A spectacular rare green comet shining in the sky? An expert on what to expect from Nishimura | Albiseyler

Soaring Green Comet Illustration

The recently discovered comet Nishimura, C/2023 P1, has attracted public attention. Although its trajectory suggests a close approach to the Sun and possible visibility with the naked eye, a brilliant display of the sky, especially something like this spectacular illustration, is unlikely.

Of all the objects in the solar system, perhaps the most remarkable are the large comets that occasionally grace our skies. If you’ve been on social media in the past few days, you’ve probably seen articles claiming that we have such a comet in the sky right now: C/2023 P1 (Nishimura).

As I write this, Comet Nishimura is passing by on its first visit in over 400 years. Japanese astronomer Hideo Nishimura discovered the comet on August 12. Soon after, pre-discovery images of the comet from January were found, allowing astronomers to determine its path.

They quickly realized that Nishimura would be leaning closer to the Sun this month than Mercury’s orbit. Given the brightness of the comet at the time of discovery, it could be bright enough to be seen with the naked eye. So, will it be a spectacular sight in our skies? Probably not.

Comet Nishimura

This image of Comet Nishimura, with its green coma and thin tail, was captured on August 18 from June Lake, California. Credit and Copyright: Dan Bartlett

Unfortunately, Nishimura’s orbit will keep it close to the Sun in the sky as seen from Earth. While it’s certainly bright enough to be visible to the naked eye in a dark sky, at best it hugs the horizon just after sunset—almost lost in the Sun’s glow.

Still, astronomers around the world are excited. Even a comet that is difficult to detect with the naked eye is worth observing. And as science writer and astronomer David H. Levy he once said:

“Comets are like cats: they have tails and do exactly what they want.”

There is a chance that Nishimura could unexpectedly lighten up. If so, we may see something special in the next few weeks. If not, there’s always next year – but more on that later.

Bright Comet Recipe

When far from the Sun, in the icy depths of space, comets are essentially dirty snowballs: clumps of ice, dust, and rock left over from the formation of the Solar System.

As a comet approaches the Sun, its surface begins to heat up. The ice near the surface heats up and “sublime‘, turning into gas and erupting outward from the comet’s surface. This gas carries dust and debris and envelops the core in a translucent cloud of gas and dust called a ‘coma’.

The solar wind then blows gas and dust away from the Sun, giving the comet its tail (or tails). Tails always point from the Sun.

The comet we see is sunlight reflected off the gas and dust in the coma and tails—the nucleus itself is hidden from view. Therefore, the brightness of a comet is usually determined by three things:

  1. nucleus size: a larger nucleus usually means a larger active region (although some comets are more active than others) and more gas and dust production
  2. distance to the Sun: the closer a comet is to the Sun, the more active (and brighter) it will be
  3. distance to Earth: the closer the comet is to us, the brighter it will be.

What about Nishimura?

This brings us to Comet Nishimura. It seems likely that Nishimura isn’t that big—otherwise we’d have seen it sooner—nor is it particularly close to Earth. However, it passes relatively close to the Sun and is expected to be very active around perihelion (its closest point to the Sun).

If visible in a dark night sky, the comet would be quite impressive. Sadly, even at its best, Nishimura will be close to the Sun in the sky.

In addition, the comet and Earth happen to be in perhaps the worst orientation for observation: Nishimura will stay close to the Sun as it moves away from us, and remain buried in the star’s glow.

Short window to see Nishimura from Australia

Nishimura peeks over the western horizon soon after sunset, but just barely. The best chance to see it from Australia comes in the week of September 20 to 27, when the comet’s head sets about an hour after the Sun. It will be furthest from the Sun in the evening sky on September 23.

When twilight ends, Nishimura will be very close to the western horizon and will soon set. This means that it is likely to be lost in the glare of the Sun.

The location of Comet Nishimura in the sky

This image shows Nishimura’s position in the sky as seen from Toowoomba, 40 minutes after sunset on the evening of September 23. At that time, in waning twilight, the comet will be only 5 degrees above the western horizon. Credit: Screenshot/Stellarium

But remember, comets are like cats. Some comets break up when they are closest to the Sun, in which case they often become significantly brighter. If this happened to Nishimura, it might be much easier to detect.

Unfortunately, the comets most likely to fragment are those that first visit the inner Solar System and move along very long orbits of tens or hundreds of thousands of years. Nishimura is a seasoned visitor, with an orbital period of around 430 years. It probably swung around the Sun many times and survived, making it less likely to break up.

However, while the comet’s head may be lost in twilight, the tail may still be visible when the sky darkens. Before the comet disappeared in the glow for Northern Hemisphere viewers, observers set its tail to be around six degrees in length – and it will likely grow as the comet approaches the Sun.

If you’re lucky, you might catch a glimpse of the tail rising proudly above the horizon as the sky darkens.

Another big comet

If Nishimura doesn’t turn out to be the show you were hoping for, there’s a chance another Comet could put on a truly spectacular show next year. Comet C/2023 A3 (Tsuchinshan-ATLAS) was discovered at the beginning of this year. It is currently almost as far from the Sun as Jupiter.

It will continue to descend towards the Sun over the next 12 months and will make its closest approach to the Sun in late September 2024. Tsuchinshan-ATLAS looks promising. If it behaves as expected, it could be a spectacular sight – but remember: comets are like cats!

Written by Jonti Horner, Professor (Astrophysics), University of Southern Queensland.

Adapted from an article originally published in Conversation.Conversation

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