For a moment four years ago, the spacecraft Cassini watched Earth from 900 million miles away. The probe had ducked behind Saturn. There, shielded from the sun's rays, the robot turned its delicate lenses toward home. On July 19, 2013, Earthlings in the know waved and smiled for the paparazzo in the sky. Everyone else went about their day. Cassini, a gracious photographer, caught the entire Earth on camera anyway.
Cassini will end its 20-year mission on Sept. 15, burned up in Saturn's atmosphere. The mission has been a major success. Cassini landed the Huygens probe on Saturn's moon Titan and sensed hydrogen in the icy plumes of Enceladus. The spacecraft took half a million photos while orbiting the sixth planet, capturing Saturn's rings and superstorms in unprecedented detail.
But perhaps no other Cassini photograph carries the emotional heft of the Day the Earth Smiled.
The greatest space photos are named. Apollo 17 gave us Earth as the Blue Marble. Voyager 1's portrait of our planet from beyond Neptune is the Pale Blue Dot. The Pillars of Creation show the hydrogen spires of the Eagle Nebula. Astronomer Carolyn Porco, the leader of the Cassini imaging team who conceived of the July 2013 photo shoot, decided to name the view from Saturn after the beaming residents of Earth.
Porco and her colleagues organized a campaign to smile into the void at 21:27 Coordinated Universal Time (accounting, of course, for light's 15-minute dash from Earth to Saturn). It would be only the third time that Earth had been photographed from such a distance, after an earlier Cassini image and the Voyager portrait. It also marked the first time that Earth inhabitants knew they were being photographed from the outer solar system, beyond the asteroid belt.
"This could be a day, I thought, when all the inhabitants of Earth, in unison, could issue a full-throated, cosmic shout-out and smile a big one for the cameras from far, far away," Porco wrote in June 2013.
"The experience itself was both exciting because so many people were interested in the event, but also a bit scary because we wouldn't know until afterward whether the images worked," said Matthew Hedman, a physicist at the University of Idaho who was involved with the project but spoke only on behalf of himself.
Zooming in on the Day the Earth Smiled, you can see sunlight scattering through Saturn's E ring, pointed out Italian astronomer Gianrico Filacchione in a recent article in Nature Astronomy. (NASA's Jet Propulsion Laboratory notes this is a composite of images taken with green, red and blue filters, which when combined offers a view in natural color.) Brighter are the F and G rings, which contrast against the darkness of Saturn's night.
It was a view of Saturn impossible to get from Earth. Cassini photographed what was, essentially, an eclipse of the sun by Saturn, Hedman said. Our planet shines beneath those massive rings. Earth is a pinprick - at that distance, the image scale is 53,820 miles per pixel, and the Earth's diameter is just under 8,000 miles.
"Among the numerous images returned by Cassini," Filacchione wrote, "this is the one that for me best collects the richness of Saturn's system."
The images came back in to Earth in pieces. "We were seeing glimpses of the entire system, one image showing Tethys, another showing some of the main rings, another showing Enceladus in the E ring, and of course a few showing Earth near the rings," Hedman said. As the scientists assembled the images into a mosaic, Hedman could see light through the broad sweep of the E ring, made of microscopic particles expelled from Enceladus. "I guess it was kind of like putting [together] a jigsaw puzzle," he said.
The picture of Earth wasn't the only image taken that day. The Cassini team ultimately stitched together 141 photos into a sweeping view of Saturn, a mosaic 404,880 miles across. Shot from the back, Saturn is a black ball suspended in ink, enclosed in the coffee-colored circles of its rings.
"On the one hand, it is a beautiful image that will serve as a reminder of all the great data Cassini obtained," Hedman said. "And on the other, it contains a lot of information about the properties of the rings that we will be trying to understand for many years to come."