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

Here sits the white dome of an observatory that runs the Mount Lemmon Survey (MLS). The survey is part of the Catalina Sky Survey - one of the most prolific surveys for detecting near earth objects. On January 3, the survey first detected a tiny moving dot on its computer screens, shifting it's position against a fixed background of stars.

Mount Lemmon Survey Observatory at dusk. Source: Wikipedia

At first it seemed to be an asteroid moving through space at 4.5 AU (418 million miles) from the sun, which is 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 daylight to be worthwhile. I decided to try capture it again.

This year, I happened to be in Boise, Idaho in the week of Oct. 21 for work. Last year, at the same time, I was face to face with another naked eye visible comet - Atlas/Tsuchinshan in the streets of of the same city where I had travelled for work. While history does not repeat itself, it rhymes. I did not get a chance to seek it out at Boise after all. My first opportunity to see it this year, visually through binoculars, was on the following weekend, from my own backyard in New Jersey.

For the photo, I wanted to capture some part of the landscape (rooftops of my neighbors' houses) along with the sky, so I used a regular DSLR lens with a moderately wide view (75mm). To avoid start trails, I also used a tracking mount - the SkyWatcher Az/Gti. The software stacking process I used after, caused the edges of rooftops in each frame to overlap creating a dreamy blur effect for the foreground. However it was already at a low separation from the sun. Shortly after astronomical twilight, which is within the next hour after sunset when the brighterst stars come out, it was dipping behind the roofs of neighboring houses. I decided to try again with a more magnified view.

The next few days were cloudy but on the 1st of November, I got the chance to shoot it again. This time with a small telescope, a 430 mm refractor (Apertura 72 ED). The image would be 6x higher magnification from the prior. The comet is clearer in view but obviously offers less context in terms of a frame of human reference.

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 unoffcial 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? Which socio-economic change? Will humanity flourish into being a multi-planet explorers reaching for the stars or regress into being a myopic, inward looking territorial species like on it's last trip? Will they recognize this comet by name or will all be forgotten and it be a rediscovery?

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