Comet C/2022 E3 (ZTF) is garnering increasing interest as it approaches Earth, with the object on the cusp of being visible to the naked eye under ideal conditions. But when might the next naked-eye comet appear following this event—and are astronomers even able to predict such phenomena?
Comets are astronomical objects made up of frozen gasses, dust and rock that orbit the sun. Sometimes referred to as “cosmic snowballs,” these objects are blasted with increasing amounts of radiation as they approach our star, releasing gases and debris.
This process forms a glowing atmosphere around the comet’s nucleus, known as a coma—which, in the case of C/2022 E3 (ZTF), appears green—and two vast tails of gas and dust.
For a comet to reach naked-eye brightness, it must approach close to the Earth or sun, while being sufficiently large.
“If the comet has a large nucleus, and lots of dust, it will reflect more sunlight and appear brighter,” Jessica Lee, an astronomer with the Royal Observatory Greenwich in the U.K., told Newsweek.
When measuring the brightness of astronomical objects, the brighter a given object is, the lower its magnitude. For example, an object with magnitude +2 is brighter than one that has a magnitude of +8.
Magnitude +6 is roughly the lower limit of what the naked eye can see, although real-world factors such as levels of light pollution and local weather can play a role in determining whether or not an object will actually be visible unaided at a given location.
While the future trajectory of comets can be determined with a high degree of accuracy, predicting the brightness of comets is a notoriously tricky business. This characteristic is difficult to forecast because it depends on how sunlight is scattered by dust particles in the coma and tail.
The density of this dust at any given moment is linked to the rate of sublimation—the transition of a substance directly from a solid to liquid state—of ice in the comet’s nucleus as the sun’s rays blast the comet. This process can be highly variable, even for the same comet over time.
“It has been said that comets are like cats—they have tails and do what they want,” Jonathan Shanklin, visual observations coordinator with the British Astronomical Association, told Newsweek.
There is always a chance that a comet will become more—or indeed less—active than expected. They may also experience an “outburst” either due to an impact with a small lump of space rock or an explosive release of gasses, causing them to brighten dramatically.
“They can also fragment, which then makes future brightness prediction impossible,” Shanklin said.
When could the next naked-eye comet appear?
Given that the brightness of comets is inherently challenging to predict, the question of when the next naked-eye comet might appear is difficult to answer with any degree of confidence.
To add to the uncertainty, there is always a possibility that a previously unknown comet with the potential to reach naked-eye brightness will be discovered.
There are two main categories of comet, based on the amount of time they take to complete an orbit around the sun. Short-period—or periodic—comets complete their orbits in less than 200 years. Long-period comets, on the other hand, take more than 200 years to circle our star—sometimes significantly more.
“Some comets have known orbits around the sun and so have regular time intervals we can measure,” Lee said. “Some comets are on very wide or very inclined orbits which can take tens of thousands of years to orbit the sun and so we are unaware of their orbital path until they approach us for the first time—or the first time since we’ve had telescopes that can spot them.”
“Sometimes comets can be deflected by interactions with other objects and their orbital path can change, so this could be the first time that a comet ever approaches the Earth,” she said.
A study published in the journal Earth, Moon, and Planets found that over the past 200 years one bright naked-eye comet has appeared every 2.5 years on average—the last one was NEOWISE, which put on a show in 2020. But it is is important to note that this does not mean a naked-eye comet appears at regular 2.5 year intervals.
Similarly, in the last 200 years there have been 20 or so comets that could have been described as “great”—meaning they were easily visible to the naked eye.
“This works out to an average of one every 10 years, but doesn’t mean we will see another one in 10 years time,” Lee said.
If, for the sake of argument, we discount the possibility of new promising candidates being discovered in the next few years—a plausible scenario but an unknown ONE—what are the known comets that are most likely to reach naked-eye visibility?
Of the previously identified long-period comets, none are currently known that might reach naked-eye brightness in the near future, Shanklin said. But there is always a chance that a known one will brighten more than expected, or undergo an outburst.
Periodic comets that fly past Earth many times are “reasonably” predictable, Shanklin said, although many undergo slow changes in brightness, either as a result of aging, or changing perihelion (the point in an astronomical object’s orbit at which it is closest to the sun) distance from one return to the next.
Most of the periodic comets are small, therefore they have to do approach relatively close to either the Earth or the sun to become visible to the naked eye. In the latter case, they are rarely well-placed for naked-eye viewing, Shanklin said.
The most likely candidate of the known periodic comets to reach naked-eye brightness in the near future is the comet 12P/Pons-Brooks, which orbits our sun every 71 years.
Some predictions indicate that this comet could become a naked-eye object by late March, 2024, as it approaches perihelion. But it is entirely possible these predictions may not come to pass.
Other than 12P/Pons-Brooks, no other comets are currently predicted to become visible to the naked eye after C/2022 E3 (ZTF) over the next few years.
“After C/2022 E3 (ZTF) fades below naked-eye visibility 12P/Pons-Brooks is likely to be the next one—and may be brighter than it,” Shanklin said, assuming that no new comet candidates are discovered and nothing unpredictable happens to any known comets. If everything goes to plan, 12P could reach a magnitude of +4.
“It is moderate in size [and] gets moderately close to the sun—so closer than the Earth is. It doesn’t get particularly close to the Earth so it is more the size and perihelion distance that makes it get within naked-eye range,” Shanklin said.
Aside from 12P/Pons-Brooks, there are a handful of other comets that are set to reach perihelion in the next couple of years that will be fairly bright, but there is “no guarantee” they will reach naked-eye visibility, Lee said. Two which will be in a good position to observe from the Northern Hemisphere are 144P/Kushida and 62P/Tsuchinshan.
The former will be at its brightest in January, 2024, with current estimates predicting its brightness in this period at about +7, meaning it would be visible with binoculars. 62P/Tsuchinshan will also reach perihelion in January, 2024, and has a current predicted magnitude of +7-8, which would again require binoculars to view.
There is always a chance either of these two comets could brighten unexpectedly, but forecasts currently indicate equipment will be needed to see them.
In the unlikely situation that no naked-eye comets appear in the next few decades, at least there is a good chance we will be able to rely on the famous Halley’s Comet, which can be seen from Earth every 75-79 years or so. Its next approach to our planet is scheduled for 2061.
“Halley’s Comet is the only short period comet that we know of currently that regularly reaches naked-eye brightness,” Lee said. As a result, it is the only naked-eye comet that can appear twice in a human lifetime.
“If you look back at all the other ‘great’ comets of the last 100 years, the rest have been long-period comets.”