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Comet ZTF: 5 questions about its passage near Earth


50,000 years after its last passage through the interior of the Solar System, comet C/2022 E3 (ZTF) is returning. It can be observed with the naked eye for a few days. This is what you need to remember about this visitor who may never come back.

Comet ZTF will be visible in France between the end of January and the beginning of February of this year 2023, long after its last passage through the region, around the Paleolithic. You can see it with the naked eye, but to fully understand what you will observe, here is everything you need to know about this celestial body.

When was comet C/2022 E3 (ZTF) discovered?

The discovery of C/2022 E3 (ZTF) is recent, as it was on March 2, 2022 that the comet was spotted by the Palomar observatory, perched more than 1,700 meters above sea level in California, north from San Diego. The site specializes in the detection of so-called transient objects, that is to say objects whose luminosity varies suddenly over a short period of time – like a comet, therefore.

This is not the first time that this comet has circulated inside the Solar System: additional observations, cross-referenced with other work, show that the comet probably already passed through the area nearly 50,000 years ago. years, notes the French Astronomical Society. But at the time, it was impossible to have such detailed sky tracking as today…

Trajectory of the comet in the solar system. // Source: Nasa screenshot

The choice of the name of the comet follows very precise rules: the letter “C” does not mean “comet”, but is a code to designate a non-periodic comet, that is to say one which takes a long time to rotate around the Sun. There are other letters, which designate other situations: comets which return regularly, others which are only passing through, etc.

Then comes the year of discovery, a letter to designate the time of year (the first half of March). The number designates the order of discovery of the object during this fortnight. Finally, we find in parentheses the acronym of Zwicky Transient Facility, which designates an astronomical survey by the Schmidt Samuel-Oschin telescope, installed in Palomar.

What is a comet?

Comets are “small” spatial bodies, whose dimensions generally reach a few (tens) of kilometers. They exist everywhere in the universe, but some attract much more attention since they cross the Solar System. The best known is, of course, Hale-Bopp, but we could also mention Tchoury, Hyakutake or Hartley.

Their particularity is to have a plume, a “tail”, when these objects are within range of the influence of a star. This hair is composed of dust and gas, very luminous under the effect of the star, and can extend over millions of kilometres. It is this trail from elsewhere that often facilitates their detection, like a signature in the sky.

Comet C/2020 F3 Neowise.  // Source: Flickr/CC/Nigel Hoult (cropped photo)
Comet C/2020 F3 Neowise. // Source: Nigel Hoult

Comets are composed of an assembly mixing ice, rock and dust. They are the subject of a hypothesis in astronomy: these are the objects that could have brought water and organic compounds to planet Earth, thus participating in the emergence of life. During the formation of the Solar System and therefore of the Earth, the planet was bombarded by celestial bodies.

Comets are to be distinguished from asteroids, which are more composed of rocks and metals, but also from shooting stars, which are very small objects entering the Earth’s atmosphere at full speed. They then become luminous by the heating of their surface and their combustion. Once on Earth, these objects are called meteorites.

Where and when to see C/2022 E3 (ZTF)?

Spotted in March 2022, it is only since December of the same year that comet C/2022 E3 (ZTF) can be observed in good conditions — first with a telescopesince December, and now with the naked eye since January. But if you want to maximize your chances, stay away from cities and avoid any source of light pollution.

Until the beginning of February 2023, the conditions should be increasingly favorable to follow C/2022 E3 (ZTF), since the comet will continue to approach the Earth. Don’t panic, however: it’s a relative proximity. The minimum distance between Earth and the comet will not drop below 42 million kilometers.

We have a recap on Numerama for where and when to see Comet ZTF with the naked eye in France. It will be between the Big Dipper and the Little Dipper, so it is towards the north that you have to turn. Note that the comet will still be visible after the first days of February, but its brightness will gradually fade as it sets sail again.

What is magnitude?

There is a term in astronomy that is used to describe the apparent brightness of a celestial object — a star, an artificial satellite, a comet, etc. It is the magnitude. It is a number that works like this: the lower the number, the brighter the object. Conversely, a high magnitude indicates a shine that is particularly difficult to detect.

For a human eye, the commonly accepted limit of perception is 6. Beyond that, more and more sophisticated instruments must be used: binoculars, telescope, terrestrial telescope, space telescope. For example, Hubble is located at 30. Note that the scale does not stop at zero either. There are negative magnitudes.

Comet 2022 E3 (ZTF).  // Source: Dan Bartlett
Comet 2022 E3 (ZTF). // Source: Dan Bartlett

A few examples: the planet Uranus is located around 5 and 6 depending on the observation conditions. The Vega star is at 0 by convention. Sirius, the brightest star outside the Solar System: -1.5. Pluto is at 15, the Moon at -12.6 when full. Venus at -4.4 when she is in her best mood. And the sun ? -26.8. We only see him.

And in the case of C/2022 E3 (ZTF), then? When it was discovered, in March 2022, its magnitude was 17, recalls the French Astronomical Society. Then, this number only decreased as it progressed towards the Sun. At the beginning of January, its magnitude was evaluated at 7. And at its maximum, it must reach 5, allowing it to be observed with the naked eye.

When will we see C/2022 E3 (ZTF) again

Probably never. In any case, no one will be alive to be sure: its trajectory is so eccentric that it will not return for tens of thousands of years (the last time was 50,000 years ago, according to calculations). The worst-case scenario even suggests that it will eventually leave the Solar System.

In astronomy, an eccentricity of 0 is a perfectly circular orbit. An eccentricity of 1 corresponds to a parabolic orbit. Earth’s orbit around the Sun has an eccentricity of 0.0167, while that of ZTF reaches 1.00027. In sum, C/2022 E3 (ZTF) follows a trajectory relative to the Sun resembling an elongated and possibly “unclosed” oval.

So this may be the last time that C/2022 E3 (ZTF) comes to see us. It’s a little sad, but take comfort: according to NASA, there were 3,743 comets in January 2023. And the American space agency estimates that the Space System has billions of them, beyond Neptune, in the Kuiper belt, and even beyond Pluto, in the Oort cloud.


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