Eighty-five years after being discovered by an astronomer named Clyde, a camera called Ralph is about to bring Pluto into sharp focus
BOULDER, Colo. — Pluto has been leaving a lot to the imagination since 1930 when a sharp pair of eyes belonging to Clyde Tombaugh spotted the long-rumored ninth planet blinking back at him in a pair of photographs taken a week apart.
Still just barely visible to ground- and space-based telescopes, Pluto has remained a head-scratching orb of obscurity for 85 years.
But that’s about to change.
After a journey of more than nine years and 5 billion kilometers, NASA’s New Horizons probe is beginning to send back eye-catching, full-color images of the far-flung globe.
Credit this time around goes to the sharp set of eyes belonging to Ralph, the small but powerful visible and near infrared multispectral camera suite at the heart of New Horizons.
Pluto, at long last, is about to get its close-up.
“This is the space event of the decade,” Alan Stern, New Horizons principal investigator said ahead of the July 14 flyby. “Forget that hackneyed saying ‘we’re going to rewrite the text book.’ For New Horizons, we’re writing it for the first time because we know so little about Pluto.
Breaking the mold
Perhaps it is fitting that the dwarf planet is being visited for the first time by a dwarf of a spacecraft.
Weighing a modest 478 kilograms at launch, the piano-sized probe has no large, protruding solar arrays — which wouldn’t work so far from the sun anyway — and relies instead on a single plutonium-fueled radioisotope thermoelectric generator to produce the 200 watts that power its onboard computers, communications systems and a suite of compact, lightweight science instruments.
New Horizons’ seven instruments — three optical instruments, two plasma instruments, a dust sensor and a radio science receiver/radiometer — run on a stingy 28 watts combined. Stern said the instrument suite’s power and mass constraints were unprecedented for a planetary probe, adding: “We broke the mold.”
The New Horizons spacecraft was designed and built for NASA by the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. Its science payload was developed under direction of the Boulder-based Southwest Research Institute — Stern’s home institute — with instrument contributions from SwRI, APL, NASA’s Goddard Space Flight Center, the University of Colorado, Stanford University and Ball Aerospace & Technologies Corp.
New Horizons is equipped to capture visible and near-infrared images of Pluto as it passes within 12,500 kilometers of its surface. “Best of” pictures enabled by New Horizon’s Ball Aerospace-built Ralph instrument are expected to reveal surface details on Pluto as small as 60 meters across.
Also to be imaged during the July 14 encounter is Charon, the largest of Pluto’s five known moons.
The full data set from the flyby will take more than a year to trickle back to Earth due to the probe’s extreme distance and limited power.
After whisking through the Pluto system, an extended mission will send New Horizons toward possible multiple encounters with Kuiper Belt Objects, small icy bodies that reside in the enormous region of space that begins 1.6 billion kilometers beyond Neptune’s orbit.
“We are really reaching out into the unknown,” said Lisa Hardaway, the Ball Aerospace program manager for Ralph.
Building the multispectral and near-infrared hyperspectral imaging instrument meant tackling a set of daunting challenges, she said. Ralph weighs only 10.3 kilograms, operates on roughly seven watts — the power of a typical incandescent night light — and uses no moving parts.
“We knew that we were going to be power-constrained from the get-go. So the name of the game was reduce power, reduce mass as much as possible. Doing so meant that we could extend the life of the whole mission,” Hardaway said.
Adding to the tough engineering requirements, she said, electrical, electronic, and electromechanical parts had to endure the nine-year cruise — a sojourn of coldness and severe radiation — and then deal with low light levels at faraway Pluto.
“We had a constraint on just about every system,” Hardaway said.
Stern said that Ralph is the mission’s “remote sensing powerhouse.” Its 250-meters-per-pixel resolution “is just going to go through the roof.”
Ralph was named after the character, Ralph Kramden, in the 1950s “The Honeymooners” television show. And like the TV character, Ralph is paired with Alice, an ultraviolet imaging spectrometer onboard that will inspect Pluto’s atmosphere.
For Hardaway, dealing with the low light levels at Pluto was fairly unique. “Light is about 1,000 times dimmer on Pluto than it is on Earth. We had to make sure our optics could handle that,” she said.
There was yet another facet to New Horizons that everyone on the project faced – time. “The schedule was _very_ compressed … a big hurdle,” Stern said. “We got our first funding in January 2002 and we launched in January of 2006. Achieving that was a huge challenge.”
Seconding that aspect of putting New Horizons — and Ralph —together is Jim Baer, a staff consultant optical engineer in Ball’s optics and detector engineering department who has been designing, building, and testing optical systems and instruments at the Boulder-based company for nearly thirty years. He led the optical engineering for Ralph.
“Time was a big factor. We had a job to do with very little time,” Baer said, recalling Ralph’s 22-months-to-launch development schedule.
Space was another constraint. Packing all the components within Ralph’s housing “was a little like building a ship in a bottle,” he said.
Cold optics at ground zero
Baer said housing Ralph’s optics required coping with extreme cold. The instrument consists of three black-and-white and four full-color imagers with telescopic resolution 10 times better than the human eye. Great care was taken to assure that the focal length of the instrument and its “prescription” has stayed the same on the long cruise. That “cold optics” demanded use of similar materials that all shrunk together, he said, to assure Ralph stays in sharp focus.
“People ask me what I do for a living. My response is that I make disposable cameras,” Baer said. “But working on Ralph I tell them that it’s taking color pictures of Pluto that will go in the textbooks … for everyone to look at them forever. It’s the capstone project of my career. In a billion years, this instrument will look just like it did when it left our hands. It’s truly our message to infinity,” he said.
SwRI’s Stern said that he hoped New Horizons “would make people feel like America is on its game.” However, getting the first spacecraft to Pluto was hard-fought from 1989 to 2003, he admitted, witnessed by 14 years of the scientific community repeatedly pushing for such a mission.
“If the Pluto mission had been a cat, it would be dead long ago because cats only get nine lives. This mission went through so many iterations … people just wouldn’t take no for an answer,” Stern said. “And now we’re at Pluto’s door step. This is ground zero … this is what we came for,” he said.