Mohawk Valley Astronomical Society

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Heat of the Moment

by Perry Pezzolanella

Venus is similar in size, mass, and location to Earth, but varies vastly in its climate, atmosphere, and surface conditions. The sizzling heat of nearly 900ºF and crushing atmospheric pressure of around 92 times Earth’s pose a formidable challenge to spacecraft landing there and trying to operate for an extended time. The longest lasting lander on the surface was Venera 13 in 1982 surviving 127 minutes while all the others fell short, most barely surviving one hour. The result is a consistent lack of approval for surface missions even though there is no lack of mission ideas. Beating the intense pressure and acid have been achieved by using thick titanium, but the heat has been a huge problem. Electronics quickly fail in oven-like heat of 400ºF and the thought of anything surviving for weeks in heat over twice that would sound impossible, until now. New breakthroughs in technology now makes it possible for a lander to survive at least 60 days in the hostile environment, and possibly much longer.

There is a lack of working knowledge about Venus’ surface conditions: temperature, pressure, winds, and chemistry. This data is critical for the understanding of the weather and the process of how chemical compounds and particles interact with each other and how they are transported throughout the atmosphere. It is also not known how the surface and atmosphere interact under the corrosive, pressurized inferno conditions. No lander has lasted long enough to make the necessary long-term measurements. Recently developed silicon carbide electronics, sensors, and other technologies have advanced to the point where a very simple, but powerful long-life scientific probe can function on Venus.

Keeping electronics cool long enough within a titanium pressure vessel is a challenge. The Soviet Venera and Vega landers were pre-chilled at -100ºF before being deployed from orbit. This brute-force approach worked good enough to do some science on the surface and transmit a few pictures, but a longer stay on the surface is necessary to do research on meteorology and seismology along with analyzing the surface and atmospheric composition in greater detail. There is finally a way to protect the brains of future landers with advances made in battery and chip technology. Creating a battery to operate long enough on Venus is close to reality. NASA is working with a company called Advanced Thermal Batteries, Inc. and together created the first battery that has demonstrated the capability to operate at Venus’ temperatures for an entire Venus solar day of 117 Earth-days. The battery is based on thermal battery systems for powering smart missiles. A single battery consists of 17 cells and uses specifically designed chemistry and structural materials. This advanced type of battery technology may provide a new energy storage device for future exploration in harsh environments across the Solar System including the hot, deep atmospheres of the giant planets. Batteries may be the only solution for powering Venus’ landers and rovers as light levels are too low for solar panels which cannot survive the intense heat. Nuclear power used on the Mars rovers Curiosity and Perseverance will not work as they generate heat, which only adds to the stress of maintaining a reasonable operating temperature. Thermal batteries on Venus can utilize the torrid conditions to heat a special high-temperature electrolyte that is solid and inert at room temperatures. They can remain operational without the need for thermal insulation.

The work on this new type of thermal battery is part of ongoing work at NASA’s Glenn Research Center, home of the Venus test chamber known as GEER (Glenn Extreme Environment Rig). This 14-ton stainless steel test tank is a 0.8 cubic meter pressure vessel that can accurately simulate the conditions on Venus. It can be set at 500ºC (932ºF), 100 Earth’s atmospheres, and filled with carbon dioxide along with the proper ratio of eight minor gases: nitrogen, sulfur dioxide, water, carbon monoxide, carbonyl sulfide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride (a nasty planet!). Even prototypes of a lander can be placed inside the chamber to be tested. There is also mini-GEER, a smaller, 4-liter vessel, used to test smaller items. It saves money as it takes less time after testing to cool down from the ferocious heat.

Microchips are being designed and tested that can withstand Venus’ conditions for months instead of a few hours without need of a protective vessel. Under extreme heat, silicon, the backbone of modern electronics, becomes a pure conductor, and makes it useless for computing, because stopping and starting the flow of electricity is how zeroes turn into ones. Silicon carbide, a hybrid of silicon and carbon, is commonly used as an abrasive in sandpaper and growing fake diamonds. It has a bigger bandgap then silicon, which means its electrons can absorb much more energy before it becomes a conductor. Silicon carbide is difficult to work with as it does not melt, and the techniques used to make large silicon wafers do not work. The process is riddled with impurities making them unreliable. Researchers led by Cree (now Wolfspeed), an electronics company, have devised ways to grow large silicon carbide crystals that are usable and reliable. These have been developed into full-fledged computer circuits by layering the chips, but they are not Pentiums. A modern silicon chip can contain seven billion transistors whereas the silicon carbide chips being tested in the Venus chamber has only 175. They can be compared to a pocket calculator, but operating on Venus they would more powerful than the chips on the Apollo flight computers. The silicon carbide chips have survived at least 60 days in the test chamber proving that it will soon be possible for lengthy stays on the surface of Venus.

GEER is the test bed for a scientific probe known as the Long-Lived In-situ Solar System Explorer (LLISSE) that can be configured for different mission profiles, payloads, and deployment options. LLISSE is intended to make three key measurements for Venus’ exploration: better knowledge of the super-rotation of the atmosphere, the climate and evolution, and surface-atmosphere interaction/weathering. LLISSE will be a fully dedicated weather station making periodic measurements of temperature, pressure, wind velocity/direction, and chemical composition; it will relay the data to an orbiter every 8 hours for at least 60 days. It is hoped that measurements can be made long enough to observe the changes that occur as day turns into night. A complete day on Venus from noon-noon is 117 Earth days making day and night each lasting 58.5 days. Twilight may last up to 2 weeks due to the dense atmosphere and slow rotation.

LLISSE is a small probe roughly resembling a box 10” on a side and weighing hardly 20 pounds. It will be battery powered and could last up to one Venus year of 225 Earth days. There will be no camera, but it could be added if one can be designed and tested in time. LLISSE is unable to land on Venus on its own as it is not equipped with a heat shield, so it will have to hitch a ride on a larger lander that will be equipped with one. Before the lander is safely on the surface it will deploy LLISSE which will land a safe distance away from the lander. Another option is that LLISSE may also be deployed from a balloon mission among the clouds. Research is also being done to determine if LLISSE can be delivered on its own using its own heat shield, but mission cost must be considered. No matter how LLISSE is deployed, it will hopefully return data for several months once it lands.

Thousands of hours of testing various components have proven that the time has finally arrived for long term exploration of the surface of Venus with the dreams of rovers, drones, and sample return missions becoming reality. At long last the exploration of Venus below the clouds will become almost as accessible as Mars and perhaps more rewarding, because when it comes to exploration, the less we know of a world, the more knowledge we gain, and we still know very little about Venus.