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Birght comets like Comet McNaught

  1. Jan 21, 2007 #1
    Some comets like Comet McNaught glow very brightly in the sky. However, they are not a sun, obviously. They are just large chunks of rock? It can't just be from the sun's reflection? Or is it as the bright ones have an ice coating around them. I know they have large kinetic energy but not enough to radiate at the wavelength of visible light.

    How do they get into orbit in the first place? Is it usually from a volcanic explosion from one of the planets in our solar system? Or do they usually come from outside our solar system.
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  3. Jan 21, 2007 #2


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    They are made of up of dust and various kinds of ice. They are leftovers from the formation of the solar system. I think there are two major reservoirs of comets in the outer solar system. One is the Oort cloud, and the other is the Kuiper belt. As they approach the sun they heat up and the vapour trail is what makes them visible.
  4. Jan 21, 2007 #3
    Why not?
  5. Jan 22, 2007 #4
    So they are bits and pieces of rock that dosen't belong to a planet in our solar system. There are billions of these things floating about but we rarely see a them? But we always see a comet. How do these rocks become a comet that glows so brightly? Do they have to go out of their usual orbit?
  6. Jan 22, 2007 #5
    One reason was that I thought there are billions of these rocks floating about but we rarely see them during the day? But we can see comets. What makes comets special?
  7. Jan 22, 2007 #6
    They're big. Really big. And very close to the sun. I guess what makes the core more special than another body is the surrounding plume of ejected material.
    Last edited: Jan 22, 2007
  8. Jan 22, 2007 #7


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    As already said, they aren't all rock. They are mostly ice and dust. That's why they have tails and get much, much bigger [looking] than an equivalent mass in an asteroid.
    Probably not billions, and no, we don't see anywhere near all of them. Most of them are too small or orbit too far outside the planets orbits (otherwise they'd already have been swept-up) to ever see.

    But the ones we do see we see because as they come close to the sun, they heat up and blow off gas and dust and water, making them reflect much, much more light than they would if they didn't off-gass.
    Yes. I guess scientists aren't exactly sure what knocks them out of the ort cloud, but it could be that there is enough left-over debris out there that close encounters with minor planets still happen enough to knock them out of their orbits. And a few that we see have probably been in reasonably stable (if very eccentric) orbits for quite a while.
  9. Jan 23, 2007 #8
    I was wondering why we only see stars and the moon in the sky with the naked eye. There are a lot of asteroids out there as well. Asteriods are defined as composed of rock, carbon or metal, which is orbiting the Sun. A comet is composed of dirt and ices. Comets are characterised by dust and gas tails when in proximity to the Sun. That is why we can usually see comets but not asteroids. And the comets are usually very far from the earh so that is also why we see so few of them?

    http://www.aerospaceguide.net/whatisanasteroid.html [Broken]

    We hardly ever see comets and when we do, they will not periodically circulate around the earth. Hence if we do see one, they must be out of their usual orbit. I wonder why there aren't any comets in a stable orbit around the earth? Maybe the earth is too close to the sun and they will melt. It could also be the case that when a comet has disappeared from our view, it has melted.
    Last edited by a moderator: May 2, 2017
  10. Jan 23, 2007 #9
    Out of their usual orbit of the earth? Must be some misunderstanding..

    I don't think the frequency of comets is so startling. You see planets too - Venus and Jupiter might frequently be mistaken as some of the brightest stars in the sky. We don't normally see asteroids because, by definition, an asteroid is basically just a planet that is much too small to see. The main exception is if an asteroid with a lot of ice happens to get very close to the sun (in which case we observe it as a comet). Since the plume can't be indefinately sustainable against the solar wind, we could only expect to see comets with highly eccentric orbits (passing near to the sun only briefly every many years).

    As for satellites of earth, you can observe many of them just after sunset as well..
  11. Jan 23, 2007 #10


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    "Very far" is very relative. So to keep it simple, we don't see many asteroids with the naked eye because they are too small relative to their distance from us to be seen.
    I'm not sure what you mean here either. Comets orbit around the sun, usually with very long (compared with most other objects in the solar system) orbital periods.

    If they orbited around the Earth, they'd be called "moons" and whatever gas and water could have blown off would have done so by now, leaving relatively small rocky/metallic objects. I guess it is possible for a planet to capture a comet, but it would probably be difficult because of the eccentricity of their orbits.
    Last edited: Jan 23, 2007
  12. Jan 24, 2007 #11
    Do we have comets that can be seen periodically on earth? Note: They may not orbit around the earth but their orbits could be so eccentric that for a brief amount of time they circulate near enough to the earth.
  13. Jan 24, 2007 #12
    We see all comets periodically, each time they pass close to the sun (to produce a bright tail). The distance from earth isn't the important factor.
  14. Jan 25, 2007 #13
    So comets are classified as periodic and nonperiodic. The nonperiodic ones are continually being discovered by astronomers and they can be named after them.
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