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Telescopes and space missions

Telescopes and space missions

A ‘plasma broom’ for cleaning on Mars

09 Jun 2017 Sarah Tesh

A “plasma broom” for sweeping away dust on Mars has been developed by scientists in Romania. The device has been designed to clean solar cells that are typically found on probes sent to the planet by forcing away dust with bursts of plasma. The team led by Catalin Ticoş from the National Insitute for Laser, Plasma and Radiation Physics in Bucharest hopes it could be an efficient solution to the problem of Martian dust.

As Earth’s closest planetary neighbour, Mars holds a certain fascination for humans. In our pursuit to know more about the red planet, multiple probes have been sent and there are currently two operational landers on the planet’s surface – NASA’s Curiosity and Opportunity rovers – as well as NASA’s Spirit, which appears to have lost contact with Earth. But the environment on Mars is very different to Earth and dust has proven to be a particular nuisance for the robots.

Fine dust particles can be found everywhere on Mars, with sizes ranging from a fraction of a micron to hundreds of microns. The planet’s winds carry the dust over long distances and then deposit it as a fine layer on exposed surfaces. There are also larger sand particles that are more than 1 mm across and are transported by saltation – the same mechanism seen in deserts on Earth when sand is blown across the ground. The deposition and accumulation of the dust can cause a number of problems for Mars rovers. “Reports from NASA show that dust is quite a problem for missions to the red planet,” says Ticoş. “The Opportunity and to a lesser extent the Spirit rovers have been affected by the Martian dusty atmosphere.” Not only can dust cover optical detectors and cameras rendering them useless, it can also decrease the power production of solar panels at a rate of roughly 0.3% per day. Furthermore, the finer particles can penetrate equipment through holes and slits, and become a frictional problem for moving parts.

Sticky surfaces

Unfortunately, it isn’t a simple case of tipping the particles off or brushing them away with a standard broom. The dust sticks strongly to surfaces due to factors such as electrification caused by high electric fields during storms, and chemical composition – the high iron content is easily magnetized, for example. Previously researchers from NASA and other groups invented an electrical method called an electrodynamic dust shield – a fine mesh imprinted on surfaces is coupled to a voltage source to create an electric field that repels the dust. “The problem appears when you have large dust grains, which are quite heavy,” explains Ticoş, “Even if they are electrically charged it becomes very challenging to lift them off and transport them over tens of centimetres.”

The “plasma broom” solution developed by Ticoş and colleagues uses bursts of plasma jet produced by a simple plasma accelerator. When a large current is passed through two electrode plates separated by a field of rarefied gas, the voltage difference between the two electrodes ionizes the gas, creating the plasma. In the broom, this is done with a coaxial gun – the two metal electrodes are arranged as an inner rod within a hollow cylindrical shell. The discharge current flowing through the central rod electrode produces a magnetic field, which, together with the electric field, exerts a Lorentz force on the ionized gas that expels it. “The trick is that you need a quite high current in order to produce a reasonable magnetic field and this can be achieved more conveniently in a pulsed operation,” Ticoş explains. “For a fraction of a second (100 µs) the current is very high (several kiloamps) and the force pushing the plasma is quite strong.” During a pulse, the plasma is expelled at a very high speed – several kilometres per second – and so can simply blow dust away from an area two to four times bigger than the diameter of the jet.

An advantage of the plasma broom is that it uses low-pressure CO2 as the gas between the electrodes. This is particularly ideal for operation on Mars as the atmospheric pressure there is 150 times lower than on Earth and the atmosphere is 96% CO2. This means the gun will be able to function in “open” Martian atmosphere without the need for a pump or gas bottle. Ticoş and colleagues have also considered the energy required for the cleaner to function on Mars. This depends on the voltage the gun operates at and can vary between a few hundred to a few thousand Joules per pulse.

Energy efficient

“We did an energy budget estimate taking into account the solar irradiance on Mars,” says Ticoş, “and it appears perfectly feasible to fire a few shots even on a daily basis for cleaning the solar panels, which will boost considerably the energy production rate.”

To test the broom’s performance, the researchers covered photovoltaic cells (area 26.86 cm2) with 1 mm of synthetic Martian soil called JSC-Mars 1 A. This is a brown powder made from volcanic ash with particle sizes up to 1 mm. The gun was pointed at the cells from a distance of 5–11.5 cm and fired shots every two minutes. “Our plasma broom is quite effective,” Ticoş proclaims, “it can remove over 90% of the dust covering a surface of tens of centimetres squared, after a few shots.”

The next stage of the team’s research will focus on ways to remove dust in other environments, such as the Moon. The problem there is that the Moon has no atmosphere so instead of a plasma jet they will try a weak electron beam. As for using the plasma broom on a Mars lander, Ticoş suggests it could be autonomous or remotely controlled. “In the first case you would use it to clean the same area while if it is attached to a controlled mobile arm it could sweep dust off of a large solar array.”

The study is presented in New Journal of Physics.

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