Comet Halley is often called "Humankind's Comet" because it has appeared throughout much of recorded human history and because its orbital period of about 76 years is roughly equivalent to a human lifespan. Comet Encke might be nicknamed "Spaceflight's Comet," for it has made the short list of targets for comet-exploring spacecraft since the 1960s. With the shortest orbital period of any comet - just 3.3 years - and an inclination relative to the plane of the Solar System of only about 10°, Encke is among the most accessible comets. It is thus ironic (and appropriate, if we call it Spaceflight's Comet) that, despite being the target of many proposed missions, Comet Encke has yet to welcome a visitor from Earth.
Comet Encke came closest to receiving a visit about a decade ago. Following its launch on 3 July 2002, NASA's 775-kilogram COmet Nucleus TOUR (CONTOUR) spacecraft (image at top of post) moved through a series of elliptical phasing orbits about the Earth designed to position it for a solid-propellant rocket motor burn on 15 August 2002. The burn would have launched it into solar orbit near the Earth. CONTOUR would then have re-encountered Earth in August 2003. The gravity-assist kick it received from our planet would have put it on course for an Comet Encke close flyby on 12 November 2003 (that is, 10 years ago today).
Instead, the CONTOUR spacecraft disintegrated during its Earth-departure burn. Observers visually tracked three objects where there should have been one CONTOUR. The CONTOUR Mishap Investigation Board determined that the most likely cause of the failure was a design oversight: that the spacecraft's solid-propellant rocket motor, embedded at its center, produced enough heat that it weakened CONTOUR's structure, causing the spacecraft to break apart under acceleration. The Board cautioned, however, that lack of telemetry during the Earth-departure burn left open the possibility of several other causes, including rocket motor casing rupture, meteoroid or human-made space debris collision, and attitude-control failure leading to a destructive tumble.
If engineers and scientists at NASA's Goddard Space Flight Center (GSFC) had had their way, Comet Encke would have received its first visitor as early as 3 December 1980. In fact, it would have received two visitors at the same time, for they envisioned launching two spacecraft to Comet Encke on a single rocket. The Encke probes, near twins, would have flown by the comet at a relatively slow speed compared with other proposed comet spacecraft; hence, in the November 1974 NASA Technical Note they wrote to describe it, they dubbed their mission a "ballistic slow flyby."
Robert Farquhar led the four-person GSFC team. In 1972-1973, he had participated in GSFC's 35-member Cometary Explorer Study Group, which aimed to explore Comet Grigg-Skjellerup in April 1977 and Comet Giacobini-Zinner in February 1979 using a single 450-kilogram spinning spacecraft. The NASA-appointed Comet and Asteroid Science Advisory Committee had endorsed Cometary Explorer as the first step in a logical program of comet exploration leading to a Comet Halley mission in 1985-1986. Unfortunately, the U.S. civilian space agency had been unable to fund Cometary Explorer. The 1980 Encke mission would, it was hoped, put NASA comet exploration back on track.
The twin Comet Encke probes were meant to depart Earth between 16 August and 30 August 1980 atop a Titan rocket with a Centaur upper stage. Ironically, given the CONTOUR's fate, the GSFC team rejected an additional solid-propellant "kick" rocket motor as too risky. The probes would travel on a curving ballistic path directly from Earth to Encke; hence the term "ballistic" in the mission's name.
Farquhar's team modeled its Comet Encke mission on the German-U.S. Helios A/Helios B Sun probe missions. Helios A left Earth in late 1974 (about a month after the GSFC group published its Technical Note, in fact). The Helios probes were designed to survive perihelion (the point in their Sun-centered orbit where they were nearest the Sun) at only 0.30 times the Earth-Sun distance, which is inside the orbit of the planet Mercury; the Encke probes would pass their cometary target as it neared perihelion at 0.34 times the Earth-Sun distance. The Helios probes would orbit in the plane of the Solar System; the Encke probes would match their target's modest orbital tilt.
The GSFC team's Encke probes, which would spin to create gyroscopic stability, would move apart immediately after they separated from their launch vehicle's Centaur stage. Farquhar's team dubbed them the "tail probe" and the "coma probe." Each would resemble the lower half of a hourglass-shaped Helios spacecraft. Solar cells on their sides would power spacecraft systems and a suite of science instruments.
If necessary, a course-correction rocket burn would take place 10 days after launch. A second burn 50 days after launch would aim the tail probe at a point in the Comet Encke's wan tail about 10,000 kilometers behind the nucleus and would aim the coma probe at a point immediately in front of the nucleus. A third, very modest, course-correction burn was scheduled for Launch +85 days. The two spacecraft would encounter Comet Encke at about Launch +102 days.
Depending on their launch date, the two Comet Encke spacecraft would reach their target between 3 December and 8 December 1980 moving at between 7.6 and 9.03 kilometers per second. Comet Encke would reach perihelion on 6 December. The Encke flybys would occur at around 1000 hours Greenwich Mean Time on each day of their arrival window so that the 100-meter antenna at Effelsberg, West Germany - the same antenna used to communicate with the Helios probes - could receive data from the Encke probes for as long as possible before they set below the local horizon.
Farquhar and his colleagues envisioned that their two probes would carry slightly different science payloads. The 375-kilogram coma probe, which would linger within 1000 kilometers of the sunlit side of the nucleus for nearly 42 minutes, would include a despun platform bearing its radio dish antenna, TV camera, neutral mass spectrometer, UV spectrometer, and Lyman-alpha spectrometer. The 325-kilogram tail probe would include a despun antenna, but would lack the coma probe's despun platform and its four instruments. Both probes would include on their spinning main sections an ion mass spectrometer, a DC magnetometer, an AC magnetometer, an electron analyzer, a plasma analyzer, an electric field detector, a dust detector, and a dust composition instrument.
The GSFC team was not the only group in 1974 that planned a 1980 Comet Encke mission. The GSFC scientists and engineers made a point of comparing their mission plan with its main rivals. They explained that, in their comparison, "the primary evaluation criteria [would] be the science value and realism of attaining mission objectives."
Their plan's leading rival, a mission design advocated mainly by the Jet Propulsion Laboratory and its contractors, was based on solar-electric propulsion. Launch would take place on 17 December 1978 and a Comet Encke flyby would occur on 6 November 1980. The GSFC team noted that the mission's 30-centimeter-diameter solar-electric (ion) propulsion thruster had yet to be developed, let alone tested; nevertheless, it would be expected to operate flawlessly for 690 days. In addition, the thruster would interfere with the spacecraft's particle-and-fields instruments. Interference would not cease when the thruster was switched off.
Assuming that its untried thruster functioned as hoped, however, the solar-electric spacecraft would pass Comet Encke moving at only 4 kilometers per second, which constituted an advantage over GSFC's ballistic slow flyby. It would do so, however, more than a month before perihelion, when Comet Encke was still about 0.5 times the Earth-Sun distance from perihelion. At that point in its orbit, the nucleus would be relatively inactive: if past observations were any guide, Comet Encke would have almost no tail.
The ballistic slow flyby's lesser rival was a ballistic fast flyby advocated mainly by NASA Ames Research Center and its contractors. A spin-stabilized spacecraft similar to the Pioneer 10 and Pioneer 11 outer Solar System spacecraft would launch on 18 August 1980 atop a relatively cheap Atlas/Centaur rocket with a solid-propellant kick stage. After a voyage of just 92 days, the spacecraft would whiz past Comet Encke on 18 November 1980 at a velocity of 20.1 kilometers per second.
Farquhar's group noted that high-speed impacts with Comet Encke dust particles could easily destroy the ballistic fast flyby spacecraft, and that its camera would likely return only motion-blurred images (assuming that it had time to locate the nucleus or any other important comet features). It would remain within 1000 kilometers of the nucleus for a mere nine minutes.
The GSFC team concluded that, compared with the solar-electric and ballistic fast flybys, the ballistic slow flyby was "superior in every respect." This assertion may well have been correct; the rivalry between the slow flyby, solar-electric, and fast flyby groups split the small community of comet exploration advocates, however, helping to ensure that no spacecraft explored Comet Encke in 1980.
Comet Encke last reached perihelion on 6 August 2010, and will next reach perihelion on 21 November. It is not expected to brighten beyond about magnitude 7. At that level of brightness, it will be at the limit of visibility for unaided eyes under very dark skies. It will be bright enough, however, to become an intriguing binocular object.
Reference
Mission Design for a Ballistic Slow Flyby of Comet Encke 1980, NASA Technical Note D-7726, R. Farquhar, D. McCarthy, D. Muhonen, and D. Yeomans, NASA Goddard Space Flight Center, November 1974.
