ISRO Docked Two 220 kg Satellites in Orbit on 16 January 2025, but the Docking Tests Behind a Space Station and Chandrayaan-4 Are Still Being Studied

ISRO docked two satellites in orbit in the early hours of 16 January 2025, which the Press Information Bureau (PIB) counts as making India the fourth country to do it, after the United States, Russia and China. The satellites, SDX01 and SDX02, weigh about 220 kg each and had been launched together on a PSLV rocket on 30 December 2024.

ISRO pulled them apart on 13 March 2025, joined them a second time on 20 April and passed electric power between them on 21 April. A written reply in the Rajya Sabha on 23 March 2026 says ISRO is still only studying the next two docking experiments.

The launch and two postponed dates

PSLV-C60 lifted off from Sriharikota at 10 pm IST on 30 December 2024, according to Spaceflight Now. ISRO’s mission brochure puts the release of the two satellites about 15 minutes later, at an altitude of roughly 477 km. The PSLV is ISRO’s workhorse, and its 104-satellite flight of 2017 is told in a separate piece. The PIB’s briefing of 28 December said the rocket’s fourth stage would also carry 24 experiments from academic institutions and start-ups.

On 31 December the Union minister Dr Jitendra Singh said docking was expected on 7 January 2025. It did not happen then. On 6 January ISRO moved it to 9 January, after identifying an abort scenario that needed more checks in ground simulations, News On AIR reported.

The 9 January attempt was postponed as well. During a manoeuvre to close the gap to 225 m, the satellites drifted apart faster than expected while they were out of ground contact. ISRO’s mission page says the satellites were safe.

In a trial run on 12 January, ISRO brought the pair to 3 m and then moved them back to a safe distance, the PIB records. The docking itself came in the early hours of 16 January.

How ISRO closed the last 15 metres

Both craft circle Earth at about 28,800 km/h, so the chaser has to bring its speed relative to the target down to 0.036 km/h, the PIB explained on 28 December. That is 10 millimetres a second, the approach speed on ISRO’s mission page.

SDX01 was the chaser and SDX02 the target. ISRO’s brochure says the two are androgynous: the docking mechanism is identical on both, and either could have been the active one. The plan took the chaser in stages. The craft first drifted to about 20 km apart, then held at 5 km, 1.5 km, 500 m, 225 m, 15 m and 3 m. From 5 km the satellites exchanged position and orientation data over a radio link, as Spaceflight Now described it.

Several sensors took over as the gap closed. ISRO lists a laser range finder working from 6,000 m down to 200 m, rendezvous sensors from 2,000 m to 10 m, and a proximity and docking sensor from 30 m to 0.4 m. A video camera recorded the last metres, and a mechanism entry sensor detected the chaser entering the target. A processor fed by satellite-navigation (GNSS) signals worked out each craft’s position and speed relative to the other.

Docking was initiated from the 3 m hold point, the PIB’s 16 January factsheet says. The motor-driven mechanism captured the target, retracted to draw the two craft together and then rigidised, which is ISRO’s word for locking them into one stiff structure. The pair could then be steered as a single object, ISRO said.

ISRO’s mission page compares its mechanism with the International Docking System Standard used by other agencies for human missions. The ISRO ring is 450 mm across and uses two motors, against 800 mm and 24 motors for the standard. The page also says that docking small spacecraft needs finer precision than docking large ones. The minister calls the indigenous system the Bharatiya Docking System.

Undocking, a second docking and a heater test

On 13 March 2025, at about 09:20, ISRO separated the satellites on the first attempt, in a 460 km circular orbit inclined at 45 degrees. Ground stations at Bengaluru, Lucknow and Mauritius tracked the operation, and ISRO reported that the health of both satellites was normal.

The second docking came at 8:20 pm on 20 April 2025. This time the approach was fully autonomous from 15 m to contact, ISRO says. In the first docking a hold at 3 m had been exercised manually.

On 21 April ISRO sent power from SDX02 to SDX01 and then back, running a heater in one satellite from the other’s supply for about four minutes. ISRO said the performance was as expected.

What the space station asks of it

The Cabinet approved the first module of the Bharatiya Antariksh Station, BAS-1, on 18 September 2024, as part of a revised Gaganyaan programme of eight missions to be completed by December 2028. The Department of Space told the Rajya Sabha on 6 August 2026 that the station will have five modules and is planned to be operational by 2035.

ISRO’s SpaDeX page says in-space docking is essential when several launches are needed for a common objective. The 23 March 2026 reply goes further for crewed flight: docking technology is needed for crew transfer and for moving propellant, fluid and power. It says proving it in orbit paves the way for docking a Gaganyaan module to a station such as BAS. SpaDeX tested one item on that list, power transfer.

What Chandrayaan-4 asks of it

The Cabinet’s approval of Chandrayaan-4 on 18 September 2024 lists docking and undocking among the technologies the lunar sample-return mission has to demonstrate. It funds the mission at ₹2,104.06 crore, which includes two LVM3 launches, and expects completion within 36 months of approval. India’s soft landing near the Moon’s south pole is covered in our Chandrayaan-3 piece.

ISRO’s SpaDeX page calls the mission a forerunner for the autonomous docking that lunar missions such as Chandrayaan-4 will need without GNSS support from Earth. The sources opened for this article do not say whether ISRO has yet shown that part.

What has not been demonstrated

ISRO’s March 2025 statement says SpaDeX showed rendezvous, docking and undocking in a circular orbit, using two small satellites. The 23 March 2026 reply describes two further experiments that ISRO is studying.

SPADEX-2 would dock two spacecraft in a highly elliptical orbit, a stretched path that is not circular, to meet the needs of Chandrayaan-4. It would also transfer a sample between two modules. SPADEX-3 would dock two pressurised modules in a circular orbit with the Bharatiya Docking System, as a precursor for docking at the station.

The reply gives no launch date and no approval for either one. The Cabinet note of September 2024 gives December 2028 as the date by which the eight human spaceflight missions, ending with the launch of BAS-1, are to be completed.