Vikram Lander Went Hippity-Hop On The Moon — Here's Why That's Significant

The idea of hopping on the lunar surface brings to mind visuals of the Apollo astronauts making their way around the Moon in the late 1960s and early 1970s.

In most National Aeronautics and Space Administration (NASA) footage, astronauts can be seen bouncing off the surface rather than taking slow, steady steps.

They hopped because of their experience of the difference in surface gravity at the Moon (1.62 metre per second squared) and Earth (9.8 metre per second squared). Moon's gravity is only one-sixth that of Earth.

Just as humans did back in the day (many times not so easily), robotic craft on the Moon, too, have hopped on the surface — but only twice (not counting lunar launches).

The second-ever instance of a lunar hop was logged only this past Sunday (3 September) by the Vikram lander.

Vikram lifted up off the lunar surface by about 40 cm and landed safely about 30-40 cm away, the Indian Space Research Organisation (ISRO) said.

The first-ever hop on the Moon occurred back in the 1960s.

The United States' (US) space agency, NASA, was then exploring the possibility of a human mission to the Moon with its Surveyor programme.

In 1967, the Surveyor-6, one of five Surveyor spacecraft to soft-land successfully on the Moon, made the first-ever hop on the lunar surface.

This feat made it the first spacecraft to be launched from the surface of Earth's natural satellite.

The Surveyor-6 moved 2.5 metre away from its landing site with a hop. The manoeuvre was executed as part of an investigation of the surface mechanical properties.

"This manoeuvre provided excellent views of the surface disturbances produced by the initial landing and the effects of firing rocket engines close to the lunar surface," the Lunar and Planetary Institute says in its overview of the Surveyor programme.

"Photography obtained after the hop contributed to the soil mechanics investigation," it added.

Notably, the Surveyor-6 also faced a lunar night soon after the hop, just as Vikram does now, over five decades later.

The Surveyor spacecraft was placed in hibernation for the two-week lunar night.

Contact with the spacecraft resumed briefly after lunar sunrise — a possibility open to the Vikram lander.

The lander and rover's future will become known around 22 September.

Hop Into The Basics

A typical landing-focused space mission could involve an impacter, a lander, a rover, or a combination of the latter two. The lunar lander-hopper is a different variation.

A 2010 study explored the novel concept of a small lunar lander-hopper.

The paper described the hopper as "a vehicle with both landing and surface mobility capabilities on a single platform."

"Unlike rovers, which traverse the lunar surface while in contact with the ground, hopping reuses the landing propulsion system to lift back off again and hop over the lunar terrain," the study's authors said in the abstract.

This is essentially what the Vikram lander did.

The study discussed two types of lunar hops — ballistic and hover hops.

In the ballistic hop, the vehicle lifts off and traces a ballistic trajectory towards its target destination.

The hover hop sees the vehicle ascend, keep a steady altitude while moving towards its target landing area, and then make the descent.

Depending on the choice of design, a lunar hopper may win out against a rover for the purpose of exploring, say, the interiors of craters or the tops of mountains.

The use of a wheeled rover down a dark crater would require a more elaborate system, such as one involving a tether.

The hopper could cover lunar terrain that might otherwise be challenging to do with a wheeled rover, while also photographing what's underneath — especially if the area beneath is unexplored terrain — and relaying new imagery and data back to Earth.

Such a hopper could be a handy all-rounder — a lander but can do more than land, a rover but unhindered by the intricacies of terrain, and an helicopter, but not quite.

The defining factor would be the amount of fuel on board, but also the extent of gravitational force. A hopper would be better suited for low-gravity environments.

Hop Ahead

Countries like the US and Israel are serious about deploying lunar hoppers.

Israeli startup WeSpace Technologies is developing autonomously flying robotic systems, also known as thruster-propelled drones or hoppers, for a range of lunar surface exploration missions.

With their hoppers, they aim to fly customer payloads over the lunar surface, especially to the more inaccessible areas of the Moon, such as the permanently shadowed regions in the polar regions.

"The hopper... is designed to fly over obstacles and navigate craters and rifts where constant radio communication is impossible (and, coincidentally, where water ice can most likely be found)," according to this report.

WeSpace's Chief Technology Officer, Yigal Harel, was the programme director for the “Beresheet” programme — Israel's first lunar mission and the first attempt by a private company to land on the Moon.

The lander unfortunately crash-landed on the lunar surface in 2019, the same year as Chandrayaan-2.

In the US, Arizona State University is working with Intuitive Machines, a lunar-focused space exploration company based in Houston, Texas, on a mini extreme mobility lunar vehicle.

Called Micro-Nova, the vehicle will hop around the Moon’s surface and take the first-ever pictures inside craters close to the lunar south pole.

NASA awarded the project, a $41.6 million contract, to develop a small, deployable hopper lander capable of carrying a 1 kg payload more than 2.5 km over the lunar surface.

Lest it be thought that the business of hopping is limited to the Moon alone, NASA in the past has also funded, under its Innovative Advanced Concepts programme, a proposed Pluto lander.

The lander will be able to hop, skip, and jump around the Moon's surface, sometimes kilometres at a time.

It helps that Pluto's gravity is lower than the Moon's and a little more than one-twentieth that of Earth.

Vikram's Hop

The Chandrayaan-3 mission has hit several milestones on the Moon already as the lunar night descends fast upon the south pole region.

After wrapping up all their work, Vikram and Pragyan now find themselves in sleep mode, awaiting the next lunar sunrise for a chance to wake up.

The hop experiment with the lander before the night falls has come as a pleasant surprise.

Besides the obvious demonstration of the capability of lift-off and landing on the lunar surface, which paves the way for India to bring lunar samples back to Earth someday and, much further in the future, ferry Indian astronauts to and back from the Moon, one can speculate on the experimental possibilities in the present with the surprise hop experiment.

For instance, the lander payload Instrument for Lunar Seismic Activity (ILSA), which measures seismicity around the landing site, may have been employed to pick up fresh data from the lander's second lift-off and landing.

Earlier, ISRO was reporting ILSA's detection of vibrations from the rover's movements across the lunar surface, as well as from a possibly natural source.

The exact nature of the follow-up experiments and scientific results in connection with the hop experiment will come to light after ISRO analyses the data and publishes the findings.

But that the lunar hop even occurred is mighty impressive.

The question of a lunar hop was broached by Indian science YouTuber "Gareeb Scientist" in an interview with the chairman of ISRO, S Somanath, less than a week before the Chandrayaan-3 launch.

At the time, Somanath had said, "See, that all depends on the health of the instruments on board. Suppose inertial system has to work perfectly, and we have to load a new programme to take off from there and land in some other place. It is a totally different guidance algorithm which is currently not on board."

He had added that ISRO would opt for the lunar hop depending on "whether we have completed all our primary mission objectives."

With the Chandrayaan-3 lander having "exceeded mission objectives," as ISRO themselves said in an X post Monday (4 September), the coast may have seemed clear for a lunar hop — only the second ever in history.

All in a good lunar day's work for the Vikram lander.