Spectral signatures of photosynthesis. II. Coevolution with other stars and the atmosphere on extrasolar worlds

As photosynthesis on Earth produces the primary signatures of life that can be detected astronomically at the global scale, a strong focus of the search for extrasolar life will be photosynthesis, particularly photosynthesis that has evolved with a different parent star. We take previously simulated planetary atmospheric compositions for Earth-like planets around observed F2V and K2V, modeled M1V and M5V stars, and around the active M4.5V star AD Leo; our scenarios use Earth's atmospheric composition as well as very low O2 content in case anoxygenic photosynthesis dominates. With a line-by-line radiative transfer model, we calculate the incident spectral photon flux densities at the surface of the planet and under water. We identify bands of available photosynthetically relevant radiation and find that photosynthetic pigments on planets around F2V stars may peak in absorbance in the blue, K2V in the red-orange, and M stars in the near-infrared, in bands at 0.93-1.1 microm, 1.1-1.4 microm, 1.5-1.8 microm, and 1.8-2.5 microm. However, underwater organisms will be restricted to wavelengths shorter than 1.4 microm and more likely below 1.1 microm. M star planets without oxygenic photosynthesis will have photon fluxes above 1.6 microm curtailed by methane. Longer-wavelength, multi-photo-system series would reduce the quantum yield but could allow for oxygenic photosystems at longer wavelengths. A wavelength of 1.1 microm is a possible upper cutoff for electronic transitions versus only vibrational energy; however, this cutoff is not strict, since such energetics depend on molecular configuration. M star planets could be a half to a tenth as productive as Earth in the visible, but exceed Earth if useful photons extend to 1.1 microm for anoxygenic photosynthesis. Under water, organisms would still be able to survive ultraviolet flares from young M stars and acquire adequate light for growth.     

一种航天器星光折射连续导航方法

针对星光折射航天器自主导航应用中观测缺失导航折射星造成误差上升甚至导航发散的情况，提出一种适用于航天器星光折射导航空白段的新方法。阐述了星光折射导航机理，给出了导航星观测窗口，进而设计基于神经网络的导航算法，该方法充分利用已有信息，有效预测并修正航天器状态信息，使星光空白段前后导航误差变化平稳，发挥星光折射间接敏感地平精度高的特点，保证了航天器高精度定位，且不需要添加硬件设备，算法简洁、实用。最后，通过计算机仿真校验了该导航方法的有效性。     

Life and light: exotic photosynthesis in binary and multiple-star systems

The potential for Earth-like planets within binary/multiple-star systems to host photosynthetic life was evaluated by modeling the levels of photosynthetically active radiation (PAR) such planets receive. Combinations of M and G stars in (i) close-binary systems; (ii) wide-binary systems, and (iii) three-star systems were investigated, and a range of stable radiation environments were found to be possible. These environmental conditions allow for the possibility of familiar, but also more exotic, forms of photosynthetic life, such as IR photosynthesizers and organisms that are specialized for specific spectral niches.     

Looking for planetary moons in the spectra of distant Jupiters

More than 100 nearby stars are known to have at least one Jupiter-sized planet. Whether any of these giant gaseous planets has moons is unknown, but here we suggest a possible way of detecting Earth-sized moons with future technology. The planned Terrestrial Planet Finder observatory, for example, will be able to detect objects comparable in size to Earth. Such Earth-sized objects might orbit their stars either as isolated planets or as moons to giant planets. Moons of Jovian-sized planets near the habitable zones of main-sequence stars should be noticeably brighter than their host planets in the near-infrared (1-4 microm) if their atmospheres contain methane, water, and water vapor, because of efficient absorption of starlight by these atmospheric components. By taking advantage of this spectral contrast, future space observatories will be able to discern which extrasolar giant planets have Earth-like moons capable of supporting life.     

On plane oscillations of a pendulum with variable length suspended on the surface of a planet’s satellite

The problem of planar oscillations of a pendulum with variable length suspended on the Moon’s surface is considered. It is assumed that the Earth and Moon (or, in the general case, a planet and its satellite, or an asteroid and a spacecraft) revolve around the common center of mass in unperturbed elliptical Keplerian orbits. We discuss how the change in length of a pendulum can be used to compensate its oscillations. We wrote equations of motion, indicated a rule for the change in length of a pendulum, at which it has equilibrium positions relative to the coordinate system rotating together with the Moon and Earth. We study the necessary conditions for the stability of these motions. Chaotic dynamics of the pendulum is studied numerically and analytically.     

Detecting tree-like multicellular life on extrasolar planets

Over the next two decades, NASA and ESA are planning a series of space-based observatories to find Earth-like planets and determine whether life exists on these planets. Previous studies have assessed the likelihood of detecting life through signs of biogenic gases in the atmosphere or a red edge. Biogenic gases and the red edge could be signs of either single-celled or multicellular life. In this study, we propose a technique with which to determine whether tree-like multicellular life exists on extrasolar planets. For multicellular photosynthetic organisms on Earth, competition for light and the need to transport water and nutrients has led to a tree-like body plan characterized by hierarchical branching networks. This design results in a distinct bidirectional reflectance distribution function (BRDF) that causes differing reflectance at different sun/view geometries. BRDF arises from the changing visibility of the shadows cast by objects, and the presence of tree-like structures is clearly distinguishable from flat ground with the same reflectance spectrum. We examined whether the BRDF could detect the existence of tree-like structures on an extrasolar planet by using changes in planetary albedo as a planet orbits its star. We used a semi-empirical BRDF model to simulate vegetation reflectance at different planetary phase angles and both simulated and real cloud cover to calculate disk and rotation-averaged planetary albedo for a vegetated and non-vegetated planet with abundant liquid water. We found that even if the entire planetary albedo were rendered to a single pixel, the rate of increase of albedo as a planet approaches full illumination would be comparatively greater on a vegetated planet than on a non-vegetated planet. Depending on how accurately planetary cloud cover can be resolved and the capabilities of the coronagraph to resolve exoplanets, this technique could theoretically detect tree-like multicellular life on exoplanets in 50 stellar systems.     

Interplanetary transfer of photosynthesis: an experimental demonstration of a selective dispersal filter in planetary island biogeography

We launched a cryptoendolithic habitat, made of a gneissic impactite inoculated with Chroococcidiopsis sp., into Earth orbit. After orbiting the Earth for 16 days, the rock entered the Earth's atmosphere and was recovered in Kazakhstan. The heat of entry ablated and heated the rock to a temperature well above the upper temperature limit for life to below the depth at which light levels are insufficient for photosynthetic organisms ( approximately 5 mm), thus killing all of its photosynthetic inhabitants. This experiment shows that atmospheric transit acts as a strong biogeographical dispersal filter to the interplanetary transfer of photosynthesis. Following atmospheric entry we found that a transparent, glassy fusion crust had formed on the outside of the rock. Re-inoculated Chroococcidiopsis grew preferentially under the fusion crust in the relatively unaltered gneiss beneath. Organisms under the fusion grew approximately twice as fast as the organisms on the control rock. Thus, the biologically destructive effects of atmospheric transit can generate entirely novel and improved endolithic habitats for organisms on the destination planetary body that survive the dispersal filter. The experiment advances our understanding of how island biogeography works on the interplanetary scale.     

Possibilities for the detection of microbial life on extrasolar planets

We consider possibilities for the remote detection of microbial life on extrasolar planets. The Darwin/Terrestrial Planet Finder (TPF) telescope concepts for observations of terrestrial planets focus on indirect searches for life through the detection of atmospheric gases related to life processes. Direct detection of extraterrestrial life may also be possible through well-designed searches for microbial life forms. Satellites in Earth orbit routinely monitor colonies of terrestrial algae in oceans and lakes by analysis of reflected ocean light in the visible region of the spectrum. These remote sensing techniques suggest strategies for extrasolar searches for signatures of chlorophylls and related photosynthetic compounds associated with life. However, identification of such life-related compounds on extrasolar planets would require observations through strong, interfering absorptions and scattering radiances from the remote atmospheres and landmasses. Techniques for removal of interfering radiances have been extensively developed for remote sensing from Earth orbit. Comparable techniques would have to be developed for extrasolar planet observations also, but doing so would be challenging for a remote planet. Darwin/TPF coronagraph concepts operating in the visible seem to be best suited for searches for extrasolar microbial life forms with instruments that can be projected for the 2010-2020 decades, although resolution and signal-to-noise ratio constraints severely limit detection possibilities on terrestrial-type planets. The generation of telescopes with large apertures and extremely high spatial resolutions that will follow Darwin/TPF could offer striking possibilities for the direct detection of extrasolar microbial life.     

Space and the fate of humanity

Current growth and consumption rates on Earth cannot be sustained into the future. Space technology is already a vital tool in the management of the planet and we should look at it to mitigate some of the problems we face. However, this should not include colonization of interstellar space. Rather we should focus on using solar energy from space and on mining asteroids, both of which would be feasible if the Moon was developed as a space base and power station. The most difficult and expensive part of getting into space is escaping Earth's gravity - something that could be avoided once a presence was established on the Moon. A lunar base would also provide the obvious site from which to reach GEO, travel to Mars or back to Earth and, ultimately, to explore the further reaches of the Solar System.     

An estimate of the prevalence of biocompatible and habitable planets

A Monte Carlo computer model of extra-solar planetary formation and evolution, which includes the planetary geochemical carbon cycle, is presented. The results of a run of one million galactic disc stars are shown where the aim was to assess the possible abundance of both biocompatible and habitable planets. (Biocompatible planets are defined as worlds where the long-term presence of surface liquid water provides environmental conditions suitable for the origin and evolution of life. Habitable planets are those worlds with more specifically Earthlike conditions). The model gives an estimate of 1 biocompatible planet per 39 stars, with the subset of habitable planets being much rarer at 1 such planet per 413 stars. The nearest biocompatible planet may thus lie approximately 14 LY distant and the nearest habitable planet approximately 31 LY away. If planets form in multiple star systems then the above planet/star ratios may be more than doubled. By applying the results to stars in the solar neighbourhood, it is possible to identify 28 stars at distances of < 22 LY with a non-zero probability of possessing a biocompatible planet.     

Progress in multilateral Earth observation cooperation: CEOS, IGOS and the ad hoc Group on Earth Observations

The Committee on Earth Observation Satellites (CEOS) coordinates international civil space-borne missions designed to observe and study planet Earth. With over 100 Earth observation satellites expected to be launched during the next 10 years, it is clear that collaborative opportunities have not been fully maximized. In 2003 CEOS has been focusing on articulating a more comprehensive satellite data utilization approach and in following up on its significant involvement in the World Summit on Sustainable Development. The CEOS Chair also serves as Co-Chair of the Integrated Global Observing Strategy (IGOS) Partnership, which seeks to reduce observation gaps and unnecessary overlaps and to harmonize and integrate common interests of space-based and in situ systems. IGOS focused in 2003 on development of a number of themes, including Carbon Cycle, Water Cycle and GeoHazards. The IGOS Ocean Theme is now in its implementation phase. NOAA, while chairing CEOS and co-chairing IGOS, has also been actively involved in organizing and hosting a ministerial-level Earth Observation Summit with a follow-on Group on Earth Observations (GEO) charged with developing the framework for a comprehensive global Earth observation system(s). All these activities demonstrate the commitment to developing more coherent and sustained Earth observation strategies for the good of the planet.     

一种利用地球敏感器的星光导航方法

针对直接敏感地平的红外地平仪-星光导航系统精度低的问题,提出通过建立扁率误差补偿函数,研究扁率引起的姿态角测量误差;建立地心矢量的姿态角误差模型,通过修正地球扁率误差来补偿姿态角,对卫星施加姿态偏转指令调整地心矢量偏移。同时基于SODERN地球红外辐射模型,考虑红外辐射强度随纬度和季节的变化建立真实红外辐射曲线,通过滤波算法获得精确的地球切线边缘的量测信息,提高红外地平仪测量精度。仿真结果表明,该方法有效提高了基于红外地平仪的星光导航定位方法的可靠性和精度,导航位置误差能达到500m左右,较原有系统提高3倍以上。     

Ozone concentrations and ultraviolet fluxes on Earth-like planets around other stars

Coupled radiative-convective/photochemical modeling was performed for Earth-like planets orbiting different types of stars (the Sun as a G2V, an F2V, and a K2V star). O(2) concentrations between 1 and 10(-5) times the present atmospheric level (PAL) were simulated. The results were used to calculate visible/near-IR and thermal-IR spectra, along with surface UV fluxes and relative dose rates for erythema and DNA damage. For the spectral resolution and sensitivity currently planned for the first generation of terrestrial planet detection and characterization missions, we find that O(2) should be observable remotely in the visible for atmospheres containing at least 10(-2) PAL of O(2). O(3) should be visible in the thermal-IR for atmospheres containing at least 10(-3) PAL of O(2). CH(4) is not expected to be observable in 1 PAL O(2) atmospheres like that of modern Earth, but it might be observable at thermal-IR wavelengths in "mid-Proterozoic-type" atmospheres containing approximately 10(-1) PAL of O(2). Thus, the simultaneous detection of both O(3) and CH(4) - considered to be a reliable indication of life - is within the realm of possibility. High-O(2) planets orbiting K2V and F2V stars are both better protected from surface UV radiation than is modern Earth. For the F2V case the high intrinsic UV luminosity of the star is more than offset by the much thicker ozone layer. At O(2) levels below approximately 10(-2) PAL, planets around all three types of stars are subject to high surface UV fluxes, with the F2V planet exhibiting the most biologically dangerous radiation environment. Thus, while advanced life is theoretically possible on high-O(2) planets around F stars, it is not obvious that it would evolve as it did on Earth.     

Strategic,technological and ethical aspects of establishing colonies on Moon and Mars

With the vast experience gained by Aerospace Community in the last five decades, the natural future course of action will be to expand Space Exploration. Our understanding of Moon is relatively better with a number of unmanned satellite missions carried out by the leading Space Agencies and manned missions to Moon by USA. Also a number of unmanned satellite missions and surface rover missions were carried out to Mars by those Space agencies generating many new details about Mars. While the future exploration efforts by global community will also be centered obviously on Moon and Mars, it is noteworthy that already NASA had declared its plans for establishing a Surface Base on Moon and developing the technical infrastructure required. Surface Bases on Moon and Mars give rise to a number of strategic, technical and ethical issues both in the process of development, and in the process of establishing the bases. The strategic issues related to Moon and Mars Surface Bases will be centered around development of enabling technologies, cost of the missions, and international cooperation. The obvious path for tackling both the technological development and cost issues will be through innovative and new means of international cooperation. International cooperation can take many forms like—all capable players joining a leader, or sharing of tasks at system level, or all players having their independent programmes with agreed common interfaces of the items being taken to and left on the surface of Moon/Mars. Each model has its own unique features. Among the technical issues, the first one is that of the Mission Objectives—why Surface Bases have to be developed and what will be the activity of crew on Surface Bases? Surface Bases have to meet mainly the issues on long term survivability of humans on the Mars/Moon with their specific atmosphere, gravity and surface characteristics. Moon offers excellent advantages for astronomy while posing difficulties with respect to solar power utilization and extreme temperature variations. Hence the technical challenges depend on a number of factors starting from mission requirements. Obviously the most important technical challenge to be addressed will be in the areas of crew safety, crew survivability, adequate provision to overcome contingencies, and in-situ resource utilization. Towards this, new innovations will be developed in areas such as specialized space suits, rovers, power and communication systems, and ascent and descent modules. The biggest ethical issue is whether humankind from Earth is targeting ‘habitation’ or ‘colonization’ of Moon/Mars. The next question will be whether the in-situ resource exploitation will be only for carrying out further missions to other planets from Moon/Mars or for utilization on Earth. The third ethical issue will be the long term impact of pollution on Moon/Mars due to technologies employed for power generation and other logistics on Surfaces. The paper elaborates the views of the authors on the strategic, technical and ethical aspects of establishing Surface Bases and colonies on Moon and Mars. The underlying assumptions and gray areas under each aspect will be explained with the resulting long-term implications.     

Strategic, technological and ethical aspects of establishing colonies on Moon and Mars

With the vast experience gained by Aerospace Community in the last five decades, the natural future course of action will be to expand Space Exploration. Our understanding of Moon is relatively better with a number of unmanned satellite missions carried out by the leading Space Agencies and manned missions to Moon by USA. Also a number of unmanned satellite missions and surface rover missions were carried out to Mars by those Space agencies generating many new details about Mars. While the future exploration efforts by global community will also be centered obviously on Moon and Mars, it is noteworthy that already NASA had declared its plans for establishing a Surface Base on Moon and developing the technical infrastructure required. Surface Bases on Moon and Mars give rise to a number of strategic, technical and ethical issues both in the process of development, and in the process of establishing the bases. The strategic issues related to Moon and Mars Surface Bases will be centered around development of enabling technologies, cost of the missions, and international cooperation. The obvious path for tackling both the technological development and cost issues will be through innovative and new means of international cooperation. International cooperation can take many forms like—all capable players joining a leader, or sharing of tasks at system level, or all players having their independent programmes with agreed common interfaces of the items being taken to and left on the surface of Moon/Mars. Each model has its own unique features. Among the technical issues, the first one is that of the Mission Objectives—why Surface Bases have to be developed and what will be the activity of crew on Surface Bases? Surface Bases have to meet mainly the issues on long term survivability of humans on the Mars/Moon with their specific atmosphere, gravity and surface characteristics. Moon offers excellent advantages for astronomy while posing difficulties with respect to solar power utilization and extreme temperature variations. Hence the technical challenges depend on a number of factors starting from mission requirements. Obviously the most important technical challenge to be addressed will be in the areas of crew safety, crew survivability, adequate provision to overcome contingencies, and in-situ resource utilization. Towards this, new innovations will be developed in areas such as specialized space suits, rovers, power and communication systems, and ascent and descent modules. The biggest ethical issue is whether humankind from Earth is targeting ‘habitation’ or ‘colonization’ of Moon/Mars. The next question will be whether the in-situ resource exploitation will be only for carrying out further missions to other planets from Moon/Mars or for utilization on Earth. The third ethical issue will be the long term impact of pollution on Moon/Mars due to technologies employed for power generation and other logistics on Surfaces. The paper elaborates the views of the authors on the strategic, technical and ethical aspects of establishing Surface Bases and colonies on Moon and Mars. The underlying assumptions and gray areas under each aspect will be explained with the resulting long-term implications.     

Optical search for extraterrestrial intelligence with Air Cerenkov telescopes

We propose using large Air Cerenkov telescopes (ACTs) to search for optical, pulsed signals from extraterrestrial intelligence. Such dishes collect tens of photons from a nanosecond-scale pulse of isotropic equivalent power of tens of solar luminosities at a distance of 100 pc. The field of view for giant ACTs can be on the order of 10 square degrees, and they will be able to monitor 10-100 stars simultaneously for nanosecond pulses of about 6th magnitude or brighter. Using the Earth's diameter as a baseline, orbital motion of the planet could be detected by timing the pulse arrivals.     

Taxonomy of the extrasolar planet

When a star is described as a spectral class G2V, we know that the star is similar to our Sun. We know its approximate mass, temperature, age, and size. When working with an extrasolar planet database, it is very useful to have a taxonomy scale (classification) such as, for example, the Harvard classification for stars. The taxonomy has to be easily interpreted and present the most relevant information about extrasolar planets. I propose an extrasolar planet taxonomy scale with four parameters. The first parameter concerns the mass of an extrasolar planet in the form of units of the mass of other known planets, where M represents the mass of Mercury, E that of Earth, N Neptune, and J Jupiter. The second parameter is the planet's distance from its parent star (semimajor axis) described in a logarithm with base 10. The third parameter is the mean Dyson temperature of the extrasolar planet, for which I established four main temperature classes: F represents the Freezing class, W the Water class, G the Gaseous class, and R the Roasters class. I devised one additional class, however: P, the Pulsar class, which concerns extrasolar planets orbiting pulsar stars. The fourth parameter is eccentricity. If the attributes of the surface of the extrasolar planet are known, we are able to establish this additional parameter where t represents a terrestrial planet, g a gaseous planet, and i an ice planet. According to this taxonomy scale, for example, Earth is 1E0W0t, Neptune is 1N1.5F0i, and extrasolar planet 55 Cnc e is 9E-1.8R1.     

Planet populations in the Milky Way and beyond

Only about 15 years ago, speculations probably as old as mankind itself about the existence of planets orbiting stars other than the Sun turned into evidence. Recent technological advances make it now possible to find planets at separations from their host star that we consider suitable for life to form and evolve. However, we neither know the necessary nor the sufficient conditions. Even the detection of another planet teeming with life would signpost a beginning rather than an end. It would not answer the deeper questions of the origin and (maybe more importantly) future of our existence. In order to understand our own role in the cosmos, we need to investigate our context, which not only contains habitable planets similar to ours, but comprises the full amazing diversity of the planetary zoo. A comprehensive picture will only arise from complementary evidence provided by several applied planet detection techniques. The most curious effect of gravitational microlensing plays a special role with its capability for inferring a census of planets within the Milky Way, involving different stellar populations, with a sensitivity reaching down to the mass of the Moon, and even in neighbouring galaxies such as M31.     

地-月低能耗转移轨道中途修正问题研究

采用地-月低能耗转移轨道的探测器从地球停泊轨道转移到极月轨道一般需要3~4个月时间,这类转移轨道对入轨精度有较高的要求。本文对地月转移轨道中途修正问题进行了研究。文中结合地-月低能耗转移轨道的特点,给出一种分段式多目标多次中途修正方案。利用显式制导结合牛顿迭代,分别以地球和月球作为中心天体求解兰伯特问题,在假设探测器各种轨道误差的基础上进行了蒙特卡罗仿真。采用该方法一般需要3~5次中途修正能够满足月球探测器环月轨道入轨精度要求,整个转移过程燃料消耗小于传统地月转移轨道。文中给出的仿真结果验证了该方案的可行性。     

Properties of an Earth-like planet orbiting a Sun-like star: Earth observed by the EPOXI mission

NASA's EPOXI mission observed the disc-integrated Earth and Moon to test techniques for reconnoitering extrasolar terrestrial planets, using the Deep Impact flyby spacecraft to observe Earth at the beginning and end of Northern Hemisphere spring, 2008, from a range of ～1/6 to 1/3 AU. These observations furnish high-precision and high-cadence empirical photometry and spectroscopy of Earth, suitable as "ground truth" for numerically simulating realistic observational scenarios for an Earth-like exoplanet with finite signal-to-noise ratio. Earth was observed at near-equatorial sub-spacecraft latitude on 18-19 March, 28-29 May, and 4-5 June (UT), in the range of 372-4540?nm wavelength with low visible resolving power (λ/Δλ=5-13) and moderate IR resolving power (λ/Δλ=215-730). Spectrophotometry in seven filters yields light curves at ～372-948?nm filter-averaged wavelength, modulated by Earth's rotation with peak-to-peak amplitude of ≤20%. The spatially resolved Sun glint is a minor contributor to disc-integrated reflectance. Spectroscopy at 1100-4540?nm reveals gaseous water and carbon dioxide, with minor features of molecular oxygen, methane, and nitrous oxide. One-day changes in global cloud cover resulted in differences between the light curve beginning and end of ≤5%. The light curve of a lunar transit of Earth on 29 May is color-dependent due to the Moon's red spectrum partially occulting Earth's relatively blue spectrum. The "vegetation red edge" spectral contrast observed between two long-wavelength visible/near-IR bands is ambiguous, not clearly distinguishing between the verdant Earth diluted by cloud cover versus the desolate mineral regolith of the Moon. Spectrophotometry in at least one other comparison band at short wavelength is required to distinguish between Earth-like and Moon-like surfaces in reconnaissance observations. However, measurements at 850?nm alone, the high-reflectance side of the red edge, could be sufficient to establish periodicity in the light curve and deduce Earth's diurnal period and the existence of fixed surface units.