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| Comet C/2022 E3 (ZTF)... |
Type: Comet
Constellation: Ursa Minor
Distance: 48.9 million miles (at 20.15, Jan 27th 2023)
Equipment: Atik 460/EFW 2, Samyang 135mm lens@ F2, Vixen GPDX mount, guiding with Lodestar X2/PHD
Subframes: 18 x 200s each for LRGB (comet): 6 x 20s each RGB for stars, flats, bias as dark frames.
Astronomers discovered Comet C/2022 E3 (ZTF) using the
48-inch (1.2-meter) Samuel Oschin robotic telescope (part of the Zwicky Transient
Facility (ZTF),
located at
Comet C/2022 E3 (ZTF) was in the morning sky, in the
direction of the constellation
The ZTF program images the whole Northern Hemisphere every two nights looking for supernovae, variable stars, binary stars, flashing merging neutron stars, asteroids and comets. Overall, it has discovered 10 comets in the past three years.
When astronomers first detected C/2022 E3 in March 2022, the comet was around 400 million miles from the sun, or just within the orbit of Jupiter. At that time the comet was a very faint 17th magnitude, initially appearing as a stellar object until its proper motion showed it moving against the sky background.
The solid, core structure of a comet is known as the nucleus. Cometary nuclei may be up to 20 miles across (Hale-Bopp was one such monster) but are generally much smaller: the nucleus of Comet C/2022 E3 (ZTF) has been estimated to be only a mile across. Because of their low mass, comet nuclei do not become spherical under their own gravity and often have irregular shapes.
Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. As such, they have been popularly described as "dirty snowballs". The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, ethane, and other, more complex molecules such as long-chain hydrocarbons and amino acids.
These nuclei normally orbit the Sun at an immense distance right on the very edge of the Solar System as part of a sphere of such objects surrounding it, known as the Oort Cloud, Occasionally, one such nucleus will undergo a gravitational encounter, perhaps with the distant giant planet Neptune, one that will send it spiralling in towards the Sun.
As the frozen nucleus approaches the Sun, solar radiation increasingly releases a huge and extremely thin atmosphere around the comet called the "coma". It is this coma that we see, rather than the nucleus of the comet itself. The force exerted on the coma by the Sun's radiation pressure and solar wind cause a cometary "tail" to form pointing away from the Sun. Both the coma and tail are illuminated by the Sun and start to become visible when a comet passes into the inner Solar System. Cometary dust reflects sunlight directly while the gases glow from ionisation by the Sun's intense radiation.
It was it this point that the researchers made out the distinct tail, or coma, proving that C/2022 E3 (ZTF) was indeed a comet rather than an asteroid.
The green colour of Comet C/2022 E3 (ZTF) is typical of comets and is believed to be due to the presence of an unstable form of carbon molecule (“dicarbon”) formed by the photolysis of the carbon compounds present in the nucleus, and which fluoresce with a green light under solar irradiation in the vacuum of space.
Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet 17P Holmes.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions - such as when Earth passes through a comet's orbital plane, an “anti-tail”, pointing in the opposite direction to the ion and dust tails, may be seen. This has been observed for Comet C/2022 E3 (ZTF) and can be seen in my image.
By Jan. 12, 2023, the comet had travelled nearly another 300 million closer to Earth, becoming visible in the night sky near the northern constellation Corona Borealis. As stated earlier, solar radiation ionises the gas emitted from the comet, giving rise to complex magnetic fields around it. While making its closest approach to the Sun, a blast of radiation and solar particles (called a “coronal mass ejection”) swept over the comet, squeezing its magnetic field lines together and leading to a disruption in the flow of gas from the comet – a "tail disconnection event" which was clearly observed from Earth.
On February 1 and 2, Comet C/2022 E3 (ZTF) will have reached its closest point to Earth (27 million miles) and can be seen overhead against the background of the rather obscure constellation of Camelopardalis, close to the borders of the better-known constellations Ursa Major and Ursa Minor. A map of the comet's progress is shown below:
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| Track of comet C/2022 E3 (ZTF) during February 2023... |
A few days later, on Feb. 5 and 6, the comet will pass across the night sky to the west of the bright star Capella and then pass across the constellation Auriga. On Feb. 11th, it lies just to the east of Mars. From there, it will descend toward Taurus, becoming ever dimmer as it moves away from Earth, back out toward the edge of the solar system.
The last time Comet C/2022 E3 (ZTF) passed near the sun was 50,000 years ago. However, the latest orbital element calculations suggest that the comet’s recent close approach to the Sun has changed its orbital path to a parabolic orbit, rather than an elliptical one. Such an orbit is not “closed”, so after it sweeps around the sun C/2022 E3 will move back out into deep space, never to return again.
Much blather has been written by the excitable English press (particularly the Daily Mail) about poor old Comet C/2022 E3 (ZTF), with headlines hyping it up to be an eyeball-searing, “one-in-a-lifetime event”. The truth, on the odd occasion that it can ever be found, is much often more mundane and is usually buried in the depths of the article. I can, however, reassure you that the comet will not affect house prices, cause cancer or lead to a rapprochement with the EU, whatever the Daily Mail may say in future.
The reality is that bright comets — the kind that can be seen easily without binoculars or telescopes — appear on average perhaps two or three times every 15 to 20 years. The last such comet to do that was Comet NEOWISE (C/2020 F3) in July 2020 – hardly a “once-in-a-lifetime” event.
Then there are the common comets, of which most are only visible either with good binoculars or a telescope. If you look at my blog (both old and new) indexes, you will find several examples of ones I have imaged over the years.
The vast majority of comets fall into this category, but C/2022 E3 (ZTF) may end up ranking as “slightly brighter than common”, since for a short while it may hover at around “fifth magnitude”, the limit of naked-eye visibility (and only for those fortunate enough to be blessed with dark, non-light polluted night skies). Additionally, the light from a comet is not a bright point, but a fuzzy, diffuse spot, one that will be increasingly difficult to pick up from a night sky containing a bright waxing moon that will be full on February 5th.
I was able to pick up the comet in 7x50 binoculars as a tiny fuzzy blob, but only because I knew exactly where to look (and I am also a fairly experienced night-sky observer armed with a computer-controlled camera set-up that was pointing the way!)
My image shows the comet displaying a distinct greenish colour and sprouting two faint tails, the gas tail being the long one to the right on the image. Sadly, such long-exposure images tend to be quite deceptive. For one thing they bring out colours and levels of detail that are not readily evident to the eye, even through binoculars or telescopes.
My CCD camera is a monochrome device, which is more sensitive than colour cameras. To get colour data, I have to take multiple exposures through colour filters. This works just fine with most celestial objects, which don’t move against the sky background.
Comet C/2022 E3 (ZTF) was “motoring” in celestial terms against the background sky, however. My set-up was able to “guide” the camera, locking it on to the comet, but this meant that the stars became trailed on the long cumulative exposures as the comet was moving quite perceptibly across the sky background. You will see the stars as faint grey (luminance), red, green then blue lines on the above image as the filters were placed in sequence during the imaging session.
To capture stars for the image that allow a frame of positional reference, I just shot a few short exposures with the camera guiding on a star as usual. Combining the separate LRGB exposures together gave the colour image of the comet, with the stars overlaid in “screen mode” to give the view you see above.
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| "Starless" monochrome version.... |
Just as an experiment, I ran the individual luminance sub-frames through Starnet and then stacked them. Although a bit noisy, the cometary tails do seem to stand out a bit better, with the "fan" of the dust tail spread out across the comet's orbital plane and the twin tail gas clearly visible off to the right.
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