As of the date at the top of this post, Comet C/2012 S1 - better known as Comet ISON - is 115 days from perihelion, its closest approach to the Sun. The comet, a "dirty snowball" a few kilometers across, will pass just 1.1 million kilometers over the Sun's surface on 28 November 2013. On 26 December 2013, as it leaves the inner Solar System, it will pass Earth at a safe distance of about 0.43 Astronomical Units (AU); that is, about 0.43 times the average Earth-Sun distance of 149,597,871 kilometers.
As it rounds the Sun, Comet ISON could become a "great" comet; that is, one that shines brightly enough that it becomes hard to miss even in the light-polluted skies of large cities. Its diffuse, glowing tail might span several full moon-widths, and its nucleus might glare brighter than Venus (normally the third-brightest celestial object after the Sun and moon). It might even become visible in daylight.
Because it is a first-time visitor to the realm of the planets, however, it is impossible to predict how it might behave. As veteran comet discoverer David Levy once observed: "Comets are like cats; they have tails, and they do precisely what they want."
Dud or delight, Comet ISON is of great interest to planetary scientists. Comets are time capsules, and first-time comets like ISON preserve undisturbed material dating from the birth of the Solar System 4.6 billion years ago. Because of this, Comet ISON has been under close observation since its discovery on 21 September 2012.
One might ask whether we will dispatch a spacecraft to explore Comet ISON; it is a fair question, given that we have launched spacecraft to comets for nearly 30 years. Alas, at today's level of space technology, it cannot be done. The very thing that makes Comet ISON so interesting - that it is passing through the inner Solar System for the first time - means that we lack sufficient time to ready a spacecraft to visit it. Comets like ISON can arrive from any direction and are nearly always extremely faint until they are well inside the Solar System. It is rare for us to discover one much more than a year before it reaches perihelion.
The comets our spacecraft have visited have had the bad luck to be captured into short-period Sun-centered orbits, typically through a close encounter with Jupiter's mighty gravitational pull. Many such comets, most of which are doomed eventually to deplete their stock of ices as the Sun intermittently broils their surfaces, have been observed for decades or even centuries. This means that we can plan a mission to fly past one years in advance.
NASA and NASA contractor engineers and scientists first became interested in launching robotic spacecraft to comets during the 1960s, but serious comet mission planning had to await the shift in U.S. robotic program priorities from Apollo support - for example, imaging potential Apollo landing sites - to Solar System exploration. Scientific meetings devoted to comet exploration in the 1971-1974 period revealed two comet science communities: an old guard of astronomers, many of whom saw spacecraft as competition for funding for telescopic comet studies, and young turks who sought spacecraft missions to comets at the earliest opportunity. The latter won an important victory in August 1974 when the Space Science Board of the National Academy of Sciences endorsed a plan to launch two flyby probes to Comet Encke in 1980.
Robert Farquhar, an engineer at NASA's Goddard Space Flight Center (GSFC) in suburban Washington, DC, was instrumental in planning the 1980 Comet Encke dual-probe flyby mission. He also participated as Mission Definition Manager in GSFC's Cometary Explorer study in 1972-1973. On 1 November 1974, a year after the Cometary Explorer study ended, Farquhar presented to the International Astronomical Union (IAU) Colloquium on The Study of Comets a hopeful blueprint for 1980s cometary exploration. It started with the 1980 Encke mission and culminated with pre- and post-perihelion Comet Halley flybys in 1985-1986.
Farquhar's blueprint was arguably all about Comet Halley. Easily the most famous comet, Halley orbits the Sun once about every 76 years. The missions Farquhar described aimed to test spacecraft and instruments and to provide NASA with comet exploration experience ahead of Halley's historic first perihelion passage of the Space Age. In his report to the IAU Colloquium, he made "a special plea" to unnamed "appropriate science advisory groups" to begin "serious planning for sending space probes to Halley. . .in the near future."
Farquhar saw his comet flyby mission series as a possible lead-in to comet rendezvous missions. A rendezvous mission, which would see a spacecraft match orbits with a comet and fly formation with it for weeks, months, or even years, would demand considerably more propulsive energy than Farquhar's flybys. He wrote that comet rendezvous would require NASA to develop a Solar-Electric (solar-powered ion) Propulsion Module or a high-energy chemical rocket stage. Flyby missions to comets with different characteristics would aid scientists and engineers in designing instruments and choosing targets for future rendezvous missions, Farquhar explained.
The Encke mission would begin on 24 August 1980 with the launch of two probes (total mass, 845 kilograms) on a single Titan-3E rocket with a Centaur upper stage. The upper stage would inject the probes into a Sun-centered elliptical orbit with a period of 0.55 years (that is, one-sixth of Encke's 3.3-year orbital period). The probes would fly past Encke at the relatively low velocity of 7.9 kilometers per second on 7 December 1980, just one day after the comet passed perihelion 0.34 AU from the Sun. If all went as planned, Probe 1 would pass no more than 824 kilometers from the Sunward side of Comet Encke's nucleus. Probe 2, meanwhile, would pass through Encke's narrow tail, which Farquhar explained lacked detectable dust. Lack of dust would reduce the probability that the probes would suffer damage during their Encke encounters.
Of the four comets visited in Farquhar's mission series, Encke would be the easiest one to explore. The 1980 perihelion passage was especially favorable for a slow flyby; Farquhar explained that an opportunity as favorable would not occur again until the distant year 2013. He noted also that a relatively small post-flyby maneuver (a velocity change of about 130 meters per second) would put the probes on course after six more solar orbits for a second near-perihelion Comet Encke flyby on 28 March 1984.
The 1984 Comet Encke dual-probe flyby would constitute only the second comet visit in Farquhar's series. The third, a visit to Comet Giacobini-Zinner, would not leave Earth until 10 March 1985, nearly a year after the second Comet Encke flyby. Though dictated by the motions and positions of the target comets, not by choice, the gap between missions would be advantageous; Farquhar wrote that it would provide ample time for engineers and scientists to use Comet Encke flyby data to optimize subsequent flyby missions.
The Giacobini-Zinner flyby would begin with probe launch from the payload bay of a Space Shuttle Orbiter. In keeping with the NASA line in the 1970s, Farquhar assumed that the Shuttle launch would be cheaper than launch on an expendable rocket. Once in Earth orbit, the Shuttle crew would check out the Giacobini-Zinner probe and its attached solid-propellant booster rocket, deploy them from the payload bay, and pilot the Orbiter away to a safe distance. The booster would then ignite to launch the probe out of Earth orbit. Farquhar noted that a Delta-3914 expendable rocket could launch the probe if no Shuttle were available.
The probe would fly past Comet Giacobini-Zinner on 11 September 1985 at a relative velocity of 20.6 kilometers per second just six days after it passed perihelion 1.03 AU from the Sun. Farquhar wrote that Giacobini-Zinner, with an orbital period of 6.62 years, was especially well observed in 1972, when it passed relatively near Earth. He noted that it expelled 50 times more dust than Comet Encke, but only 1/1000th as much as Comet Halley.
The Giacobini-Zinner probe's initial orbit would have a period of exactly one year, so the probe would fly past Earth at a distance of about 32 Earth diameters - just beyond the moon's orbit - on 10 March 1986. This gravity-assist flyby would alter its orbital path, though not as much as would a second Earth flyby at a distance of about 1.5 Earth diameters on 20 August 1987. The second Earth gravity-assist flyby would nudge the probe into an orbit with a period of 1.61 years and place it on course for a close encounter with Comet Borrelly on 25 December 1987. The Borrelly flyby would occur seven days after it passed perihelion at a distance of 1.36 AU from the Sun.
Borrelly, Farquhar noted, had been gravitationally perturbed in 1936 by Jupiter into an orbit that made it difficult to observe. Fortunately, a second Jupiter perturbation in 1972 had nudged it into a new orbit, again placing it within reach of Earth-based telescopes and flyby spacecraft. It would, he wrote, be readily observable in 1981, six years ahead of the planned flyby. Because of its decades spent incognito, less was known of Borrelly than of the other comets in Farquhar's proposed mission series. Farquhar could not, for example, report on the amount of potentially damaging dust the probe would encounter as it darted past Borrelly at 17.3 kilometers per second.
Between the Giacobini-Zinner and Borrelly flybys, NASA would carry out its Comet Halley flybys using a pair of 500-kilogram probes launched together in a single Shuttle Orbiter payload bay. The probes would each carry sub-probes that they would release just before they reached Halley. The Halley probes would leave Earth orbit on 4 July 1985, each following a different path. Probe 1, the Pre-Perihelion Probe, would pass Halley on 8 December 1985, 63 days before the comet reached perihelion on 9 February 1986. At the time of the pre-perihelion flyby, spacecraft and comet would be located 1.37 AU from the Sun and 0.71 AU from Earth. Because Comet Halley has a retrograde orbit - that is, its orbital motion is "backward" relative to the planets and most other Solar System bodies - the Pre-Perihelion Probe would fly past Halley at a blistering 55.3 kilometers per second.
Probe 2, the Post-Perihelion Probe, would fly past Halley on 20 March 1986, 39 days after perihelion, at a distance of 1 AU from the Sun and 0.8 AU from Earth. Its velocity relative to Halley - 64.5 kilometers per second - would shatter all flyby speed records.
Halley nucleus imaging would dominate the Pre-Perihelion Probe's science program. This was in part because the probe's lower flyby speed would facilitate tracking. In addition, Halley would be farther from the Sun during the Pre-Perihelion flyby, so could be expected to have expelled less dust. This, Farquhar wrote, would facilitate imaging and reduce the likelihood of spacecraft damage from dust impacts. In addition to its science value, imaging the nucleus would assist flight controllers; knowing the precise location of the nucleus would help them to plot the best course for the Post-Perihelion Probe.
Of the comet encounters Farquhar proposed, only one - the 11 September 1985 encounter with Comet Giacobini-Zinner - took place, and then neither at the hoped-for close distance nor with a purpose-built comet exploration spacecraft. In March 1986, the Soviet Union, European Space Agency, and Japan all explored Comet Halley with flyby spacecraft. The scientific and political wrangling that blocked preparation of a U.S. Comet Halley spacecraft will be examined in subsequent Beyond Apollo posts.
Reference
Mission Strategy for Cometary Exploration in the 1980's, NASA TM-X-70804, Robert W. Farquhar, NASA Goddard Space Flight Center, November 1974.
