Sodium glue of Mercury

Image credit: NASA

The largest comet in the Solar System is actually a planet. It is Mercury and sometimes its observation is possible

Researchers have known for years that Mercury has a huge tail. Last week, Andrea Alessandrini photographed her from the balcony of his home in Veroli, Italy.

“I took the picture on May 5 using a 66mm (2.5-inch) refracting telescope and a Pentax K3-II camera,” says Alessandrini, an amateur astronomer who works by day as an aerospace engineer. “This is a 7 minute exposure at ISO 1000.”

First preceded in the 1980s, Mercury’s tail was discovered in 2001. Its origin is Mercury’s super-thin atmosphere.

Mercury is so close to the sun that the pressure of sunlight can push atoms out of the atmosphere into space. The escaping gas forms a tail more than 15 million miles long.

The key to detecting the tail of Mercury is sodium. There are many elements in the tail of Mercury; sodium is just one. But because sodium is so good at scattering yellow light, it is the best element for tracking the long column of gas. “I use a special 589 nm filter tuned to the yellow glow of sodium,” says Alessandrini. “Without that filter, Mercury’s tail would be invisible.”

NASA’s MESSENGER spacecraft spent years observing Mercury’s tail closely.

For reasons related to the Doppler shift of sodium absorption lines in the solar spectrum, Mercury’s tail is brightest when the planet is ± 16 days from perihelion (the closest approximation to the sun). Read the investigation in “Final Reference”.

That special date is this week: On May 13, 2021, Mercury will pass 16 days after perihelion and the tail could be up to 10 times brighter than what Alessandrini saw last week. Coincidentally, that same day the crescent Moon will pass by Mercury in the evening sky.


Imaging the sources and full extent of the sodium tail of the planet Mercury. Jeffrey Baumgardner, Jody Wilson and Michael Mendillo. American Geophysical Union / Geophysical Research Letters.