The exploration of the Moon has regained momentum in recent years, with various countries undertaking missions to unravel the mysteries of our nearest celestial neighbour. After the Apollo missions in 1960’s, plans are afoot to revisit the moon under the Artemis programme with more advanced technology for crucial navigation and docking functions. Private companies like Space-X and Blue Origin are also actively involved in the planning and execution for crewed moon landings and long duration stays. After the successful Chandrayaan-1 mission during 2008, Indian space research has made further strides in carrying out a number of space science missions including the Mars Orbiter Mission (2013), ASTROSAT (2015) and Chandrayaan-2 (2019).

Coming up soon is Chandrayaan-3, planned to be launched with an improved lander and rover configuration. Generally, a lunar robotic spacecraft consists of an orbiter, a lander and a rover, all carrying scientific instruments to make various observations including that of the surface and sub-surface composition, surface emitted volatile gases like water vapour, and high-resolution photography for close-up and panoramic views.

The lander and rover are docked to the orbiter before launch. After acquiring a lunar orbit, the lander/rover separates from the lunar orbiter and is suitably slowed down to descent on moon’s surface. In Chandrayaan-2, such an integrated lunar craft payload stack was accommodated inside India’s most powerful GSLV MK3’s (Geosynchronous Satellite Launch Vehicle MK-3, now renamed as LVM3) nose cone during the process of integration with the launch vehicle.

The LVM3 first launched Chandrayaan-2 into an elliptical orbit around earth. In each circle around the moon, the lunar craft raised its highest point of the orbit (apogee) using the earth’s gravitational force near the lowest point (perigee) by gaining additional momentum there. Then it suitably oriented itself towards the moon and escaped from the earth’s gravity.

On-board thrusters or small rockets are used for changing the direction of motion and providing thrusts as necessary for placing the whole module in an orbit around moon. Many countries are currently involved with a variety of ongoing and planned missions to the Moon. The USA has several ongoing missions including the Lunar Reconnaissance Orbiter (LRO), operational since 2009 that has been instrumental in studying the Moon’s surface and resources. Other countries such as China, Japan and Russia have also sent missions to the Moon in recent years.

Japan’s Selene mission provided new insights into the Moon’s origin and evolution, while Russia’s Luna missions have been studying the Moon’s geology and environment since the 1960s.

These efforts are also crucial from the point of view to search for the abundance of water/ice required for a sustained human presence on moon and resources like 3???? (Helium-3) for generating clean nuclear fusion energy for earth-based applications. Private enterprises have also been involved in lunar missions, with several companies announcing plans for Moon missions in the coming years. SpaceX has announced plans to send its Starship spacecraft to the Moon for a round trip in 2023.

Blue Origin is developing its Blue Moon lunar lander, which it hopes to use for private missions. Pittsburgh-based Astrobotic company has signed contracts with several organisations, to deliver scientific instruments and other payloads to the Moon. Given the rich history of lunar expeditions, particularly the Apollo programme, which provided groundbreaking technologies for human landings, the ambitious goal of returning humans to the Moon appears to be within reach.

The Apollo-11 mission marked a historic milestone in 1969 and the Apollo 17 concluded the series as the final human lunar mission in 1972. While the focus in recent years has been on robotic missions for scientific experiments, the excitement of landing and working on Moon adds a different dimension to it.

An understanding of the factors that enabled the successful human moon landings five decades ago under the Apollo programme, is valuable for technical insights into the human landing missions being planned now. From the Apollo10 to Apollo 17 missions, the basic design concept and the commensurate production of necessary hardware remained more or less the same except for some minor improvements in the onboard auto-navigation system.

There is considerable literature available in the public domain from NASA, like the mission report and various technical evaluation reports providing the configuration and design details of Apollo missions. The Apollo-11 mission’s lunar spacecraft mainly consisted of the Command Module (CM), the Service Module (SM), and the Lunar Module (LM).

The Saturn V rocket launched this spacecraft to space, providing enough energy to leave earth’s gravitational force and touch down on the lunar surface (see figure for more details). On 20 July 1969, Neil Armstrong and Buzz Aldrin became the first to set foot on the Moon, while Michael Collins orbited above. The LM carried the astronauts from lunar orbit to the Moon’s surface, utilizing radar and visual cues for guidance. The auto-navigation and docking methods with astronaut’s manual interventions if and when required proved crucial for mission success. The Apollo and the present Artemis programmes of NASA have different design philosophies.

The main one being that while the Apollo missions utilised a direct ascent approach, where a single spacecraft was launched from earth to the moon’s orbit and then the lunar module separated from the command module to land on the moon, the Artemis mission adopts a lunar orbit rendezvous approach where the Orion spacecraft of the Artemis mission carries astronauts and docks with the Lunar Gateway, the space station orbiting the Moon.

A separate Human Landing System (HLS) docks to this space station for carrying the crew down to the lunar surface, while the Orion spacecraft remains docked with the Lunar Gateway waiting for the HLS to return to go back to earth. The first uncrewed Artemis1 mission has been carried out successfully by orbiting Orion spacecraft around the moon and bringing it back to earth. Subsequent Artemis launches may establish a reliable and safe space transportation system for astronauts/moon visitors.

The Saturn V rocket, used for the Apollo missions, has not been utilised since 1973. NASA has shifted its focus to the Space Launch System (SLS) rocket, which is presently being used for launching heavy payloads like Artemis-1. The SLS is a more advanced and partially reusable rocket with greater thrust, payload capacity, and modern technology, ensuring higher reliability, safety, and other amenities essential for human space exploration. The SLS is designed to accommodate a team of 3-4 astronauts in its Orion spacecraft, along with other payloads such as landers and habitat modules, to support long-duration missions beyond earth’s orbit.

Achieving the goals of the Artemis programme and more complex robotic missions to moon would need further scientific investigations, technological innovations, and international collaboration. This will enable humanity to reach new frontiers in space with the Moon as a promising gateway to the vast universe beyond.

(The writer is Senior Professor, Indian Centre for Space Physics (ICSP) and Ex-ISRO Scientist and Professor; Ex-Visiting Faculty, Physics Department, Bangalore University)