• November 27, 2013

    ISON Q&A: Perihelion

    Today we answer questions about Comet ISON and perihelion. What do you need to know about ISON? Tell us on Facebook, Twitter and Google+, and we'll try to find the answers.


    So, everyone seems pretty excited.

    This is true.


    What’s the deal?

    Comet ISON is about to shimmy up to the Sun and see if it wants to be friends.


    Does it?

    No. The Sun is uncaring and emotionally closed off. Also, it is a giant non-sentient ball of burning hydrogen and helium. All its relationships end in tears and/or total annihilation.


    Never learned to love. Credit: SDO


    Well, that sounds bad.

    Yes. It’s going to be terrible. Pass the popcorn.


    What’s this talk I hear of ISON reaching “perihelion”?

    OK, if you want to be all official and technical, perihelion is when ISON reaches the point in its orbit that’s closest to the Sun. After a journey of millions of years from the Oort Cloud at the outer edge of the solar system, ISON will finally come within 0.012 astronomical units (AU) of the Sun – closer than Mercury – on Nov. 28.


    ISON begins to have doubts about the basic competence of its travel agent. Credit: NASA/STEREO


    What happens then?

    There are several scenarios, but here are the main ones: ISON survives with its nucleus, or solid core, still whole. ISON is weakened by its encounter with the Sun and breaks up slightly, but its pieces are still there. ISON undergoes something called cataclysmic disruption, is completely torn apart by the Sun’s gravity, and essentially disintegrates. Or finally, it breaks up to the point where a fragment remains, but it’s very small.

    In the first two scenarios, we would see ISON from Earth, shining brightly in the night sky and streaming a tail of sublimated gas, as it reemerges from behind the Sun. In the last two, there would be nothing left for naked-eye viewers to see.


    We don’t know for sure?

    No. Comet ISON is right on the edge of what astronomers call the Roche Limit, which is the distance at which a solid body can pass an object with massive gravity, like the Sun, before the gravitational field rips that object apart. Comets aren’t completely solid objects, however – they’re loosely bound collections of ice and dirt. Astronomers have done their best to estimate ISON’s density and strength, and those calculations put the comet almost exactly at the Roche Limit. So it could go either way.


    Know what's hard to find? Images of the Roche Limit. Here is a picture of a beagle who is concerned about our lack of a definitive answer. Credit: Wikimedia Commons user Mariano Szklanny


    How will I know what happened?

    You can watch the Solar Terrestrial Relations Observatory (STEREO) coverage of Comet ISON. STEREO is an orbiting NASA probe that studies the Sun. If you look here, you can get a thorough, step-by-step accounting of the observations STEREO is able to make during perihelion. The Solar Dynamic Observatory (SDO), hoping to see extreme-ultraviolet emission from ISON as it pulls up to the Sun, will also provide real-time images during the event. STEREO reports how you can watch the event as it happens or shortly after here.


    Can I use this information to get out of awkward conversations at Thanksgiving dinner?

    Absolutely. We recommend the phrasing, “That’s an interesting point/question/observation on my love life/child-rearing/political affiliation, but if you’ll excuse me, I really need to go see if Comet ISON has survived perihelion. It was right at the Roche Limit, you know, and we see Oort Cloud comets so rarely!” And then run.