What is Chandrayaan-3 is doing while orbiting around the moon

Introduction

Exploration of space beyond the near-Earth regime has been one of the most challenging and fascinating ventures of humankind and continues to capture the imagination of generations. Over the ages, several space-faring nations have undertaken numerous missions to explore most of the planets in the solar system, their natural moons, various minor planets/ asteroids, comets and even interplanetary voyages. The Moon and Mars are the most explored and also comparatively more crowded planetary bodies at present. India’s Chandryaan-3 (CH3) is the latest entry into the lunar orbit. More intensified activities around the Moon are foreseen in the next few years due to the renewed interest in lunar exploration, heralded by Artemis missions for return to the Moon and preparations for colonisation of Mars. 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 theenvironment is needed to formulate reasonable mitigation practices to avoid close-approach threats in planetary orbits.

The 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 of deep space objects

Observation and tracking of deep space objects are inherently more complex compared to that in the near-Earth regime, 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.

Lunar orbits

Orbital evolution in lunar orbit is primarily influenced by lunar gravity, gravity of the Sun and the Earth, and the Sun Radiation Pressure. For orbits lower than 500 km, non-uniformity of lunar gravity due to mass concentrations dominates, which along with the third body perturbations due to the Earth and the Sun causes the 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 100 km circular orbit is about 160 days.

The major types of lunar orbitsinclude 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.

The current situation around the moon

As of July 2023, there are 6 active lunar orbiters (see Fig-1). Two of the five probes of NASA’s THEMIS mission have been re-purposed under ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun) as ARTEMIS P1 and ARTEMIS P2, both operate in eccentric orbits of low inclination. NASA’s Lunar Reconnaissance Orbiter (LRO) orbits the Moon in a nearly polar, slightly elliptical orbit. Chandrayaan-2, the second lunar mission of ISRO and Korea Pathfinder Lunar Orbiter (KPLO) also operate in polar orbits of 100 km altitude. NASA’s Capstone operates in a 9:2 resonant southern L2 NRHO, its perilune passes over the lunar North pole at 1500-1600 km altitude, while the apolune is over the South pole at a distance of nearly 70,000 km. The Japanese spacecraft Ouna which was placed in lunar orbit as part of Kaguya/SELENE mission in 2009 and Chandrayaan-1 launched in 2008 are the two defunct spacecraft. All the other orbiters have been either moved out of the moon-bound orbital regime or have landed/impacted the lunar surface, either deliberately or due to failure to land softly. For example, Chang’e 4 mission’s data relay satellite Queqiao, launched by China in May 2018, was later moved to a halo orbit near the Earth-Moon L2 point. Currently, the only operating rover is China’s Yutu-2 rover released by Chang’e 4, which operates on the far side. From the available media sources, it is expected that Luna-25 of Russia with a Lander and Rover will be in Lunar orbit of 100 km by August 16, 2023 and will be landing on South pole of the moon by August 21-23, 2023.