The Camera Behind the New Pluto Photos
For decades after its discovery in 1930, Pluto looked like nothing more than a gray smudge in the abyss of space. We knew it was there—even knew its size and gravity—but, without better images, we could not answer seemingly basic questions about it. This week there was breaktrough. So how did they build a camera that travels billions of miles from Earth, and what type of camera did NASA use?
When you see Pluto looking tan- and sepia-toned in the new, high-resolution photos, you’re looking at data captured by a camera called Ralph. It captures visible and some infrared light.
Since it captures visible light, Ralph is in many ways comparable to the camera found in a phone or fancy DSLR. In conventional camera terms, it’s a 75mm lens at f/8.7. But it was far harder to built than a normal camera. The team was working under a number of big constraints.
Many of them came down to this: 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,” engineer Lisa Hardaway, from Ball Aerospace Calorado 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.
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. 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. We don’t have any of those options so 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.
Hardaway says that, though difficult, her team could devise solutions to space’s frigidity and Pluto’s relative dimness. Once these problems had been solved too, they stayed that way.
It was far harder to work under the two most important constraints on the mission: mass and power. Ralph needed to be light, and it needed to use very little power.
“We were given a requirement we could use, but they asked us to be even better,” Hardaway says. Any mass not used by Ralph could be used to store fuel. And any power not required by the device could be directed toward steering the probe to new subjects, beyond Pluto. Together, this conservation of weight and power could potentially extend New Horizon’s usefulness to researchers by years.
“I knew NASA really, really wanted to get as much fuel onboard as possible, and even though we met our original requirements, we really wanted to do better, as good as we could do,” she said.
They did. Ralph went on New Horizons weighing about 23 pounds, below NASA’s requirements. It uses seven watts of electricity, about as much as “the power of a standard night light,” according to Ball.
It was also developed much faster than Ball would prefer. Hardaway says that normally teams hope to have 36 months to design, build, and test a new optical device, but Ralph was completed in just 22 months. That timescale pales to how long it took the camera to actually reach Pluto—almost 113 months—and how long it will have to study the dwarf planet up close—only a couple days.
Next, the probe will turn toward other objects in the Kuiper Belt, the ring of rocky asteroids and dwarf planets beyond Neptune. It will take years to reach them—but at least it will have enough fuel.
And, for now, scientists are celebrating what they have accomplished.
“Up until now, Pluto was just a point of light. Many times you couldn't even separate Pluto from its large moon Charon,” says Cathy Olkin, a planetary scientist and one of the principal investigators with New Horizons. “From Earth, there was just one pixel of Pluto… I’ve been just thrilled to see these images come down.”