SpaDeX facts for kids

Quick facts for kids
Space Docking Experiment

SpaDeX Chaser (SDX01) and Target (SDX02) spacecrafts during testing
Mission type	Rendezvous and Docking
Operator	ISRO
Spacecraft properties
Bus	Modified IMS-1
Manufacturer	U R Rao Satellite Centre (ISRO) Ananth Technologies
Start of mission
Launch date	30 December 2024, 10:00 PM IST (16:30 UTC)
Rocket	PSLV-CA C60
Launch site	Satish Dhawan Space Centre FLP
Contractor	ISRO
Orbital parameters
Altitude	460 kilometres
Inclination	45°

The SpaDeX mission, short for Space Docking Experiment, is a special project by the Indian Space Research Organisation. It uses two satellites that work together. The main goal of SpaDeX was to test and show off new technologies. These technologies help spacecraft meet up in space (called rendezvous), connect together (called docking), and fly close to each other in a group (called formation flying).

These skills are super important for future space missions. They will help with sending humans into space, fixing satellites while they are orbiting Earth, and other close-up operations in space.

SpaDeX uses two satellites, each weighing 220 kg. They are a special type called modified IMS-1. When they perform their close-up operations, one satellite acts as the "Chaser" and the other as the "Target."

Both satellites were launched together on December 30, 2024. They lifted off from the Satish Dhawan Space Centre using a Polar Satellite Launch Vehicle. After launch, they were put into slightly different orbits.

Once in space, the two satellites performed special moves to come back together. The Chaser satellite (SDX01) carefully approached the Target satellite (SDX02). Then, it carried out very precise maneuvers to successfully connect. With this success, India became one of the few countries in the world to achieve in-space docking using its own technology.

What Were the Goals of SpaDeX?

ISRO had several important goals for the SpaDeX mission:

• To show how satellites can meet and dock on their own, using a specific method called the V-bar approach.
• To check if electrical power could be sent from one docked satellite to the other.
• To control both satellites as one unit after they docked. This was done using the Attitude Control System of one of the satellites.
• To make sure the satellites could work as separate units again after undocking. They were planned to operate their science tools for two years.

How SpaDeX Was Developed

The idea and early studies for SpaDeX started in 2016. The Indian government approved the Space Docking Experiment (SpaDeX) in 2017. It received initial funding to begin the project.

By July 2022, SpaDeX had received more funding to continue its development.

Many different centers and companies helped design and build the SpaDeX mission. The UR Rao Satellite Centre (URSC) led the design of the Chaser and Target satellites and their docking systems. Other ISRO centers like Vikram Sarabhai Space Centre (VSSC) and Space Applications Centre (SAC) also helped.

Ananth Technologies provided important parts for the SpaDeX satellites. They also made 29 key components for the PSLV-C60 rocket that launched the satellites. Ananth Technologies quickly put the satellites together, tested them, and delivered them to URSC.

The former head of ISRO, S. Somanath, said that docking is a very important step for future missions like Chandrayaan-4. He explained that SpaDeX was a test mission to prepare for these bigger projects.

The success of SpaDeX is crucial for two future ISRO projects. These are the planned Bharatiya Antariksha Station (India's own space station) and the Gaganyaan missions, which will send Indian astronauts into space.

Indian government officials, including Minister Jitendra Singh Rana and Prime Minister Narendra Modi, have said that SpaDeX's success is a big step towards building the Bharatiya Antariksha Station by 2035.

Experts like Nambi Narayanan believe that mastering docking is key for future deep space missions. Mylswamy Annadurai, who led the Chandrayaan-1 project, also noted that this docking technique will help manage space debris in the future.

On May 23, 2025, ISRO announced that the main goal of docking was achieved. The mission then moved into an "extended phase." This means they will do more experiments using the remaining fuel.

Working with Other Countries

An Italian company called Leaf Space worked with ISRO on this mission. Leaf Space helped set up communication links with the POEM-4 platform and the SpaDeX satellites. This partnership allowed ISRO to send commands to the satellites and receive data from them throughout the mission.

How the SpaDeX Satellites Work

The two satellites are called SDX01 (the "Chaser") and SDX02 (the "Target"). Both satellites have docking devices that can either actively connect or passively receive a connection.

The rocket launched the satellites separately, creating a gap of about 20 kilometers between them. After launch, the spacecraft performed maneuvers to stop them from drifting too far apart. Then, they began the rendezvous operation to meet up. When they were about 11 kilometers apart, the satellites were orbiting at a speed of about 28,400 kilometers per hour.

The Chaser satellite then started to get closer to the Target. It slowly reduced the distance between them to just a few meters. After they docked, the mission successfully showed that electrical power could be transferred between the two connected spacecraft.

The Space Applications Centre developed a tiny, high-resolution camera for SDX01. This camera could take photos and videos. SDX02 carried a Multi-Spectral Payload (MMX) to observe plants and natural resources on Earth. SDX02 also had a radiation detector. This tool collected data on radiation levels in space, which is important for planning the Gaganyaan human space missions.

After they separated, both satellites continued to operate their science tools for two years.

ISRO developed its own docking system, called the Bhartiya Docking System (BDS). They based it on the International Docking System Standard (IDSS) after facing challenges in getting docking technology from other countries.

For the satellites to dock on their own, their relative speeds were slowed down to just 0.036 km/h (10 mm/s). This was done using small retrorockets and a set of sensors. The BDS uses only two motors, unlike the IDSS which uses 24. The docking port on SpaDeX is 450 mm wide. The docking ports for the Gaganyaan mission and the Bharatiya Antariksha Station will be larger, at 800 mm.

The two-motor design helped make sure the connection was secure and aligned correctly at slow speeds. Sensors like proximity sensors, laser rangefinders, and special cameras helped with real-time alignment and navigation. When the satellites were docking, an Inter-Satellite Communication Link (ISL) sent data smoothly between them. This made the system more reliable and able to work on its own. Once the satellites were within 5 km of each other, they could use ISL to share information about their position and direction.

New Technologies Used

The SpaDeX mission used several new processes and technologies:

• The rocket was put together in a new facility called the PSLV Integration Facility (PIF). This helped reduce the time needed between launches.
• A special docking system was developed. It allowed for a gentle connection at a speed of 10 mm/s.
• Various sensors were used to measure distance and speed. These included laser rangefinders for distances from 6,000 to 200 meters, and proximity and docking sensors for very close distances. An image sensor captured the docking event.
• An advanced computer system helped figure out the satellites' positions and movements. It used GNSS (like GPS) to get accurate location data.
• VHF/UHF radios allowed the satellites to talk to each other and share location data.
• A special trigger mechanism was designed to capture and hold the other satellite securely for docking.
• Technology to transfer power between satellites was developed.
• Built-in artificial intelligence helped the satellites understand their own condition.
• Special computer programs (algorithms) were created for rendezvous and docking. These were tested in a simulation lab.

Mission Timeline

Docking and Undocking Events

• On December 30, 2024, the PSLV-C60 rocket launched the SDX01 (Chaser) and SDX02 (Target) satellites. They went into a circular orbit 475 kilometers high.
• Over the next few days, ISRO increased the distance between the two satellites to more than 20 km.
• On January 6, the docking maneuver was delayed from January 7 to January 9. This was because ISRO needed to do more tests on the ground.
• By January 8, the two satellites had reduced their separation to 600 meters. ISRO started a maneuver to bring the chaser satellite even closer, from 500 meters to 225 meters. However, the drift was larger than expected. So, the docking was delayed a second time.
• On January 9, ISRO put the chaser on a slow path to bring it closer to the target. The docking was expected to start on January 10, but it was delayed again for several days.
• By January 12, the SDX01 and SDX02 satellites were within 15 meters of each other. ISRO turned on their cameras so the satellites could take pictures and videos of each other.
• On January 13, the chaser tried to get within 15 meters and then 3 meters of the target. But there was a delay in receiving signals from the proximity and docking sensors. These sensors were very important for the satellites to line up correctly. Because of the delay, an automatic safety mode turned on to prevent any accidental crash. This stopped the docking attempt and increased the distance between the satellites to 8 km.
• On January 16, a second docking attempt was made and it worked! The target satellite successfully captured the chaser for the first time. ISRO also managed to control the two satellites as one combined unit after they docked.
• On March 13, ISRO successfully undocked and separated the SpaDeX satellites. This process involved several steps, including releasing special levers. All the necessary steps for meeting, docking, and undocking in a circular orbit were successfully proven by ISRO. Both SDX01 and SDX02 then orbited Earth independently.
• ISRO planned to do more experiments and another undocking/re-docking by March 15, 2025. This re-docking was planned to happen automatically. Such experiments are only possible during a short 15-day window every two months.
• On April 20, 2025, ISRO successfully showed the docking of SDX01 and SDX02 for a second time. This time, the docking was fully automatic from 15 meters away, unlike the first time which needed some manual steps at 3 meters.

Other Experiments

• On March 28, 2025, V. Narayanan confirmed that the first docking had not fully transferred electrical power between the satellites. This experiment was moved to a later time because of a small possible misalignment of the power ports. The SpaDeX satellites still had enough fuel to complete this experiment.
• ISRO completed a "rolling experiment" on March 28, 2025. In this test, one satellite circled the other and returned to its starting position while staying in sight. This helped ISRO test many things, including software, sensors, and ground control. The experiment will help ISRO understand how to dock satellites from different angles. It will also help them see if vertical docking is possible.
• On April 21, 2025, the power transfer from SDX02 to SDX01 and vice versa was successfully completed. In this experiment, one satellite powered a heating element on the other. Both satellites worked perfectly during the 4-minute power transfer. The spacecraft were also undocked into a 460-km circular orbit with a 45-degree inclination.
• On May 3, 2025, ISRO successfully completed a high-speed satellite rendezvous. This showed their skill and readiness for orbital defense. SDX01 and SDX02 were guided into a coordinated, high-speed contact at an orbital speed of 28,800 km/h. The satellites tested their control, communication, and automatic systems as they slowly got closer. This experiment was like training for aircraft combat in space. It showed the precision needed in space warfare. This test was done after comments from US Vice Chief of Space Operations Gen. Michael Guetlein about China's similar activities in space.

Challenges Faced

ISRO did not do a trial mission for the space docking experiment because of money limits. The two satellites and their equipment cost about ₹125 crore to build. The rocket launch cost an additional ₹250 crore. Before launch, the rocket had to wait to avoid possibly getting too close to other satellites.

Pre-Launch COLA for the PSLV-C60 flight

There were some reports that SpaDeX had technical difficulties with the undocking procedure that was supposed to happen on January 16, 2024. V. Narayanan announced on January 29 that ISRO was reviewing the undocking procedure. They did not want to undock and then lose control of the satellites. With 60–70% of the fuel remaining, and the project costs fully used, more docking, undocking, and power transfer experiments were planned. V. Narayanan told Press Trust of India on February 8, 2025, that there were no issues. ISRO planned to take its time, study, and organize more experiments before trying the undocking procedure again.

What's Next for SpaDeX?

After the SpaDeX mission launch on December 30, 2024, ISRO Chief S. Somanath said that more SpaDeX missions would be launched. These future missions would be bigger and more complex. They would likely test larger docking systems for the Gaganyaan human spaceflights and the Bharatiya Antariksha Station.

ISRO plans to launch SpaDeX-2 between 2025 and 2028, if the government approves it. It is expected to take about 18 months to prepare after approval. The goal of SpaDeX-2 is to dock two satellites in an elliptical orbit, which is a more challenging task than a circular orbit. In a circular orbit, the satellites' path and speed stay the same. But in an elliptical orbit, they are always changing.

See Also

• ETS-VII or KIKU-7, a Japanese mission from 1997 with similar goals.
• Kosmos 186 and Kosmos 188, the first Soviet docking attempt.
• Gemini 8, the first docking done in space.
• Shenzhou, a Chinese docking mission.
• Jules Verne ATV, the first European docking in space.
• List of Indian satellites