LUVMI - The Lunar Volatiles Mobile Instrumentation

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The Objective: Uncover the moon's darkest secret

The lunar south pole, with its many scars and craters, hides a number of permanently shadowed regions (PSR). Source: moontrek.jpl.nasa.gov

The moon, an arid desert?

Since the Apollo missions, the moon appeared as a hostile, barren landscape. With its surface covered in dry regolith and with the absence of a tempering atmossphere, it is subject to temperature fluctuations of up to +/- 140 K. It was believed, that water, even if it had once existed, would have evaporated into the vacuum of space.

A secret hidden in darkness

The low inclination of the lunar orbit of only 1°32' allows depressed areas in high latitudes to remain in eternal darkness. These permanently shadowed areas are never reached by heating sunlight and therefore remain extremely cold. Infrared observations have shown that in some craters, especially near the lunar south pole, the temperature never exceeds 100 K. This is so cold that water ice could exist, despite the vacuum of space. Such locations were dubbed "cold traps" for their ability to capture volatile elements and gases like H, HO, H2, H2O, CH4, CO2, SO2 and others, that were released by meteroite impact or the solar wind.

A glimpse from space

As these cold traps are never illuminated by the sun, they remain too dark to be observed directly. Nonetheless, recent years have brought new light into the shadow: Remote sensing satellite missions like Chandrayaan-1 or Clementine have provided strong indications that significant amounts of volatiles, among them water, are frozen in cold traps at the lunar poles. In 2009, the NASA LCROSS mission crashed a spent upper rocket stage into the Cabeus crater near the lunar south pole. In the resulting ejection plume, a water content of   5.6 +/- 2.9 % was observed.

Fuel for thought

If these indications were to be revealed as true, huge amounts of water ice could exist on the moon. The Cabeus crater alone, with its 95km diameter, could provide thousands of tons of easily accessible water, hydrocarbons and more. These resources could be used to produce rocket fuel, oxygen, potable water, grow plants and much more - making a permanent human presence on the lunar surface possible.

The challenge

In-situ measurements are necessary as a definitive proof for the existence of lunar water.But so far, no mission has investigated permanently shadowed craters from the ground. Project LUVMI intents to change that.

Project Description:

LUVMI Prototype (Credits: Martyna Hodges, Open University)

For project LUVMI, an international consortium of 5 partners from science and space industry have come together to design a mobile laboratory to access and analyse permanently shadowed regions (PSR). The system consists of a mobile payload support platform, developed by Space Application Services, that can carry a number of instruments for the analysis of the lunar pole environment.

The core instruments of LUVMI will be the Volatiles Sampler (VS), which is developed by the TUM Chair of Astronautics and OHB System, and the Volatiles Analyser (VA), developed by the Open University. The VS is a miniature soil sampling instrument, that can be inserted into the lunar regolith. Once inserted, a heating element will raise the regolith's temperature to release any bound volatiles. These released gases will be captured and guided to the VA, a miniature mass spectrometer. Even though the VA weighs less than 500g, the VA is capable of measuring the amount and composition of most relevant volatiles.

In combination, these instruments will have a total mass of less than 1,5kg. Yet they replace an entire laboratory of ovens, mechanisms and instruments.

To support the rovers navigation and sample site selection, LUVMI further features a novel cutting-edge micro-lens array camera. This technology, developed by Dynamic Imaging Analytics, bears great potential for space applications, as it allows to generate 3D-Images from a single camera, provides excellent signal to noise ratios and operates without an active focusing mechanism.

The Volatiles Sampler

Render image of the Volatiles Sampler

Basic idea:

 

The VS employs a novel approach to soil sampling: Instead of extracting a sample from the soil, which is then placed into an oven and heated for the extraction of volatile compounds, the VS brings the oven to the sample, that is into the ground. The VS consists of a central heating rod, enclosed by a tubular shell, which is inserted into the ground to enclose a cylindrical volume of regolith. The heating of the enclosed sample releases the bound volatiles. The released volatiles increase the gas pressure inside the tubular shell. This gas will either be lost downwards through the soil or upwards through the orifice to the Volatiles Analyser, which is used to analyse its chemical composition. To calculate the total amount of released gas, and thus the original abundance of volatiles in the soil, the amount of lost gas must be known. To this end, two pressure sensors measure the gas pressure before and after the orifice. This allows the calculation of the amount of gas that has passed from the shell to the VA. To determine the amount of gas lost through the regolith, the VS can be pressurized with a reference gas to a specific pressure. The observation of the pressure loss over time then allows to calculate the gas permeability of the soil. This allows to infer the original abundance of volatiles in the soil. This process is illustrated in the video below:

 

 

 

 

Areas of research

Heating moist dust? Sounds Simple!....right?

Not quite. It is true, the purpose of the VS is to heat dust... but, as is always the case in the field of aerospace engineering, things turn out to be slightly more complicated than they appear on first sight.

Mechanical Insertion:

Before the lunar regolith can be heated, the VS needs to be inserted into the ground. However, the VS cannot simply be pushed into the soil. In lunar gravity, the relativly small LUVMI rover does not have enough vertical force available. In other words, if the LUVMI rover would try to push the VS into the ground, it would simply lift itself off the surface. For this reason, the VS is equipped with a drill like shell. The rotating shell, which is equipped with an auger and a cutting edge, can be softly screwed into the ground. This way, the necessary vertical force is reduced, but the soil inside the drill shell remains relativly undisturbed.

Thermal Simulation of Lunar Soil Heating:

Optimising the design of the heater poses a significant challenge. The VS will operate in the extremely cold porous lunar regolith in near perfect vacuum. This poses two significant problems: First, experimental verification of the heater needs to be performed in thermal vacuum chambers, requiring expensive equipment and complicated procedures. Second, due to the extremely low thermal conductivity of regolith in vacuum, it is difficult to measure temperature profiles inside the soil. For these reasons, the heating process is simulated in COMSOL multiphysics to reduce the amount of tests needed and to better interpret test results.

Thermal-Vakuum Testing

While Simulations can reduce the amount of required tests, testing doesn't become obsolete. Tests are still needed to very simulation results. For the LUVMI VS, a thermal vakuum test setup is developed that allows the end-to-end testing of a fully functional VS prototype in a representative environment. To achieve this, liquid nitrogen is used to cool the lunar regolith simulant to -196°C in high vacuum.

 

 

 

Image of a heating Rod prototype during thermal-vacuum testing.
Orifice pressure drop measurement setup.
Volatiles Sampler static prototype after assembly in the vacuum chamber.
Liquid nitrogen cooling of the vacuum chamber.
Simulated temperature field in COMSOL Multiphysics.
3D printed drill shells.
Insertion testing of the VS in lunar regolith simulant.
The Volatiles Sampler/Volatiles Analyser prototype in the thermal vacuum chamber.

Consortium

  • Space Applications Services (Belgium) - project coordinator
  • The Open University (UK)
  • Dynamic Imaging Analytics (UK)
  • Technical University of Munich (Germany)
  • OHB System AG (Germany)

Funding

The project is funded by the European Commission (EC) under the EU Horizon 2020 program.

Publications

  • Biswas et al, "Application of the LVS Subsurface Probe on the LUVMI Rover for a Lunar Volatiles Exploration Mission", European Lunar Symposium, 2017