The much-awaited comet C/2012 S1 (ISON), has been prematurely called "comet of the century" but is, at least, a highly unusual visitor from the edge of the solar system. It has been teasing professional and amateur astronomers alike in the past two months, when it has been invisible in the glare of the Sun. This is a purely geometrical effect: ISON is still 2.5 times farther from the Sun than the Earth, but is on the other side of the Sun.
Some time in August, eager amateur observers should be able to pick the comet out of bright dawn skies, as its angular distance from the Sun grows throughout this month from 12 to 30 degrees. At perhaps 13th magnitude, ISON will still be an exceedingly faint object, in reach of only die-hard astrophotographers. But there are more eyes in the solar system than those on or near Earth, and at an international workshop to coordinate observations on August 1 and 2 at the Johns Hopkins University Applied Physics Laboratory in Maryland, team members from several interplanetary missions joined colleagues from ground-, balloon-, plane-, rocket- and orbit-based telescopes to share their plans.
The two components of ISON
On June 13, 2013 the Spitzer Space Telescope imaged comet ISON with its two available filters around 3.6 and 4.5 micrometers wavelength. The image on the left is the 3.6-micron image, dominated by dust reflecting sunlight, which is also drawn out as a little tail. On the right this dust component has been subtracted from the 4.5-micron image, leaving only a spherical shell of glowing gas. Contrary to a NASA press release we still have no clue whether it is carbon dioxide or carbon monoxide: both gases have emission lines in this spectral window, and other comets far from the Sun have been dominated by different volatiles. In any case water vapor should take over as the main "driver" of ISON's activity in late August when the comet crosses a magical "ice line".
One spacecraft that will be called into action later is NASA's MESSENGER in orbit around Mercury: it will have an interesting vantage point when ISON approaches the Sun in November. But the first deep space mission to recover ISON - right now, actually - is Deep Impact's old fly-by spacecraft which, after visiting two comet nuclei and doing some exoplanet astronomy with its camera, had already observed ISON in January. Now the "DIF" is in a much better position than the Earth and is supposed to observe ISON, mainly for infrared spectroscopy, on August 8 and 9. The data downlink is slow, as Deep Impact is now more than twice as far from Earth as the Sun, and will follow on August 10. More observations are possible until August 16 in the infrared and September 12 in visible light. Then the comet is too close to the Sun from Deep Impact's unique vantage point and can only be observed again way into 2014 -- assuming that the comet even survives its close brush with the Sun, of course, something no one at the meeting was willing to bet on.
NASA / JPL / UMD
Comet ISON from Deep Impact
Deep Impact observed comet ISON on January 17 and 18, 2013, taking more than 150 photos. The comet was more than 5 AU away from Deep Impact at the time. These were the first space-based observations of comet ISON.
Next in line is the Mars Reconnaissance Orbiter (MRO), which will be in a perfect spot in late September when comet ISON approaches Mars to within 11 million kilometers -- six times closer than it will ever come to Earth (64 million km on December 26). While MRO's HiRISE camera in particular has occasionally been used as a telescope to look for example at the Earth - Moon system, it is optimized in every respect for scanning the Martian surface. So while both HiRISE and the infrared instrument CRISM will be used during the comet's approach, a lot of guessing is involved in planning the exposures. Therefore the MRO team is happy to join Deep Impact on August 20 for some early remote observations of ISON: the viewing angle is unproblematic as Mars is still on the far side of the Sun and thus closer to ISON than the Earth, but the distance will still be 151 million kilometers. This translates into a poor resolution of HiRISE on the comet of 150 km per pixel, the signal-to-noise ratio won't be great, and CRISM might not see anything at all - still both instrument teams are happy for this opportunity of a little trial run.
NASA / JPL / U. Arizona
HiRISE viewing distant worlds
The planned use of the HiRISE camera on the Mars Reconnaissance Orbiter for imaging comet ISON as it approaches and hurries closely past Mars is not unprecedented: while optimized for scanning the Martian surface, a brightly lit high-contrast target, the 50-cm telescope can also be used as an improvised astronomical telescope. This quite convincing portrait of the Earth - Moon system was acquired on October 3, 2007, over a distance of 142 million kilometers - comparable to ISON's distance during the observations coming on August 20. In late September ISON will be much closer, but HiRISE will still not be able to resolve the comet's small nucleus. But a year later another comet will come even closer to Mars - and then a detailled image of its nucleus can be hoped for.
In the -- still growing -- consolidated timeline of planned observations, DIF and MRO are the only contributors from the professional side in August. But, come September, the ISON campaign will take off -- literally! -- as even a balloon and a rocket launches are in preparation and numerous astronomical satellites will be called to duty in addition to ground-based telescopes (of the nightly, radio and even of the solar kind) and amateur astronomers invited to contribute to specific projects. It is a matter of debate if this is the greatest astronomical campaign devoted to a single object of all time: the spread of observing eyes from Mercury to Mars certainly is. But what's missing is the fleet of spacecraft actually going to visit the comet itself that defined the also very huge campaign awaiting Halley's comet in 1986. The return of Halley's comet had been predicted for centuries, though, while ISON was only discovered in September 2012.
This comet has moved into the crosshairs of science not so much because of its potential -- but not in any way guaranteed -- coming great brightness and impressive tail that may be visible for a few days in December. It is the never-before-observed combination of a fresh comet from the Oort cloud -- way beyond even the planets and the Kuiper belt -- that will come to within 1.2 million kilometers of the solar surface! And was discovered 14 months before that encounter with destiny, also unheard of for such a "sun-skirter". ISON's behavior in the past year has been puzzling, with a stupendously flat lightcurve in the first half of 2013; interpretations at the workshop varied widely and ranged from a mostly dry dustball, soon to crumble, to an ice-rich body that already underwent a prolonged outburst just fading back to normal. Survival of the tiny nucleus -- less than 4 kilometers in diameter, according to the best Hubble images -- closest to the Sun is not guaranteed either. Not only the solar heat but even tidal forces will be uneasily close to destructive. For that reason, plans for another emergency workshop or video conference in December were already mulled at the August meeting so that the ISON campaign could react promptly to the comet's fate.
A color image ... that isn't
Probably the most publicized image of comet ISON to date was taken with the Wide Field Camera 3 of the Hubble Space Telescope on April 30 - not for science but to get a pretty picture of the comet in front of the deep canvas of the distant Universe. But the complex interplay of ISON's motion along its orbits and Hubble fast movement in its Low Earth Orbit ruined all attempts to stack the exposures through two filters into something presentable. To save the effort, the black-and-white image of ISON with the least trailing was pasted -- in grey -- onto a color composite of the sky background, resulting in a hybrid black-and-white comet / color sky image that continues to confuse.
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