Comet McNaught’s Tail
Published on Jan 15, 2007 at 1:08 pm.
6 Comments.
Filed under comets.
A reader posted a question on my earlier Comet McNaught post about the tail of the comet. Rather than answering that in the comments of that post, I decided to devote a whole posting to answer the question. John’s question was why Comet McNaught’s tail didn’t seem to be pointed away from the Sun in the SOHO latest SOHO photos. Now, that is a good question, because comet tails are supposed to point away from the Sun. In fact, I mentioned that several times in my followup posts to the earlier post. So, why does the tail not point away from the Sun? The image above was taken by the LASCO (Large Angle and Spectrometric Coronograph Experiment) instrument aboard the SOHO (Solar and Heliospheric Observatory) spacecraft. The Sun is at the center of the image (the surface of the Sun is blocked by a disk so that the instrument can image the much fainter corona). The comet is the big bright smear to the lower left. Clearly, the tail doesn’t look like it is pointing at the Sun.
I mentioned a bit about comet tails both in my Comet McNaught update, and even more in the post immediately prior to this one. Basically, there are two types of tails, the Type I ion, or gas, tail, and the Type II dust tail. The ion tail is typically bluish, and it is carried almost straight outward away from the Sun by the solar wind. But, the dust tail is a bit more complicated. It is pushed outward by light pressure. When the dust shed by outgassing of the nucleus of the comet, it forms a cloud around the nucleus. Most people don’t realize it, but light can actually push on things. So, the light from the Sun is able to push the tiny dust grains outward. But, these dust grains are still orbiting the Sun. So, a push outward causes them to be pushed into farther orbits. The farther an orbit is from the Sun, the slower it moves. So, these dust grains lag ever so slightly behind the nucleus of the comet. The farther they are pushed, the more they lag, so the tail gently sweeps backward, though still largely pointing away from the Sun. The result is a bifurcated tail, one bluish Type I and one yellowish Type II. Tom has posted an image on his site taken from the STEREO spacecraft showing the two tails. I’ve included a (not particularly well drawn) diagram to show this.
But photographs are inherently two dimensional. The tail is curved in three dimensions. So, projecting this onto a two dimensional photograph, taken from the perspective of the Earth, not the Sun or some observer external to the Solar System, can often do some strange things. For example, in 1957, Comet Arend-Roland passed near the Sun, and the dust tail curved so much that it swung around behind the head of the comet, as seen from Earth. 
Though the tail was most definitely curving away from the Sun, from the perspective of the Earth, you could see the back end of it appear to swing around onto the other side of the head and appear as a spike pointing towards the Sun as seen in this photograph from the Armagh Observatory. We call this sunward appearing spike an antitail. Not all comets show an antitail, only those who happen to be just the right position for the orbital geometries to give this perspective. The tail definitely is not pointing sunward. It just seems to be from our perspective.
Now, let’s get back to Comet McNaught. Here the comet’s orbit is extremely inclined with respect to the ecliptic. That is one reason that the Northern Hemisphere got all the good views the past week or so, and now it will be the Southern Hemisphere’s turn to see the comet. It should be even more impressive now that it has swung past the Sun and shed even more material. Typically the dust tail is more pronounced after perihelion than before. But, this makes for some interesting viewing geometry as seen from Earth. In my update posting on Comet McNaugth a few days ago, I included some diagrams showing the orbit of the comet. Go back and look at them, and you will see that the comet is a bit off to the side and below the Sun, as seen from Earth. It is also rather between Earth and the Sun, and off to the side. The dust tail will be pointing away from the Sun, but curved. From our perspective, that means that the tail will be curved upwards, but oriented so that it is coming somewhat towards us but off to the side a bit. Projecting that onto two dimensions gives an arc that doesn’t look like it is coming from the Sun, but rather to a point somewhat below the Sun.
Try this. Look at a lamp, or some object in front of you. Hold your clenched fist out at arms length towards the lamp, but slightly off to the side. Now imagine something sticking out from your fist, away from the lamp, but curved upwards. Now, wouldn’t that look something like what you see in the SOHO photo at the top of this page?
So, something like that is what is going on with Comet McNaught’s tail, and that is why it doesn’t appear to point away from the Sun as seen in the photograph. So, thanks, John, for the question! I got a whole post out of it! And, there will be more Comet McNaught followup posts to come over the next several months.
-Astroprof
Image Credits: NASA (SOHO), Armagh Observatory
Astronomy Buff - Comet McNaught: So bright you can see it during the day on January 15, 2007 at 2:13 pm: 1
[…] Another good writeup by AstroProf on why the tail isn’t pointed directly away from the Sun […]
A Ler…-- Rastos de Luz on January 16, 2007 at 8:16 am: 2
[…] “Comet Mcnaught’s Tail“, no Astroprof’s Page. […]
dean williams on January 25, 2007 at 5:52 am: 3
That really was very good. BUT, nowhere can I find anything about the speed this thing is traveling at. I know speed is relative, but what is it’s speed through space away from the sun
Astroprof on January 25, 2007 at 9:51 am: 4
At present, the comet is moving at about 63km/s. I don’t know the exact orientation of its motion with respect to the Sun, but it is moving almost directly away from it, so the speed that it is moving away from the Sun would be just a shade under its speed through space, so still about 60km/s.
Andrew Sass on January 26, 2007 at 2:35 am: 5
Great answer to a great question - ‘Viva’ we say to you from Africa
Astroprof’s Page » Comet McNaught Update on February 28, 2007 at 3:22 pm: 6
[…] It’s been a while since my last postings on Comet McNaught (here and here). And, a reader asked a question on another post about where the comet is heading. I thought that others might be interested, so I looked up some information to share with everyone. […]