Two satellites peering at the sun have snapped photos of Mercury's long, comet-like tail, but it took an amateur astronomer to bring the pictures to light.

The twin satellites are part of NASA's Stereo (Solar Terrestrial Relations Observatory) mission. They zip around the sun in Earth's orbit, one behind our planet and one ahead of it. Their main job is to monitor the sun and its atmosphere.

Since Mercury is so close to the sun, the planet occasionally wanders into the satellites' photos. Some of these images show a long, comet-like tail streaming off the planet, away from the sun. [New photo of Mercury tail.]

Astronomers didn't notice the tail in the photos right away. But it didn't escape the eyes of Australian medical researcher Ian Musgrave.

Musgrave was poring over an Internet database of images when he discovered Mercury's wispy tail. He then asked scientists at Boston University to take a look, university researchers said.

The scientists presented their findings at the European Planetary Science Congress meeting in Rome today (Sept. 22).

Mercury's tail

Scientists have known for years that Mercury has a long tail. From Earth it can be seen by analyzing light from sodium gas sputtered off Mercury's surface. The sun's intense radiation pressure pushes many sodium atoms off into space, creating a tail that extends far beyond the planet.

Mercury also has several smaller tails made of other gases. NASA's Messenger satellite recently detected these as it flew by Mercury in preparation to orbit the planet, researchers said.

But there are many details still to be worked out about these mysterious tails, and the new solar-satellite data should shed some light, researchers said.

"What makes the Stereo detections so interesting is that the brightness levels seem to be too strong to be from sodium," said study researcher Carl Schmidt of Boston University.

What's it made of?

The current focus of the team is to sort out what the gases in the tail might be.

The researchers are working to refine their brightness-calibration methods, and they're trying to determine the precise wavelengths of light that would get through the Stereo cameras' filters.

"The combination of our ground-based data with the new Stereo data is an exciting way to learn as much as possible about the sources and fates of gases escaping from Mercury," said researcher Michael Mendillo, also of Boston University.

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