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RGClark
2007-Jul-27, 04:26 PM
Am I the only one who didn't know the rovers contained radiothermal heaters?

Update: Mars Rovers Weather the Storm
By Dave Mosher
Staff Writer
posted: 27 July 2007
10:30 am ET
"Each rover has eight radioactive heaters in addition to electric heaters for keeping batteries and electronics within safe operating temperatures. While the plutonium-powered heaters aid the rovers' survival on low-power days and through extremely cold nights, the electric heaters are also necessary."
http://www.space.com/news/070727_rovers_update.html


Bob Clark

01101001
2007-Jul-27, 04:59 PM
They are RHUs (http://en.wikipedia.org/wiki/Radioisotope_heater_units) rather than RTGs. Close cousins, though.


Radioisotope heater units are small devices that provide heat through radioactive decay. They are similar to tiny radioisotope thermoelectric generators (RTG), and normally provide about one watt of heat each, derived from the decay of a few grams of plutonium 238.

Los Alamos National Lab: Lab technology helps power Rover on Mars (http://www.lanl.gov/news/index.php/fuseaction/nb.story/story_id/4763)


A little bit of plutonium from the Laboratory is keeping NASA's Mars rovers warm and ready to rove despite the frigid Martian temperatures.

In fact, the Spirit and Opportunity rovers can stay warm and keep collecting data for nearly five times longer, thanks to about an ounce and a half of Los Alamos plutonium-238.

Los Alamos' Pu-238 Science and Engineering (NMT-9) Group made eight lightweight radioisotope heater units each for the Spirit and Opportunity rovers. Each of the 16 units contains just under one-tenth of an ounce of plutonium, and each pumps out a continuous one watt of heat as the plutonium decays.

Housed inside the rover fuselages, called Warm Electronic boxes because they provide a temperature-controlled environment, the heater units keep electronic and mechanical components warm enough to function reliably in the bitter cold of space. They transfer heat directly to the rover systems and instruments, without moving parts or electronic components.

The heater units are the latest in a long line of plutonium heaters and thermal batteries fabricated at Los Alamos for all of NASA's deep space probes, as well as for the Sojourner rover, which explored the red planet for three months as part of NASA's Pathfinder mission in the summer of 1997.

NASA JPL: Mars Exploration Rover Mission :: In-situ Exploration and Sample Return: Technologies for Severe Environments (http://marsrovers.nasa.gov/technology/is_severe_environments.html)


Heat inside the warm electronics box comes from a combination of electrical heaters, eight radioisotope heater units and heat given off by electronics components.

Each radioisotope heater unit produces about one watt of heat and contains about 2.7 grams (0.1 ounce) of plutonium dioxide as a pellet about the size and shape of the eraser on a standard pencil. Each pellet is encapsulated in a metal cladding of plutonium-rhodium alloy and surrounded by multiple layers of carbon-graphite composite material, making the complete unit about the size and shape of a C-cell battery. This design of multiple protective layers had been tested extensively, and the heater units are expected to contain their plutonium dioxide under a wide range of launch and orbital-reentry accident conditions. Other spacecraft, including Mars Pathfinder's Sojourner rover, have used radioisotope heater units to keep electronic systems warm and working.

djellison
2007-Jul-27, 05:16 PM
Sojourner had them, other spacecraft have had them. They're a very simple way of injecting some heat into systems.

MER has six 1W RHU's for the batteries and a further two on the rover electronics module.

Doug

novaderrik
2007-Jul-27, 08:00 PM
good thing the anti nuk-yu-ler crowd didn't hear about this before launch..
of course, now the enviro freaks are gonna come out of the woodwork and protest the way we are polluting Mars..

01101001
2007-Jul-27, 08:13 PM
good thing the anti nuk-yu-ler crowd didn't hear about this before launch..

They did.

Nukes-in-Space in Columbia's Wake (http://space4peace.net/articles/columbiaswake.htm):


Indeed, this May and June [2003] NASA is planning to launch two rockets from Florida carrying rovers to be landed on Mars equipped with heaters powered by plutonium. The Global Network Against Weapons & Nuclear Power In Space (www.space4peace.org) has been conducting demonstrations to protest these launches.

JonClarke
2007-Jul-27, 10:40 PM
The lunokhods were the earliet spacecraft to have such units that I am aware of.

Jon

Nicolas
2007-Jul-30, 12:51 PM
Just a random thought: do they need high grade plutonium in these, or could they place nuclear waste plutonium or other nuclear waste in it?

Would be a nice one :).

01101001
2007-Jul-30, 01:36 PM
Just a random thought: do they need high grade plutonium in these, or could they place nuclear waste plutonium or other nuclear waste in it?

It's plutonium 238, not the less-hot weapons grade plutonium 239.

Wikipedia: Plutonium-238 (http://en.wikipedia.org/wiki/Plutonium-238)


Plutonium 238, is a radioactive isotope of plutonium with a half-life of 86.41 years and is a very powerful alpha emitter. Because of its high level of alpha activity, it is used for radioisotope thermoelectric generators and radioisotope heater units.
[...]
The United States currently has limited facilities to produce plutonium-238. Since 1993, the U.S. has purchased all of the plutonium-238 it has used in space probes from Russia. 16.5 kilograms total have been purchased.

I think it's a matter of its having just the right half-life, hot enough to provide good power, cool enough to last a couple decades.

Atomic Energy Insights newsletter (http://www.atomicinsights.com/sep96/materials.html):


Plutonium 238 is a non-fissile, alpha emitting isotope with a half life of 87 years. A sample of pure material would produce approximately 0.54 kilowatts/kilogram of thermal power. In some configurations, the surface temperature of a Pu-238 fuel element can reach 1050 degrees C.

These characteristics make Pu-238 the most capable heat generating isotope. It will outlast most customers; even after 20 years a Pu-238 based power source will produce 85% of its initial power output. It has a high energy density, allowing power system mass and volume to be minimized. It is also easy to shield and its emissions will not interfere with sensitive instrumentation.

Unfortunately, Pu-238 is difficult to manufacture, making it extremely expensive. An accurate price is difficult to determine because of the lack of an open market, but the recent estimates by experts in the field indicate that the material costs several thousand dollars per gram in kilogram sized lots - if it is available at all. Since RTG conversion efficiency is on the order of six to eight percent, this puts the price of a 50 W power supply at close to a million dollars.

They list Strontium 90 as an alternative.

Guy Webster
2007-Jul-30, 05:46 PM
For those interested, there is more information about radioisotope power for Mars exploration at these URLs: http://www.jpl.nasa.gov/news/fact_sheets/mars-power-heating.pdf
http://www.ne.doe.gov/space/neSpace2f.html (DOE page on RHUs)
http://nuclear.gov/pdfFiles/SRPS_safety.pdf (space nuclear systems & safety fact sheet)