Oil continues to gush from a well underneath the Gulf of Mexico at a rate of more than 200,000 gallons per day. Some newspapers have noted that the spill is now visible from space, after NASA released satellite images of the oil slick, which reached 40 miles in width over the weekend. (It is now 130 miles across.). How big does something have to be before we can see it from space?
Just a few inches across. The familiar trope that something is so big that it's visible from space has no technical significance. It depends on both how far from Earth the observer is located and what viewing tools he's using. The best commercial satellites can currently pick out objects down to 16 inches across—about the size of a home plate in baseball—from more than 400 miles above earth's surface. (The resolution isn't good enough to identify the object as home plate, but merely to detect its presence.) And the Pentagon's satellites can reportedly see down to 6 inches, although Uncle Sam won't officially disclose the resolution of his cameras. NASA's oil spill pictures are notable not because they mark the spill's visibility from space but because they were taken by a moderate-resolution imaging device designed to detect large-scale environmental disasters like wildfires and volcanic eruptions. The two radiometers that snapped the oil spill pictures record at 1 kilometer and 15 meter resolutions, from about 438 miles above earth.
The oil spill can surely be seen from space by the naked eye at this point. By convention, space begins about 62 miles above Earth's surface. From twice that distance, astronauts can pick out trains. From the International Space Station, floating approximately 250 miles overhead, you can see a number of manmade objects, including the Pyramids at Giza and caravans of cargo ships. (The Great Wall of China is visible, but it's not the easiest structure to spot.) Residents of the space station can even see light reflecting off of individual oil rigs, so the oil spill was surely evident to them fairly early if they were in the right orbit.
The good news is that it's highly unlikely the spill will ever be visible from the moon, at 238,900 miles from Earth. None of mankind's handiwork, including the Great Wall, is visible to the naked eye from that distance.
Satellites are getting more and more powerful every year. The resolution of the best commercial satellites has improved by 50 percent in a decade. In a couple of years, commercial satellites will be able to see objects less than 10 inches across. Unfortunately, the technological improvements will make little difference to civilians. Earth-imaging companies operate under contracts with the government that force them to degrade their pictures to a peak resolution of one-half meter, or 19.7 inches, before selling them to private customers.
Interested in ordering a satellite image? Just contact a private Earth-imaging company like GeoEye and tell them where you want them to aim their lenses. Every spot on Earth passes within their field of vision within three days, and they can pivot the camera in a 60-degree field. A shot of your neighborhood will run you around $12 per square kilometer, with a minimum order of 100 square kilometers. You might also be able to buy imagery out of their archives, which holds pictures covering 420 million square kilometers.
Got a question about today's news? Ask the Explainer.
Explainer thanks Mike Abrams of NASA's Jet Propulsion Laboratory and Mark Brender of GeoEye, Inc. Thanks also to reader Steve Hoefer for asking the question.