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Ballistic Missions to Comet Encke in 1980 -A New Phase of Solar System Exploration

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Ballistic Missions to Comet Encke in 1980 -A New Phase of Solar System Exploration View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Embry-Riddle Aeronautical University 1974 (11th) Vol.1 Technology Today for The Space Congress® Proceedings Tomorrow Apr 1st, 8:00 AM Ballistic Missions To Comet Encke In 1980 -A New Phase Of Solar System Exploration William J. Bursnall Staff Engineer, Martin Marietta Corporation, Denver, Colorado Follow this and additional works at: https://commons.erau.edu/space-congress-proceedings Scholarly Commons Citation Bursnall, William J., "Ballistic Missions To Comet Encke In 1980 -A New Phase Of Solar System Exploration" (1974). The Space Congress® Proceedings. 1. https://commons.erau.edu/space-congress-proceedings/proceedings-1974-11th-v1/session-4/1 This Event is brought to you for free and open access by the Conferences at Scholarly Commons. It has been accepted for inclusion in The Space Congress® Proceedings by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. BALLISTIC MISSIONS TO COMET ENCKE IN 1980 - A NEW PHASE OF SOLAR SYSTEM EXPLORATION -* William J. Bursnall Staff Engineer Martin Marietta Corporation Denver, Colorado ABSTRACT A prime mission candidate for initiating explo­ Space-flight missions to Comet Encke have been the ration of both a comet and asteroids with unmanned subject of a number of NASA-sponsored studies over spacecraft is available during the 1980 apparition the past several years (see Bibliography in of the short-period Comet Encke. Direct investi­ reference 1). In these studies, a variety of gation of such bodies is expected to provide the mission options and spacecraft configurations have best insight into conditions existing during the been presented and analyzed. The purpose of this early periods of solar system evolution. paper is to summarize the results of a recent study performed by the Martin Marietta Corporation's A discussion is presented on low-cost ballistic Denver Division for the NASA/Ames Research mission options to Comet Encke. Particular Center^'. This study was undertaken to determine emphasis is given to the results of a recently- the feasibility and desirability of using current- completed study of ballistically-launched, spin- technology, light-weight, spin-stabilized space­ stabilized spacecraft for 1980 missions to Comet craft for ballistic flyby missions to Comet Encke Encke and the asteroids Geographos and Toro. during the 1980 apparition. Characteristics and utilization of a small, separable probe to enhance comet science return are As a basis for the definition of scientific also described. In conclusion, it is shown that objectives and measurements, the physical charac­ the Encke ballistic flyby can provide the necessary teristics of Comet Encke will be summarized first, technical foundation to support the planning of and a typical instrument complement will be more advanced Encke follow-on missions as well as described. The spectrum of mission options for the challenging flyby of Comet Halley in 1986. Encke encounters in 1980 will be reviewed with emphasis on pre-perihelion, short-flight-time ballistic opportunities. Also included are options INTRODUCTION for subsequent flyby of the asteroids Geographos and Toro after Encke encounter. The variations of The exploration of Earth's neighboring planets with comet encounter conditions affecting scientific unmanned spacecraft is systematically building a accomplishment will be discussed in relation to the fund of knowledge on the characteristics of Mars, selected mission. The design features of a typical Venus, Jupiter, and, most recently, Mercury. By Atlas/Centaur-class spacecraft are described, and the end of the present decade, it is projected that the utilization of a separable probe to enhance U.S. spacecraft will also be enroute to Saturn and comet science is also discussed. The impacts of Uranus. The scientific results of the Pioneer and combined Encke/asteroid encounters on spacecraft Mariner programs have been impressive and, in some design and mission are then evaluated. In con­ cases, very surprising, but they are still snap­ clusion, key technical issues are discussed to show shots of bodies as they have evolved since their the direct relationship of the 1980 Encke flyby formation. During their evolution, they have mission to the successful accomplishment of follow- experienced many catastrophic and metamorphic on exploration of Encke and to flyby of comet changes as a result of the actions of gravity, Halley in 1986. volcanism, solar activity, and other dynamic events. As a result of such changes, however, planets offer limited data on solar system origins and formative COMET ENCKE CHARACTERISTICS processes. Potential sources of additional infor­ mation on the early stages of solar system In 1819, Johann. Franz Encke, then at the observa­ development are the comets and asteroids which are tory at Seeburg, Switzerland, proved that the considered to be very close to the state in which apparently different comets observed in 1786, 1795, they were formed. The short-period Comet Encke, in and 1818 were actually the same comet that now particular, is currently the primary NASA candidate bears his name. Based on Encke's calculated for initiating small-body exploration. epheraeris, the comet was successfully acquired at its apparition in 1822. Since 1818, Comet Encke * Based on a research report prepared for the Ames has been observed at each apparition with the Research Center, NASA under Contract NAS2-7564. exception of 1944. At its brightest, Encke's comet 4-1 is ordinarily slightly fainter than the naked-eye Recent observations of comets in ultraviolet by the limit. The 3,3-year orbital period of this well Orbiting Astronomical Observatory (OAQ) spacecraft observed comet still remains the shortest known for show a half intensity Lyman-alpha halo out to about any comet. This frequent accessibility offers the oae million kilometers. opportunity for an orderly program of progressive exploration,, The solar wind interacts with the coma to produce a tail downstream and the upstream phenomena shown in Whipple"', in a recent periodical article,, gives Figure 1, The solar wind is decelerated, either an excellent summary of comet characteristics and gradually or suddenly, a turbulent, transition discusses the uncertainties as well as the facts region follows, and then there is a flow around the that have evolved over the many years of comet coma. A type of contact surface exists and observations. For the purposes of this paper, only separates the solar wind and cometary plasma,, but a 'brief discussion, of comet physics will be given its maintenance and processes are not clearly known. to establish the basis for defining science The solar wind may, in addition, contribute to instrument and mission-design requirements. In the dissociation and ionization of neutral parent following discussion, the three basic structural molecules. Ionized constituents of the coma are parts of a comet (nucleus, coma, and tail) will be swept into the tail. Observations have shown, that described, and the quantitative parameters for Eneke Encke exhibits a Type I tail of ionized plasma but will be summarized. has no significant Type II dust tail. This is con­ sistent with its aging and earlier depletion by In general, the existance of a cometary nucleus is mass losses. The maintenance of the tail and its 'based on circumstantial evidence. More direct observed fine structure may involve electrodynamic evidence, however, was obtained by Dr. Elizabeth processes not clearly understood. Roemer in photographs of Comet Encke near aphelion £n 1972^', In these observations, Encke appeared Table I is a summary of the primary physical as a "stellar object" with a magnitude of about properties of Comet Encke. A more detailed 20*5* In accord with Whipple's model of an icy- discussion of these properties, their derivation, conglomerate (or "dirty snowball") nucleus, it is and associated references may be found, in reference inferred that such a central body exists to provide 2. the source for gases and dust found in the coma and tail. The nucleus contains complex parent molecules such as H20, NH3, CH^, and C02 in the form of com­ COMET SCIENCE OBJECTIVES AMD INSTRUMENTATION plex ices. As the comet approaches the Sun, these ices sublimate at temperatures of a few hundred In spite of the long history of comet observations, degrees Kelvin, and! provide "daughter" molecules many of the basic physical properties and processes 'lib, ich are s een in c one t s pec t r um me a s ur erne nt s. Also can only be inferred, fr om gr ound - b a s e d. s t ud i e s , interspersed in the nucleus ices is meteoric materi­ Direct measurements in the comet environment will al which is the source of dust in the comet. It is be required to understand more completely their estimated that Encke's nucleus is about 3.2 km in o r ig i n a nd e vo 1 u t i o n. In a c c or d a. :nc e w 1, th t he w id e " diameter,,, range of observables and energy processes possible at the comet, considerable breadth in scientific The coma is the "atmosphere" of the comet and is a objectives is required on initial flyby missions in near-spherical region around the nucleus in which p r 1 e p a r a t i on f or 1 a t, e 1 r, mor e a, mb i, t i ous f 1 i gh t s * ioniza t ion , pho t o-d i s s oc ia t ion, and other proc esses Since the first flyby of a comet is in every sense occur i n the ma ter 'ia, 1 ema na t ing f r om the nu cl eu s . an exploratory venture,, there is no strong The coma diameter 'varies with the solar distance of rationale that special emphasis be given, to only the comet* At 1 AU, Encke*s mean diameter is about one s c ie nt i fie d is c ip 1, ine , The sc ience o'b j ec t ive s , 1,05 km and decreases to about 40,000 km, at 0.5 AU.
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