(ORDO NEWS) — The largest comet in the solar system is actually a planet. This is Mercury. Researchers have known for years that Mercury has a huge tail. Last week, Andrea Alessandrini photographed him from the balcony of his home in Veroli, Italy.
“I took the picture on May 5 using a 66mm (2.5”) refractive telescope and a Pentax K3-II camera, ”says Alessandrini. “This is a 7 minute exposure @ ISO 1000.”
First predicted in the 1980s, Mercury’s tail was discovered in 2001. Its source is the super-thin atmosphere of Mercury. Mercury is so close to the Sun that the pressure from sunlight can push atoms out of the atmosphere into space. The escaping gas forms a tail over 24 million kilometers long.
The key to finding the tail of Mercury is sodium. There are many elements in the tail of Mercury; sodium is just one of them. But because sodium scatters yellow light so well, it is the best element to track a long plume of gases. “I use a special 589 nm filter tuned to sodium yellow,” says Alessandrini. “Without this filter, the tail of Mercury would be invisible.”
NASA’s MESSENGER spacecraft has observed Mercury’s tail at close range for several years. This film shows that the brightness of the tail changes as Mercury orbits the Sun:
For reasons related to the Doppler shift of sodium absorption lines in the solar atmosphere, Mercury’s tail is brightest when the planet is ± 16 days from perihelion (closest approach to the Sun).
This special date comes this week: On May 13, Mercury will pass perihelion in 16 days, and the tail could be 10 times brighter than Alessandrini saw last week. Coincidentally, on the same day, the crescent moon will pass Mercury in the evening sky.
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