Questions Posed by ISRO’s SpaDeX Mission

Mission SpaDeX of ISRO

Mission SpaDeX has been planned by ISRO to prove a technology wherein additional spares or supplies could be delivered to an orbiting satellite of space station for varied reasons including those of the planned “Bharatiy Space Station (BSS)” or Chandrayan-4. ISRO had launched its SpaDeX mission through its PSLV C60 on 30 Dec 24. The launch of the vehicle, all its four stage motor operations were copy book style with perfection and so was the injection of the payloads into the selected orbit. A big congratulation came from all quarters of people both from India and abroad and justifiably so. For ISRO, it was last space launch of the year 2024. The primary payloads for the mission were the two satellites of SpaDeX and an ISRO Robotic Arm apparently meant to catch space debris although it could have many other applications too.

The mission of this launch was to inject the multiple payloads for the following purposes: -

• To Demonstrate Space Docking and Undocking of two satellites for given objectives injected in 476 Km altitude Orbit.
• To Try ISRO Robotic Arm functions in orbit mounted on POEM-4 at 482 Km altitude.

As ‘SpaDeX’ name of mission implies, the primary objective aim was the Space Docking Experiments in the Lower Earth Orbit. These experiments included the following subsets: -

1. The Docking Experiments between Satellites SDX01 (Target) and SDX02 (chaser)
2. After injecting in orbit, induce 300m drift/separation in Chaser.
3. Smooth Rendezvous of the Chaser to the Target for satisfactory docking.
4. Test the composite spacecraft maneuvering after docking.
5. Undocking of the Chaser

The Robotic Arm along with some other equipment were mounted on PSLV-Orbital Experimental Module-4 (POEM-4) that included a total 14 Payloads of ISRO. POEM-4 was injected into a higher orbit (at 482 Km) after which the remaining 10 payloads of agencies/institutions other than ISRO were maneuvered to lower altitude 365 Km and deployed. For these 10 payloads, ISRO’s obligation was only to inject in its slotted orbit with subsequent care taken by the respective agencies.

The Launch and Orbital Injections

As mentioned above, injection of all payloads of C60 rocket was in a typical copy book style of perfection. However, a slight problem started in the SpaDeX payload SDX02 while inducing a slow drift of 300 meters in subsequent 3 days or so. This drift for some reason got exaggerated to approx. 3 Km from where rendezvous to the Target got difficult and hence delayed by a few days. On the other hand, the other payload of POEM-4 having Robotic arm performed satisfactorily that was considered a proud moment for ISRO as they intimated it on 4th Jan 25. They also shared tethered debris capture by the robotic arm on 6thJan 25.

On Jan 6th ISRO experienced an abort scenario for SDX02 rendezvous and thus postponed the docking to 9th Jan 25 which was originally scheduled for 7th Jan 25. They also mentioned that the parameters acquired needed validation and called for some more ground simulations. Indian media was abuzz with speculations and rumours of possible failure. However, experts called for caution as unexpected situations do crop up in space explorations. On 7th Jan 25 ISRO mentioned of possible docking on 9th Jan 25 at 8 AM onwards. However, this was not to be. On 9th Jan when the spacecraft was maneuvered for reducing the drift from 500m to 225m, it actually got exaggerated to 1.5 Km on 10th Jan. On 11 Jan they reported the Inter-satellite Distance (ISD) of 230m and 105m,15m and 3m on 12 Jan 25.

Confusions on Unreported Drifts

There was no update whatsoever from ISRO from 13-15 Jan 25 when two other Space agencies ‘Digantara’ an Indian start-up and ‘s2a systems’, a US space agency had created a flutter in India when they mentioned in social media ‘X’ that SpaDeX ISD on 13 Jan was 4 Km, 14 Jan 1.32 Km and 15 Jan 1.5 Km while making passes through their respective skies. ISD of both agencies were by and large matching BUT there was no comment or update from ISRO either confirming or denying their findings. If these data are correct, ISRO needs to do serious soul searching as to how its ISD drifted to as much as 4 Km on 13 Jan, 1.5 Km on 14thand 1.2 Km on 15 Jan 25 from the stated 3-meter separation that ISRO had mentioned on 12 Jan 25.

Confirmation of Docking

On 16 Jan 25, ISRO intimated of a successful docking in the early morning and earned the distinction of becoming the 4th nation in the world to achieve the feat. The docking was also confirmed by other sources and the world bodies congratulated ISRO and India for this success. Actually, words of compliments were showered on ISRO like drops of rains from both India and abroad. Undoubtedly, it was a big scientific achievement.

ISRO will remain busy in the coming days in checking the various maneuvers of the combined SpaDeX, transfers of electricity and other data from the Chaser to the Target followed by another important process of ‘Un-Docking’. ISRO has not clarified if they intend to conduct a controlled de-orbiting and atmospheric re-entry of the Chaser in future, a step that will be required to be carried out by any space capsule carrying the spares, supplies, consumables or astronaut crews for the “Bhartiya Space Station (BSS)” in future. 

Some Questions to be Addressed by ISRO

While the technology of docking has been successfully demonstrated, many questions have also arisen during the mission that must be addressed by ISRO with a robust answer. These questions include the followings: -

1. What made the two SpaDeX satellites to repeated drifts and that too, far beyond specified distances? Is there a need to relook at the sensors and control systems used in the present mission?
2. Could the fuel within the ‘Chaser’ been sufficient to make the docking possible if another drift of varied distance could have been induced?
3. Is the 2-motor propulsion adequate for Chaser for a ‘Precision Docking’?
4. Does the specified drift anywhere correlate with an actual capsule carrying spares, consumables or crews being carried in a separate launch that may not have identical height, inclination or velocities as the C60 rocket provided while injecting SpaDeX into orbit.
5. Is the present design of Docking System having thin rods and lever hooks for locking of the docked system protruding out of the chaser compatible with minor misalignment during docking? Could the same docking system be used for a pressurized capsule carrying astronauts to the future BSS?

 Bottom Lines

ISRO’s SpaDeX mission was launched successfully aboard PSLV C60 rocket with several payloads of both ISRO as well as other agencies/institutions. All satellites were injected successfully in their specified orbits. The main purposes were to demonstrate space docking and performance of Robotic Arm in picking up a space debris. While the latter performed satisfactorily the former had few hiccups that were subsequently sorted out thus culminating a successful docking. However, many questions were posed by unpredicted drifts induced in SpaDeX system that needs to be addressed.