ISON Image of the Week

Swift Science (July 15, 2013)

A 5.5-minute exposure of C/2012 S1 (ISON), recorded by the NASA Swift satellite in late January 2013.
Last week we highlighted the early space-based observations of Comet ISON from the NASA EPOXI (formerly Deep Impact) mission, the results from which gave us a very early indication of the activity level of the comet, and marked the beginning of space-based studies of this potentially exciting object. That article also highlighted the fact that our Comet ISON Observing Campaign is both space and ground-based, and that the CIOC intends to use comet-watching spacecraft like EPOXI as much as is possible over the coming months. But the CIOC is both ambitious and resourceful, and we have no intention of limiting our space-based studies of Comet ISON to only those missions designed for studying comets. Space is full of extraordinary telescopes, imagers and detectors designed for looking at stars, galaxies, planets, the Sun, rocks, dust, gases, ice, and numerous other features of our solar system, and we hope to take complete advantage of as many of these as possible over the next several months.

This trend began early, when in late January a team of cometary scientists (including the CIOC's own Dr. Michael Kelley) used an instrument on the NASA Swift satellite to get valuable data about Comet ISON. The Swift spacecraft is not designed for comet observations. It was launched in 2004 to study "gamma ray bursts", or GRBs - the result of extraordinarily energetic explosions in distant galaxies. On board Swift are a series of instruments that are designed to work together and scan the skies in optical, ultra-violet, x-ray and gamma-ray wavelengths, with a mission goals primarily involved in understanding the origins and evolution of GRBs, classifying and looking for new types of GRBs, and seeing what they can teach us about the early universe.

Comets can be spectacular and often unpredictable, but they are unquestionably not sources of GRBs! But in spite of this, the aforementioned research team, led by Dr. Dennis Bodewits (Univ. of Maryland), recognized that Swift's Ultraviolet/Optical Telescope (UVOT) could be pointed at Comet ISON and used to give valuable insight into the comet, which at the time was just over 4 A.U. (~372 million miles) from Earth. So this they did, on January 30, 2013, and the returning images like that shown opposite, which was a 5.5-minute exposure recorded by the UVOT instrument. The data returned allowed Bodewits and his co-investigators to make determinations about the water and dust production rates of Comet ISON, and also make the first early estimates of the size of the comet's nucleus. Specifically, they found that the comet's nucleus was on the order of 5 kilometers (3 miles) in diameter and was shedding far more dust than water: approximately 51,000Kg per minute of the former, compared to only about 60Kg per minute of the latter.

The size estimate of the nucleus means Comet ISON is an average-to-small sized comet. Comet Hale-Bopp, for example, had a 60km (36 miles) nucleus, though that was an unusually large comet. But when it comes to Sungrazers, of which ISON certainly is one, size doesn't matter so much, with Comet C/2011 W3 (Lovejoy) putting on a spectacular show in December 2011 despite a very small nucleus of only about 0.5km (0.3 miles) diameter.

In terms of dust/water production, again Comet ISON was clearly not as "active" as the extraordinary Hale-Bopp, but still much more active than an average comet would be at such a great distance from Earth. The reason the dust production of Comet ISON was far greater than its water production is that water-ice does not "sublimate" (melt/vaporize) very efficiently at such enormous distances from the Sun so the dust was primarily being lifted off the nucleus by something less volatile like carbon monoxide (CO) or carbon dioxide (CO2). However, as Comet ISON gets closer to the Sun in the coming weeks and months, the effects of solar radiation will begin to take their toll on the comet's ices, and we expect the water production rate to increase substantially.

The Swift observations of Comet ISON were performed before the CIOC had actually been formed, so we can't really claim credit for having instigated those observations. But over the past few months we have contacted numerous "non-comet" space missions and encouraged them to adapt their instrumentation to observing the comet at opportune times this year. These missions include planetary mission to Venus and Mercury, and those at or on Mars, as well as several Sun-watching missions, space-based telescopes (e.g. Hubble and Spitzer) and various other astronomical spacecraft like Swift. When all is said and done, we anticipate the CIOC being one of the broadest and most comprehensive cometary observing campaigns in history, regardless of how bright Comet ISON turns out to be!

Every week this year we will put up a new image related to Comet C/2012 S1 (ISON). If you have a cool image you'd like us to consider, please send it to, along with a description and any credits you would want applied. We'll contact you if we choose to use your image on the CIOC Website.

See our ISON Image of the Week Archives for earlier picks!