Thanks to a camera affectionately known as “Ralph,” NASA’s New Horizons spacecraft has delivered stunning natural-color images of Pluto and its moons.
Black-and-white pictures coming from New Horizons’ Long Range Reconnaissance Imager (LORRI) have been combined with Ralph’s color data to create sharp, higher-resolution images of Pluto — a planet that has remained a mystery for some time.
The color images have allowed the New Horizons scientists to investigate both Pluto’s atmosphere and surface topography, including mountain ranges, canals, and craters.
In some ways, the Ralph camera is simple. Ralph has a 75-mm lens aperture, weighs about ten kilograms, and operates on an estimated 7 watts. The camera, essentially, is a three-axis telescope with primary, secondary, and tertiary mirrors.
Designing for the harsh conditions of Pluto, however, made the device more complex.
Pluto, for example, is about 1000 times dimmer than Earth, and the planet’s low light conditions presented challenges for Lisa Hardaway, lead systems engineer and program manager at the Boulder, CO-based Ball Aerospace and Technologies Corp., and her design team.
The instrument, having to make the most out of the available photons on Pluto, is very susceptible to the light reflected by the Earth, Sun, and the moon. Ralph therefore had to be covered during much of the New Horizons launch.
“All of that light could actually saturate or fry the detector,” said Hardaway.
A door — Ralph’s only moving part — shielded the detectors and was opened after New Horizons had passed Mars, a distance where sunlight is weaker.
The camera and its electrical parts were also designed specifically to withstand thermal radiation and the nine-and-a-half-year cruise to Pluto.
Ralph’s focal length, for example, needed to remain the same, and not shrink under the planet’s extreme cold conditions.
To achieve a consistent focal length, the engineers designed the optical bench and mirrors out of the same material: aluminum. In the ice-cold temperatures of Pluto, the mirrors and the chassis shrank, but did so together so that the focal length did not change.
The Two Sides of Ralph
There are two sides to Ralph: the Multi-spectral Visible Imaging Camera (MVIC) and the Linear Etalon Imaging Spectral Array (LEISA). LEISA forms images in every infrared “color” between 1.25 and 2.5 microns; for each pixel, a spectrum can be assembled and each chemical’s “fingerprint” identified.
The Multi-spectral Visible Imaging Camera includes three separate systems. A framing camera captures pictures in black and white, mostly for navigational purposes, said Hardaway.
A Time Domain Integration (TDI) panchromatic imager creates a picture that is 5.7° wide, and as long as needed.
The third system, a color TDI imager, works like the panchromatic imager but has four lines, each with a different “color”: blue, red, near-IR (NIR), and a wavelength strongly absorbed by methane.
The New Horizons team can then take the four colors of the Ralph image and make a reasonable approximation of a color image as a person would see it.
Ralph also utilizes its multiple bands to characterize the gases in Pluto’s atmosphere. Each pixel strip picks up on specific wavelength bands.
In late July, the New Horizons spacecraft used Ralph’s compositional data to discover nitrogen, carbon monoxide, and methane ices on Pluto.
The Ralph camera sits right next to “Alice,” another instrument named after a character from the popular 1950’s sitcom, The Honeymooners. Alice, a UV spectrometer, takes its own images and combines Ralph’s data into one photo.
The color data coming from Ralph has delivered its share of unforeseen discoveries, even to its own engineers. Hardaway, for example, expected to see more impacted craters on Pluto.
Hardaway was also surprised to learn that Pluto and its moon Charon exhibited different colors, demonstrating that the two bodies were made from different materials and debunking a popular idea of Pluto’s origins.