Astro Bob: Comet ZTF will NOT streak across the sky (but it's still pretty cool)
A lot of wild things are being said about this comet. We'll get the facts straight and help you find it in binoculars.
I've read recent online news media posts about Comet ZTF with a mix of humor and alarm. All this talk about a rare, green comet streaking across the sky that was last seen by the Neanderthals.
First, lots of comets are green — at least in the telescope. The color comes from carbon, which glows a striking aqua green when energized by the sun's ultraviolet (UV) light. The hue shows up beautifully in time exposure photographs, and it's occasionally visible in brighter comets when viewed through a telescope. But as far as the naked eye or even binoculars, it's a rare sight.
While it's true the comet last passed this way some 50,000 years ago, it's very unlikely any Homo sapiens or Neanderthals noticed it unless ZTF made an exceptionally close approach to Earth. This time around it passes relatively nearby but won't be bright in the traditional sense like Comets NEOWISE (in 2020) and Hale-Bopp (1997). With those, you could just step outside, allow a few minutes to get used to the dark and ba-boom — there they were!
Comet ZTF is visible in binoculars right now but only from a reasonably dark sky with a minimum of light pollution. Through my 10x50s it's a small, fuzzy glow about 1/3 the size of the full moon with a brighter center and whiff of a tail pointing north.
Through a 6-inch or larger telescope, the comet becomes increasingly more impressive. In my 15-inch scope it reveals a bright, dense core (called the nuclear region) buried in a fuzzy coma that sprouts a lovely, fan-shaped tail. No color is visible.
At least for now, Comet ZTF is still too faint to see without optical aid, but as you'll learn, that may change later this month. Observers estimate its current brightness at magnitude 6.5-7, still below the naked-eye limit of 6.
Finally, comets don't streak. Meteors streak. Comets release dust and shards of rock that over time run into the Earth — or the Earth runs into them. Striking the atmosphere at high speed they vaporize in a brief and dramatic flash. Basically, it's one and done. Comets are typically a kilometer or two across and located millions of miles away. They creep instead of streak across the sky, moving slowly night to night as they orbit the sun just like the planets.
With luck, Comet ZTF will continue to brighten as predicted, peaking around 5.5 magnitude — bright enough to see faintly without optical aid from a dark-sky site. I hope you get a few chances to spot and follow it across the sky in the weeks ahead. As far as predicted comets, it's the brightest expected for the year.
ZTF stands for Zwicky Transient Facility , an observing program based at Palomar Observatory in California that robotically surveys the entire northern sky about once every 2 days with a 48-inch telescope. When discovered March 2, 2022, the object's star-like appearance indicated it was likely an asteroid. But followup images captured a bit of fuzz — called a coma — around the blip, confirming it as a comet instead.
Since that time, ZTF has worked its way into the inner solar system, passing closest to the sun on January 12 at 103 million miles (166 million km). Solar heating has vaporized some of comet's dust-laden ice, causing the coma to expand and brighten and two tails to blossom. The short, yellow one is made of dust that reflects sunlight, while the other — narrow and blue — glows from fluorescing carbon.
Normally, comets fade after departing from the sun, but ZTF happens to be headed toward Earth and will continue to brighten into early February, passing 26 million miles (42 million km) from your rooftop on Feb. 1. Then it's outta here, headed back into the deep freeze of the Oort Cloud , a journey that will take millions of years.
Because it's near the Earth right now, Comet ZTF's apparent motion across the sky will be quite noticeable night to night especially later in the month. Currently, it's headed north into Draco the Dragon, moving about 1.5° a night, which is equal to the span of three full moons. That will expand to 6.5° nightly by month's end. That's fast enough to see its motion against the background stars in binoculars in about 2 hours.
Like cats, comets have tails and can be unpredictable. It's possible that ZTF won't quite reach naked-eye brightness. Or random cracks could split its crust and release pent-up gas and dust in a massive outburst that would render it plainly visible to the naked eye. Being composed of fragile ices, the comet could also break apart and suddenly fade. For these reasons I try to observe current comets every clear night I can. I find them dynamic and beautiful.
In the coming few weeks, before the moon is full again on Feb. 4, plan an outing to see ZTF. Although low in the northeastern sky at mid-month, the comet quickly climbs up the northern sky. From Jan. 16-24, the best time to view it will be on the late side — after midnight. But that quickly changes as the comet picks up speed.
Circle Jan. 25-28. Not only will ZTF be well placed in the northern sky during the evening hours, but the moon will be a thick crescent and not a bother. Although I set the chart time for 11 p.m., the comet climbs high enough above the horizon by Jan. 25 to start observing around 8 p.m. local time. After Jan. 28, the waxing moon will become a problem, so from Jan. 29-Feb. 2, best viewing will be after moonset in the morning sky.
I'll provide updates on the comet's brightness and appearance in the coming weeks. I wish you good luck in your hunt for this dusty butterfly.