Stirling Cycles Will Cool Future Venus Rovers

Previous explorers only lasted for 2 hours

By Gabriel Gache on November 12th, 2007 14:15 GMT
Venus, along with Mars, is Earth's 'sister planet', but there's a big difference between the two. While temperature on Mars is -65 degrees Celsius on average, Venus has a temperature of about 450 degrees, due to an out of control greenhouse effect, hot enough to melt lead. The URSS and the US have sent a few probes over the years to study the planet, which sent back valuable information, but which only lasted about 2 hours in the blazing heat, before their electronic components failed.

NASA is now designing a new rover, to be sent on Venus, that has a high-tech cooling system to protect its electronics, and maintain it functional for a few weeks, enough time to better explore its surface. The electronic parts are vital for the mission, so NASA thought of packing them in a ceramic base insulator placed inside a metal sphere about the size of a grapefruit, through which cool gas will be pumped.

The system used to pump cool gas inside the sphere, being relatively well known, and is called the Stirling cooler. Cooling gas is drawn into a cylinder, and compressed. By compressing the gas, it heats up, the heat is absorbed from the cylinder and it dissipated in Venus atmosphere. The compressed gas expands and cools the metal sphere, therefore protecting the vital electronic systems, and the rover could crawl the surface of Venus up to 50 Earth days.

So far, so good. But there is a problem. Physics says than heat moves from a hot body, to a cooler one, never the other way around. On Earth, it is relatively simple to produce low temperature, because our atmosphere is not so hot. Bringing the temperature of the sphere somewhere below 100 degrees Celsius is not possible on such a small rover.

Though Stirling coolers were invented in 1816 by Reverend Robert Stirling, they were mostly ignored until the middle of the 20th century, when the impressive energy efficiency that they displayed become easier to understand. Today, the Sterling cooling system is used in most of the refrigerating devices produced on Earth, such as the refrigerator.

To cool down the metal sphere which houses the electronics, they had to design it to run at the relatively cool temperature of 200 degrees Celsius, so the radiator would reach a temperature of 500 degrees Celsius, and cool in Venus atmosphere. Such a device would need about 240 Watts of power to run, which could be provided by plutonium batteries that produce power from the heat of radioactive decay.

So far, NASA has no Venus rover mission running, but it could become a reality within a decade, due to the high priority to investigate the Venus surface. Active cooling is essential for such a mission, but even so, it would be very difficult. The radioactive isotope power source is the best way to run the Stirling cycle cooler.

Studying Venus is essential for the understanding of how the planet formed, and could answer some questions about its past, and perhaps the most important of them all, if indeed Venus was cooler and housed oceans of liquid water, or even had a friendly life environment.
Artistic impression of the rover on Venus surface
   Artistic impression of the rover on Venus surface

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