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41.
Daniel Goldin set the record for longevity as administrator of NASA, serving from 1992 to 2001. Active and controversial, he was unusually visible as a self-proclaimed ‘agent of change’. Coping with a turbulent political environment, Goldin directed NASA from the end of the Cold War to the early 21st century. He effected change in the multitude of programs under his aegis, especially the Space Station and robotic Mars program. He was hailed at one point as a miracle worker and poster boy of government reinvention for his ‘faster, better, cheaper’ strategy of ‘doing more with less’. But Goldin left the agency under fire for cost overruns and reforms that reached too far. Using a policy innovation process approach, this paper traces Goldin's eventful years at NASA, his policy ends and administrative and technological means. It analyzes the record of success and failure of one of the most, influential administrators in NASA history. That record provides useful lessons for how an administrator gains, uses, and loses power in the US space policy system. 相似文献
42.
Large-scale, long-term technological programs funded by government are extraordinarily difficult to begin, maintain, and complete. They must negotiate not only technical, but also political hurdles. International connections add to the complexity. This paper analyzes the course of the International Space Station (ISS) from the perspective of the NASA administrator. The roles of James Beggs, Dan Goldin and Sean O’Keefe are discussed as illustrative of how top federal executives can influence such programs. Much depends on when in the program's life cycle an administrator happens to serve and how long he or she serves. There are times when major decisions are possible, and the course of a program set for years. However, these strategic decisions also require tactical choices along the way. The function of the NASA administrator is to successfully harmonize the political environment and technical realities of a huge program like ISS. Administrators influence the birth and execution of a program through choices they make, resources they acquire, and coalitions of support they build for the program. As a program like ISS extends over many years, program success requires NASA administrators to move a technological enterprise forward, rather than to let it drift or be terminated. 相似文献
43.
Tarter JC Backus PR Mancinelli RL Aurnou JM Backman DE Basri GS Boss AP Clarke A Deming D Doyle LR Feigelson ED Freund F Grinspoon DH Haberle RM Hauck SA Heath MJ Henry TJ Hollingsworth JL Joshi MM Kilston S Liu MC Meikle E Reid IN Rothschild LJ Scalo J Segura A Tang CM Tiedje JM Turnbull MC Walkowicz LM Weber AL Young RE 《Astrobiology》2007,7(1):30-65
Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute. 相似文献
44.
The approximate theory of ELF propagation in the Earth-ionosphere transmission line described by Booker (1980) is applied to a simplified worldwide model of the D and E regions, and of the Earth's magnetic field. At 1000 Hz by day, reflection is primarily from the gradient on the underside of the D region. At 300 Hz by day, reflection is primarily from the D region at low latitudes, but it is from the E region at high latitudes. Below 100 Hz by day, reflection is primarily from the gradient on the underside of the E region at all latitudes. By night, reflection from the gradient on the topside of the E region is important. There is then a resonant frequency (~ 300 Hz) at which the optical thickness of the E region for the whistler mode is half a wavelength. At the Schumann resonant frequency in the Earth-ionosphere cavity (~ 8 Hz) the nocturnal E region is almost completely transparent for the whistler mode and is semi-transparent for the Alfvén mode. Reflection then takes place from the F region. ELF propagation in the Earth-ionosphere transmission line by night is quite dependent on the magnitude of the drop in ionization density between the E and F regions. Nocturnal propagation at ELF therefore depends significantly on an ionospheric feature whose magnitude and variability are not well understood. A comparison is made with results based on the computer program of the United States Naval Ocean Systems Center. 相似文献