How the Chandrayaan-3’s little ‘hop’ changed our insight into building lunar habitats

The first historic panoramic photograph from the lunar surface, transmitted by Luna 9 to Earth between 01:50 and 03:37 GMT on February 4, ended a bitter ten-year argument. American and Soviet experts had been fighting over whether the Moon's surface was a deep trap of fluffy dust, metres and metres of it, that would swallow any landing craft.

Luna 9 proved the ground was firm enough to hold a spacecraft.

The only other tool it carried was a simple radiation counter. That device measured the daily radiation dose on the Moon: 30 millirads. One millirad is a tiny unit, about the radiation you get from eating a banana. So 30 millirads a day is quite safe for human travellers. Most of that radiation came from cosmic rays, which are high-energy particles zipping in from deep space. A smaller part came from the lunar soil itself, excited by those very cosmic rays.

These discoveries shook the Americans. They abandoned their spider-like six-legged experimental lander. They built a lighter, four-legged design, the one that eventually carried astronauts in the Apollo missions. They also changed their radiation protection design.

Fast forward to the present. The Indian Space Research Organisation (ISRO) launched Chandrayaan-3 and performed an unexpected feat on September 3, 2023. After finishing its main work, when it was at the brink of the end-of-life, scientists commanded the Vikram lander to fire its engines one more time, using leftover fuel.

The lander lifted itself about 40 centimetres. It flew a short distance (roughly 30 to 40 cm away) and soft-landed again. Instruments aboard, including the ChaSTE (Chandra's Surface Thermophysical Experiment), a thermometer-like probe that measures soil heat, were folded back safely. Once it safely landed, they were redeployed at the new spot.

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This little hop did two big things. First, it proved that engines can be restarted on the Moon, a critical skill for prospective missions that bring back rock samples or land humans. Second, it revealed a hidden secret in the top few centimetres of regolith.

The Moon's soil, a mix of crushed rock and fine powder called regolith, is not uniform. At the Chandrayaan-3 landing site, there is a loose, airy top crust. Just two to six centimetres below that lies a much harder, compacted layer.

That two-layer cake structure has upset many engineering plans for permanent Moon habitats. For example, how do you design thermal insulation when the upper dust is so porous? And how do you anchor a habitat into a surface that is soft on top but stiff just a few centimetres down?

A new paper in the prestigious Astrophysical Journal earlier this year now brings these data together. Researchers from the Physical Research Laboratory (Ahmedabad), Andhra University's Department of Engineering Physics (Visakhapatnam), and ISRO's Space Applications Centre (Ahmedabad) used the hop experiment data to confirm this cake-like layering.

Understanding this layered soil is not simply academic. It will shape how engineers design, build, and operate future human shelters on the Moon, from foundations to heat shields.

The Moon has no atmosphere to shield it. So space rocks – from tiny grains to small pebbles – constantly slam into its surface. This endless bombardment must eventually break every pebble, even every grain, into finer and finer powder.

Two questions begged for an answer: What is this fine moon dust actually like? And how deep does the dust layer run?

In the mid-1960s, a fierce argument over the nature of lunar dust became one of the highest-stakes debates of the Space Race. Would a lander sink into a kilometre-deep ocean of loose powder? Or touch down on solid ground?

Engineers could not build a craft without an answer.

The fight pitted a Western school of thought against a Soviet one. Western theorists, led by British-American astrophysicist Thomas Gold, predicted deep, dangerous dust traps. The Soviets argued the Moon's surface was rigid and porous, like a hard sponge.

Gold's theory sounded alarming. He said cosmic erosion had turned the Moon's rocks into fine powder over the course of billions of years. Solar wind and tiny meteorites pulverised the surface into sub-micron dust, particles smaller than a speck of talcum powder.

In a vacuum, Gold added, the Sun's radiation charges dust particles, causing them to repel one another. They then drift downhill into craters and low plains (called maria). He warned that some maria could be miles thick, a soft trap that would swallow a heavy lander whole.

Not all Western scientists agreed. Gerard Kuiper, another well-known astronomer, insisted that lava flows on the Moon were solid enough for landing.

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The Soviets had a different model: the “Volcanic Slag” idea. Their argument came from decades of measuring how the Moon reflects light, a technique called photometry. Think of it like shining a torch on a fine chalk powder versus a mirror. Loose powder scatters light smoothly in all directions. The Moon, however, throws light sharply back toward the Sun.

That behaviour told the Soviets the surface was not a deep powder bed. They reasoned that without an air blanket, micrometeorites hit the Moon at insane speeds, tens of kilometres per second. The heat of each impact instantly vaporises and melts local rock, creating a bubbly, slag-like crust. Not dust.

Nikolai Pavlovich Barabashov, a leading Soviet selenologist (a moon geologist), concluded that the Moon's ‘Ocean of Storms’ – an expansive lunar plain – was a solidified, rough, pumice-like mass. Scattered across it were broken rocks ranging from sand grains to small pebbles.

Legend says spacecraft engineers went to Sergei Korolev (the Soviet chief designer who built the first satellites and launched Yuri Gagarin) with a blunt question: “How should we design the lander’s legs? Assume fine dust or solid ground?” Korolev reportedly signed an administrative order in late 1964 that settled the dispute inside the Soviet team. The order purportedly read: “The Moon must be considered to have a solid ground.” So the Soviets built their Luna landers. On February 3, 1966, an inflatable ball bounced across the lunar surface. Inside that ball was Luna 9, the first spacecraft ever to land softly on another world.

After it stopped rolling, the lander opened four protective petals like a flower. A small camera peeked out and snapped the first picture ever taken from the Moon’s surface.

Those photographs depicted a strange landscape. The ground looked somewhat porous, pitted with tiny holes, and littered with oddly shaped rocks.

Thomas Gold still disagreed with the interpretation. But the evidence was hard to dismiss: a 99 kg capsule had bounced, rolled, settled, and operated without sinking for about 6 days, 11 hours, and 10 minutes. The surface had enough strength to hold a spacecraft. That dealt a heavy blow to the extreme versions of the deep-dust theory.

Later probes clarified the full picture. The Moon is indeed covered in a layer of dust (the regolith), a loose top crust. But billions of years of solar vacuum packing and cosmic ray bombardment have compressed the deeper layer so tightly that just a few centimetres down, it becomes a rock-hard foundation.

For years, space scientists have argued about the nature of the lunar surface soil. Was it a deep, shifting sea of dust or a solid slab of rock?

New data from ISRO now shows both sides were right, but at a tiny, layered scale no one had measured before.

During Vikram’s hop experiment, the lander’s rocket thruster acted like a giant hair dryer pointed at the ground. It blew away the top three centimetres of loose, airy dust.

What was underneath was a surprise: a tightly compacted, rigid layer.

So the Moon has Thomas Gold's ultra-fine, fluid-like dust on top, and right below it, the solid foundation that Soviet scientists had predicted.

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Earlier Soviet missions, like Luna 24, brought back soil samples and noticed that the dust gets denser as you go deeper. But they could not see how suddenly that change happens in the first few centimetres of untouched ground.

Another Soviet probe, Luna 13, used a little cone-pushing tool called a penetrometer. Imagine pressing a finger into wet sand to feel how hard it is. This is how this instrument worked. That gave the first rough engineering numbers: the loose top dust had a density of about 0.8 grams per cubic centimetre, a bit lighter than table salt.

Now, the hop experiment has revealed a sharp, cake-like two-layer structure within just a few centimetres of the surface. Billions of years of micrometeorite impacts have altered the Moon's uppermost skin in ways different from those in the layers below.

Chandrayaan-3 provided the first direct, on-site view of this extreme change near the South Pole. At the immediate surface, the dust behaves like dry flour, loose and slippery. But just 6.5 centimetres down, it becomes twice as dense and five times stickier, acting like damp, stiff clay.

To put this in numbers: the strength jumps from 300 Pascals (a unit to measure pressure; 300 pascals is roughly the load of a chocolate bar pressing on your fingertip) to 1600 Pascals (like a heavy brick). That is a dramatic shift in just two or three inches of lunar soil.

The nature of lunar soil is not only a geological puzzle. It also influences how heat moves through the soil. We already knew the Moon's surface can get sizzling hot, around 120°C, when the Sun is overhead. But which lay beneath, temperature-wise, was a complete mystery.

Then ISRO’s ChaSTE (Surface Thermophysical Experiment) probe drilled in and measured it. The ChaSTE instrument (part of the Chandrayaan-3 mission) is a motorised needle that can penetrate lunar soil without smashing or hammering.

Imagine pushing a thin, sharp straw into a layered cake, slowly and with a twisting motion, so the layers do not mix. Yet, you can probe whether the inner layers have baked adequately. That is how this probe worked.

Developed by the Space Physics Laboratory at VSSC (Vikram Sarabhai Space Centre) and PRL (Physical Research Laboratory), the needle used a rotation-based drive instead of the old hammering method. This meant it could penetrate 10 centimetres into untouched lunar soil without disturbing the natural dust layers.

Inside the needle were ten tiny temperature sensors, stacked one centimetre apart like beads on a string. As the needle pushed down, each sensor measured the temperature at its own depth, from the blazing surface (0 cm) to 10 cm below.

The instrument worked in two modes. First, in passive mode, all ten sensors simply measured the surrounding dust's temperature every second. Their accuracy was impressive: within half a degree Celsius. That gave a real-time snapshot of how temperature changes with depth.

Then emerged the active mode. A small heater at the tip of the probe released a measured pulse of heat into the deeper soil. The sensors tracked how fast the heat spread into the surrounding dust. This told scientists how well the soil conducts heat, or in this case, how poorly.

When ISRO plotted the numbers, they saw a dramatic plunge. The surface temperature was above 50°C – sufficiently hot to fry an egg. But just eight to 10 centimetres down, the temperature had dropped to minus 10°C. That is a 60-degree swing over the length of a pencil.

Why such a steep drop? Because the top few centimetres of lunar dust are a phenomenal thermal insulator. The uppermost three centimetres consist of jagged, loosely packed grains with almost no contact between them; no thermal conduction is possible. In a vacuum, there is no air to carry heat away, so no convection. The loose powder is so fluffy that heat barely trickles down.

So the Moon’s surface bakes under the Sun, while the dense layers just centimetres below stay locked in a permanent deep freeze.

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Chandrayaan-3's hop experiment last month and the ChaSTE temperature probe have forced a rewrite of the old manuals. So, how do these conclusions change the way engineers will build permanent homes on the Moon?

Take insulation first. Engineers used to worry that protecting astronauts from the Moon's brutal heat (120°C during lunar daytime) and biting cold (-130°C during lunar nighttime) would require shipping heavy synthetic blankets from Earth. Now ChaSTE has shown that just 10 centimetres of loose surface dust acts as a near-perfect thermal barrier. Future habitats will likely use robots to pile local soil over inflatable domes, either by bagging the dust or 3D-printing it. This gives a stable indoor temperature using nothing but what the Moon already has.

Now consider the construction of a habitat. The old thinking was that the Moon's surface was like a sandy beach, semi-uniform, requiring deep drilling to build pillars for building heavy structures. However, the hop data show that within the first 6.5 centimetres, the soil becomes stiff and grippy, like damp clay. So launch pads, blast walls, and habitat bases do not need deep anchors. Engineers can simply remove the top three centimetres of loose, flour-like dust and build directly on the hard sub-layer just inches below.

Excavation machines will also need to be redesigned. The prototypes, such as NASA's RASSOR (Regolith Advanced Surface Systems Operations Robot), were designed to scoop a near-uniform dust layer across the lunar surface. However, the top two to three centimetres is abrasive, fluid, and prone to popping with static electricity. The layer below is five times tougher, denser, and tightly packed. Future digging robots must change modes quickly, adjusting torque, tooth shape, and drum speed as they transition from scraping the "flour" to cutting the "hard clay".

An additional challenge is dust. The lunar surface is dusty, and it enters everywhere. The previous plan was to use wipers to keep loose dust out of moving parts. However, the hop experiment showed that it will not work. When the thruster blew away the top layer, it sent jagged, micro-porous shards into the air. These dust particles are sticky and razor sharp. These particles cannot be brushed off. Therefore, airlocks, spacesuit joints, and hatch seals will need active shields, either magnetic or electrodynamic, to fend off this clingy, abrasive dust.

On the Moon, a single day stretches nearly 14 Earth days long, and the night that follows is just as long. ChaSTE found that the dusk does not fall gradually and gracefully like on Earth. In a snap, sunlight is gone, and the ground plunges from blazing heat into freezing darkness.

Because there is no atmosphere to hold or circulate heat, the moment a shadow falls, the surface radiates its warmth straight into space. The temperature does not taper because there is no atmosphere to hold or circulate heat. ChaSTE probe captured this sharp “twilight transition” in precise detail. Early engineers believed the warm ground would act as a soft buffer for temperature control in the living environment. Now it is clear that, as soon as a site moves from sunlight into shade, systems must switch immediately to full internal heating mode.

Also, in the earlier design, the solar panels were flat and were to be positioned slightly tilted, similar to solar farms on Earth. Now that Chandrayaan 3 has found that at the South Pole region, the Sun moves low and fast, projecting long horizontal beams and sharp shadows. Flat panels lose power long before the actual sunset. The new design requires tower-based, vertical solar arrays on swivelling mounts that follow the Sun. And here is the clever design: the heavy battery banks will be buried underneath those towers at depths of 3-10 cm, as ChaSTE mapped. That layer stays naturally insulated, keeping batteries warm while the towers above endure the sudden twilight freeze. Using this knowledge, engineers are designing a system called Regolith Thermal Energy Storage (RTES) that will act as a pump. During the day, thermal energy can be stored in the subsurface regolith, and at night it can be retrieved to heat and power systems.

Finally, choosing where to land. In the past, mission planners selected large, flat areas based on satellite imagery. But now we know that at the South Pole region, soil properties can change completely within a span of centimetres. Broad maps are no longer safe. Future habitat missions will first send small scout rovers or drop thermal probes to test a specific 50 metre plot – centimetre by centimetre – before the heavy construction modules even touch down.