Mercury Appears With Spectacular Comet-Like Tail in Night Sky

Comets aren't the only celestial object that can carve a stunning tail across the sky, as pictures snapped of Mercury as it passed close to the sun show a long tail dragging out behind the planet.

The tail was captured by astrophotographers in mid-April, including Sebastian Voltmer, who snapped a picture of the comet-like tail from Spicheren in northeastern France.

"Comet-like Mercury on April 12, 2023 (Yuri's Night)," he captioned his Instagram post. "The solar wind and micro-meteorites eject sodium atoms from Mercury's surface. This creates a yellow-orange tail of sodium gas that is around 24 million kilometers [15 million miles] long."

Yuri's Night commemorates the first human in space, Soviet cosmonaut Yuri Gagarin, who orbited the earth on April 12, 1961.

Comets are the most famous tail-owners in space, with passing comets recognizable by the streak of light trailing out behind them.

Comets orbit the sun, often in large, elliptical orbits, spending most of their time out in the cold outer solar system and passing by Earth's orbit as they approach the sun. As the comet heats up, its surface begins to melt, streaming ice and dust behind it in its wake.

"This leaves it with lots of loose particles, both charged and neutral, that come away from it while it is moving," Ian Whittaker, a senior lecturer in physics at Nottingham Trent University in the U.K., told Newsweek in January. "The neutral particles come off in a cone behind the comet as it moves—a bit like being behind a big truck on the motorway while it's raining, all the excess water hits whatever is directly behind it."

Mercury, the first planet from the sun, also has a tail, which forms for a similar reason as it reaches its closest point to the sun. Scientists first predicted Mercury had a tail in the 1980s, and the tail was confirmed to exist in 2001, with more details having been learned by NASA's Messenger mission that orbited the planet between 2011 and 2015, NASA explains.

The tail was found to shorten and lengthen based on how far the planet is from the sun. Mercury's distance from the sun varies greatly during its orbit, between 28.5 million miles at its closest point (perihelion) to 43.4 million miles at its furthest (aphelion). At perihelion, its tail is its longest, reaching around 15 million miles long.

The tail is formed by Mercury's surface being sheared off behind it by solar winds and meteor impacts. While comprised of a number of different elements, sodium is the element responsible for the characteristic yellow color of the tail, as it scatters most of the sunlight hitting it.

The tail is not visible to the naked eye, however, which is why it took scientists so long to discover it. Instead, special filters that only allow the wavelengths of the sodium to glow through are required to spot the tail in its full glory.

"I captured this stacked CCD image from my backyard using a 135 mm lens and a 589 nm filter," Voltmer wrote in his Instagram post.

Mercury's tail gets longer and brighter as it approaches its perihelion, or its closest point to the sun in its 88-day orbit, due to increasingly intense solar winds battering its surface. However, the tail appears brightest from Earth at 16 days before or 16 after perihelion.

Mercury's last perihelion was on April 1, and therefore it was 16 days past perihelion on April 17. The next perihelion will occur in late June, with the tail next appearing the brightest in July.

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