Backyard Voyages: ATLAS Shrugged

Stop!

Is this really it?

I stopped walking, squinting to focus, eyes scanning the area again cautiously. In all these years of looking, the moment of the find has not changed much in character. An uncomfortable mix of uncertainty and optimism, maybe even a subconscious holding of breath in case I disturbed the universe, usually culminating in a moment of relief.

That indeed was it.

Like a faint apparition in the nautical twilight of Boise, Idaho, beyond an empty parking lot next to a tree, hung the unmistakable form of a comet. My first naked eye comet with the ungainly name of C/2023 A3 Tsuchinshan-ATLAS (more on that later).

Comet C/2023 A3 as seen from Boise, Idaho on Oct 14th (handheld, cellphone shot)

It was a dark, deserted street corner two blocks from my hotel. I was visiting the unfamiliar town all week for work. Past sunset - and while I took dinner served in the room watching a Netflix show, Venus shone low in the western sky visible from my hotel window like a guiding light. This was the week of best visibility of the comet, and having missed it in the morning skies in the days prior, I was keen on finding it now, since by the time I would be back home again, it would be past prime. The adjacent part of the sky where it was supposed to be, was obscured by buildings. As the sky darkened after sunset I decided to take a walk - to find this slightly darker nook away from street lights around my hotel with a clear line of sight to the west. It paid off.

I took out my phone, because that’s all I had with me, and took a few pictures. The one here is the first shot, taken handheld, upon seeing it with unaided eyes.

Over the last two weeks my social media feed has been lit up with beautiful pictures of the comet. Breathtaking images of it with backgrounds of vast flats, rugged mountains, reflective lakes, the Milky Way, and some creative ones pretending it to be their flashlight, a tennis ball or cigar smoke. In contrast, my first picture of this comet is mundane, in front of a traffic STOP sign. That evening of Oct 14, I took several “cleaner” shots, but In hindsight, that first one with the awkward stop sign is what makes it special. It reminds me that once in a lifetime celestial visitors on an 80,000 year long journey can show up quietly, not only at exotic locations but also in our ordinary neighbourhoods, at home or wherever life may take us.

Sometimes all we got to do is stop for a moment and look.

Sentinels

The first to discover this visitor was Tsuchinshan or Purple Mountain observatory in Nanjing, China in 2023. It was then picked up a month later again by a telescope in South Africa which is part of an early warning network system known as ATLAS - Asteroid Terrestrial impact Last Alert System. The comet bears the names of both.

While Purple Mountain is a standalone observatory, ATLAS is a system involving 4 sites, two in Hawaii and one each in Chile and South Africa. Each site has a robotic telescope that scans the night sky on clear nights looking for space anomalies that may threaten our existence.

Robotic sky surveys are not new. There are even hallowed ones such as the Sloan Digital Sky survey that serve different purposes such as mapping deep space in three dimensions. But the NASA funded ATLAS commissioned in 2015 is specially built to look for NEOs (near earth objects).

At great distances, comets and asteroids look just like stars or planet in the sky, as dots, though dimmer, shining only in reflected light. The amount of light they reflect depends on a property known as albedo. In other words, all cats are grey in the dark. To get around this, the system looks for movement. It scans large swaths of the sky in low magnification revisiting every patch of the sky up to four times per night. Computers relentlessly compare the images, looking for movement of one of the dots as the background stars stay the same.

The Anti-tail

Back home the week of October 20, I got to image C/2023 A3 using more sophisticated equipment, a DSLR camera with a zoom lens set on a star tracking mount. The comet was no longer visible by naked eye but would readily reveal itself in a pair of astronomical binoculars. Comets are known to have two tails - one is the most visible dust tail and the second is the more elusive ion tail which is a blue hued fainter tail caused by ionised gases that are created by the solar wind. As the comet rose higher up each day, entering the constellation of Ophiucus, I could see the third “tail” faintly appearing in my image. This is the anti-tail that seems to emanate from the head/coma and extends towards the opposite direction to the dust tail. This, I learnt, is debris left by the comet core on its orbital path that becomes visible when the earth crosses its orbital plane.

The comet seen on Oct 20, from my front yard in New Jersey. DSLR on star tracker mount.

Oort Origins

As newly discovered bodies are reported, their orbital path is computed repeatedly based on the observed arc of the journey. Initially it was predicted that the comet has an extremely elongated orbit with a period of 80 thousand years. More refined calculations now predict that the comet may be a one time visitor. After its current visit it may exit the solar system altogether. Based on the eccentricity of the orbit, it is believed to have originated in the Oort Cloud. A large spherical shell of icy debris that is believed to extend from the outer reaches of the solar system about 1000 times the earth-sun distance (astronomical unit or A.U.) to about 100,000 A.U., or halfway the distance to our nearest star. This cloud is estimated to contain billions of floating objects of which some like our comet, and asteroids, get perturbed and fall into an orbit around our sun.

Source: NASA / JPL-Caltech - http://photojournal.jpl.nasa.gov/catalog/PIA17046,
Public Domain, https://commons.wikimedia.org/w/index.php?curid=28366203

ATLAS Shrugged

So in the end, this ancient spacefaring visitor turned out to be a harmless beautiful comet at a safe distance and not an earthbound asteroid. On another night 60 million years ago, the dinosaurs did not get so lucky. One such innocuous looking dot was on an orbital intersect with the earth and wiped out their species from existence.

For objects on a collision path, the ATLAS system is sensitive enough to provide a one week warning for City Killers (asteroids with a size of ~20 meters). For bigger, brighter objects, fondly called Country Killers (~100 meter size), it can provide an advance warning of one month.

Will we get lucky the next time a dot shifts anomalously in the distant skies?

ATLAS shrugged.