SKY REPORTER: Center Stage for a Comet

Hubble Space Telescope view of comet ISON (C/2012 S1) on April 10, 2013.
NASA, ESA, J.-Y. Li (Planetary Science Institute), and the Hubble Comet ISON Imaging Science Team

Comet ISON reminds me of a theatrical production playing off-Broadway with wonderful reviews, but then developing issues and some negative buzz. Tweaking goes on behind closed doors but, as time nears for its Broadway run, critics are eager to see changes. 

On the celestial stage, astronomers are scheduling dates at observatories great and small and the curtain of morning twilight will soon draw back revealing the current state of Comet ISON, latest claimant to the title “Comet of the Century”.

Although ISON hasn’t brightened as expected, NASA’s Comet ISON Observing Campaign kicked off August with an Observers Workshop in Laurel, Maryland to discuss extensive research plans. Astronomical facilities including Keck Observatory, InfraRed Telescope Facility, National Solar Observatory, Big Bear Solar Observatory, Hubble Space Telescope, and many private observatories world-wide are currently accepting ISON observing proposals.

At the end of November Comet ISON, formally known as C/2012 S1, is expected to pass less than one solar diameter from the Sun. It then will be subject to extreme heat and gravitational stress and its reactions will be of great interest. That information will be seen in light of what by then will have been gleaned about the object’s composition and structure as it moves past Mars then races toward the Sun this autumn.

At the beginning of August, Comet ISON is in the constellation of the Crab, rising about an hour before the Sun for observers in mid-northern latitudes, with a predicted brightness near magnitude 14—much too faint to be seen against the glow of morning twilight. By midmonth, the comet rises about two hours before sunrise, still is too dim for its glow to compete with advancing morning light.

However, by the last day of this month, comet ISON rises above the east-northeast horizon at 3:30 am, three hours before sunrise. In that early morning sky ISON will be five degrees of arc from Mars but 64 million miles more distant from us than the Red Planet. Current predictions by NASA are for a total brightness of magnitude 13 around that date. From dark sky observing sites the comet’s fragile light might be glimpsed in large telescopes as it ascends higher above the horizon before twilight washes away the view.

Very soon afterward we’ll know more about the validity of exuberant predictions made before May when seasonal twilight overwhelmed the comet’s glimmerings. Perhaps we really will have a block-buster comet to savor during this holiday season, remembered and talked about the rest of our lives. Maybe ISON will join the ranks of other great comet contenders noted in my earlier Hayden blogs. Or, it might be a complete bust. Compilations of ISON observations made before the Sun intervened in May indicate the hoped for acceleration of brightness had not yet happened. Happily, we’ll soon know if the comet has changed much over the summer. 

Capriciousness and curiosity have big roles in our fascination with comets. Previews are over, the curtain of twilight will soon vanish, and observers are keen to get on with their work. As of August ISON remains a developing story.

The Moon

Perseid Meteor Shower

This best known “shooting Star” event is expected to peak between late evening August 12 and twilight the next morning. Perseid meteors are dusty fragments released by Comet Swift-Tuttle along its orbit, encountered by earth about this time each year.

Planets

Venus and Saturn are the only planets easily visible during evening hours of August. Both appear as twilight fades, with Venus low in the southwest and descending beneath the horizon about an hour after sunset. Saturn doesn’t set until later in the evening. Jupiter is becoming more apparent in early morning hours. Mars is still quite faint compared with Jupiter, the Red Planet’s current neighbor in Gemini.

The planets for August 15, 2013: