APOD: Two Comets in Southern Skies (2022 Oct 08)

[APOD Robot]

Two Comets in Southern Skies

Explanation: Heading for its closest approach to the Sun or perihelion on December 20, comet C/2017 K2 (PanSTARRS) remains a sight for telescopic observers as it sweeps through planet Earth's southern hemisphere skies. First time visitor from the remote Oort cloud this comet PanSTARRS sports a greenish coma and whitish dust tail about half a degree long at the upper left in a deep image from September 21. It also shares the starry field of view toward the constellation Scorpius with another comet, 73P/Schwassmann-Wachmann 3, seen about 1 degree below and right of PanSTARRS. Astronomers estimate that first time visitor comet C/2017 K2 (PanSTARRS) has been inbound from the Oort cloud for some 3 million years along a hyperbolic orbit. Schwassmann-Wachmann 3 is more familiar though. The periodic comet loops through its own elliptical orbit, from just beyond the orbit of Jupiter to the vicinity of Earth's orbit, once every 5.4 years. Just passing in the night, this comet PanSTARRS is about 20 light-minutes from Earth in the September 21 image. Seen to be disintegrating since 1995, Schwassmann-Wachmann 3 was about 7.8 light-minutes away.

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[orin stepanek]

is it normal that the tails of two different comets point in different
directions? Aren't they determined by where the sun is?

[bystander]

is it normal that the tails of two different comets point in different
directions? Aren't they determined by where the sun is?

I'll take a stab at this. Comets have two types of tails: an ion or plasma tail, and a dust tail. The ion tail points directly away from the Sun while the dust tail lags a bit and is curved due to motion (???).

Here, with C/2017 K2 we primarily see the ion tail, but if you look closely to the left and behind it you can detect the dust tail. Periodic comet 73P/SW3 is older and disintegrating with most of the volatiles boiled off. There is no ion tail (or very faint). All we see is the dust tail pointing in much the same direction as C/2017 K2's dust tail.

Just a guess, but this is what makes sense to me.

[Ann]

is it normal that the tails of two different comets point in different
directions? Aren't they determined by where the sun is?

I'll take a stab at this. Comets have two types of tails: an ion or plasma tail, and a dust tail. The ion tail points directly away from the Sun while the dust tail lags a bit and is curved due to motion (???).

Here, with C/2017 K2 we primarily see the ion tail, but if you look closely to the left and behind it you can detect the dust tail. Periodic comet 73P/SW3 is older and disintegrating with most of the volatiles boiled off. There is no ion tail (or very faint). All we see is the dust tail pointing in much the same direction as C/2017 K2's dust tail.

Just a guess, but this is what makes sense to me.

I disagree that we see the ion tail of C/2017 K2 (PanSTARRS).

APOD Robot wrote:

First time visitor from the remote Oort cloud this comet PanSTARRS sports a greenish coma and whitish dust tail about half a degree long at the upper left in a deep image from September 21.

I expect gas tails (ion tails) to be blue, or at the very least slightly bluish:

The Tails of Comet NEAT (Q4) Credit & Copyright: Chris Schur.

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You can easily see the greenish (or cyan-colored) coma of Comet NEAT in the picture above, as well as its (here) almost reddish dust tail and blue ion tail.

As for comet PanSTARRS in today's APOD, I'd say that there is no trace of a gas tail. We are seeing the dust tail and the coma and nothing else.

So Orin is right that the two comets in this picture have their tails pointing in opposite directions.

Ann

[Chris Peterson]

2017K2_2022-09-21_web-label.jpg
is it normal that the tails of two different comets point in different
directions? Aren't they determined by where the sun is? :shock:

Dust tails generally follow the orbital path of the parent body, although the smaller the particle the more it can be pushed outward by solar wind and radiation pressure. So the actual dust tail is usually curved outward along the orbital path. In this case we have a comet that is orbiting fairly close to the ecliptic plane, and a comet that is orbiting almost perpendicular to it, so it's unsurprising to see very different apparent tail directions.

[Ann]

How comet tails change during their orbit of the Sun. Image: NASA.

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As we can see from this illustration, ion tails always point straight away from the Sun, but dust tails are affected by the orbital speed and direction of the comet as well as by the solar wind. So it makes sense that two comets may have dust tails pointing in different directions.

Ann

[orin stepanek]

Thanks; Bystander; Ann; and Chris!