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The Phoenix Mars Lander has whapped the Web world upside the head with its captivating images from the Red Planet. But without the successful generation of energy by its two solar arrays (with the help of lithium-ion storage batteries) to power its science experiments, the mission would be an abysmal failure.

People, it's time to give it up for the photovoltaic components of the latest Martian expedition!

As the Phoenix descended toward the Martian surface on May 25, I contacted NASA's Jet Propulsion Lab for additional specifics about the whimsical pair of PV panel fans. Before and after the craft landed, I spoke with Randii Wessen, a JPL scientist handling media duties who admitted he was not a solar expert, but because the team was really busy at mission control, he "was going to fake it." He did better than that.


The Phoenix's Ultraflex solar array soaks up some Martian rays.
(Photo courtesy: JPL/NASA/U of AZ; remix: TC)

He told me that the flexible arrays, which deployed successfully minutes after the lander touched down, were made by ATK (Alliant Techsystems) at its Goleta, CA, factory north of Santa Barbara, using gallium-arsenide triple-junction cells processed by Boeing unit Spectrolab at its fab in Slymar, CA, not too far up the 210 freeway from JPL. Although he didn't know the conversion efficiency, the number of cells per panel, or the total number of panels, he said that each 4.2-square-meter decagon-shaped array had about 770 W in power (though much less under Martian solar conditions) and that the mission could get by on one array, "but we would prefer to have both [of them] working."

The Phoenix's PV system is stationary, with no tracking mechanism. "Mechanical devices have a tendency to fail," Wessen explained, "so if you have a really short prime mission--and this one is 90 sols, basically 90 earth days--you don't want to add any more complexity than you really have to." Also, he "doesn't know of any landed [space] vehicle that had articulatible solar panels."

The deposition of Martian dust degrades the output of the arrays, according to the scientist, to the tune of 0.3% over the first 30 days. There's actually an upside to a dust devil swirling over the Phoenix: Wessen related that experiences with the Martian rovers had shown the little tempests actually help clean the dirt off the solar panel's surface. The output is also affected because the sun does not fully illuminate the panels, but rather has a grazing illumination effect since the craft landed with a 1% tilt, and because it is situated in a polar area, where the sun stays lower in the sky and is less intense.

It took almost a week to get additional info from ATK, while Spectrolab, after much haranguing by yours truly, finally declined to comment. ATK did file a short press release the day after the Phoenix landed, which had a few paragraphs about its Ultraflex solar array (and other company gizmos) on board the spacecraft.

Mark Anderes, one of ATK Space's marketing and communications guys, replied to my questions with the following details. "The solar cells used on ATK's Mars Phoenix Landed array are Spectrolab's standard 28.3% efficiency UTJ [ultra-triple-junction] cell. This cell efficiency is currently about the highest available for a space-rated solar cell.

"Each Ultraflex array is made up of 10 pie-shaped flexible substrates (gores). There are 126 cells per gore and 1260 cells per wing. Power production in space, at earth-to-sun distance (1 AU), is about 750 W per wing. On Mars during the Phoenix mission, the peak power production is less than 200 W per wing due to the increased distance from the sun, low sun angle at the northern latitude of Mars, and solar attenuation from the Martian atmosphere.

"The Phoenix Ultraflex arrays are the highest performance space-rated solar arrays ever used. The specific power for these arrays is greater than 105 W/kg, compared to a typical rigid panel performance of approximately 50 W/kg. This means that for a given power output, the Phoenix Ultraflex arrays are approximately half the mass of a typical space-rated rigid panel solar array. This high performance is one of the main reasons why ATK's Ultraflex arrays were selected for use on NASA's next-generation manned spacecraft Orion."

A quick search of the Spectrolab Website revealed a tech sheet for the UTJ solar cells. (There's also one for the Ultraflex on the ATK site.) They have been superseded by the company's NeXt XTJ triple-junction devices, which are rated at 29.9% conversion efficiency, but the UTJ are still pretty powerful cells. Up to 32 square cm in size, the PV units are made on 140-micron-thick germanium wafers, with a gallium-indium-phosphide/gallium arsenide/germanium structure, featuring a stack of two terminal triple junction cells, interconnected with two tunnel junctions. The fabrication process's key enabler is a huge metal organic vapor phase epitaxy system, in which the critical GaAs films are grown.

Spectrolab pioneered solar power in space, placing the first panels in orbit on Pioneer 1 in 1958 and the first cells on the moon with Apollo 11. In 2006, the company celebrated the completion of its two-millionth multijunction cell. It claims to have its PVs on at least 60% of the satellites in orbit (more than 675 KW in all) and to be responsible for core solar-power components on all the recent missions to the Red Planet.

I guess that means Spectrolab has a 100% share of the installed solar-cell market on Mars.