• November 18, 2013

    The Latest Hubble Image of Comet ISON

    by Josh Sokol

    As of mid-November, ISON is officially upon us. Using Hubble, we've taken our closest look yet at the innermost region of the comet, where jets of sublimating ice are fueling a spectacular tail. 


    Visit Hubble Heritage for high-resolution images. Credit: NASAESA, and the Hubble Heritage Team (STScI/AURA)


    Made from observations on November 2nd, the image combines pictures of ISON taken through blue and red filters. As Hubble follows the close, fast-moving comet, background stars are streaked in the direction of ISON's motion. The combined effect, which has been removed from the final image, can be seen here:


    A stacked color composite of the November 2nd observations. Since 

    Hubble tracked the comet, each red and blue image of ISON itself adds up

    in the same place, producing a clear, colorful image of the comet. The

    smeared-out background stars don't fare so well. Also visible are cosmic

    rays, which can be seen as small streaks angled in random directions, and

    the chip gap: a horizonal dead zone in the middle of the image. 

    Credit: Hubble Heritage Team


    As we expect, the spherical coma around ISON's nucleus is blue and the tail has a redder hue.  That's in part because of reflected sunlight: dust grains in the tail reflect more red light than blue light, while gas in the coma reflects more blue light. But the coma is also glowing blue-green on its own, as its atoms catch solar energy and re-emit light according to their unique spectral characteristics. 


    This is most color separation we've seen so far in ISON. The comet, nearer than ever to the Sun, is brighter and more structured than ever before. We've certainly come a long way since Hubble started observing Comet ISON, way back in April. The following panel is a sort of comet triptych: we're comparing three Hubble images of ISON, all set to the same scale:


    Comet ISON, looming closer. Since we started observing it, ISON has taken up more real

    estate on the sky, both because it's physically growing and because it's getting nearer to us. 

    Credit: Hubble Heritage Team


    Of course, our eight-month retrospective pales in comparison with ISON's own journey, which started some 10,000 years ago in the Oort cloud -- and might end in a fiery wreck as ISON hugs a tight turn around the Sun on November 28.


    With that long trek in mind, it's time to appreciate where we are. We're in the Golden Age of Comet ISON. Thanks to a jump in brightness late last week, the comet can now be seen with unaided eyes for the first time. Barring the aforementioned catastrophe, it looks to stay visible until mid-December, although it will stay close to the Sun, which will make it tougher to see.


    Thankfully, ISON's proximity to the Sun hasn't stopped astrophotographers, who are busy taking amazing images. These gorgeous photos make it clear that ISON has "outgrown" Hubble -- here's Hubble's field of view superimposed over Michael Jeager's November 8 image of the comet:


    An image of ISON's full extent, with an imprint showing the area of our newest Hubble image.

    Credit: Michael Jaeger


    What's remarkable here is that the entire ISON, this awesome, shimmery space tadpole, is being produced from a dusty ball of ice estimated to be a few kilometers in diameter. Compared to ISON's full extent, Hubble's latest image is tiny. It only shows the very base of the tail. Yet even in this closest closeup we've ever had, a single pixel spans 24 km across the comet.


    Now that Comet ISON is nearby, amateur astromers rule the day.  But Hubble observations, including this latest image, are still providing key insights into the science and spectacle of a comet we hope will continue to impress.


    Our previous coverage of Hubble's ISON images, for your perusal: