The collection of amateur astro photos posted on the internet could make up a vast untapped sky survey to rival even professional telescopes. A new study uses photos culled from an online image search to reconstruct the orbit of a comet.
"This is one area where groups of amateur astronomers – distributed, diverse, and taking images at rapid cadence and with good time resolution – can contribute data that are complementary to what the big telescopes produce," astronomer Dustin Lang of Princeton University told Wired.com in an e-mail. "We're thinking about what we could discover if we could slurp in every astronomical image on the web."
Two years ago, Lang and colleagues launched astrometry.net, a website that automatically analyzes the patterns of stars in astronomical images and uses them to figure out which part of the sky the image shows. The site generated a lot of data with the lofty goal of helping "organize, annotate and make searchable all the world's astronomical information," but Lang wasn't sure how all that information would be useful.
"With that hammer in hand, we started looking around for nails," he said. "One idea was to look at calibrating data that otherwise wouldn’t make it into the scientific realm."
On April 1, 2010, Lang and New York University astronomer David Hogg searched online images for photos of Comet 17P/Holmes, which famously exploded in October 2007 and briefly became the brightest object in the sky.
Their search returned 2,241 unique images, of which 1,299 actually turned out to be usable photos of Holmes. The others included diagrams, charts or illustrations of the comet's orbit, as well as the moon, a Greek statue and two pictures of cats.
The researchers used astrometry.net to determine the images' positions on the sky. But to figure out which part of the image contained the actual comet, Lang had to come up with a model of astrophotographers themselves. For instance, most photographers place the interesting object in the center of the frame, so Lang's algorithm looked for the comet there first.
"Because this set of images was collected by photographers, you need to be thinking about how they decide to point their cameras," Lang said. "It’s not like we designed the experiments to collect data. We need to kind of bottle the fact that people are doing the observing."
Next, the researchers laid the images carefully on top of each other to create a montage of the comet moving across the sky.
When they compared the resulting orbit to the one published by NASA's Jet Propulsion Lab, it fit pretty well, the researchers reported Friday on arXiv.org.
Lang and Hogg are well aware of the implications of accessing the images and posting their paper on April 1.
"Our paper is sort of an April Fool's Day joke," he said. "It has become a tradition – papers that have an aspect of silliness, or a punch line, but also real content."
But, he added, "All the results are real, and while this result per se is intentionally a bit silly, we're exploring the extent to which we can do real science using very heterogeneous data of unknown provenance."
Rediscovering the comet's orbit without any conscious effort from the photographers is "really, really, really cool," said astronomer Kevin Schawinski of Yale University, who co-founded the crowdsourced science powerhouse Galaxy Zoo but was not involved in the new work.
Galaxy Zoo relies on nonscientist volunteers who devote their own time to categorizing images of galaxies. By contrast, the astrophotographers who snapped shots of Comet Holmes had no idea they were participating in a citizen-science project, and most still don't.
"It shows you what you can do with a large crowd of people. Even with unwitting participants they did some really incredible science," Schawinski said. And as with Galaxy Zoo – which has veered from galaxies to supernovas to old weather data – the same methods used for Comet Holmes could be applied to other photos of the natural world.
"I think there’s enormous potential to essentially create a massive online-based collaboration to do science," Schawinski said.
Images: Dustin Lang and David W. Hogg.
*Citation: "Searching for comets on the World Wide Web: The orbit of 17P/Holmes from the behavior of photographers." Dustin Lang and David W. Hogg. arXiv.org, 2011.