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Mike Ravine, Advanced Projects Manager for Malin Space Science Systems  (Source: torynfarr/flickr)

A two-image panorama shot with the rover's 1024 x 1024 mono navigation cameras  (Source: NASA)
Reasons included the amount of data produced, the fact that they had to meet the needs of different cameras, and the team's familiarity with these sensors

NASA recently accomplished a huge feat by landing its Mars rover Curiosity on the Red Planet, but one of the questions on the minds of many was why such a sophisticated machine used 2 MP cameras.
Mike Ravine, a project manager from Malin Space Science Systems, was happy to answer that in an interview with Digital Photography Review. He said the main reasons for using 2 MP sensors in the cameras were the amount of data produced, the fact that they had to meet the needs of different cameras, and the team's familiarity with these sensors.
"There's a popular belief that projects like this are going to be very advanced, but there are things that mitigate against that," said Ravine. "These designs were proposed in 2004, and you don't get to propose one specification then go off and develop something else. Two MP with 8 GB of flash didn't sound too bad in 2004. But it doesn't compare well to what you get in an iPhone today."
The amount of data produced is a large reason for using 2 MP cameras. There just isn't enough bandwidth for anything more powerful because the cameras must share with other instruments. Curiosity sends data back to Earth via the UHF transmitter, which transmits to two spacecraft orbiting Mars. The data is then sent back to Earth, and this system only allows for 250 megabits per day to be shared amongst various instruments.
The 2 MP camera sensors also were the tools of choice for the use of four different cameras, including the MAHLI, MARDI and two Mastcams. Having four different sensors for each camera would be expensive and more difficult to maintain rather than having one type of sensor all across the board. 
The team's familiarity with the sensors was crucial, too. The team knew the behavior of Truesense imaging chips and Kodak's KAI-2020 chip, so it makes sense that they'd work with what they know. 
"We know how to clock them and drive them," said Ravine. "They're a very easy CCD to drive."
Other issues, like the low pixel count, are not an issue either since the two Mastcams will create images from multiple exposures. 
NASA rover Curiosity landed successfully on Mars earlier this week after launching from Cape Canaveral, Florida on November 26, 2011. Curiosity is a one-ton, nuclear-powered, Mini Cooper-sized science laboratory that will explore the Martian surface for the next two years. 

Source: Digital Photography Review

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RE: Still no reason why they couldn't put 8MP
By DennisB on 8/9/2012 2:20:50 PM , Rating: 2
Astronomy CCDs (cooled by NO2) have been in the Megapixel range since the early 80's so size is not the problem.
The problem is as mentioned the RAM, processors and UHF.
Not mentioned but related is the power consumption of higher power transmitters. Curiosity probably cannot have enough to spare. The instruments or getting scientific information have higher priority. That's the mission after all. Not to mention the power needed to move it.

RE: Still no reason why they couldn't put 8MP
By Solandri on 8/9/2012 6:46:53 PM , Rating: 1
So how much will it cost to deliver a replacement tank of liquid nitrogen to Curiosity on Mars?

The liquid nitrogen-cooled CCDs can be high res because the cryogenic cooling reduces the noise you typically get from high-MP small commercial CCDs. If you can't cool it, you want a bigger lower-res sensor to reduce noise, just like a 16 MP DSLR produces cleaner pixels than a 8 MP point and shoot.

By m51 on 8/10/2012 11:03:59 AM , Rating: 3
Astronomy CCD's are cooled to reduce dark current in the CCD so very long exposures can be used.

Cooling does not affect the resolution usable.

Lower resolution sensors with larger pixel elements are used in Astronomy CCDs so there is more light gathering area per pixel and shorter exposures can be used.

These are all the result of needing to take images at extremely low light levels. This is not the case with the cameras on MSL.

The MSL cameras are already cooled since Mars is damn cold to begin with.

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