Indian lunar probe may help solve one of physics' biggest problems

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On August 23, India became the fourth nation in history to achieve a successful landing on the Moon, and also broke the record for the highest latitude for such a feat. During the lunar day, the Vikram descent module and the Pragyan rover conducted numerous experiments, until they were finally put in suspension with a slim hope of waking up on September 22. In this relatively short time complete, a test that could be key to the future of missions to our satellite.

Photograph before and after the lunar jump, focused on the rover's descent ramp. Credits: ISRO.
Before and after picture of the moon jump taken from Vikram's front camera. Credits: ISRO.


Vikram and Pragyan depend on solar energy to function and keep its sensors and circuitry in the correct condition to operate in the inhospitable lunar environment. Once the Sun sets below the horizon, the electronics are subjected to extreme temperatures close to absolute zero.

Before putting Vikram into sleep mode, the controllers tried a risky test. Using some of the fuel left over from the landing, on Sunday 3, the order was given to start the engines and give a jump. The lander rose 40 centimeters vertically, and moved 40 centimeters horizontally.

This is a first demonstration for future sampling missions to return to Earth.. However, one of the aspects of greatest scientific value is related to a seemingly unrelated branch of physics.


Sand behavior

In physics, it is desired to know the state of a system in the past and future. In rare cases it is possible to obtain exact expressions to determine, for example, the position and velocity of a particle under a certain force.. However, when considering many objects interacting with each other, it is impossible to accurately determine the state of each component. For this purpose, it is necessary to resort to statistical approximations to get a general idea.

Landing on the Moon requires retro-thrusters that shoot hot gases at high velocity into the regolith.. Since both lie within the near-perfect vacuum of space, approximations that on Earth allow us to understand the flow of material are rendered useless.

The biggest problem is the so-called viscosity. Although at the collision site between the engine boom and the regolith may work, beyond that small region, the concept itself is lost.. And with it the ability to model the behavior of the system.

Knowing the speed at which small rock fragments are ejected is relevant for future lunar missions. It is believed that some of them may reach escape velocity and impact future orbital space stations. On the other hand, when bases are built on the surface, new spacecraft descending may affect the integrity of the structures with the fired material.

Different frames of the lunar landing process. It is possible to observe the laminar and continuous behavior of the regolith, making it difficult to estimate the velocity of each fragment.

New science

Although the effects of regolith shot at high speed can be avoided with relative ease, there is the problem that a rocket engine could dig a large hole under the vehicle and affect stability. This is especially challenging when considering vehicles such as Starship or Blue Moon Lander.

The images from the Indian experiment have the potential to provide relevant information on regolith behavior and flow. Accompanying the videos captured by the Chinese Chang'e probes and the Apollo missions. Being a key factor for future permanent settlements and constant supply.

Francisco Andrés Forero Daza
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