“While the previous missions were essentially aimed at scientific explorations, upcoming ventures will likely involve multiple actors of diverse interests, including those primarily driven by resource utilisation for commercial purposes. A better understanding of the environment is needed to formulate reasonable mitigation practices to avoid close-approach threats in planetary orbits,” Isro said.
Current Space Debris Mitigation Guidelines by the UN and Inter-Agency Space Debris Coordination Committee (IADC) are applicable to spacecraft and orbital stages “that will be injected into Earth orbit.
“Currently space debris pose a major threat to the long-term sustainability of outer space activities in the ever-increasingly congested Earth orbits. Therefore, based on the lessons learnt while operating in the near-Earth regime, it is interesting and desirable to undertake studies related to close approaches in view of the increasing number of objects in the lunar orbits.
Tracking deep space objects
Observation and tracking of deep space objects are inherently more complex compared to that in the near-Earth regime, Isro said, adding that this was mainly due to the vast distance involved between the object and the observer which introduces considerable latency, signal attenuation and associated complexities.

“Functional assets like spacecraft/landers/rovers are tracked by active and passive means. Typical active techniques involve range and doppler measurement, very long baseline interferometry (VLBI)/Delta Differential One-way Ranging (DOR), and laser ranging with retro-reflectors. Optical transponders have also been demonstrated for missions like the Messenger, Mars Global Surveyor, and Hyabusa-2 which can give better accuracy,” Isro said.
Lunar orbits
Elaborating on lunar orbits, it said: Orbital evolution in lunar orbit is primarily influenced by lunar gravity, gravity of Sun and Earth, and Sun radiation pressure.
“For orbits lower than 500km, non-uniformity of lunar gravity due to mass concentrations dominates, which along with the third body perturbations due to Earth and Sun causes orbit eccentricity (without any change in the semi-major axis) to increase. As a result, the perilune altitude is gradually lowered leading to eventual impact with the lunar surface. For example, the expected orbital lifetime of a spacecraft at a 100km circular orbit is about 160 days,” it said.

The major types of lunar orbits are: Halo orbit around Langrange’s point, Nearly Rectilinear Halo Orbit (NRHO), Low Lunar Orbit (LLO), and Distant Retrograde Orbit (DRO).
“NRHO orbits offer the advantages of being stable and requiring less orbit maintenance, maintaining continuous communication with Earth and other lunar orbiting crafts, eclipse avoidance etc. and are highly suitable to host lunar gateways. Several forthcoming missions may also be placed in similar orbits, but given the vast spatial extent of such orbits (far larger than the GEO belt), no congestion is anticipated in the foreseeable future. Majorityof the currentlyorbiting lunar probes operate in LLO,” Isro said.

Current situation
Chandryaan-3 is the latest entry into lunar orbit. As of July, there were six other active lunar orbiters: Two of the five probes of NASA’s THEMIS mission have been re-purposed under Artemis, operating in eccentric orbits of low inclination, NASA’s Lunar Reconnaissance Orbiter (LRO) orbits Moon in a nearly polar, slightly elliptical orbit, Chandrayaan-2 orbiter and Korea Pathfinder Lunar Orbiter (KPLO) also operate in polar orbits of 100km altitude and NASA’s Capstone operates in a Nearly Rectilinear Halo Orbit (NRHO) that is more stable.
Japan’s Ouna which was placed in lunar orbit in 2009 and Chandrayaan-1 (2008) are the two defunct spacecraft, while all the other orbiters have been either moved out of the moon-bound orbital regime or have impacted the lunar surface, either deliberately or due to failure to land softly.

“It is expected that Russia’s Luna-25 (lander and rover) will be in lunar orbit of 100km by August 16 and will be landing on the lunar South pole of the moon by August 21-23, 2023,” Isro said.
Risk of close approaches & mitigation
“Even with a handful of orbiting spacecraft, frequent conjunctions are experienced by LRO, KPLO, and Chandrayaan-2 orbiter as their orbital regimes in Low Lunar Orbit (LLO) overlap. Sometimes such conjunctions even warrant CAMs to be on the safer side because of the uncertainty associated with orbital estimates,” Isro said.
Pointing out that effective co-ordination happens among agencies to avoid critical conjunctions, Isro added that for the Chandryaan-3 mission, the propulsion module is expected to orbit around Moon in a circular LLO of about 150km altitude for many years to come.

“Majority of the lunar landers are also likely to temporarily reside in LLO (typically for a few days or weeks) before landing. Under the ambit of Isro System for Safe and Sustainable Space Operations Management (IS4OM) critical analysis of each of the lunar-bound manoeuvres for orbit lowering are being performed to assess potential risks of close approaches with the other lunar orbiters before executing the manoeuvres,” Isro added.
The way forward
Reiterating how space object population has been growing for space beyond Earth, especially in the lunar environment, Isro said: “Greater uncertainty associated with orbital knowledge entails collision risk assessment to be a necessity for safe operations for a spacecraft orbiting Moon.”
India has proactively taken up initiatives in collaboration with international organisations like Inter Agency Space Debris Coordination Committee (IADC), which include studies related to future evolution of space object environment in cislunar and lunar region to bring out specific guidelines and best practices for space operations to be sustainable in these regions.

Watch Chandrayaan-3’s Vikram nears landing, only 1,437 Km away from Moon