For a quarter-hour today, some of us on Earth can look up and know that almost a billion miles away, above the sky, a set of robotic eyes is looking right back. The Cassini spacecraft will be passing into Saturn’s shadow at that time, slewing its cameras to catch the planet’s majestic rings backlit by our star.

Built, launched and operated at a total cost of more than $3 billion, Cassini will spend 15 minutes capturing a natural-color photographic mosaic of Saturn’s “E” ring, an annulus of dust and water ice fed by plumes of vapor venting from somewhere warm, wet, and potentially habitable deep within the icy moon Enceladus. Another attraction will lurk in the far distance, barely visible just outside the E ring by virtue of Saturn’s deep shadow: The blue crescent of the sunlit Earth, bearing an untold number of upturned gazes from potential fans in the continental United States, the Caribbean, Central America, or the eastern half of the South Pacific. With a bit of luck, Cassini might also glimpse Earth’s adjacent Moon.

Cassini snapped a similar mosaic in 2006, and, more than a half-century into the Space Age, there are many pictures of our planet from deep space. The difference now is that, for what may be the first time ever, the scientists planning this latest interplanetary stunt are publicizing their intentions beforehand.

The mosaic we took in 2006 was a wasted opportunity,” says Carolyn Porco, leader of Cassini’s imaging team and the mastermind of today's observations. “No one knew that picture would be taken, but if we had told them in advance, we could’ve made it a great opportunity for global celebration. People could have been high-fiving all over the planet!”

Porco, who worked with Carl Sagan and others in 1990 to snap Voyager 1’s famed “Pale Blue Dot” image of Earth from nearly 4 billion miles away, calls tomorrow “The Day the Earth Smiled.”

This will be a moment of global cosmic self-awareness, when everyone can pause at the same time and consider our beautiful, tiny planet floating through the never-ending blackness of space,” she says. “Even though the whole Earth won’t be in sunlight, everybody on the planet will still be in this photo.”

Factoring in the hour and twenty minutes that it takes light to travel between Earth and Saturn, Cassini will observe our planet from 5:27 to 5:42 pm, Eastern Daylight Time. Mission scientists plan to release a final mosaic of its observations some six to eight weeks later. Astronomers Without Borders is coordinating celebratory Saturn-observing “star parties” for the event, and NASA’s Jet Propulsion Laboratory has created a “Wave at Saturn” Web page detailing where in the sky the ringed planet will be located during Cassini’s snapshots. Porco, through her company Diamond Sky Productions, is also organizing a contest for the public.

Pretend you have encountered an alien from another world orbiting a star on the other side of the galaxy,” she says. “Now imagine you have only one picture you can show to this being to demonstrate what is special about your planet. What would that picture be? Cassini, our robot, is taking this picture of us, and we’d like to take a picture of typical life on Earth to be sent out to our fellow galactic citizens.” Anyone can submit a single snapshot, which must be taken today. Eventually, the winning image will be transmitted into the Milky Way from the world’s most powerful radio telescope, Arecibo Observatory in Puerto Rico.

This is likely the last time Cassini will observe the Earth. The spacecraft is due to take a fiery, mission-ending plunge into Saturn’s atmosphere in 2017, and researchers are scrambling to squeeze as much science as possible into its remaining time.

It’s not clear when we’ll ever do this again,” Porco says. “Missions this big and ambitious aren’t really in vogue anymore, so maybe we’ll only have a spacecraft this sophisticated out there once every hundred years. I do see this as probably our last great, gorgeous look back at Saturn and at the Earth before Cassini goes dark.”