Remnants of Comet ISON fading fast

November 30th, 2013 by Roy W. Spencer, Ph. D.

As I surmised yesterday, it appears that the nucleus of Comet ISON was mostly destroyed during its close approach to the sun, and what remains is fading fast, now estimated to be magnitude 5 in brightness:

According to Karl Battams’ recent blog post,

“…during its passage through the Sun’s million-degree corona, its dusty/gassy coma got very much burned away, though clearly some fine dust survived (which is the fine cloudy stuff you see being pushed away from the Sun).”

(His post also has a couple of very cool animations from the STEREO spacecraft, so I encourage you to take a look.)

Why does the comet appear the way it does now, after perihelion? Here’s a semi-technical explanation…what I’ve surmised based upon my knowledge from atmospheric science.

Basically, the material being flung out sideways from the comet’s normal orbital path is very fine, whereas the particles in the “head” are larger. Although the reasons for this are not discussed in other blogs posts about the comet, I suspect it is the same reason why tiny particles of rock can float in the air as aerosols, whereas large particles would fall to the Earth. Or, why fine sediment particles are suspended in a river, while large particles fall to the bottom.

Gravitational forces act on the mass of an object, whereas atmospheric or liquid viscous pressure forces act on the physical size (cross-sectional area) of the object. Because mass increases as the 3rd power of the particle radius, and physical size (or cross-sectional area) increases as only the 2nd power, the larger a particle is, the greater the gravitational effects are relative to pressure forces.

In the case of the comet, the pressure forces are from radiation pressure and the solar wind. As the nucleus of Comet ISON got pulverized during perihelion, the tiniest particles were more affected by the radiation pressure and solar wind than by gravity, and they got “blown away” from the normal orbital path. As I mentioned in my last post, they appear to be “flung outward”, away from the normal gravitationally-dominated orbital path of the comet nucleus.

If you watch the above video, you will see this fine material being blown in the solar wind, away from the nucleus. Presumably, what is left in the nucleus are somewhat larger particles whose path is still dominated by gravity, but there is so little left that there is not much there to reflect sunlight that we can see.

I suspect that in the coming days only the better telescopes will be able to see what is left of the comet. Astrophotographers like me will see very little if anything at all.

22 Responses to “Remnants of Comet ISON fading fast”

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  1. OssQss says:

    Thanks Doc

    It is amazing!

    A little more than a year from discovery to the apparent demise of a comet.

    A few millimeters of additional gravitational influence a couple light years away and things could have been very much different.

    Reminds me of lyrics from an old song,,,,,,,,,

    “out of the blue and into the black”

    Name that tune? 🙂

  2. The artist has the same initials as the state New York!!!! 🙂

  3. Massimo PORZIO says:

    I’m absolutely ignorant about this, so maybe that mine it’s a silly question, but just to know:

    does anybody know why while the comet was approaching the Sun atmosphere its head shows an horizontal line?

    The line show up about at 11/27 19:30, it had its maximum at about 9:30 of the day after and it disappeared at about 12:54 just before entering the Sun atmosphere.

    Is that an artifact of the satellite’s sensor or what else?

    Have a nice day.


    • Massimo PORZIO says:

      Ok, it was a silly question, I googled by myself and discovered that that line is called over-saturation spikes and are due to the brightness of the head of the comet that saturates the satellite detector.

      It seems that it was very hot there 🙂

  4. Stuart L says:

    Is there any possibility that the dispersed particles will eventually come back together again by mutual gravity.

    • Adam Gallon says:

      Unlikely, what’s left will be blown apart by the solar wind. The masses of the individual particles will be too low to gravitationally attract each other.

  5. Bri says:

    I believe the exiting comet behaves the way is dose because we are seeing an explosion with most of the ejecta on the side opposite the sun. The tail following the trajectory of the comet is the stuff that has been melted away but still follows its original Corse but is spreading out because each molecule now has sideways momentum. My theory is the comet was heated from one side and the temperature differential caused an explosion on the sun side giving the other side the extra momentum to shoot out. It still has the same orbit but now it is just a wide band of micro comets.

    • Dr. Strangelove says:

      From NASA website

      “The comet will approach within about 730,000 miles (1.2 million km) of its visible surface, which classifies ISON as a sungrazing comet. In late November, its icy material will furiously sublimate and release torrents of dust as the surface erodes under the sun’s fierce heat…”

      “Sungrazing comets often shed large fragments or even completely disrupt following close encounters with the sun, but for ISON neither fate is a forgone conclusion.”

      “We estimate that as much as 10 percent of the comet’s diameter may erode away, but this probably won’t devastate it,” explained Knight. Nearly all of the energy reaching the comet acts to sublimate its ice, an evaporative process that cools the comet’s surface and keeps it from reaching extreme temperatures despite its proximity to the sun.”

      Apparently if heat did not destroy ISON, gravity did it.

  6. Dr. Strangelove says:

    I dont agree with your explanation. I think radiation pressure and solar wind are too weak to break up a comet. Radiation is massless photons while solar wind is subatomic particles. Their impact force is too small. I think the cause of the breakup is difference in gravitational force. The part of the comet nearer the sun is pulled by greater force than the part farther away.

    The comet is a relatively small body. But the sun is a very big body. When near the sun, the comet is pulled by great forces. Strong enough to break it. Imagine if the sun has a solid surface. When you stand on it, your legs will break because you weigh hundreds of tons. I think thats what happened to comet ISON.

    • Dr. Strangelove says:

      Radiation pressure is measured in micro pascal. Solar wind in nano pascal. They are too weak to break a solid object. I computed the stress due to sun’s gravity at perihelion (1.86 million km) assuming the comet is 2 km in diameter, 2 trillion kg in mass and spherical shape. It’s about 13 pascal. Since the comet broke up, it’s cohesive strength is less than 13 pascal or equivalent to very soft clay. I surmise it’s not a solid rock but more like a pile of rubble.

    • Bri says:

      I agree that gravity must have had an important role in the break up but we have seen a comet break up due to gravity with Shoemaker Levy and it was quite different than this comet. The comet broke into a string of comets as the parts closer to Jupiter got pulled free and changed their orbit slightly compared to the rest of the comet. Assuming the comet was completely ripped apart you would end up with a string of very small pieces that would evaporate almost instantly. This plasma would retain its original orbit and freeze into dust. The problem is the comet has two tails when it comes out of the sun and nether one is pointing the right direction one appears to be a 90 deg from the trajectory and one if following the comet. So I think there are much more complex interactions going on here and I will look forward to more info from NASA assuming the remnants can be tracked.

      • Dr. Strangelove says:

        If the comet disintegrated into many pieces due to sun’s gravity, I suspect the small pieces near the sun will get sucked by the sun and disappear. The bigger pieces farther out may retain their trajectory but wider orbit. But they may be too small to be visible.

        BTW the comet’s tail points not necessarily along its trajectory but away from sun in the direction of the solar wind. If you see two tails, it could be two big pieces in different locations in space.

        • ri says:

          Yes the tail should be pointing away from the sun but in the clip above there are two and nether one is pointing in the right direction.


    • fonzie says:

      Objects in orbit are essentially weightless… Recall an object in orbit is merely “falling” into the larger object but “missing” the object because of a fast lateral motion. HOWEVER,this is a long eliptical orbit so maybe it’s not quite as simple as that. Anyone care to comment on that?

      • fonzie says:

        on second thought i don’t even think a long elliptical orbit should matter…

      • Dr. Strangelove says:

        I calculated the gravitational force between the sun and comet. It exists even if the comet is weightless. It is not the weight that breaks it but the centripetal force due to sun’s gravity. When the stress between two points in the body exceeds the body’s cohesive strength, it will break.

  7. coturnix says:

    I bet tbe comet evaporated because of the terrible runaway greenhouse effect caused by all the co2 degassed from it as it flew past the earth orbit.

  8. Dr. Strangelove says:

    The solar wind and radiation pressure exert 250 micro pascals on the dust particles. Wind on earth surface exerts 101 kilo pascals. One billion times greater. The acceleration due to sun’s gravity (g) at perihelion is 38 m/s^2. On earth surface g = 9.8 m/s^2. Dust of ISON is 4 times heavier than dust on earth. I suspect most dust will fall to the sun than blown away by solar wind.

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