As of the date 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, which is paying its first and only visit to the inner Solar System, will reach perihelion just 1.1 million kilometers above the Sun's surface on 28 November 2013. On 26 December 2013, it will pass Earth at a distance of 64.2 million kilometers. There is, of course, no risk of collision; that distance is equal to nearly half the Earth-Sun distance.
As it rounds the Sun, Comet ISON could become a "great" comet; that is, one that shines brightly enough that it is hard to miss even in the light-polluted skies of large cities. Its diffuse tail might span several full moon-widths, and its nucleus might glare brighter than Venus (normally the third-brightest celestial object in Earth's skies, after the Sun and moon). Because it is a first-time visitor, however, it is impossible to predict exactly 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 like ISON, first-time visitors from the space beyond the planets, are time capsules dating from the birth of the Solar System 4.6 billion years ago. Comet ISON has been under close observation by instruments both on and off the Earth since its discovery on 21 September 2012.
One might ask whether we will to dispatch a spacecraft to Comet ISON; it seems a fair question, given that we have launched spacecraft to comets for nearly 30 years. Unfortunately, at today's level of space technology, it simply cannot be done. The very thing that makes Comet ISON so interesting - that it is visiting the inner Solar System for the first time - means that we had insufficient time to prepare a mission to visit it. Comets like ISON can arrive from any direction and are extremely faint while far from the Sun, so it is rare for us to discover one much more than a year before it reaches perihelion.
The comets our spacecraft have visited, on the other hand, have had the misfortune to be captured into short-period Sun-centered orbits, typically by one or more close encounters with Jupiter's mighty gravitational influence. Such comets, most of which are doomed eventually to deplete their stock of ices or to disintegrate as the Sun intermittently boils their surfaces, have been observed for decades or even centuries. This means that engineers and scientists 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 robotic program emphasis from Apollo support - for example, imaging the moon to enable scientists to choose Apollo landing sites - to Solar System exploration in its own right. 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 (SSB) of the National Academy of Sciences endorsed a plan to launch two probes to fly past Comet Encke in 1980.
Robert Farquhar, an engineer at NASA's Goddard Space Flight Center in suburban Washington, DC, was instrumental in planning the 1980 Comet Encke dual-probe fluby mission. On 1 November 1974, he presented to the International Astronomical Union's Colloquium on The Study of Comets a hopeful blueprint for 1980s cometary exploration. It started with the Encke mission and culminated with pre- and post-perihelion Comet Halley flybys in December 1985 and March 1986, respectively.
In fact, 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 instrumentation and to provide NASA with comet exploration experience before Halley's historic first perihelion passage of the Space Age. He made "a special plea" to unnamed "appropriate science advisory groups," calling upon them to begin "serious planning for sending space probes to Halley. . .in the near future."
Farquhar saw his comet flyby mission series also as a lead-in to more ambitious comet rendezvous missions. A comet 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 ballistic flybys. He wrote that comet rendezvous would require NASA to develop a Solar-Electric Propulsion module or a high-energy chemical rocket stage. In addition, the initial series of flyby missions to comets of different characteristics would aid scientists and engineers in designing instruments and choosing targets for future rendezvous missions.
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. They would enter an elliptical Sun-centered orbit with a period one-sixth that of Comet Encke's 3.3-year period and would fly past Encke at the relatively slow velocity of 7.9 kilometers per second on 7 December 1980, just one day after the comet reached perihelion at a distance from the Sun of 0.34 Astronomical Units (AU) (that is, 0.34 times the Earth-Sun distance). If all went as planned, Probe 1 would pass no more than 824 kilometers from the Sunward side of Comet Encke's nucleus. Meanwhile, Probe 2 would pass through Encke's narrow tail, which Farquhar explained lacked detectable dust. This reduced the likelihood that the probes would suffer damage during their Encke encounter.
Of the four comets visited in Farquhar's mission series, Encke would be the easiest 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 (velocity change = about 130 meters per second) would put the probes on course for a second near-perihelion Comet Encke flyby on 28 March 1984.
The 1984 Comet Encke dual-probe flyby would constitute only the second mission in Farquhar's series. The third, 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 explained that it would provide ample time for engineers and scientists to use Comet Encke flyby data to optimize subsequent flybys.
The Giacobini-Zinner mission would begin with probe launch from the payload bay of a Space Shuttle Orbiter. Farquhar assumed that Shuttle launch would be cheaper than launch on an expendable rocket. 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 Giacobini-Zinner 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 reached 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 would fly past Earth at a distance of about 32 Earth diameters - just beyond the moon's orbit - on 10 March 1986. This 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 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 flyby would occur seven days after Borrelly passed perihelion at a distance of 1.36 AU from the Sun.
Borrelly, Farquhar explained, had been gravitationally perturbed in 1936 into an orbit that made it difficult to observe. Fortunately, a second Jupiter perturbation in 1972 had nudged it into a new orbit about the Sun, 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 probe flyby. Because of its long period 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 encounters, NASA would carry out its Comet Halley flybys using a pair of 500-kilogram probes launched together in a single Space Shuttle Orbiter payload bay. Each probe would carry sub-probes it would release just before it 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. During the flyby, spacecraft and comet would be positioned 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 "backwards" relative to the planets and most other Solar System bodies - the Pre-Perihelion Probe would fly past it 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.
Farquhar proposed that imaging science dominate the Pre-Perihelion Probe's flyby because it would pass Halley at a slower speed than the Post-Perihelion Probe. In addition, Halley would be farther from the Sun during the Pre-Perihelion flyby, so could be expected to have expelled less dust. This would, he wrote, facilitate imaging of the comet's nucleus region. Imaging the nucleus would, of course, be vital for science, but also would enable flight controllers to precisely locate it. This would assist them as they sought to plot a 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 not with a purpose-built comet exploration spacecraft. Soon after, 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.
Mission Strategy for Cometary Exploration in the 1980′s, NASA TM-X-70804, Robert W. Farquhar, NASA Goddard Space Flight Center, November 1974.
I research and write about the history of space exploration and space technology with an emphasis on missions and programs planned but not flown (that is, the vast majority of them). Views expressed are my own.
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