Mission strategy for cometary exploration in the 1980's

A sequence of ballistic intercept missions to comets is proposed. The mission set is composed of a well-known group of periodic comets whose physical properties are dissimilar. In addition to full descriptions of the nominal mission profiles, earth-based sighting conditions and estimates of cometary ephemeris errors are presented for each target comet. The first mission of the sequence is a slow flyby (approximately 8 km/sec) of Encke's comet near its perihelion in 1980. Because of a near resonance in the orbital periods of Encke and the spacecraft, it is possible to retarget the spacecraft for a second Encke encounter in 1984. The second mission of the sequence also consists of two cometary encounters but in this case different comets are involved; Giacobini-Zinner in 1985 and Borrelly in 1987. The final mission of the sequence calls for a simultaneous launch of two spacecraft towards Halley's comet in 1985. One spacecraft is targeted fo a pre-perihelion intercept at a heliocentric distance of 1.37 AU

Utilization of multi-body trajectories in the Sun-Earth-Moon system

An overview of three uncommon trajectory concepts for space missions in the Sun-Earth-Moon System is presented. One concept uses a special class of libration-point orbits called 'halo orbits.' It is shown that members of this orbit family are advantageous for monitoring the solar wind input to the Earth's magnetosphere, and could also be used to establish a continuous communications link between the Earth and the far side of the Moon. The second concept employs pretzel-like trajectories to explore the Earth's geomagnetic tail. These trajectories are formed by using the Moon to carry out a prescribed sequence of gravity-assist maneuvers. Finally, there is the 'boomerang' trajectory technique for multiple-encounter missions to comets and asteroids. In this plan, Earth-swingby maneuvers are used to retarget the original spacecraft trajectory. The boomerang method could be used to produce a triple-encounter sequence which includes flybys of comets Halley and Tempel-2 as well as the asteroid Geographos

Solar sail orbits at artificial Sun-Earth libration points

In this Note a new family of solar sail orbits will be investigated in the sun-Earth circular restricted three-body problem. It will be shown that periodic orbits can be developed that are displaced above or below the plane of the restricted three-body system. Whereas traditional halo orbits are centered on the classical libration points, these new orbits are associated with artificial libration points. The orbits are retrograde, circular orbits with a period half that of the orbit period of the two primary masses of the problem. Numerical analysis of stability and controllability of the orbits shows that the orbits are unstable but completely controllable with both lightness number (sail areal density) and sail attitude

Halley's comet 1985-86: space exploration

A coordinated program to explore Halley's comet in 1985 to 86 is proposed. The program employs a variety of observational systems for remote observations and utilizes spacecraft encounters with the comet to obtain in-situ measurements. Included in the observational network are groundbased observatories, the Space Telescope, a Spacelab cometary observatory, small astronomical satellites, and experiments carried on airborne observatories and sounding rockets. It is assumed that a ballistic flythrough technique will be used to carry out the spacecraft encounters. The proposed strategy calls for the simultaneous launch of two spacecraft towards an intercept with Halley in March 1986. Following the Halley encounter one spacecraft is retargeted to intercept comet Borrelly in January 1988, while the other spacecraft proceeds to an encounter with comet Tempel 2 in September 1988

A Unification of Models of Tethered Satellites

In this paper, different conservative models of tethered satellites are related mathematically, and it is established in what limit they may provide useful insight into the underlying dynamics. An infinite dimensional model is linked to a finite dimensional model, the slack-spring model, through a conjecture on the singular perturbation of tether thickness. The slack-spring model is then naturally related to a billiard model in the limit of an inextensible spring. Next, the motion of a dumbbell model, which is lowest in the hierarchy of models, is identified within the motion of the billiard model through a theorem on the existence of invariant curves by exploiting Moser's twist map theorem. Finally, numerical computations provide insight into the dynamics of the billiard model

Mission design for a ballistic slow flyby Comet Encke 1980

Preliminary mission analyses for a proposed 1980 slow flyby (7-9 km/s) of comet Encke are presented. Among the topics covered are science objectives, Encke's physical activity and ephemeris accuracy, trajectory and launch-window analysis, terminal guidance, and spacecraft concepts. The nominal mission plan calls for a near-perihelion intercept with two spacecraft launched on a single launch vehicle. Both spacecraft will arrive at the same time, one passing within 500 km from Encke's nucleus on its sunward side, the other cutting through the tail region. By applying a small propulsive correction about three weeks after the encounter, it is possible to retarget both spacecraft for a second Encke intercept in 1984. The potential science return from the ballistic slow flyby is compared with other proposed mission modes for the 1980 Encke flyby mission, including the widely advocated slow flyby using solar-electric propulsion. It is shown that the ballistic slow flyby is superior in every respect

Lunatics.

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