I mentioned in a blog last week that Mars Exploration Rover B (FIgure 1), more affectionately called Opportunity, recently celebrated the 12th anniversary (in Earth years) of its landing on Mars. 12 years without maintenance on the hostile surface of another planet is incredible, considering the original operational time was planned for only 3 months. Mars Science Laboratory Curiosity currently gets the most press, with its larger tool library and fancier cameras, but there are still scientists and engineers at JPL piloting Opportunity from one scientific site to the next. In the last blog, I mentioned the main engineering hazards that the rover’s designers had to contend with: extreme cold, omnipresent fine dust, constant radiation bombardment, an 8-24 minute light-delay between Mars and Pasadena, and the aforementioned inability to physically maintain or repair the rover’s equipment. Today I’d like to look at the materials science on the Red Planet, the technology that has allowed Opportunity to weather those conditions so well.
Opportunity’s photovoltaic panels are of special interest to us at Glew Engineering, since we have a long history with the semiconductor industry. While MSL Curiosity uses a radioelectric thermal generator (RTG) to supply it with constant power, Opportunity uses a set of high-efficiency solar panels, as shown in Figure 2. These panels utilize a triple-junction structure, composed of three layers of semiconductor materials. In Opportunity’s case, the layers use compounds of gallium indium phosphorus (GaInP) and gallium arsenide (GaAs) on a germanium (Ge) substrate. By varying the concentrations of gallium in each of these layered compounds, materials scientists created a photovoltaic cell with multiple bandgap energies ranging across the sun’s energy spectrum. If a photon doesn’t have enough energy to excite an electron across the material’s bandgap, then it simply passes through the material, unabsorbed (see Figure 3). Stacking materials with decreasing bandgap energies, however, lets unabsorbed photons pass through the cell until they reach a layer with a low-enough bandgap that it absorbs the photon. Furthermore, GaInP and GaAs have direct band gaps, a property which makes them even more efficient than indirect gap crystallized silicon. With higher absorptivity and efficiency, Opportunity’s triple-junction solar cells generate more energy than a standard cell at a fraction of the weight.
Figure 3: An electron cannot exist within the bandgap; to jump to the conduction band it’s energy must be higher than the bandgap energy
By Pieter Kuiper (Own work) [Public domain], via Wikimedia Commons
Radioisotope Heater Units
As I mentioned in the last blog, Mars is very cold. While spacecraft in orbit actually have a difficult time staying cool, considering they have no atmosphere to dump their heat into, on Mars the atmosphere and ground will quickly leach away a surface dweller’s internal heat. The temperature on Mars at night can drop to -105°C (-157°F), but the lithium-ion batteries that store Opportunity’s power must be kept above -20°C (-4°F) to prevent damage. While the rover uses a number of small electric heaters to keep its electronics and joints warm during operation, using the batteries to power their own heaters at night, when the panels can’t recharge them, would quickly starve the rover. As such, Opportunity is equipped with eight radioisotope heater units (RHU), each containing 2.7 g of plutonium dioxide (PuO2) (see Figure 4). As the plutonium radioactively decays, each RHU emits about 1 Watt of heat. These heaters operate continuously, and will continue to emit heat long after the rover has been buried by Martian sands. PuO2 is the same compound used in the RTG power generators on MSL Curiosity and the New Horizonsspaceprobe that recently completed its spectacular Pluto flyby. RTGs are slightly more complicated, as they use thermocouples to translate that constant heat into electrical energy.
Opportunity also uses thermal insulation to retain its heat. The rover’s outer surface is covered in a layer of sputtered gold, which is highly effective at blocking thermal radiation (consider the gold foil surrounding satellites and space probes). Between the gold and the rover’s body is a layer or aerogel, a fantastic material that is both ultralight and has an extremely low thermal conductivity. Aerogel is manufactured by supercritically drying a gel, which keeps its structure intact while removing all liquid. This is actually the same process is used to decaffeinate coffee and dry spices without destroying the flavor. The result is a transparent, rigid material (see Figure 5) with a volume comprised of over 98% gas. This means aerogel’s thermal conductivity is essentially that of the gas contained within. If the cavities in the aerogel are small enough, they can actually interfere with the movement of individual particles and reduce the thermal conductivity even further. The rigidity, low conductivity, and ultralight weight make aerogel the perfect insulator for a space project where every gram counts in lowering launch costs.
Only the Best Materials
Although they serve different functions, the above materials all have a few things in common:
- They are expensive. These materials either use rarer materials or complex manufacturing processes.
- They are highly advanced. Even 12 years later, these materials still aren’t used in common civilian applications.
- They are lightweight. This is absolutely essential for space probes and rovers, as it costs around $10,000 to put a pound of payload in Earth orbit [ii]
While expensive, these materials have obviously proven they were worth the cost. For work like this, materials scientists and mechanical engineers always have to make a tradeoff between price, complexity, safety, reliability, and functionality. It certainly looks like they found the right balance withOpportunity’s materials design.
[i] NASA. (2003). Mars Exploration Rover Launches [Press release]. Retrieved fromhttp://www.jpl.nasa.gov/news/press_kits/merlaunch.pdf [ii] NASA. Advanced Space Transportation Program:
Paving the Highway to Space. Retrieved from http://www.nasa.gov/centers/marshall/news/background/facts/astp.html