• November 28, 2013

    ISON Breaks Up


    ISON appears to have shattered as it reached its closest point to the Sun today, torn apart by the Sun's intense gravitational field.

    Three solar observatories were monitoring the comet as it approached the Sun. ISON grew faint in the view of the Solar Terrestrial Relations Observatory and Solar and Heliospheric Observatory. The comet was not visible at all via the Solar Dynamic Observatory, indicating that it may very well have evaporated by that point.

    ISON's orbit brought it extremely close to the Sun, but its mass was right at the limit at which it could survive such a trip, making the Thanksgiving observations a nail-biter. Viewers on Earth were hoping for ISON to survive, putting on a show in the night sky as it reemerged from behind the Sun. But it looks now like the comet will have been totally destroyed, leaving nothing left to light up the night.


    NASA's Solar Terrestrial Relations Observatory and the ESA/NASA Solar and Heliospheric Observatory show Comet ISON dimming as it journeyed toward the Sun. 


    Few sungrazing comets survive their trip to our solar system's home star. Not only must these small, loosely bound collections of ice and dirt withstand the Sun's gravity, the extraordinary heat of the Sun's corona weakens a comet as it warms it, making it much easier for gravity to stretch and break the nucleus -- not unlike the way a popsicle basking in the Sun of a hot day is more likely to slide off its stick than one fresh from the freezer.

    A white plus sign shows the location where Comet ISON would have appeared in this Solar Dynamics Observatory image if it survived its encounter with the Sun. Credit: SDO


    ISON may be no more, but it will live on in the data meticulously collected by astronomers over the past few months. New sungrazing comets from the far-distant region called the Oort Cloud are infrequent, and the observations of ISON will help us understand much more about these comets, the Oort Cloud where they reside, and the conditions in which they formed. 

  • November 28, 2013

    ISON Reaches Perihelion

    Comet ISON will hit its closest point to the Sun today this afternoon. Here's where you can follow the action:

    • NASA will monitor the approach live with a Google Hangout today from 1-3:30 p.m. EST. 
    • The Solar Dynamics Observatory expects to have images of ISON begin appearing between 12:45 and 1 p.m. EST.
    • Watch the Solar Terrestrial Relations Observatory (STEREO) coverage.
    • The Solar and Heliospheric Observatory will have images as well.
  • 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.