Now that Chandrayaan-3’s Vikram lander has touched down successfully on the moon and its rover, Pragyan, has been rolling around on the lunar surface, sending home some fabulous pictures and intriguing data, such as the temperature variation between the surface and near-surface, here’s a peek into an alternative history.

This is the story of the selection of an alternative landing site (ALS), the backup site for the mission in case the primary landing site (PLS) proved elusive.

A paper by K Durga Prasad et al, titled ‘Chandrayaan-3 Alternate Landing Site: Pre-landing Characterisation’, describes the selection methodology, which was based on factors such as proximity to the PLS, conditions for safe landing, and whether it was of interest for scientific purposes. The ALS is an approximately 4 km x 2.4 km area in the same latitude (about 69 degrees South) as the PLS and about 450 km away. A hazard map was plotted using data from the Chandrayaan-2 orbiter’s high-resolution camera, which confirmed that 75 per cent of the ALS was hazard-free, making it suitable for landing and rover operations. Variability within the local terrain, illumination, and surface temperatures were studied to enable safe operations.

The ALS was found to be a scientifically interesting site with scope for sampling ‘ejecta materials’ from the lunar craters Tycho and Moretus formed during the Eratosthenian age (3.2-1.1 billion years ago).

An ‘ejecta blanket’ is formed when material ejected from a crater during an impact fall back on the lunar surface. It can extend for several kilometres beyond the rim of the crater and can be studied to determine the geology of the surrounding area.

The Eratosthenian age is a geological epoch of the Moon that occurred between 3.2 and 1.1 billion years ago. It represents a period of intense geological activity on the Moon, including the formation of large impact basins and the emplacement of volcanic deposits. 

To help interpret observed in-situ data, a set of studies were carried out:

Geomorphological characterisation, which was based on observations from the Chandrayaan-2 orbiter, particularly the best spatial resolution (25 cm) images from its high-resolution camera and the derived digital elevation model.

Thermophysical characterisation, which is the study of how materials respond to changes in temperature, was used to determine the temperatures and thermal behaviour of the ALS. The study used datasets from the Diviner radiometer aboard the lunar reconnaissance orbiter and a three-dimensional thermophysical model to understand the temperatures and thermal behavior of the alternate landing site.

Mineralogical and compositional characterisation, which is the study of the minerals and other chemical composition of a material, to understand the variations within the ALS. The study used data from the Moon Mineralogy Mapper (M3) of Chandrayaan-1.

Given that the same level of rigour would have been applied to the PLS, now known as Shiv Shakti Point, it was inevitable that ChaSTE (Chandra’s Surface Thermophysical Experiment) was the first direct measurement of the topsoil and subsoil near the lunar south pole.

As we wrap-up the story of where Shiv Shakti might have been, a quick appreciation of why the mission is important: knowledge from these missions help us better in better lunar exploration planning including designing better equipment and calibration; exploring resources and solar power generation; and establishing energy bases for future space missions.