Many of them came down to this: “We were cruising for nine and a half years,” Hardaway told me. “So our system would have to handle the space environment, specifically radiation and thermal fluctuation, for nine and a half years.”

Objects in space are often designed to be bombarded by the cosmic rays. But a probe like New Horizons traveling as far out as the outer solar system would be tested by the environment in other ways, for the farther something gets from the sun, the colder space becomes.

“Going out that far, there are some fluctuations,” Hardaway says. “It can get quite cold, and materials will shrink as they get colder. But different materials shrink at different rates.”

The answer, then, was to build almost the entire camera out of just one type of material.

“We actually built the mirrors and the chassis out of aluminum so that as they shrink, they would shrink together, to maintain the same focal length. We could do a reasonable test on Earth and still expect the same quality image,” she says.

Even the camera’s mirrors were made out of aluminum. (To turn dull aluminum into mirrors, Ball sharpened it with diamonds.) The lens was one of the few pieces of the camera that could be safely made out of glass.

Another constraint on the mission was that Ralph had to take photos using only the sun’s dim light that reaches Pluto. During its flyby, New Horizons will photograph the side of Pluto that’s turned away from the sun. This side is lit solely by the sun’s light reflecting off Charon. This is like taking a photo using just the light from a “quarter moon” on Earth, a lead optical engineer for the mission told me in an email.

So Hardaway and her team designed Ralph for the exact light conditions that New Horizons would have to operate in. “This camera isn’t adjustable. It’s designed very specifically for conditions at Pluto,” she says.

“In a standard camera, whether it’s digital or analog, you have to adjust the aperture size and change the f-stop reading so that you can get the most out of the light available. When we used to have film camera, it would go to a primary speed of film, like 400 or 800,” she told me.

“We don’t have any of those options,” she says. “We had to get it so that the detectors could take a minimal amount of photons and turn it into an image.”

That made Ralph unusually susceptible to very bright light, which required its own precautions.

“We actually had to put a cover on it for launch, because once it separated from the ferrying of the rocket, if you got a glimpse of the sun, or the sun reflected in the moon or Earth, you could saturate the detector and potentially destroy it. We didn’t open the aperture till we were closer to Mars,” she says.