You probably know that NASA released its highest-ever resolution image of Mars in March 2020. The panorama showed a landscape, not unlike a desert canyon on Earth, but features like a three-mile-wide impact crater are reminders of how completely different the Red Planet is.
The 1.8-billion-pixel composite is actually
more than 1,000 images stitched together. All were taken by the Curiosity rover
between Nov. 24 and Dec. 1, 2019, using the telephoto lens on its Mast Camera (Mastcam),
mounted to the rover at a height of 6.5 feet.
Now. What you probably don’t know is that
Mastcam is a 2MP camera that captures images not more than ~1600 X 1200 pixels.
The Mars
rover Curiosity, despite being one of the most impressive human
achievements of the last decade and costing taxpayers around $2.5 billion,
uses a 2-megapixel sensor in its main camera array. That detail might have
raised eyebrows with anyone familiar with digital imaging as it’s surprisingly
low, especially considering that today’s affordable digicams have at least 10
megapixels and “expensive” models pack upwards of 40.
The sensor used on Curiosity was the KAI-2020 from
TrueSense Imaging. It shoots at 1600×1200 and is an interline CCD (as opposed
to full-frame CCD, frame transfer CCD, or CMOS). Interline sensors are
characterized by small pixels, the use of microlenses to aid the pixels, snappy
imaging times, and the use of an electric shutter as opposed to a mechanical
one.
So why go with just 2MP? The choice was made
for a number of reasons, some of which might not be immediately obvious. First
and foremost is the distance over which data must be transmitted. The rover
sends data to two satellites that orbit Mars, which then relay it back to
Earth. This stream of data is quite limited, to something like 256 megabits
(32MB) a day. And images aren’t the only thing that Curiosity is sending back —
there are all sorts of other recordings and measurements that need to be
transmitted.
The next hurdle to more modern equipment is
simple logistics: Curiosity has been in the works since 2004. At that time, a
2-megapixel sensor was fairly bleeding-edge, but it’s obviously been eclipsed
since then. But design and testing take time and components can’t just be
swapped out like on your computer. Parts need to be able to stand up to the
rigors of the Martian environment, and ensure that they can take time.
There were other logistical reasons as well,
such as the fact that the 2MP sensor isn’t only used in one camera — it’s in
four on the MSL. These include the Mars Hand Lens Imager (MAHLI), Mars Descent
Imager (MARDI), and both MastCams. By using the same sensor NASA was able to
keep certification time to a minimum and not have to learn the ins and outs of
each sensor (such as the effect of radiation on the individual pixels).
The final reason why a 2MP sensor made the cut
is also the most practical: nothing on Mars is moving. That means that multiple
images can be easily stitched together into panoramas. This will have the same
effect as a higher resolution camera and have none of the drawbacks.
And one perk of the KAI-2020 is that it can take video. The chip can handle 720p HD (1280×720) and up to 6 fps if called upon to do so.
Source: Curiosity
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