Spacecraft technology portfolio: Probabilistic modeling and implications for responsiveness and schedule slippage

Addressing the challenges of Responsive Space and mitigating the risk of schedule slippage in space programs require a thorough understanding of the various factors driving the development schedule of a space system. The present work contributes theoretical and practical results in this direction. A spacecraft is here conceived of as a technology portfolio. The characteristics of this portfolio are defined as its size (e.g., number of instruments), the technology maturity of each instrument and the resulting Technology Readiness Level (TRL) heterogeneity, and their effects on the delivery schedule of a spacecraft are investigated. Following a brief overview of the concept of R&D portfolio and its relevance to spacecraft design, a probabilistic model of the Time-to-Delivery of a spacecraft is formulated, which includes the development, Integration and Testing, and Shipping phases. The Mean-Time-To-Delivery (MTTD) of the spacecraft is quantified based on the portfolio characteristics, and it is shown that the Mean-Time-To-Delivery (MTTD) of the spacecraft and its schedule risk are significantly impacted by decreasing TRL and increasing portfolio size. Finally, the utility implications of varying the portfolio characteristics are investigated, and “portfolio maps” are provided as guides to help system designers identify appropriate portfolio characteristics when operating in a calendar-based design environment (which is the paradigm shift that space responsiveness introduces).     

Optimization of low-energy resonant hopping transfers between planetary moons

In response to the scientific interest in Jupiter's Galilean moons, NASA and ESA have plans to send orbiting missions to Europa and Ganymede, respectively. The inter-moon transfers of the Jovian system offer obvious advantages in terms of scientific return, but are also challenging to design and optimize due in part to the large, often chaotic, sensitivities associated with repeated close encounters of the planetary moons. The approach outlined in this paper confronts this shortcoming by exploiting the multi-body dynamics with a patched three-body model to enable multiple “resonant-hopping” gravity assists. Initial conditions of unstable resonant orbits are pre-computed and provide starting points for the elusive initial guess associated with the highly nonlinear optimization problem. The core of the optimization algorithm relies on a fast and robust multiple-shooting technique to provide better controllability and reduce the sensitivities associated with the close approach trajectories. The complexity of the optimization problem is also reduced with the help of the Tisserand–Poincaré (T–P) graph that provides a simple way to target trajectories in the patched three-body problem. Preliminary numerical results of inter-moon transfers in the Jovian system are presented. For example, using only 59 m/s and 158 days, a spacecraft can transfer between a close resonant orbit of Ganymede and a close resonant orbit of Europa.     

Comparative cost and utility analysis of monolith and fractionated spacecraft using failure and replacement Markov models

Failure of a single component on-board a spacecraft can compromise the integrity of the whole system and put its entire capability and value at risk. Part of this fragility is intrinsic to the current dominant design of space systems, which is mainly a single, large, monolithic system. The space industry has therefore recently proposed a new architectural concept termed fractionation, or co-located space-based network (SBN). By physically distributing functions in multiple orbiting modules wirelessly connected, this architecture allows the sharing of resources on-orbit (e.g., data processing, downlinks). It has been argued that SBNs could offer significant advantages over the traditional monolithic architecture as a result of the network structure and the separation of sources of risk in the spacecraft. Careful quantitative analyses are still required to identify the conditions under which SBNs can “outperform” monolithic spacecraft. In this work, we develop Markov models of module failures and replacement to quantitatively compare the lifecycle cost and utility of both architectures. We run Monte-Carlo simulations of the models, and discuss important trends and invariants. We then investigate the impact of our model parameters on the existence of regions in the design space in which SBNs “outperform” the monolith spacecraft on a cost, utility, and utility per unit cost basis. Beyond the life of one single spacecraft, this paper compares the cost and utility implications of maintaining each architecture type through successive replacements.     

The prospects for enhanced international polar-orbiting satellite cooperation

With robust polar satellite missions demonstrated and/or operational in the USA, Europe, Japan, China, Russia, India, and Brazil, and with other nations also likely to develop such capability, this paper examines the prospects for as well as the potential challenges to closer and perhaps more formal international polar satellite cooperation. In suggesting that now is the time to explore the development of a coordinated polar observing system, the present paper identifies potential contributors, assesses the relevance of existing space-based Earth observation international coordination mechanisms, and considers three potential models for a future system.     

Direct measurement of the composition and spectra of cosmic rays above 1 TeV/amu from Jacee

Direct measurements on cosmic ray protons through iron above about 1 TeV/amu have been performed in a series of balloon-borne experiments with emulsion chambers. The measured energy spectra of protons and helium are power laws with exponents of 2.77 ± 0.09 and 2.72 ± 0.11 in the energy range 5 to 500 TeV and 2 to 50 TeV/amu, respectively. The proton spectrum shows no evidence of the steepening near 2 TeV which was reported by other experiments. Helium has a slightly higher intensity compared to extrapolations from lower energy measurements. The heavier elements, carbon to sulfur, show a small tendency for intensity enhancement in the relative abundance above 10 TeV/amu.     

A simplified data assimilation method for reconstructing time-series MODIS NDVI data

The Normalized Difference Vegetation Index (NDVI) is an important vegetation index, widely applied in research on global environmental and climatic change. However, noise induced by cloud contamination and atmospheric variability impedes the analysis and application of NDVI data. In this work, a simplified data assimilation method is proposed to reconstruct high-quality time-series MODIS NDVI data. We extracted 16-Day L3 Global 1 km SIN Grid NDVI data sets for western China from MODIS vegetation index (VI) products (MOD13A2) for the period 2003–2006. NDVI data in the first three years (2003–2005) were used to generate the background field of NDVI based on a simple three-point smoothing technique, which captures annual features of vegetation change. NDVI data for 2006 were used to test our method. For every time step, the quality assurance (QA) flags of the MODIS VI products were adopted to empirically determine the weight between the background field and NDVI observations. Ultimately, more reliable NDVI data can be produced. The results indicate that the newly developed method is robust and effective in reconstructing high-quality MODIS NDVI time-series.     

NEOs as stepping stones to Mars and main-belt asteroids

Human interplanetary missions are constrained by the problem of astronaut exposure to galactic cosmic radiation. This paper surveys the existing on-line near-Earth object (NEO) data base in an effort to identify NEOs that cross both Earth's ad Mars’ orbits and could be used as cosmic ray shields by interplanetary voyagers. The search concentrated on low-inclination Mars-crossing NEOs that approach Earth, Mars, and main-belt asteroids in the 2020–2100 time frame. Both outbound and return transfers were searched for. Several candidates for Earth–Mars, Mars–Earth, and Earth–Vesta transfers have been found from the very incomplete August 2008 data base. Other aspects of this interplanetary transfer option are considered.     

The measured radiation environment within Spacelabs 1 and 2 and comparison with predictions

To measure the radiation environment in the Spacelab (SL) module and on the pallet, a set of passive and active radiation detectors was flown as part of the Verification Flight Instrumentation (VFI). SL 1 carried 4 passive and 2 active detector packages which, with the data from the 26 passive detectors of Experiment INS006, provided a comprehensive survey of the radiation environment within the spacecraft. SL 2 carried 2 passive VFI units on the pallet. Thermoluminescent dosimeters (TLDs) measured the low linear energy transfer (LET) dose component; the HZE fluence and LET spectra were mapped with CR-39 track detectors; thermal and epithermal neutrons were measured with the use of fission foils; metal samples analyzed by gamma ray spectroscopy measured low levels of several activation lines. The TLDs registered from 97 to 143 mrad in the SL 1 module. Dose equivalents of 330±70 mrem in the SL 1 module and 537±37 mrem on the SL 2 pallet were measured. The active units in the SL 1 module each contained an integrating tissue-equivalent ion chamber and two differently-shielded xenon-filled proportional counters. The ion chambers accumulated 125 and 128 mrads for the mission with 17 and 12 mrads accumulated during passages through the South Atlantic Anomaly (SAA). The proportional counter rates (1 cps at sea level) were 100 cps in the middle of the SAA (mostly protons), 35 cps at large geomagnetic latitudes (cosmic rays) and 100 cps in the South Horn of the electron belts (mostly bremsstrahlung). Detailed results of the measurements and comparison with calculated values are described.     

V: SEA LEVEL: Benefits of GRACE and GOCE to sea level studies

The recently published Third Assessment Reports of the Intergovernmental Panel on Climate Change have underlined the scientific interest in, and practical importance of past and potential future sea level changes. Space gravity missions will provide major benefits to the understanding of the past, and, thereby, in the prediction of future, sea level changes in many ways. The proposal for the GOCE mission described well the improvements to be expected from improved gravity field and geoid models in oceanography (for example, in the measurement of the time-averaged, or ‘steady state’, ocean surface circulation and better estimation of ocean transports), in geophysics (in the improvement of geodynamic models for vertical land movements), in geodesy (in positioning of tide gauge data into the same reference frame as altimeter data, and in improvement of altimeter satellite orbits), and possibly in glaciology (in improved knowledge of bedrock topography and ice sheet mass fluxes). GRACE will make many important steps towards these ‘steady state’ aims. However, its main purpose is the provision of oceanographic (and hydrological and meteorological) temporally-varying gravity information, and should in effect function as a global ‘bottom pressure recorder’, providing further insight into the 3-D temporal variation of the ocean circulation, and of the global water budget in general. This paper summaries several of these issues, pointing the way towards improved accuracy of prediction of future sea level change. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Mercury Laser Altimeter Instrument for the MESSENGER Mission

The Mercury Laser Altimeter (MLA) is one of the payload science instruments on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission, which launched on August 3, 2004. The altimeter will measure the round-trip time of flight of transmitted laser pulses reflected from the surface of the planet that, in combination with the spacecraft orbit position and pointing data, gives a high-precision measurement of surface topography referenced to Mercury’s center of mass. MLA will sample the planet’s surface to within a 1-m range error when the line-of-sight range to Mercury is less than 1,200 km under spacecraft nadir pointing or the slant range is less than 800 km. The altimeter measurements will be used to determine the planet’s forced physical librations by tracking the motion of large-scale topographic features as a function of time. MLA’s laser pulse energy monitor and the echo pulse energy estimate will provide an active measurement of the surface reflectivity at 1,064 nm. This paper describes the instrument design, prelaunch testing, calibration, and results of postlaunch testing.