SDX01 and SDX02, the two satellites of ISRO’s Space Docking Experiment (SpaDeX), re-entered the atmosphere on Monday 12 October, seven hours apart. SDX01 came down over the southern Indian Ocean, west of Australia, at 16:08 UTC, and SDX02 over the North Pacific, south of the Aleutian Islands, at 23:05 UTC, according to Space-Track.

A chaser and a target

The pair lifted off together from Sriharikota on 30 December 2024, on a PSLV. Each weighed about 220 kg, and they were placed in a 470 km circular orbit at 55° inclination, SDX01 as the chaser and SDX02 as the target. ISRO built the mission to demonstrate rendezvous, docking and undocking, and the transfer of power between two docked spacecraft, technology it calls essential for an Indian on the Moon, a sample return from the Moon and the Bharatiya Antariksh Station, India’s planned space station. It was also to be a forerunner for the autonomous docking that Chandrayaan-4 will need.

Two boxy satellites stand side by side on a glass-topped test frame in a laboratory. Each is wrapped in crinkled gold foil on top, with a large black side panel laced with thin white cables and dotted with small fittings; pink protective covers hang from the top corners, and cables trail across the floor between them
SDX01 and SDX02 on the ground before their launch. Photo: ISRO, reproduced under ISRO's copyright policy

ISRO docked the two in the early hours of 16 January 2025. That made India the fourth nation to demonstrate docking in space. Between 15 January, the eve of the docking, and the undocking on 13 March, whenever Space-Track issued Elements for both at the same moment, they were identical, as for a single object.

On 13 March 2025, ISRO undocked them at the first attempt, in a 460 km circular orbit. On 20 April it docked them a second time, with full autonomy from 15 m apart, where the first docking had needed a hold commanded by hand at 3 m. On 21 April power flowed from SDX02 to SDX01 and back, for about four minutes, to run a heater on the other satellite. ISRO also lists a circumnavigation of the two satellites among its achievements of 2025.

Twenty-one months of sinking

ISRO had planned for the two satellites to work for up to two years once undocked. Space-Track’s Elements show neither climbing after late April 2025, and the fall quickened as the orbit got lower. Their mean altitude went from about 460 km at the undocking to 413 km on 1 January 2026, 312 km on 1 September and 268 km on 1 October, and about 155 km in their last Elements on 12 October. They lost more height in those last 11 days than in the 15 months after the undocking.