If you leave your camera at home on a long vacation, you can buy a disposable one. But that's not an option if you have traveled 422 million miles (679 million kilometers) to another planet—especially if that world's extreme conditions present a challenge for the average camera.
So to chronicle Phoenix's trip to the Red Planet, NASA had to come up with a special device, based on the experiences they've had with other Mars landers and rovers: the Surface Stereo Imager (SSI), which acts as Phoenix's main set of "eyes." Built out of titanium to withstand the daily Martian temperature swings from –22 degrees Fahrenheit (–30 degrees Celsius) to –112 degrees F (–80 degrees C), the imager is also designed to perform in low atmospheric pressure.
This $7-million, football-size instrument pivots on a trellis that extends about six and a half feet (two meters) above the Martian terrain. (For those who appreciate science fiction, the SSI looks like a cross between '80s cinematic robot icon Johnny 5 and the new Star Wars prequels' nemesis General Grievous.) It features two openings set about the same distance apart as our own eyes, and detects colors in a manner similar to human vision. The imager even has eyelashlike brushes to clean Martian dust off its lenses each day.
The camera helped researchers inspect the spacecraft after its May 25 touchdown as well as look around the landing site to find suitable digging areas. It has already scored hundreds of exotouristy snapshots of the stark, rust-colored landscape. The highlight, thus far, of course, has been the water ice, which suggests that Mars may have once been (or maybe still is) a habitable planet—at least for microbes.
Scientific American spoke to Patrick Woida, who helped develop the imager to learn more about what makes the device see as we do. Woida is the SSI downlink engineer for the Phoenix lander, as well as a senior staff engineer at the University of Arizona's Lunar and Planetary Laboratory in Tucson. An edited Q&A transcript based on telephone and e-mail interviews follows.
How is the way the Surface Stereo Imager sees similar to the way that a human would see the surface of Mars?
Well, the imager is two meters off the ground, so that makes it about my height. [Woida is six feet, seven inches tall.] Also, the imager has two lenses that are set apart like eyes are on our faces. This arrangement allows for the imager to have depth perception like we have. Also, we aren't recorrecting the colors in images when they get sent back here to Earth. In other words, if you were standing there on Mars looking out, that's what you would see.
How does the Surface Stereo Imager make color images?
It's like the cone cells in our eyes. We have three kinds that detect blue, green and red light. What we have on Phoenix are called charged coupled devices, along with a sensor that can detect [light] on the spectrum that's from near-ultraviolet all the way out into the infrared. We have a wheel of different filters that will only let one specific wavelength, or color, through. We don't have to just use three colors; we can mix in extra filters to get a little more richness and accuracy in our colors. For the first 90 days, the sun won't set where Phoenix is on Mars. But later on, when it's dark out, the imager is sensitive enough to do astronomy. So, in a sense, it's similar to how your eyes can dark-adapt and look out at the stars and constellations.