On 16 January 2025, two Indian satellites joined in orbit. The first successful docking of SDX-01 and SDX-02 gave ISRO an in-flight demonstration of a capability that launching spacecraft alone cannot supply: bringing separately flying vehicles together and connecting them. [1]
The Space Docking Experiment, or SpaDeX, had left Earth aboard PSLV-C60 on 30 December 2024. The launch put the equipment into space; the central experiment still lay ahead. [2]
Each satellite weighed about 220 kilograms. ISRO’s design called for them to approach in stages, using sensors, relative-position calculations and an inter-satellite communications link. The final contact had to be gentle: the docking mechanism was designed for an approach speed of roughly one centimetre per second. [3]
An approach that could be stopped
The flight did not follow its first announced docking timetable. ISRO postponed the attempt scheduled for 9 January after finding greater-than-expected drift during a manoeuvre towards a separation of 225 metres. Its update said the satellites remained safe. The eventual success followed that interruption, rather than an uninterrupted countdown to contact. [3]
The spacecraft were developed by the UR Rao Satellite Centre with other ISRO centres. Integration and testing took place at Ananth Technologies in Bengaluru under URSC supervision. The achievement belonged to that engineering network, including the teams responsible for navigation, mechanisms, software and ground operations. [3]
Docking was only part of the assignment. On 13 March, ISRO separated the spacecraft, reporting that both were healthy and flying independently. The agency said it had analysed their performance while joined and prepared the separation through ground simulations. Stations in Bengaluru, Lucknow and Mauritius monitored the operation. A connection intended for repeated space operations also has to come apart safely. [4]
Four minutes of shared power
The satellites docked again on 20 April. This time the final approach from 15 metres was fully autonomous. ISRO contrasted that with January’s attempt, when operators had manually commanded an additional pause at three metres. The second flight test therefore established something more specific than a repeat photograph of two joined spacecraft. [5]
On 21 April, electricity passed between the satellites in both directions. One spacecraft supplied a heater on the other during a test lasting about four minutes. That was a small load and a brief demonstration, but it showed the connection could carry power as well as hold the vehicles together. [5]
The next requirements are different again. In a March 2026 parliamentary reply, the Department of Space described studies for SpaDeX-2, involving docking in an elongated orbit and transferring samples, and SpaDeX-3, involving pressurised modules for space-station applications. These were proposed follow-on experiments at that checkpoint. [6]
A lunar sample must eventually pass between vehicles; a crewed station needs connections through which people can move. SpaDeX’s two small satellites supplied experience in approaching, joining, separating and sharing electricity. Turning those operations into sample transfer or a passage for astronauts remains further engineering work. [6]