• July 15, 2013

    Hubble's First Glimpse of Comet ISON

    by Josh Sokol

    Credit: NASAESA, and the Hubble Heritage Team (STScI/AURA)

    For eons, Comet ISON has drifted in the cold, far reaches of our solar system’s Oort Cloud. We don’t yet know ISON’s fate, but we do know this: the comet’s long sleep is over.

    When first imaged by Hubble on April 10, ISON was only just inside the orbit of Jupiter. Even then, the Sun was warming ISON’s surface, causing a coma of gas to form as ice sublimates – transitions directly from a solid to a gas. Researchers hope the changes ISON was undergoing this spring will shed light on what the comet has in store for stargazers this fall.

    This early image doesn’t appear to show much. The Hubble filter we used lets in light across the visible spectrum, not in specific colors – the blue and white in this image just show how the comet’s overall brightness falls off as you get farther away from the nucleus. With clever analysis, though, this image can speak volumes.

    Comet ISON model-subtracted

    The same ISON image, minus a computer model of what a simple cometary coma might look like.

    The above image is tricky. It’s not what Hubble saw. Instead, it’s a comparison of what Hubble saw with what researchers assume a comet might look like. Since a coma is a cloud of dust and gas shed from a comet’s nucleus, this model assumes that a coma will be uniform, fading only as you get farther and farther away from the core.

    But real comet comas don’t work that way, ISON included. The uniform model has been subtracted from the actual coma, so bright areas show where the coma is brighter than expected from the model. It’s obvious that the real coma is brightest where it faces the Sun (which is way, way off in the upper right).

    From this comparison, we can infer that the nucleus feeds the coma by producing a jet of gas and dust where the sunlight strikes it. This makes sense: ice sublimates into plumes of gas from the sunny side, which then exposes new pockets and crevices to the Sun. 

    Comet ISON is awake, and it’s warming up.