Goodbye, Comet C/2022 E3 [ZTF]!

 … and Welcome, Comet C/2023 A3 (Tsuchinshan-ATLAS)!

A comet's tail is the closest thing to nothing that anything can be…. and still be something, even visible!

-        this blogger

[An edited and abridged version of this article featuring the outward bound comet appeared in the Star of Mysore evening newspaper on 20th Feb 23]

Comet C/2022 E3 [ZTF] was in the news for quite some time, though very few people had actually seen it with the unaided eye.  Its sighting from the outskirts of the city was first reported in Star of Mysore on 30^th Jan 23. Now, on its outward journey from the solar system, after its closest approach to the Earth on 1^st Feb 23, some of us (see picture below) got together to get a farewell telescopic glimpse of the comet on the night of 13^th Feb 23 at Mr Krishnamurthy’s home, not too far from the city. Though its brightness was diminishing noticeably day by day, we were able to see it quite clearly, close to the well-known bright star Aldebaran (Rohini) in the constellation Taurus, through his Celestron 8” catadioptric computerized telescope, with a wide field eyepiece.  Its precise location at that time is shown in the accompanying star chart. However, it was not possible to see it with the naked eye, though barely visible through binoculars.

Picture:  Pointing his finger up in the direction of the comet is the host Mr Kishnamurthy at his residence; seated left and craning his neck is the blogger. Seated right is wild-life photographer Mr Ravishankar G S.  Others from left are: Dr Shanthakumari, Narasimhamurhy, Mahantesh Ashok, Sarthak P Gowda and Mahesh M B. 

For four of us, Krishnamurthy, Chiranjeeevi, myself and a newcomer Ms Vidya Priya, the ‘discovery’ of this comet had come two weeks earlier, on 29th January, when we had all gathered together on the roof of a tall school building located in the outskirts of Mysore.  We had spent a whole night in rather chilly conditions, zeroing in frequently on the comet with Krishnamurthy’s 8” Celestron telescope fitted with a wide field eyepiece, interspersed with observations of a number of Messier and NGC catalog objects best visible through this telescope.

On both these occasions, we were not equipped to capture a presentable photograph. The picture of the comet shown below was taken around the same time on 13^th Feb by Mr Vikas Shukla near Pune and is reproduced with grateful thanks to him.

Vikas Shukla’s photograph of the comet shown alongside a map indicating the location of the comet about the same time on the night of Feb 23    

Comets are described quite literally as ‘dirty snow balls’, made up of a mixture of dirt, water ice, methane, ammonia, carbon dioxide and other frozen gases. They originate in the vast Oort cloud (see illustration below) surrounding the solar system at its outer reaches, about a light year away. The Oort cloud lies far beyond the Kuiper Belt which itself lies well beyond the outermost planet, Neptune, and comprises of thousands of icy dwarf planets, including Pluto. While the planets of our solar system orbit in a nearly flat plane, the Oort Cloud is believed to be a giant spherical shell surrounding the Sun, the planets and Kuiper Belt objects. It's like a big, thick bubble around our solar system, made of icy, comet-like objects. 

When they get close to the Sun during their motion through the solar system, the frozen gases making up the head of the comet sublimate and pushed away by solar radiation pressure to form the distinctive cometary tails pointing away from the Sun.  Two types of tails are noticeable, the straight ones being plasma (ionic) tails and the curved ones the dust tails. A recent NASA picture of Comet C/2022 E3 [ZTF] reproduced below shows both forms of tails clearly. The generally irregular and large head or ‘coma’ of the comet consists mainly of the frozen stuff, with rocky cores.  The mean inter-particle distance in a comet's coma and tail are so exceedingly large compared to anything we can produce on Earth by way of a vacuum that they can be described as the closest thing to nothing that anything can be…. and still be something, even visible!

Comet C/2022 E3 [ZTF] – A recent NASA picture

Comets follow widely different paths around the Sun with varying periodicities, large eccentricities, and almost all possible inclinations to the ecliptic belt to which planets and satellites are confined.  Short period comets generally have periodicities of up to a few hundred years. Halley’s comet is perhaps the most famous short period comet, with a periodicity of about 76 years.  With a period of just 3.30 years, comet Encke has the shortest period of all known periodic comets in the solar system.  In contrast, the comet of present interest appears to have a periodicity, if at all, of about 50 thousand years! This appears to be the main reason for its popular fascination despite its difficult visibility.

Path of Comet C/2022 E3 [ZTF]

Though comets are plentiful in the solar system, those that get close enough to become visible to the naked eye are quite rare.  The last really good naked eye comet was Hale-Bopp in 1997. The 1965 comet Ikeya-Seki was a spectacular one while the much-heralded Halley’s comet of 1986 was a huge disappointment in relation to it. The expected increase in brightness of a comet as it approaches the Sun or the Earth is rarely predictable with any degree of reliability, much to the annoyance of most observers, both professional and amateur.  

Orbital path of the comet

As we bid good bye to the current comet of our curiosity, hoping that such visitors from outer space continue to entertain us, and more frequently too, despite the popular fears and myths associated with their visitations, we can get ready for our next visitor.

Comet C/2023 A3 (Tsuchinshan-ATLAS)

Astronomers have found a new comet labelled C/2023 A3 (Tsuchinshan-ATLAS).  It is something to look forward to eagerly as it makes its closest approach to the Sun well over a year from now. It may be worth the wait because, early estimates of the comet’s brightness suggest it might be as good as, or even better than, our departing comet. Perihelion (closest approach to the Sun) for this comet will be on 28^th September 2024. 

Of course, as with all comets, the brightness expectations can be hugely uncertain. A classic example is comet Kohoutek in 1973.  Like Tsuchinshan-ATLAS, Kohoutek was discovered far in advance of its approach to the Sun. Astronomers predicted that it could be the comet of the century, bright enough to rival the brightest of night sky objects.  But it fizzled out, perhaps as a disappointment of the century in the annals of cometary visitations. We should hope that history will not repeat itself.

The graphic above shows the comet’s path through the solar system and its position during its expected closest approach to the Earth [Sky&Telescope, Bob King, March 16, 2023]

The discovery of this comet has been credited jointly to the Asteroid Terrestrial-impact Last Alert System (ATLAS) robotic survey facility in South Africa and the Purple Mountain Observatory in Nanjing, China, early this year. Incidentally, Tsuchinshan is the Chinese term for Purple Mountain.