Combining Solar Electric Propulsion and chemical propulsion for crewed missions to Mars
This paper documents the results of an investigation of human Mars mission architectures that leverage near-term technology investments and infrastructures resulting from the planned Asteroid Redirect Robotic Mission (ARRM), including high-power Solar Electric Propulsion (SEP) and a human presence in Lunar Distant Retrograde Orbit (LDRO). The architectures investigated use a combination of SEP and chemical propulsion elements. Through this combination of propulsion technologies, these architectures take advantage of the high efficiency SEP propulsion system to deliver cargo, while maintaining the faster trip times afforded by chemical propulsion for crew transport. Evolved configurations of the Asteroid Redirect Vehicle (ARV) are considered for cargo delivery. Sensitivities to SEP system design parameters, including power level and propellant quantity, are presented. For the crew delivery, liquid oxygen and methane stages were designed using engines common to future human Mars landers. Impacts of various Earth departure orbits, Mars loiter orbits, and Earth return strategies are presented. The use of the Space Launch System for delivery of the various architecture elements was also investigated and launch vehicle manifesting, launch scheduling and mission timelines are also discussed. The study results show that viable Mars architecture can be constructed using LDRO and SEP in order to take advantage of investments made in the ARRM mission.
Electric Propulsion for Solar System Exploration
The use of ion propulsion for deep-space missions will become a reality in 1998 with the  ight of the ion-propelled, New Millennium Deep Space 1 (DS1) spacecraft. The anticipation of this event is stimulating the call for improved ion propulsion technologies, a trend that is expected to continue. This paper describes the evaluation of possible advanced solar electric propulsion technologies and their potential bene ts to projected near-term and midterm solar system exploration missions. The advanced technologies include high-performance derivatives of the DS1 ion propulsion technology, scaled-down DS1 systems, and direct-drive Hall-effect thruster systems. The results of this study indicate that signi cant near-term bene ts can be obtained by the development of improved versions of the DS1 ion propulsion system (IPSs) components. In addition, if the current trend to smaller planetary spacecraft continues, then missions  ying these smaller future spacecraft will bene t substantially from the development of scaled-down IPSs that incorporate advanced technologies in the ion engines and the propellant feed systems. The performance of the direct-drive Hall thruster systems is potentially superior to that of all other midterm options, but this technology has the highest development risk. Signi cantly reduced trip times to small bodies and the outer planets may be possible if technology programs work to retire these risks.
Developmental aphasia: The perception of brief vowels and extended stop consonants
Abstract Developmental dysphasics and matched controls were examined for their ability to discriminate (a) two synthesized vowel-vowel syllables and (b) two synthesized consonant- vowel syllables. For both vowels and consonants, dysphasics were impaired when the discriminable components of the two stimuli were brief (43 msec) but unimpaired when these components were 95 msec or longer. It is concluded that developmental dysphasics have no difficulty in discriminating transitional auditory information as such and that their impaired discrimination of synthesized stop consonants is attributable solely to the brief duration of the discriminable components.