Astro Bob: Italian amateur captures amazing photo of Mercury's tail

"Astro" Bob King is a freelance writer for the News Tribune. Read more of his work at

Comet or …? On May 27, 2020, Andrea Alessandrini used a small, 2.6-inch refracting telescope equipped with a special filter to take this remarkable photo of the planet Mercury and its glowing tail of sodium. His exposure lasted 7½ minutes at an ISO of 1000. This is a composite of three images taken over three days. (Andrea Alessandrini)
We are part of The Trust Project.

If you didn’t know better you’d guess this was a photo of a comet taken in twilight. That’s a coma and tail, right? Nope. What you’re actually seeing is the planet Mercury trailing a tail of sodium atoms! Italian amateur astronomer Andrea Alessandrini was looking for a new challenge. He wanted to try to photograph something new, not on the radar of most astrophotographers. Alessandrini recalled an article about Mercury’s sodium tail, and it got him thinking.

“I studied a little deeper and asked myself: is it possible (to photograph),” he said in an email communication.

The innermost planet Mercury orbits the sun once every 88 days at an average distance of 36 million miles (57.9 million km). (ESO)

No planet knows the sun’s ferocity like Mercury, the closest planet. Our star appears about three times larger there and almost seven times brighter. Mercury not only feels the full intensity of the sun’s radiation but also the incessant blizzard of particles that stream from it called the solar wind. With no atmosphere to sweeten the climate daytime temperatures top out around 840° F (450° C) and plummet to –275° F (–170° C) at night.


I don’t mean to imply Mercury has zero atmosphere, but it’s pretty close. The solar wind not only contributes hydrogen and helium but it also strips heavier atoms from bare rock like sodium, potassium, oxygen and calcium. Micrometeorites slam unimpeded into the surface and go poof in a haze of rock vapor, further adding to the mix.

Compare Mercury’s sodium tail in October 2008 and Sept. 2009. Differences in the pressure of sunlight depending on where Mercury is in its orbit result in changes in the size of the tail. Images from the MESSENGER spacecraft. NASA

Sodium scatters sunlight well and glows a lovely shade of yellow-orange . But what makes it form a tail? Sunlight exerts pressure. When you admire a comet you’re seeing the pressure of sunlight at work blowing back dust particles from the comet’s head into a lovely tail. In a similar manner, sunlight pushes atoms away from Mercury into a delicate tail that extends for at least 15 million miles (24 million km) behind the planet. The feature would be visible to someone standing on Mercury’s night side at the right time of year as a weak orange glow similar to a city sky illuminated by sodium lamps!

Mercury mimics a comet but can you guess what this is a photo of? Clue: it’s not what you think. A thank you to Piqui Díaz for the image.

You or I can’t just take a telescope and point it at Mercury and see its tail. It’s MUCH too faint. To attempt a photo Alessandrini needed a special filter that would block nearly all the other light streaming from Mercury and the twilight sky except for the sliver of buttery radiance from sodium atoms. He tracked the glass down at Edmund Optics , adapted it to his telescope-camera setup and then waited good weather and a favorable “apparition,” when Mercury stood high in the evening sky.

On May 27th he nailed it using nothing more than a 2.6-inch (66 mm) telescope, standard digital camera and a 7½-minute guided time exposure.


I would never even think to attempt something like this, assuming it required a professional telescope or orbiting spacecraft. Alessandrini thought otherwise. He took the risk, made the attempt and broke new ground. Just like that. We admire both his contribution to astronomy and pluck!

What to read next
Forum Communications Company recently asked subscribers to complete a reader survey. Those who participated were automatically entered for a chance to win a $250 prize. Seven lucky winners were drawn at random from all of the eligible entries. Read more to find out who won.