The 2020 Mars Rover launched July 30th, with a planned landing on February 2021. Every time we go to Mars, decisions of what to bring become a balance of what scientists would like, and what engineers feel will reliably work. This makes each mission part of a continuing chain of innovation, with the learning from past missions informing proven technologies and innovations to future missions.
Over the last twenty years, one perfect example has been CCD sensors. It has been the go-to standard for cameras in space. “A CMOS sensor would probably die very rapidly as soon as it went into space,” points out Robert Groulx, Product Manager for CCD technology at Teledyne Dalsa Semiconductor. Extremes of heat and cold can wear out electronics, and radiation and cosmic rays can wreak havoc on electronic integrated circuits by altering the states of the elements inside them. On five of NASA’s missions to Mars the CCD image sensors on its cameras were made by Teledyne, in collaboration with NASA’s Jet Propulsion Laboratory (JPL). The camera on the InSight lander helped scientists and engineers choose where to place instruments on the surface of Mars. This included the Instrument Deployment Camera, which is placed under the lander and pointed at 120° with a fisheye-type lens, and the Instrument Context Camera, which is on the lander’s robotic arm, looking at a 45° angle. Besides their lenses, both cameras are identical, collecting 1MP images to help with the placement of other instruments. The Insight cameras were monochrome frame-transfer CCD models left over from an earlier program. “But this time JPL wanted to take color photos. Color cameras are much larger, and new models couldn’t be qualified in time for launch,” says Groulx. The solution was to put color filters over the monochrome sensors. “We knew from the users of those color filters that the potential for qualification was good. We had used them on Teledyne Dalsa CCD sensors for many years but we had never sent them to space! JPL had to make sure that those color filters would endure the trip to Mars.” “For the color filters, we use a technology that is unique to Teledyne Dalsa that we’ve been doing since 2002/03,” says Groulx. “It uses color pigments that are evaporated onto the CCD. It’s more expensive but produces very good results in terms of color separation and reproduction, better than the typical dyed resist method.For this mission, our colleagues at Teledyne e2v have contributed their CCD42-10 image sensor for SuperCam and Sherloc. These instruments will help to determine evidence of wateras an influence on the Martian environment.”
Choosing the right camera
The four cameras on Curiosity used a different solution, the KAI-2020 (originally from Kodak, now OnSemi), an interline CCD sensor that shoots at 1600×1200 pixel. The cameras went into the Mars Hand Lens Imager (MAHLI), Mars Descent Imager (MARDI), and two MastCams. Curiosity launched in 2011. Why did it only have a 2MP sensor, smaller than a lot of mobile phones even then? Once again, the sheer logistics and complexity of a Mars mission defined what was possible. “There’s a popular belief that projects like this are going to be very advanced but there are things that mitigate against that. These designs were proposed in 2004, and you don’t get to propose one specification and then go off and develop something else. 2MP with 8GB of flash didn’t sound too bad in 2004. But it doesn’t compare well to what you get in an iPhone today,” said Mike Ravine, project manager at Malin Space Science Systems, responsible for the cameras’ development in a 2012 interview, “And the state of CMOS sensors wasn’t credible in 2004. They’re an interesting option now, but they weren’t then.” By using the same sensor NASA was able to minimize the effort needed for mission qualification, and more easily optimize the sensors for their operating environment, such as accounting for the effect of radiation on individual pixels. There’s also the type of photos the rover would be taking: landscapes. With nothing on Mars moving, it was possible to take multi-shot panoramas with the help of the MastCam arm, creating extremely high-resolution results. One disappointment in the Curiosity imaging system was the cancellation of 6.5 to 100mm zoom lenses that would allow the cameras to catch additional detail, as well as cinematic 3D images. Zoom lenses would have to wait until the next rover in 2020.