Sunday, May 26, 2013

Cooling a Venus Rover

An ocean flowed on Venus eons past
Before a body blow reversed her spin
And now alas, unlike her earthly twin
Her waters to the heavens have been cast.
Tectonic plates, unoiled, locking fast
And no sure passage frees the heat within
The skin and core are thermally akin
No dynamo protects from cosmic blast.
And lighter gas is swept away by rays
Ten miles deep, from pole to pole she's wrapped
In densest greenhouse gas, her body steeps.
Each blistered night, a hundred plus earth days
In Vulcan's ashy forge forever trapped
And with sulphuric acid tears she weeps.
- Diane Hine, Sister Planet (10 March 2012)

Figure 1: Artist’s impression of the Venusian surface with lightning in the background. Credit: Greg S. Prichard.

With a surface temperature of 450 °C and a surface atmospheric pressure of 92 bars (equivalent to the pressure a kilometre under the Earth’s ocean), the surface of Venus is a hostile environment. Although sending a rover to explore the surface of Venus is expected to yield results of great scientific value, the high surface temperature on Venus will wreak havoc on any electronic components. The longest-lasting lander on the surface of Venus was the Russian Venera 13 lander which touched down on 1 March 1982 and survived for 127 minutes. In addition, the thick cloud layers in Venus’ atmosphere allow only 2 percent of the sunlight to reach the planet’s surface and this severely limits the potential of solar energy to power surface operations. The light level on the Venusian surface is comparable to a rainy day on Earth.

Figure 2: Atmosphere of Venus. Credit: Pearson Education Inc.

Due to the extreme and unique surface conditions on Venus, a rover designed for long-duration surface operations on Venus will have to tackle challengers not faced by Martian or Lunar rovers. A study conducted by two researchers at NASA’s John Glenn Research Centre in Cleveland, Ohio, investigates the power and cooling systems for a rover designed to last more than 50 days on the surface of Venus. The study evaluated a nuclear-powered rover than derives its energy from the decay of radioactive plutonium-238 that is encapsulated in the form of seven general purpose heat source (GPHS) modules. Each GPHS module provides 250 W of thermal energy and weighs approximately 1.5 kg.

Heat from the seven GPHS modules are used to power a Stirling engine. One side of the Stirling engine is in contact with the GHPS modules and serves as the hot-sink with a temperature of 1200 °C. The other side end of the Stirling engine is exposed to the ambient environment on Venus and it is at a temperature of 500 °C. Using helium as a working fluid, the temperature difference drives a pressure difference between two chambers to produce a total mechanical power output of 480 W. To accommodate some level of uncertainty, 400 watts of mechanical power is assumed to be available for use. From the 400 W, 100 W of electrical power is generated to drive the rover and power the electronics, while 280 W of mechanical power is available to actively cool the electronics.

The rover’s electronics are enclosed within a 10 cm spherical vault that is surrounded by a 5 cm thick ceramic-based insulator. Of the total heat load on the electronics vault, 77 W comes from the high temperature environment on Venus and 10 W comes from heat being generated from electronics and sensors. Based on this heat load, the rounded up estimated heat rejection requirement is 100 W. To do that, a Stirling cooler is used to provide active cooling of the rover’s electronics vault. Using 240 W of mechanical power for active cooling, the Stirling cooler can keep the temperature within the electronics vault under 300 °C. This temperature is sufficiently cool for high temperature electronics to operate for long durations.

Figure 3: A rendering of Venus without its atmosphere.

“Understanding the atmosphere, climate, geology, and history of Venus could shed considerable light on our understanding of our own home planet. Yet the surface of Venus is the most hostile operating environment of any of the solid-surface planets in the solar system,” wrote Dr. Geoffrey Landis of NASA’s John Glenn Research Centre who was one of two researchers involved in the study. “Putting a long-lived rover on the surface of Venus could revolutionise our understanding of the planet, helping to answer such questions as why Venus ended up so different from Earth,” says Mark Bullock of the Southwest Research Institute in Boulder, Colorado. “Many scientists suspect Venus was much cooler in the past and was perhaps even covered with oceans of liquid water where conditions could have been friendly to life.”

G.A. Landis and K.C. Mellott, “Venus Surface Power and Cooling Systems”, Acta Astronautica 61 (2007) 995-1001