With this achievement, ISRO joins the elite group of nations that have mastered rendezvous, docking, and undocking operations. This technology will play a critical role in future space missions, including human spaceflight, lunar exploration, and building a space station. India's space capabilities continue to expand, bringing the nation closer to its ambitious space goals.