In space, everything is in motion. From a spacecraft orbiting the Earth to moons dancing around planets and star systems on epic voyages across our galaxy, nothing ever truly stops moving.
We find patterns in these movements, such as the ecliptic, the great highway along which the sun moves across the sky and the planets take turns traversing our nights. From these patterns astronomers can work forward to predict when the next eclipse will occur or backward to understand how our solar system formed. However, we often find objects that move in odd ways, such as a comet that orbits opposite the standard movement of planets and most asteroids, known as a retrograde orbit. These movements indicate that something strange is either happening or previously happened in the distant reaches of our sun’s sphere of influence.
The Oort Cloud is a vast sphere of cold, dark objects loosely held in a shell by the sun’s gravity. Reaching from about 2,000 to 200,000AU (astronomical unit, equal to the average distance between Earth and the sun, about 93 million miles), the Oort Cloud is where most of our long-period comets come from, taking hundreds or even thousands of years to complete an orbit. This is far past the orbits of Neptune and Pluto, and this distant region is where science fiction and science fact have attempted to explain strange orbits with concepts like a Planet X.
Recent AI models suggest that in the past billion years or so it’s likely that a wandering star careened through the outer edges of our solar system, leaving gravitational chaos in its wake. The lasting effects of such a close stellar encounter could explain the movements of many objects in the outer solar system, including retrograde comets.
Multiple observatories around the world spotted one such comet in early 2023, designated C/2023 A3 (Tsuchinshan–ATLAS). It follows a retrograde orbit, and its progress toward the inner solar system suggests that its orbital period may be millions of years, from the farthest reaches of the Oort Cloud, and likely set on its current path by the gravitational pull of a passing star. Its projected outbound trajectory indicates that it may leave our solar system entirely after its swing around the sun, destined to traverse the cold depths of the galaxy for millions or billions of years.
October offers backyard astronomers throughout the Northern Hemisphere great opportunities to see this once-in-a-lifetime comet. Projection is difficult, but it’s shaping up to be one of the brightest comets in decades — it could reach magnitude-0 brightness in mid-October.
On around October 15 look to the southwest just after sunset. Venus will be low on the horizon, with the red star Antares to its left and comet C/2023 A3 just above and to the right. You should be able to see the comet with the naked eye, but even a good pair of binoculars or small telescope should give you a great view of the tail stretching across the constellation Serpens, directly away from the point where the sun just set. It will then dim over a few weeks as it moves higher in the sky and away from the Sun, returning to the dim, frigid fringes of our system.
If you would like to learn more about the sky, telescopes, or socialize with other amateur astronomers, visit us at prescottastronomyclub.org or Facebook @PrescottAstronomyClub to find the next star party, Star Talk, or event.
Adam England is the owner of Manzanita Financial and moonlights as an amateur astronomer, writer, and interplanetary conquest consultant. Follow his rants and exploits on Twitter @AZSalesman or at Facebook.com/insuredbyadam.
