October, 2024
Comet C/2023 A3 (Tsuchinshan-ATLAS) – aka Comet A3 but I will use “Tsuchinshan” – eases its way through the solar system, falling toward the sun under the iron grip of gravity. The comet’s origins lie in the Oort Cloud, a vast region of ice and rocky debris at the far edge of the gravitational influence of the sun, some 2000 to as much as 100,000 times farther from the Sun than Earth, nearly one half the distance from our Sun to Proxima Centauri, our nearest neighbor star in the Centauri system. By comparison, exiled planet Pluto’s orbit varies from 30-50 times the Earth/Sun distance. An artist’s rendition might view the cloud as defining a cell-like membrane indicating the boundary of the star system it encloses.
Perhaps Proxima Centauri has a similar cloud surrounding it. Computer and mathematical simulations done by exo-planet specialists suggest that conditions favorable to Oort cloud formation are dependent on many variables – mass and age of the star; number, placement, mass and orbit of planets; stability of the star system; distance to nearby stars – and therefore the situation would be completely different for each star system. We have our version. We should ask the locals when we meet them…
The name “cloud” suggests a roughly spherical shape. We do not know for sure, as it is too far away to observe directly. The prevailing theory hypothesizes an inner puffy disk shape extending out to a spherical shell at the outer limits. It is thought to contain perhaps trillions of icy objects, all possible comets, most of which originally inhabited the early planetary spaces before the solar system was in its current stable condition. Over billions of years and gravitational interactions with the larger outer planets, early comets either crashed into those planets, the sun, or were kicked to the outer reaches of the solar system through gravitational scattering, most lost forever to interstellar space. The ones that did not quite get away created the Oort cloud.
Objects within the cloud vary in size from grains of sand to Everest size chunks of ice. To make one revolution around the sun at that distance it might take a chunk of ice anywhere from 100,000 years if it’s near the inner edge of the cloud to roughly a million years near the outer edge. Most never come within the orbit of Pluto. For those that venture close – nearly 150 have been recorded within the orbit of Jupiter – orbital periods vary between 200 years, for those that come closest to the sun, all the way up to thousands of years for those that miss the sun by as much as the radius of Jupiter’s orbit. Within the cloud, collisions and near misses between comets and nearby passing stars can alter trajectories, nudging a comet more directly toward the sun so that it’s orbital period decreases dramatically, bringing its path closer to the sun and our observations.
A closer look at Tsuchinshan’s journey: It’s not coming in on the ecliptic, the plane that all the planets move in, but from above, at an angle of about 40o, in a long elliptical path, accelerating as it falls, slowly thawing, warming, finally beginning to glow as it reaches the orbits of the inner planets. Ionized particles radiating out from the sun – the solar wind – blast the comet. Electromagnetic radiation from the sun – visible and ultraviolet light and x‑rays on the high energy side; infrared, micro and radio waves on the lower energy side – heat the icy surface to a glowing gaseous vapor that flows off the comet and away from the sun, following the outward path of the solar wind and visible to us earthbound observers as a glowing tail streaming out behind the comet. The tail always points away from the sun, even though the path of the comet does not point directly at or away from the sun. Because of this, the direction toward the sun is plainly visible to us, but the actual path of the comet must be determined over time by direct observation, then reckoned by calculation.
Tsuchinshan entered our naked eye visibility here on planet Earth in late September 2024, first appearing as a morning star that steadily gained brightness until it out-shown even Venus. It reached perihelion, the point in its orbit where it swings dramatically close to the sun and slingshots out the other side, on October 11, becoming an evening presence as it began the long voyage home to the Oort cloud. The skies appear favorable for a viewing either tomorrow or the next day.
We’ve gathered at Keith and Selma’s farm atop a west-facing hill. A few wispy clouds threaten the view but, if fortune holds, the gaps between them will expose the comet we have all come to see. Internet searches tell us to expect it around 45-60 minutes after sunset west of southwest and just above the horizon. But that’s if you live in England at 40 degrees north latitude on Greenwich time. We do not and so must translate to our local time and place. A daunting task under any circumstances. Ah, but there is a marker in the skies for us to latch on to: Venus is emerging as an evening star and Tsuchinshan should be somewhere to the right of the planet. Armed with this information the hoped for position lies just between two of the wispy clouds noted earlier. Finger crossing allowed!
As it turns out, the comet appears to the right of Venus, but much higher in the sky than we anticipated and nearly half an hour later. But, lo and behold, thar she blows, just bright enough to make out with the naked eye if you squint properly, a skill in which all amateur stargazers are well-practiced.
Donning binoculars provides a most gratifying sight, the hazy head of the comet with a long tail streaming out at about 11:00, were you looking at a clock. I’m completely in awe. Here is a visitor to our skies from significantly far away, far enough that the time it took to travel here is measured in multiple centuries. And unlike other comets with shorter orbital periods, likely, this one won’t be back for tens of thousands of years, if ever. Flabbergasting just thinking about it.
Hold on! I can explain everything! It’s not what you think! It might be worse. So here’s how it all began. The back story. As a kid, sleeping out in the backyard, watching the stars throughout the night, I began taking long exposure pictures with my Brownie Hawkeye camera. The perfect camera for a ten-year-old. Point and shoot, roll film forward, repeat. What could be simpler? The most intriguing feature to me was the shutter speed. A simple lever adjustment allowed exposure times from milliseconds to infinity, i.e., always open. I would set it on infinity and point it at the sky for up to six continuous hours. I had to wait days before I got the film back from the developer, but it was worth it to see the star tracks as they moved through the night in long graceful curving arcs. The north star was particularly nice since it stayed in the same place as the other stars made circles around it.
In 8th grade, I decided to build a telescope; I needed better views of the heavens. The project required me to grind the flat surface of an 8” round 2” thick piece of glass into a paraboloid – the perfect shape to take rays of light from distant objects and focus them to a point, from there to be magnified with an eyepiece – then silver the glass to create a mirror. The grinding worked as follows: The mirror blank was placed on top of a second piece of glass called the tool. Varying sizes of grit mixed with water were placed between them and I pushed the mirror blank back and forth across the tool as I walked around the barrel I used as a worktable. The grit scraped against the glass like sandpaper. Here’s the trick! As the top piece of glass moves out from the center, the tool gets more pressure on the outer edges, which are ground away, while the mirror gets more pressure on the center, which grinds it away. The overall effect creates a spherical concave hollow on the mirror and a spherical convex hill on the tool. And just like with sandpaper, the courser grit takes off a lot of glass quickly and is used to achieve the proper shape. Subsequent finer grits smooth the glass until eventually, a fine jeweler’s rouge on pitch is used for the final polish. In the end it looked just like a piece of glass again, smooth and transparent, no scratches, and with a slight bowl shape, no more than one eighth inch deep in the center. It took me several months to accomplish. What a project! Then I sent the glass away to be silvered to reflect 99.5% of the light that shines on it. When it came back as a true mirror it was beautiful. Shiny, bright, and ready for action. Eager to get it pointed to the sky, I housed it in a plywood tube on a plumbing pipe mount. Eyepiece holders, focusers, etc. were acquired and finally, after 2 years in total, there it was. A thing of beauty and boy was I excited.
The first time I turned it to the skies I fell in love with what I saw. Hundreds of stars, thousands of stars, strange blurry patches, planets, craters on the moon. And much much more just out of reach. I would get lost just grazing through the skies sampling the ever new vistas. I needed to get out of the city’s light pollution. Where was I going to find dark skies? In no time at all I had decided 1) I needed a bigger telescope (eventually I ground a 12½“ diameter mirror, but that’s a story for another time) and 2) I needed to take pictures of what I was seeing. #2 is still in the works, requiring some sort of mechanism to keep the telescope moving such that it follows the motion of the stars and planets as they move across the sky: a clock drive. The clock drive would have to rotate the telescope at the rate of one rotation per day, which is, of course, how long it takes the sky to return to where it is now 24 hours from now. Then, with the image of the star locked in place, I could take long exposure pictures of any object in the sky that I could get the scope to focus on. This was my dream, this is my dream.
Sounds easy enough in principle and there are certainly plenty of examples out there from large scientific research telescopes to backyard amateur scopes to simple hand crank devices for holding a camera so that it follows the stars. I began pondering how to build my clock drive. Would it be gravity driven, perhaps a ticking pendulum clock that ratcheted the scope around very slowly, or maybe a computer-controlled electric motor. I didn’t know which way to go.
It was at that point it really began to sink in that the stars do not move: the Earth rotates! Duh!! If I could only find a way to counteract the motion of the earth, I could see everything as standing still, the universe at rest. Remove the need for the clock drive! This project nibbled at my brain and captivated me through my early adult years. But I didn’t have the kind of money to buy an electric drive for my telescope nor the tools to build one. Nor did I come up with a way to stop the rotation of the Earth…
Then things get really sticky. Stopping the Earth’s rotation effect is insufficient. The Earth not only rotates on its axis, but orbits the sun at very large speeds, though maybe not as large as would be apparent relative to the fixed stars. I need a more nuanced clock drive, with not just one motion to account for, but also the motion of the Earth around the sun. But then the Sun is located in one of the spiral arms of the our Milky Way galaxy, moving in the general direction of the constellation of Cygnus the swan. The spiral arms are orbiting the super massive black hole at the center of the galaxy. That’s an even higher speed than our movement around the sun. Oh, and of course, our galaxy is drifting within a group of other galaxies, and all the galaxies of the universe are growing farther and farther apart as the universe expands from the big bang 14 billion years ago. You can see the problem I face. Now my clock drive has to be super-super-sophisticated. But I’m undaunted. I can do this!
I fire up my trusty laptop, open a spreadsheet and start making calculations. Ha, not enough programming power in Excel. I need better tools. I learn Mathematica, a super powerful mathematics program. Uh oh, add in differential geometry and tensor calculus, whatever the heck they are, to account for navigating these large motions. I can do this! I enroll in a university program and spend ten years getting enough math and computer programming tools together to stuff a horse. I can see the light at the end of the tunnel.
Problem is, I’m also learning more new things. Now I know that the atmosphere is going to cause distortion in all my pictures. I study adaptive optics, a technique that uses a laser/computer feedback system to alter the shape of the telescope mirror in real time to account for atmospheric turbulence. Whoa, who knew! But wait! Wouldn’t it be easier just to get above the atmosphere? Like the Hubble Space Telescope? I could build my own rocket, launch my homemade telescope and its now super fancy clock drive into space, land on the moon and take my pictures there. No atmosphere. But wait again, my clock drive will need to be calibrated for additional motion due to the orbit of the moon around earth. Better to stay in space.
Once in space I’ll stop my orbital motion around the Earth. Just blast my rocket thrusters backward along my orbital path until I am at rest relative to Earth. At that point, Earth will rotate beneath me at the rate of one revolution per day. (Caveat: I’ll need to be far enough away from Earth that once I stop orbiting, my rocket doesn’t start to fall back to the ground.) While I’m at it, why not stop my motion around the sun so that I’m only riding along with the solar system as it moves through the galaxy. I know what you’re thinking! Why not stop the motion within the galaxy too!?!? Then I would be sitting still in the galaxy as it rotates around me and moves wherever it’s moving in the local cluster of galaxies. I could go on, but this is getting seriously complicated for my rocket, albeit simpler for my telescope drive. If I take it one motion at a time, then perhaps it’s doable, in principle.
All this sounds fine and dandy, I suppose, and a pipe dream to you. Surely you’re wondering how a 72 year old retired physics teacher, amateur volleyball player, and weekend hiker could build that rocket on his fixed income social security check and pittance IRA. I honestly don’t know, but it sounds like fun!! In the end what I really have is an obsession with the night sky and the mysteries it holds. Comets are a very fun part of that.
There’s no telling for sure whether Tsuchinshan will visit us again. The latest astronomical measurements indicate a parabolic fly-by path rather than an elliptic closed orbit. Sadly, assuming no major interference from other gravitational sources, Tsuchinshan traces a path of no return.
Einstein understood that all things in the universe move relative to each other, that no place serves as “the center of the universe.” People originally thought Earth the center of the universe, then Copernicus claimed Sun the center, later we discovered our place in the galaxy, our galaxy’s place in the universe, and the shifting confounding properties of the universe itself. It’s mighty complicated.
Last night we witnessed a comet in the sky, a chunk of ice and dirt from the very edge of our sun’s influence. It traveled here, brightening our skies and our imaginations for a few weeks and now returns to the great “out there.” We were lucky for the opportunity. May there be more!