Backyard Voyages: A6 Lemmon - A Millennial Deja Vu

Comet C/2025 A6 Lemmon, through a DSLR lens on Oct 27th, New Jersey

Date: January 3, 2025
Location: Santa Catalina Mountains, Coronado National Forest near Tucson, Arizona.
Altitude: 9,000 feet above sea level

Far from human settlements, in the forests of central Arizona, sits the white dome of an observatory that runs the Mount Lemmon Survey. The survey is part of the Catalina Sky Survey - one of the most prolific surveys for detecting near earth objects. On the night of January 3, its 60 inch Cassegrain telescope diligently scanning the night skies, detected a tiny moving dot a few pixels wide, shifting its position against a fixed background of stars.

Mount Lemmon Survey Observatory at dusk. Source: Wikipedia

At first it seemed to be a usual asteroid moving through space. At 4.5 AU (418 million miles) from the sun, it was just inside the orbit of Jupiter. At this distance from the sun, it is common for icy bodies to start releasing volatile material as they get heated up by the sun. By February, follow up observations found it to have a small condensed coma and a short tail, the tell tale signs of a comet. It was named C/2025 - A6 Lemmon.

The next task would be to compute an orbit. Orbits of comets are specially difficult to compute compared to other celestial objects since they are susceptible to planetary perturbations and also forces due to their own gaseous emissions. But fortunately there is someone who specializes in just this - Syuichi Nakano of the Central Bureau for Astronomical Telegrams (CBAT) of Harvard. Mr. Nakano computed an orbit using 117 positional observations gathered over time between Nov. 12, 2024 (prediscovery data from PanSTARRS survey) and Aug. 14. He also computed the perihelion date for this comet to be November 8, 2025, which will be a few days from the date of this writing.

Deja Vu

In October, the comet was back in the news as it was getting nearer to earth and was estimated to be naked eye visible. It was expected to be closest to the earth on Oct. 21. My first attempt at shooting it was in the pre-dawn hours of Oct 19. But it was too washed out by the rising sun to be worthwhile. I decided to try capture it again at dusk in the following days.

During the week of Oct. 21, I happened to be in Boise, Idaho for work. Coincidentally, last year, at the same time I was also in the city, and came face to face with a naked eye visible comet - Tsuchinshan-Atlas. While history does not repeat itself, it rhymes. This time I did not get a chance to seek it out while at Boise after all. My first opportunity to see this comet was on the following weekend, from my own backyard in New Jersey.

Some landscape in addition to the sky always gives astrophotos unique character. Unfortunately there are no stunning natural features visible from backyard. So unlike those pristine shots on Facebook where the comet hangs over a mountain amid rolling fields, my neighbors' rooftops would have to do for a backdrop. A moderately wide DSLR lens (75mm) helped get both sky and ground in frame. To avoid start trails from 30 second frames, everything had to be placed on a star-tracking mount. Unexpetedly the star tracker produced a special touch. During post processing the image stacking process caused the roof edges in each frame to overlap creating a dreamy blur effect which was unique. Misty dark outlines of human habitat appeared painted in watercolor bordering an evening sky with a comet.

The next clear night - 1st of November, I shot it again. This time zoomed in with a small telescope, a 430 mm refractor. The image was 6x higher optical magnification and gave me more time to shoot. The comet turned out clearer but expectedly had less context in terms of a frame of human reference. This is the image at the bottom.

Tales of a Future's Past

In a way this discovery is a re-discovery. This comet would have been seen from the earth some 1350 years ago. It was then late 7th century. At the time, the world was a very different place. Islam was expanding rapidly after the death of Muhammed. The Caliphates were on the rise and clashing with the Byzantine empire in Constantinople (modern day Istanbul). Bulgaria was newly established on the edge of the Black Sea gaining recognition from the Byzantines. The kingdom of England did not exist yet - but Christianity was spreading among the Anglo Saxons in the region. In China, Buddhism was on it's way to becoming state religion, resulting in an unofficial persecution of Christians. In the world of commerce, the banknote made its first appearance in response to the desire of merchants to avoid transporting heavy copper coinage.

On it's current visit, Constantinople no longer exists and neither do the Byzantines or the Caliphate. China is a communist country vying for top spot againt a country that did not even exist then. And people use newfangled things called computers and smartphones to transfer electronic money over the internet instead of using dirt roads and donkeys.

After perihelion on Nov 8, this comet will swing around the sun's gravity well, making its way towards the edge of the solar system again. Some of it's energy has been sapped by Jupiter so the next time it will take 200 less years to return to in another 1150 years from now, in the earth year 3175 CE.

A thousand years from now, which tales of our present day triumphs and tribulations will make it to the pages of history to be remembered by future humans? What technological advancements will leave a mark? Which socio-economic change? Will humanity flourish into being multi-planet species or regress into being a myopic, inward looking territorial tribes like on its last trip? Will they recognize this comet by name or will all be forgotten to be rediscovered again?

This cosmic visitor will know the answers for sure, on it's next earthly rendezvous.