The U.S. Postal Service is producing a separate stamp issuance on Pluto called "Pluto—Explored!" It will focus on NASA’s New Horizons mission that in 2015 succeeded in undertaking the first exploration of this distant body by a spacecraft.
When a University of Arizona planetary scientist persuaded NASA to point the Hubble Space Telescope at Jupiter and Uranus to gather clues about the atmospheres of the two giant gas planets, he had no idea that his images might end up years later in mailboxes across the nation and possibly the world.
On March 28, 2004, three of Jupiter's largest moons — Io, Ganymede and Callisto — came together in a rare alignment, casting their shadows onto the Earth-facing side of the planet to form a triple eclipse.
Erich Karkoschka, a senior staff scientist in the UA's Lunar and Planetary Laboratory, had been waiting for this moment and he was ready. NASA had granted him 40 minutes — the viewing time offered by Hubble orbiting Earth once — to observe how the three moons danced around their host planet.
"Having three shadows and two moons transiting the disk of Jupiter at the same time is very rare," Karkoschka said. "It hadn't happened for 60 years when I made the proposal to use Hubble to observe this event."
Hubble took that image with its "infrared eyes," a camera developed by a team of UA astronomers and engineers, using three different optical filters. Because human eyes can't see infrared light, those wavelengths are translated into false-color renditions of blue, green and red.
"You can make the different features visible that way, and once you are able to see those differences, you can do better science as well," Karkoschka explained. "For this particular color image, the rationale was mostly to get a pretty image of this rare event."
Most images of Uranus, like those taken by the Voyager 2 space probe during its flyby in 1986, revealed a hazy, pale blue disk that looked rather bland and not too exciting, said Karkoschka, who took his image of Uranus in 2003, this time using Hubble's Advanced Camera for Surveys.
Because his image combined three filters, he was able to get a more colorful and revealing rendition of features in Uranus' atmosphere, such as three bright red glowing clouds in the planet's northern hemisphere.
Together with his equally colorful image of Jupiter, Karkoschka's Uranus picture features in a special edition of stamps, "Views of Our Planets," which the U.S. Postal Service officially will release on Tuesday at the World Stamp Show, the world's largest philatelic event, held in the U.S. only once every decade.
"During the modern era of space exploration, the planets of our solar system have been viewed with increasing clarity, thanks to the distant voyages of unmanned spacecraft and the development of ever-more powerful telescopes," according to the design brief. "With this pane of stamps, the Postal Service showcases some of the more visually compelling full-disk images of the planets obtained during this era."
Said Karkoschka: "I was granted one orbit in one specific year, and so I looked for the best occasion in that year, and then I realized if it was going to be on that date, I could get something that really doesn't happen very often. So I was kind of lucky."
Karkoschka has observed the giant planets on telescopes around Tucson and in Chile, and using the Hubble Space Telescope, to understand the structures of their atmospheres. From this work, he determined the vertical and horizontal distribution of hazes, clouds and methane.
"On the gas giants, there is no surface anywhere close to where we can see," he said. "There may be a very small, solid core, perhaps similar in size to Earth, while most of the outer part is hydrogen, helium and other gases including methane."
On all those planets, Karkoschka has been researching the cloud levels and what the clouds are like. Once he had images taken over several years, he could see how the clouds and their altitudes changed.
Jupiter's clouds consist mainly of ammonia crystals, while similar features spotted on Uranus are made up of frozen methane. Because it takes Uranus more than 84 Earth years to complete a trip around the sun, studying seasonal changes takes generations of researchers.
"In the 1990s, we were seeing the southern hemisphere, and it didn't look too interesting," Karkoschka said. "But with the equinox in 2007, more of the northern hemisphere came into view and there were really unique clouds that appeared red in those images. Those clouds were the first really interesting features that we could see on Uranus."
Gradually, atmospheric details first spotted by Voyager 2 became more and more interesting, and tracking them allowed Karkoschka to accurately determine the planet's spin for the first time.
"We really could see activity on Uranus, and that was exciting," he said. "The bluish part is the southern hemisphere, and the three red clouds are in the northern hemisphere, rising above the general haze spreading out below them. In the meantime, we have seen even brighter clouds. You can follow them for a few weeks or months, but after that they change or disappear."
Scientists are still a long way away from understanding the causes of those changes.
"The northern hemisphere of Uranus appears to be much more active now than the southern hemisphere," Karkoschka said. "We don't know why. If we wait for another 40 years for the season to change, and if then only the southern hemisphere is active, we know it's a seasonal effect. But if the activity is still only in the northern hemisphere, then we'll know there really is some intrinsic asymmetry at work."
By then, mailing letters may well be a thing of the past, with the stamps depicting our planets having found their way into stamp collections around the world.
Watch the three moons move in front of Jupiter during the "triple eclipse" captured in the image featured on the stamp. This animation was created based on the images taken during the event. While Hubble took 20 images during the event, the hundreds of images needed for the animation were created from the original images using the measured rotation of Jupiter and the motions of the satellites and their shadows. (Credit: NASA, ESA, E. Karkoschka and L. Barranger)