Comet ISON, a FAQ for the Perplexed

My first, rubbish image of ISON.

Comet C/2012 S1 ISON has come out from behind the sun, and around the world amateur and professional telescopes alike are focusing on it, trying to follow the evolution of this visitor from the frigid depths of space. But faster than the information comes from these scopes comes misinformation, misunderstanding and outright hoaxes.

 For many of us this is Deja Vu. Virtually everything that is being said about ISON was said about comet C/2010 X1 Elenin.

But it is worth dispelling some of the misapprehension about ISON so people can enjoy the spectacle of the comet without fear.

So, I heard comet ISON has broken up.

No, it was found where it was predicted to be and it’s brightness is pretty much as predicted.  The hoax images going around of comet ISON’s “breakup” are from the breakup of comet 73P.

Will it hit the Earth? 

No, it comes nowhere near the Earth, the closest it comes is 0.4 AU from Earth. This is just under half the distance between the Earth and the Sun. Venus comes closer to us than the comet. To give you a feel of the distances involved, get a soccer ball, a pea and a sesame seed (or a grain of salt). Go to your back door, then place the pea there. That represents the Earth. Now walk towards your back fence with 10 full strides. Place the soccer ball there. That represents the Sun. Pace out 4 full strides at right angles to the soccer ball pea axis (you may need to go into your neighbours yard to do this) and place the sesame seed/grain of salt. That’s the comet (and this exaggerates the size of the comet). See the illustrations below for more context.

But surely the comets encounter with Mars and the Sun will alter its orbit! 

This is already accounted for it the orbital calculations. We have a very good understanding of gravity and the interactions of bodies with each other, and use these interactions to slingshot spacecraft very precisely into the depths of space. There is some uncertainty involved, we don’t know the precise mass of ISON, and non-gravitational effects from the gas and dust blasting from the comet will alter the orbit slightly. However, from our experience with past sun grazing comets we can estimate the range of orbits we could have, and none of these come anywhere near Earth.

Again, go back to your sesame seed/grain of salt, move it 10 millimetres closer to the pea. That’s the level of variation involved. When comet C/2011 W3 Lovejoy, a smaller comet than ISON, slingshoted around the Sun in 2011, it turned up pretty much bang where it was predicted to be.

What if the comet breaks up? 

Comet 73P after breaking up. Image Credit: NASA/Spitzer via Wikipedia Commons 

Hollywood script writers should be smacked along side the head with the thickest astronomy textbook you can find. They are responsible for the idea that comets (or asteroids) break up and shower chunks in every direction.

What will happen is that the chunks will carry on in the same orbit as the original comet. Just as Ikea-Seki did. It may be possible we will see a spectacular “chain of pearls” effect as seen with comets 73P and Shoemaker-Levi. But the chunks will come nowhere near us, just like the original comet. Even if it completely disintegrates, like C/2010 X1 Elenin or C/2011 W3 Lovejoy, the cloud of dust will continue on the same orbit just as it did for these comets. Again, it will come nowhere near us.

What about all the objects following ISON? 

There are no objects following ISON, people have been misinterpreting hot pixels on some animations. In most CCD camera chips, there are always a few pixels that are permanently on, these are the “hot” pixels.

What about the “UFO” image?

Combination of three separate Hubble images of ISON aligned on the background stars. Image credit NASA/Hubble.

That was a combination of three separate images taken when Hubble was tracking the stars. Not only does ISON move in regard to the background stars, Hubble is also orbiting the Earth, so it’s view of ison while it is pointing towards the stars changes considerably with time. if you just combine these separate images so that all the stars are aligned, the comet will be seen in different places. See here for a more detailed explanation with images.

Will we go through the comet ISON’s tail? 

No.

But the tail will be enormous! 

And nowhere near us. The comet crosses above Earths’ orbit on November 1, at that time the comet and its tail will be 0.023 AU above Earths’ orbit, that’s 10 times the distance from Earth to the Moon. On our backyard model, the sesame seed will be the length of your foot away from the line that defines the Earth’s orbit. But at this time Earth itself is over 12 full strides away from the comet and its tail. The tail always points away from the Sun so it can’t come near Earth. Replace the sesame seed with a piece of string one stride long, that will give you a sense of the scale of the tail within the solar system (and it’s relation to Earth)

Tails actually, there is the gas tail, which points directly away from the Sun, and the dust tail, which points away from the Sun but follows the comets orbit (so it looks curved to us). The particles in the dust tail follow the comet in its orbit, so two and a half months, when Earth reaches the point in its orbit where the comet crossed above it, all the dust and gasses will be long gone. Also remember that the tail will not be as big in November, the tails of comets only reach their fullest extension after perihelion, when the comet is closest to the Sun and the rapidly sublimating ices blast lots of dust and gas into space.

See this post on the tail of ISON and the animations in it. Also, scroll back up to the illustrations of the comets orbit.

So, no meteor shower then?

No. we might get some increase in noctilucent clouds, but not a meteor shower (NASA PR flacks should be whacked with an astronomy textbook for describing noctilucent clouds as a meteor shower).

But it would be really bad if we went through the tail wouldn’t it? 
 
No. Remember that a comets tail is a fairly good vacuum, it’s less of a vacuum than the interplanetary medium and solar wind, but still it’s mostly nothing. People get upset at the idea there is cyanogen in the gases of the comets tail, but cyanogen only represent 0.5% of the gases, and 0.5% of what rates as a pretty good vacuum isn’t dangerous.

We have been immersed in comets tails before, notably comet Halley, the Great Comet of 1861 and we were in the tail of the Great Comet of 1853 for two whole days. Nothing happened.

To give you a feel for how little material is in a comets tail, I’ ll use the gas density (dust density is similar, but the gas density is more easily comparable. In one cubic centimetre of air in Earths’ atmospher there at around 10²² air molecules, in the dense coma of a comet the average density is 10¹¹ molecules, just under a trillion times less, in the tail (depending on how far you go out, the further along the tail, the less gas there is) around 10² molecules, just short of a billion trillion times less.

So if you took a volume of comets tail equal to ten volumes of Earths’ atmosphere, and dumped it in Earths’ atmosphere, you would have much much less than a trillionth of the gas coming form the comet (and most of that is water). A similar proportionality holds for comet dust, so the answer to “how much dust or gas will we get from ISON’s tail” is “almost unimaginably tiny, and well below the threshold that could possibly cause any harm”

What about Earthquakes?

No, just no. Just like comet C/2010 X1 Elenin (which didn’t cause any earthquakes) Comet C/2012 ISON has less than a billionth of the tidal force of the Moon at closest approach (as well as a negligible magnetic field). If the Moon can’t cause the poles to tip, cause massive tidal floods or earthquakes, C/2012 ISON won’t (and the previous Comet of Doom Comet 2010 X1 Elenin didn’t). We’ve been closer to other comets before with no ill effect. See this link for comprehensive discussion of solar system objects and Earthquakes.

Giant Solar Flares?

No. Comets don’t cause solar flares.

What is the size of the comet? Some say it is bigger than Mars

Comet ISON as imaged by Hubble in May 2013, Image Credit NASA/Hubble

Independent measurements from the Spitzer telescope and the Hubble telescope put a maximum size of around 5 kilometres in diameter for comet ISON’s nucleus, it is probably a bit smaller.

Some people are confusing the size of the coma, the thin envelope of dust and gas around the comet nucleus, with the enormously smaller chunk of dust and ice that is the nucleus. Remember that the coma, like the tail, is still a pretty good vacuum by Earth standards.

One commentator is claiming the nucleus is as big as Mars, but its is easy to see that it is not. Comet ISON is almost at the orbit of Mars. If you go out around 5 am this morning Mars is easy to see at magnitude 1.5, ISON is around magnitude 12 (just below Mars), many times fainter the threshold the unaided eye can see (magnitude 6).

Even if ISON was as dark as charcoal (like the Moon is) it should be an easy unaided eye object if it was the same size as Mars. For comparison, the 400 Km diameter asteroid Vesta is currently 3.3 AU from earth in comparison to ISON’s 2.7 AU, and it is magnitude 7.8, so ISON must be much, much smaller than Vesta.

Oh, Okay. So, brighter than the full Moon at Perihelion then?

No. When it was first seen some tentative predictions suggested it could get as bright as the full Moon. But it turns out that ISON is an Oort cloud comet on its first visit to the solar system, and these don’t brighten as much as comets that have been around several times. Current predictions based on over 3000 observations by amateur and profession astronomers suggest it will probably get as bright as Venus, maybe a bit more, when it is closest to the Sun.

While that is bright enough to see in the daytime, the comet is so close to the Sun that only very seasoned observers have a chance to catch it. See this post on seeing comet ISON at its brightest.

So, a bit disappointing then.

No, provided that comet ISON does not break up before perihelion (when it is closest to the Sun), it will be a nice little comet, even with the most pessimistic of projections. We can only make tentative guesses about how long and bright the tail post perihelion will be, as it depends of details of ISON’s composition we don’t know yet (like the exact ratio of dust to gas).

It might sport a spectacular tail like C/2006 P1 McNaught, or a thin searchlight like C/2011 W3 Lovejoy. If it breaks up into chunks post perihelion it might become as spectacular as Ikea-Seki, if it falls apart completely it might be more like C/2011 W3 Lovejoy.

All we can do is watch and wait as it evolves.

So where can I get good information on comet ISON then?

Well, this blog for a start. Then there is

Comet ISON news
Waiting for ISON (with good northern hemisphere spotters charts)
ISON Observers Page  and its blog.
ISON Atlas
Hubble ISON Blog
Comet ISON ephemeris
Comet ISON Factsheet

Keep watching the skies and happy observing!