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.

In what ways is the Surface Stereo Imager "better" or different than human vision? What can it see on Mars that we cannot?
Our eyes can't pick up the longer wavelengths of infrared light but the imager can. Infrared is very important for figuring out the chemical composition of rocks and soil and so forth. Plus, having infrared means we can take advantage of different lighting situations. The imager can also see polarized light, just like those polarized sunglasses that eliminate glare. This allows us to do analyses of aerosols and dust composition in the atmosphere. We also use other filters to look at the sun to get direct readings of how much sunlight is reaching the surface. This is very important because Phoenix is solar-powered. We can tell just how clear the atmosphere is and know how much energy the solar panels will get that day. We can get very high accuracy humidity readings, too, so we can tell how much water vapor is in the air.

Has the Surface Stereo Imager done what it was supposed to do so far? Any glitches?
The imager is all day, every day. In general, we shoot about 150 to 200 images daily and all the other science teams really rely on it. Before the robotic arm digs anywhere, it has to know where the ground is. We need to know when the arm is moving and where we left it at the end of the day. The geology and chemistry people need multispectral images so they know where the most interesting place to dig is. There have been some communication problems—not with the imager per se; they have been more on the sense of that we can't quite always get the images we've commanded back from Mars. They have to be relayed through one of the satellites we have in orbit around Mars, and then there's a communications delay [of 15 minutes] as they're beamed back to Earth. There are literally hundreds of things that have to go right to get a picture, and sometimes 98 out of 100 go right, and it's not quite the right 98 for us to have that image back home.But that's what you expect, and it's hard to be down about it. If I don't get my pictures today, I will get them tomorrow morning. I still have an instrument that's working and happy and a spacecraft that's working and happy. Phoenix is beating its little heart out to find cool things for us to look at on Mars.

So what does it feel like to have your baby land on a planet millions of miles away?
The imager was always on a one-way trip. It had to be done right the first time, since we couldn't fix it, and she has worked splendidly. I'm proud of our team that designed, built, tested and delivered this camera to Mars. That first day that Phoenix landed and the imager started taking pictures, everyone else was all gaga about the solar panels unfolding, and for good reason. [The first pictures the imager snapped were of the panels.] But then the second image came back of the footpad on Mars, and that was the one that I was waiting for. The first place we touched Mars was with that footpad, and the same went for the Viking landers back in 1976. It really gives you perspective. I grew up being a Trekkie and wanting to go to the stars, and when I put that into something that I could really do in my life, it was where I am right now. It provides this continuity, this thread that goes from the dreams of when I was a kid to where the reality today is that I'm actually doing it. I mean, jeez, it gives your soul a lot of worth.

*Correction (7/21/08): The image of Phoenix's Surface Stereo Imager has been added since publication. The original picture was of the 1997 Mars Pathfinder imager.