A reappraisal of the habitability of planets around M dwarf stars

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.     

Application of a simplified theory of ELF propagation to a simplified worldwide model of the ionosphere

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.     

Leading change at NASA: The case of Dan Goldin

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.     

制造商的新角色:航空维修综合者

剥离、合并、整合将是今后几年航空维修市场的主旋律. 一些航空公司希望将工程技术和管理工作分离出去,交第三方处理.但维修供应商必须将修理、航材,后勤和管理工作集合在一起,才能为航空公司担此重任.