So you’ve waved at Saturn and had your picture taken by Cassini from nearly 900 million miles away. Now what?
At the time of the long-distance photo shoot, Earth and Cassini were about 898,500 million miles apart, which means it will take 1 hour, 20 minutes and 24 seconds for the photons that will go into the image to reach Cassini’s wide-angle camera.
"If we are to capture your photons, we would need you to be waving 80.4 minutes prior to your photons reaching the spacecraft cameras," Scott Edgington, deputy project scientist for the Cassini mission at the Jet Propulsion Laboratory, explained in this blog post. “That's how we came up with the Earth-waving window,” which was between 2:27 and 2:42 p.m. Pacific time.
If that sounds like a long delay, consider how much longer it would take for something besides light to cover that distance. In a webcast from the JPL mall in La Canada Flintridge, the lab's Gay Yee Hill put things into perspective:
* A 747 jet would take 180 years to fly to Saturn.
* A cheetah, the fastest animal on land, would need 1,467 years to run to Cassini’s location.
* An average human would have to spend 25,667 years, 128 days and 6 hours to cover that distance on foot. It’s the equivalent of making 18,075 round trips between the Earth and the moon.
The image containing Earth will be combined with roughly 30 others and pieced together into a mosaic of the entire Saturn system. Only the tiniest bit will include the planet where Cassini was built.
“The Earth’s photogenic image will span approximately 1.9 pixels across,” Edgington wrote. “However, since not all of Earth is illuminated at this time — less than half — the pale blue shine that we'll see of Earth will be less than a pixel.”
JPL expects it will take a few days to process the part of the mosaic that includes us, and the full picture won’t be ready for several weeks.
Joseph Burns, a member of the Cassini imaging team based at Cornell University, said the final image will be valuable to scientists because Cassini and the sun are on opposite sides of Saturn. That means the planet and its rings will be illuminated from behind with forward-scattered light instead of lit directly with reflected light, as is usually the case.
With forward-scattered light, “we get to see these very small particles and see quite a different view of the planet and its surroundings,” he said during the Wave at Saturn webcast. Those particles can be as fine as dust, smoke or cooking flour, he said. The last time Cassini took a back-lit image, in 2006, scientists discovered three small moons, he added.
Enthusiasts who smiled for Cassini’s camera are encouraged to share their photos on social media with the #waveatsaturn hashtag. You can also print a personalized “I Waved at Saturn” certificate by going to this JPL site.
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