The Distributed CO in Comet Halley

Examination of the spatial distribution of CO intensity of Comet Halley indicates that a large fraction of CO originates from the refractory organic component in the coma, rather than directly from the volatiles in the nucleus. Based on the fluffy aggregate interstellar dust comet model, we have estimated the upper limits of the total amount of CO provided by coma dust. The implications from the comparison of the predicted results with the observed value have been discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Leadership in space transportation

Rather than aiming at overall preemenence in space, as in the past, it is important for the USA now to specify goals and objectives for its space programme. In the space transportation market, the USA faces long-term competition. Planned properly, the Space Shuttle, Space Station and transfer stages can be used as a unique transport system, forcing other suppliers to compete on US terms. Such a total transport service system could also include ELV support. In entering into international space agreements, the government must take into account the objective of maintaining the competitiveness of the US commercial sector.     

Fusion of sensors with dissimilar measurement/tracking accuracies

The case of data fusion of sensors dissimilar in their measurement/tracking errors is considered. It is shown that the fused track performance is similar whether the sensor data are fused at the track level or at the measurement level. The case of a cluster of targets, resolved by one sensor but not the other, is also considered. Under certain conditions the fused track may perform worse than the worst of the sensors. A remedy to this problem through modifications of the association algorithm is presented     

Chemical evolution in space--a source of prebiotic molecules.

In Laboratory Astrophysics at Leiden University a laboratory analog for following the chemical evolution of interstellar dust in space shows that the dust contains the bulk of organic material in the universe. We follow the photoprocessing of low temperature (10 K) mixtures of ices subjected to vacuum ultraviolet radiation in simulation of interstellar conditions. The most important, but necessary, difference is in the time scales for photo-processing. One hour in the laboratory is equivalent to one thousand years in low density regions of space and as much as, or greater than, ten thousand to one million years in the depths of dense molecular clouds. The ultimate product of photoprocessing of grain material in the laboratory is a complex nonvolatile residue which is yellow in color and soluble in water and methanol. The molecular weight is greater than the mid-hundreds. The infrared absorption spectra indicate the presence of carboxylic acid and amino groups resembling those of other molecules of presumably prebiological significance produced by more classical methods. One of our residues, when subjected to high resolution mass spectroscopy gave a mass of 82 corresponding to C4H6H2 after release of CO2 and trace ammounts of urea suggesting amino pyroline rings. The deposit of prebiotic dust molecules occurred as many as 5 times in the first 500-700 million years on a primitive Earth by accretion during the passage of the solar system through a dense interstellar cloud. The deposition rate during each passage is estimated to be between 10(9) and 10(10) g per year during the million or so years of each passage; i.e., a total deposition of 1O(9)-10(10) metric tons of complex organic material per passage.     

Evaluating the economic impact of design alternatives on domestic communication satellite ventures

What are the economic consequences of establishing a domestic communication satellite venture? Has the best design alternative been selected? Should investments be made to develop new technology? How much should be invested? These are but some of the questions facing those concerned with providing domestic communication satellite services. Because of the demand, performance and cost uncertainties and the unpredictability of the need for and timing of large capital expenditures (due to less than perfect reliability of transportation systems and satellites) providing answers to these and other related questions is a complex process.This paper describes a methodology for providing answers to the above questions. The methodology, by explicitly considering demand, performance and cost uncertainties and satellite and transportation system reliability, develops probabilistic pro forma financial analyses of domestic communication satellite system ventures. In so doing, the risk dimension is quantified and alternative courses of action can be evaluated and compared taking into account expected values, risk and risk aversion preferences. It is also important to quantify and consider the risk dimension since many R &; D projects are specifically aimed at risk reduction.     

Exploration and protection of Europa's biosphere: implications of permeable ice

Europa has become a high-priority objective for exploration because it may harbor life. Strategic planning for its exploration has been predicated on an extreme model in which the expected oceanic biosphere lies under a thick ice crust, buried too deep to be reached in the foreseeable future, which would beg the question of whether other active satellites might be more realistic objectives. However, Europa's ice may in fact be permeable, with very different implications for the possibilities for life and for mission planning. A biosphere may extend up to near the surface, making life far more readily accessible to exploration while at the same time making it vulnerable to contamination. The chances of finding life on Europa are substantially improved while the need for planetary protection becomes essential. The new National Research Council planetary protection study will need to go beyond its current mandate if meaningful standards are to be put in place.     

On the possible detection of solid O2 and O3 interstellar grains with ISO

In various models of interstellar grain chemistry, solid O₂ is formed by accretion as well as by surface reactions on grains. In dense molecular cloud models, at a later stage of the evolution, the O₂ molecule may become a substantial grain mantle constituent. Since IR dipole vibrational transitions for the homonuclear diatomic molecule O₂ are forbidden, the abundance of this potentially important grain mantle component can not be determined. However, embedded in a dirty ice matrix, the fundamental vibration of O₂ at 1550 cm⁻¹ becomes observable at 10 K, due to interactions with surrounding molecules, which break the symmetry of molecular oxygen. This process might be applicable for the dust mantle environment of interstellar grains. We have studied the role of solid O₂ and O₃ in astrophysically relevant ice mixtures and discuss the possible detection of solid O₂ and its major photolysis product O₃ in interstellar grains, in dense molecular clouds. Both molecules represent a specific target to be observed by the ISO satellite in the near future.