The coming weeks will be your last chance to catch Comet A3 Tsuchinshan-ATLAS as its orbit caries it away from Earth and its tail fades from view, but will this be the last time the comet sees Earth as well?
The comet has delighted stargazers for months, but if you look up Comet A3 Tsuchinshan-ATLAS online, there is one key detail that most reports get wrong – its orbital period, or how long it takes the comet to orbit our Sun.
While many report that the long-period comet last visited our planet 80,000 years ago, the truth is that its orbit is much, much longer.
So long in fact, that this could be the comet’s first – and perhaps last – visit to the inner Solar System.
“A3 almost certainly has never been near Earth before, nor will it again,” says Karl Battams from the Naval Research Laboratory in Washington, who has operated the NASA funded Sungrazer Comets Project since 2003.
“On its way in towards the Sun, the comet was following what appeared to be a very, very long elliptical path, with an orbital period somewhere up in the hundreds of millions of years long.
“This places its origins far outside the gravitational pull of our Sun. So it absolutely didn’t spend its whole existence on the orbit we saw, but instead was nudged onto it, probably gravitationally, a long time ago.”
What is Comet A3 Tsuchinshan-ATLAS's orbit?
This, however, does not mean that Comet A3 Tsuchinshan-ATLAS’s orbit is still hundreds of million years long however.
When the comet passed through its closest approach to the Sun, known as perihelion, on 27 September 2024, our star’s gravity altered the comet’s trajectory, changing its eccentricity – meaning how ‘squished’ its elliptical path is.
“Now it’s following an orbit that appears more like a few hundred thousand years long,” says Battams. “But even at a couple of hundred thousand years, this would ultimately place A3 back so far out into space that it would probably begin to feel minor gravitational effects from other star systems, and these would sway it off-course or steal it entirely.”
Why is there so much uncertainty when measuring a comet's orbit?
The main issue is that we can only observe comets during the short window when they are within the inner Solar System and close enough to see.
“All we can do is look at the apparent path it is following during the relatively very short window we’ve been observing it and use that information to guess at its complete orbit,” says Battams.
By measuring the current motion of the comet on the sky, astronomers like those at the Sungrazer Comets Project are able to back track the comet’s path and calculate its orbit.
“For objects on short-period orbits this is usually a very precise measurement, but for comets like this that go to/from such insanely distant regions of space, uncertainties are unavoidable regarding its position in the distant past and future.
“We’ve only followed the comet for a distance of maybe 7 or 8 AU [where 1 AU is the distance between the Earth and the Sun], on a path that is likely hundreds of thousands of AU in length – if it’s even a closed loop to begin with.”
With such long scales, even though the team have been able to measure the path to six decimal places of precision, there’s enough wiggle-room that the path of the comet could be parabolic – meaning the comet was only passing Earth by, and will not return.
All this uncertainty means that the comet’s orbital parameters are constantly being updated as more observations are taken.
With such changes constantly happening, its highlight likely that many media outlets picked up on an outdated measurement of the comet’s orbit.
“It’s conceivable that they saw a very early orbit prediction, shortly after the comet was discovered and we had little data to work with, but that was well over a year ago.”
Where can I find the latest information on Comet A3 Tsuchinshan-ATLAS’s orbit?
One of the biggest problems is that there’s no centralised repository of information.
“For its orbit, I think NASA's JPL Horizons is the most reliable source of up-to-date data,” says Battams.
“For data about its past, present, and predicted future brightness, the Comet Observation Database is very nice.
“And of course, there’s a legion of highly skilled astrophotographers who are going to continue to have fun with this one for at least a few more weeks, despite it being too faint to see naked-eye now.”
Take a look at our gallery of some of your best images of Comet A3 Tsuchinshan-ATLAS.
Where did Comet A3 Tsuchinshan-ATLAS come from?
While the comet may have been stolen from another star, this is not the only explanation of its origin.
“By far the most likely scenario is that this comet has spent its entire life floating around in the Oort Cloud – a theorised reservoir of comets that sit at the very distant reaches of our Sun’s gravity,” says Battams.
“Something, maybe a passing star, another comet, or even a collision, gave it just enough of a nudge that it started to fall into our Solar System.”
This is a common occurrence.
Such comets are ‘dynamically new’, meaning they have never been up-close and personal with the Sun, and so their ices and gases have remained untouched since the formation of the Solar System.
“A3’s new orbit likewise points it right back into the “comet limbo” from whence it came, where it will be so weakly bound to our Sun that it may just end up leaving the Solar System forever,” says Battams.
“In short: I think an alien civilization is more likely to see this comet again than Earth is!”
