SpaDeX. SpaDeX or Space Docking Experiment is a twin satellite mission developed by the Indian Space Research Organisation (ISRO) to mature and demonstrate technologies related to orbital rendezvous, docking, formation flying, which will have future applications in areas such as human spaceflight, in-space satellite servicing and other proximity operations.

SpaDeX consists of two modified IMS-1 class satellites weighing 220 kg each. During proximity operations one spacecraft acts as a Chaser and other acts as a Target.

Both spacecraft were launched together from the First Launch Pad of Satish Dhawan Space Centre aboard a dedicated Polar Satellite Launch Vehicle on 30 December 2024 at 16:30:15 UTC and subsequently injected into slightly different orbits.

After deployment, the two spacecraft then executed manoeuvres to bring them together again. The Chaser (SDX01) approached the target (SDX02) and then carried out precision manoeuvres to complete a successful docking. With this success, India became only the fourth country to achieve successful space docking —following the United States, Russia and China.

Objectives 
According to ISRO, the SpaDeX mission had the following objectives:
 To demonstrate an autonomous rendezvous and docking using the V-bar approach.
 To verify electrical power transfer between two docked spacecraft.
 To execute a composite spacecraft control using the Attitude Control System of another spacecraft while both are in a docked configuration.
 To act as independent satellites after undocking, and to operate their payloads for a period of two years.

History 

The research and development (R&D) needed to complete the project began with preliminary studies in 2016. The Space Docking Experiment (SpaDeX) was approved by the Government of India with an initial funding of ₹10 crore, cleared in 2017.

, SpaDeX was sanctioned  in funding.

The Chaser and Target satellites, along with related docking technologies, were designed and implemented for the SpaDeX mission by UR Rao Satellite Centre (URSC) with assistance from Vikram Sarabhai Space Centre (VSSC), Liquid Propulsion Systems Centre (LPSC), Space Applications Centre (SAC), ISRO Inertial Systems Unit (IISU), and Laboratory for Electro-Optics Systems (LEOS).

For the SpaDeX satellites, Ananth Technologies provided Rendezvous Processing Units (RPU) and DC-to-DC converters. Additionally, the company provided 29 essential components for the PSLV-C60 launch vehicle, such as data acquisition units, transmitters, power modules, NavIC chip and control modules.

In less than three months, Ananth Technologies completed the satellite assembly, integration, and testing for ISRO, and delivered them to the U R Rao Satellite Centre (URSC). As per the previous head of ISRO, S. Somanath, docking is a crucial component for Chandrayaan-4 and that SpaDeX is intended as a demonstrator and forerunner.

Two future projects of ISRO, the planned Bharatiya Antariksha Station and Gaganyaan human space flights missions depend on the mission's success.

According to Jitendra Singh Rana, Minister of State for Science & Technology and Earth Sciences, and Prime Minister Narendra Modi, SpaDeX's achievement represents a significant step forward in the establishment of Bharatiya Antariksha Station by 2035.

Mastering docking techniques is essential for future deep space missions, according to Nambi Narayanan. As per Mylswamy Annadurai, the project director of the Chandrayaan-1, the demonstated docking technique will aid in the future management of space debris.

International collaboration 
The Italian company Leaf Space, which offers ground segment-as-a-service (GSaaS) solutions, partnered with ISRO to create communication linkages with the POEM-4 platform and the SpaDeX satellites. It made telemetry, command operations, and ongoing SpaDeX mission telemetry monitoring possible.

Description 
The designations SDX01 and SDX02 are assigned to the "Chaser" and "Target" satellites, respectively, though both host docking devices capable of active or passive function.

The launch vehicle deployed the spacecraft separately in order to create a separation between them of about 20km. After the spacecraft executed manoeuvres to prevent them from drifting further apart and then to begin the rendezvous operation.

The Chaser then began to approach the Target, gradually lowering the separation between them to a distance of a few metres. After docking was completed, the mission planned to establish and demonstrate the transfer of electrical power between the spacecraft.

The Space Applications Centre developed a miniaturized high-resolution camera with photo and video capability that was carried aboard SDX01. SDX02 was equipped with a Multi-Spectral Payload (MMX) for vegetation and natural resource monitoring. In order to properly plan the Gaganyaan missions, SDX02 has a radiation detector that will collect data on radiation levels in space.

After separation, the two spacecraft will run their payloads for a period of two years.

The Bhartiya Docking System (BDS) was developed by ISRO based on the International Docking System Standard (IDSS) after unsuccessful attempts to import the docking technology from abroad.

For autonomous docking, the relative velocities of SDX01 and SDX02 were reduced to 0.036 km/h or 10 mm/s using retrorockets and a sensor suite. In contrast to the 24 motors used in IDSS, the BDS only uses two. The docking port on SpaDeX is 450 mm in diameter, whereas the docking port at the Gaganyaan and Bharatiya Antariksha Station will be 800 mm.

The dual-motor actuation design was designed to help ensure secure connections and accurate alignment at low speeds. The real-time alignment and navigation was supported by proximity sensors, laser rangefinders, and rendezvous cameras. During docking operations, an Inter-Satellite Communication Link (ISL) provided smooth data transmission, improving the system's dependability and autonomy. As soon as the satellites were under 5 km apart, they were able communicate via ISL to share orientation and location information.

New process and technologies 

 Instead of being assembled and integrated at Vehicle Assembly and Launching Facility (VALF) and Mobile Service Tower (MST) respectively, the launch vehicle was assembled in a new PSLV Integration Facility (PIF) and transported to the launch pad to reduce the lead-in time between missions.
 A low-impact, androgynous peripheral docking system with an approach velocity of 10 mm/s was developed and employed.
 Laser rangefinder and corner cube retroreflectors for position vector and velocity in 6,000 to 200 m range. Rendezvous sensor for position vector in 2,000 to 250 m and 250 to 10 m range. Proximity and docking sensor for position vector and velocity in 30 m to 0.4 m range. An image sensor for 20 to 0.5 m range to capture the docking event. Mechanism entry sensor from 8 cm to 4 cm to detect SDX01 entry into SDX02. Accelerometers for velocity measurement, and star tracker for attitude determination. 
 Integrated processor for Relative Orbit Determination and Propagation (RODP) with differential GNSS-based Satellite Positioning System (SPS) for Positioning, Navigation and Timing (PNT). VHF/UHF transceivers to transfer and synchronize position vector and velocity data by Inter-Satellite Communication Link.
 A trigger mechanism to capture the other satellite and securely hold it in place for docking. 
 Power transfer technology, and built-in artificial intelligence to comprehend satellite conditions.
 Rendezvous and Docking Algorithms. The Rendezvous Simulation Lab to validate the algorithms through real-time simulation, the Docking Mechanism Performance Test for last stage of docking, and the Vertical Docking Experiment Lab to test the docking mechanisms under controlled settings.

Timeline 
 PSLV-C60 successfully launched the SDX01 (Chaser) and SDX02 (Target) into 475-kilometer circular orbit with a 55° inclination on December 30, 2024. SDX01 and SDX02 were kept in the same orbit above the Earth but were injected with slightly differing forces to enable them to achieve a separation of roughly 20 km.
 As of January 2, 2025, ISRO increased the distance between SDX01 and SDX02 to begin the rendezvous phase.
 The final docking manoeuvre was originally planned for 7 January 2025 but it was postponed to 9 January due to need for additional validation through ground simulations based on an abort scenario found on January 6th. The docking window is set for January 7–10, according to URSC Director M. Sanakaran.
 The satellites were separated by 600 meters on January 8. To get the chaser satellite closer to the target, ISRO started a drift maneuver that moved it from 500 meters to 225 meters. However, the drift was greater than anticipated after the non-visibility period. The docking was delayed for the second time.
 In order to bring both satellites closer, ISRO placed the chaser on a slow drift trajectory towards the target on January 9. It is anticipated that the docking sequence will attain its initialization condition on January 10.
 The two satellites are 1.5 km apart and in hold mode on the night of January 10. By the morning of January 11th, the distance will be reduced to 500 meters. The distance will be further reduced to 15 meters, and then to 3 meters, before the docking sequence is initiated.
 The inter-satellite distance on January 11th is 230 meters. All the sensors are operating as intended and the satellite health is normal.
 While docking preparations are underway, SDX01 and SDX02 are 105 meters apart in orbit as of 12 January IST 3:10 a.m.
 The SDX01 and SDX02 imaging sensors were activated to take pictures and videos of one another when the inter-satellite distance was further decreased to 15 meters.
 A successful trial attempt was made to reach up to 15 meters and then 3 meters. Before the docking process started, both satellites were repositioned to a safer distance so that alignment data and other factors could be examined. A delay in receiving signals from the proximity and docking sensors was observed during the inter-satellite distance reduction maneuver from 15 meters to 3 meters while stabilizing the drift at 0° alignment. For the SDX01 and SDX02 to correctly align throughout the docking process, these sensors were crucial. Because of the delay, onboard safe mode operation was automatically activated to prevent any unintentional collision. This increased the inter-satellite distance between SDX01 and SDX02 to 8 km. The third docking attempt had to be canceled, and ISRO is currently troubleshooting the issue with a thorough investigation. If sensor problems continue, it can postpone further docking attempts until visibility improves.
 Manoeuvre from 15m to 3m hold point completed on January 16. Docking initiated with precision, leading to successful spacecraft capture. Retraction completed smoothly, followed by rigidisation for stability. The ISRO Telemetry, Tracking and Command Network's Mission Operations Complex verified that the docking process was successful. India became the 4th country — after USA, Russia and China — to achieve successful Space Docking. ISRO also successfully managed to control two satellites as a single entity after docking.
 In order to improve the accuracy of automated docking and obtain more data, ISRO is getting ready for a few more docking attempts using SDX01 and SDX02. Several docking maneuvers will be tried with a maximum inter-satellite distance of 100 meters in order to save fuel. The docking procedure and final approach will be the primary areas of attention.

Challenges 
ISRO did not undertake a trial mission for the space docking experiment due to financial constraints. The two satellites and associated equipment were built by ISRO for ₹125 crore, while the launch vehicle cost an additional ₹250 crore.

Future development 
During the SpaDeX mission's post-launch briefing on December 30, 2024, ISRO Chief S. Somanath declared that additional SpaDeX missions with greater size and complexity would be launched, showing larger docking systems, most likely for Gaganyaan and Bharatiya Antariksha Station.

See also 

 ETS-VII or KIKU-7, also known as Orihime/Hikoboshi, a Japanese mission in 1997 with similar objectives.
 Kosmos 186 and Kosmos 188, the first Soviet docking effort.
 Gemini 8, the first docking conducted in space.
 Shenzhou, Chinese docking mission.
 Jules Verne ATV, the first European in space docking.
 List of Indian satellites

References 

2024 in spaceflight
Robotic satellite repair vehicles
2025 in spaceflight
ISRO space probes
Space missions
Spacecraft launched by PSLV rockets
Spacecraft launched by India in 2024
ISRO programs
Space programme of India