The choice of orbit for docking satellites, particularly with human missions in mind, is a critical aspect of mission planning. Recent developments, such as India's SpaDeX (Space Docking Experiment) mission, highlight the importance of orbital parameters in facilitating successful docking operations.

Key Considerations For Docking Orbits

1. Orbit Type and Altitude

Low Earth Orbit (LEO): Most docking missions are conducted in LEO, typically at altitudes ranging from 300 km to 600 km. For example, the SpaDeX mission operates at a circular orbit of approximately 470 km. This altitude allows for easier access to space and facilitates rendezvous with other spacecraft.

Inclination: The inclination of the orbit is also vital. SpaDeX was launched into a 55-degree inclined orbit, which is suitable for a variety of missions and allows for coverage over different latitudes.

2. Relative Velocity Management

Docking requires precise control over the relative velocities of the spacecraft involved. The "chaser" satellite is often placed in a slightly lower orbit than the "target" satellite, allowing it to catch up due to higher velocity at lower altitudes. In SpaDeX, after an initial separation of 10-20 km post-launch, the chaser gradually reduces its distance to the target through controlled manoeuvres, ultimately achieving a docking distance of mere meters.

3. Autonomous Docking Technology

The development of autonomous docking systems is essential for human missions. These systems rely on advanced sensors and algorithms to navigate and dock without direct human intervention. SpaDeX utilizes various sensors, including laser range finders and proximity sensors, to ensure accurate positioning during docking manoeuvres.

Applications For Human Missions

The advancements in docking technology have significant implications for future human spaceflight missions:

Space Station Construction: For missions aimed at constructing space stations (e.g., India's planned Bharatiya Antariksha Station), multiple modules will need to dock in space. The ability to autonomously dock will streamline this process and enhance safety.

Lunar Missions: Future lunar missions, like Chandrayaan-4, will require multiple docking manoeuvres between various modules (e.g., propulsion modules and landers) to transport samples back to Earth. Successful demonstrations like SpaDeX pave the way for these complex operations by validating the technologies required for autonomous docking without reliance on Earth-based navigation systems.

In-Space Servicing: The capabilities developed through these missions can also extend to satellite servicing in orbit, where one spacecraft may need to dock with another for repairs or upgrades.

The choice of orbit for docking satellites is influenced by factors such as altitude, inclination, relative velocity management, and the need for autonomous technology—all crucial for supporting future human missions and enhancing capabilities in space exploration.

ISRO Planning Additional Docking Attempts Based On Fuel In Spadex Satellites

India's Indian Space Research Organisation (ISRO) is planning additional docking attempts with its SpaDeX satellites based on the fuel availability in the spacecraft. The SpaDeX mission, which successfully launched two small satellites on December 30, 2024, is a critical step towards India developing its space docking technology, a capability currently held by only a few countries including the US, Russia, and China.

ISRO