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NASA NTRSDate not supplied

TECHEDSAT-7 and 10: The Little Spacecraft That Could

The NOW (Nanosatellite Orbital Workshop) of NASA Ames Research Center (ARC) has two cubesats in orbit at this time: 6 U TechEdSat-10 (T-10) and the 3U TechEdSat-7 (T-7). T10 was jettisoned from the ISS via the NANORACKS system 7/13/2020, and T-7 was launched via Virgin Orbit 1/17/2021. Both were built by the Nano-satellite Orbital Workshop (NOW) at NASA ARC, and designed and fabricated by interns and students in collaboration with educational institutions. Prototyping novel technologies for non-powered re-entry and communications from orbit are primary research interests, however all subsystems including power generation and distribution, subsystem control, navigation, positioning, heat management etc. extend current technologies. Use of distributed processors using open software platforms and standards other based technologies and software is integral to all segments of spacecraft design. Here, we will present an overview of the spacecraft, experiments, and accomplishments – as well as the next three flight experiments. Some of these experiments include: The exo-brake re-entry system is being developed to enable sample return and end of life disposal; Internal communications for sensors, inter-subsystem and experiments uses both a Zigbee based PAN and internal Wi-Fi for high-speed inter-device communications; The Iridium small message LEO system (Short Burst Data) is used to both command the spacecraft and send data to the ground; Experimental use of the Global-Star system for L-band system comparison and back-up; Collaborative NOAA an experiment to communicate from LEO to the GOES geostationary satellite using the DCS (Data Collection System) with on-board Doppler correction; Mars and Lunar experimental communication systems for future cis-lunar and interplanetary nano-satellites; First demonstration of the NASA Near Earth Network systems with nano-satellites at NASA/Wallops Island; Solar array design and implementation for unique future flexible structures; Power distribution using Tardigrade rad-hard processor omni-board (designed by the team); Distributed processors with internal Wi-Fi connectivity; and Initial experiments with AI/Machine Learning.

M Murbach↗

NASA NTRS2005

A Search for Meteor Shower Signatures in the LDEF IDE Data

For 346 days after the deployment of the LDEF satellite on April 7, 1984, the tape recorder belonging to the Interplanetary Dust Experiment (DE) stored information on over 15,000 impacts made by submicron and larger-size particles on its metal oxide silicon (MOS) detectors. These detectors were mounted on trays facing in six orthogonal directions - LDEF ram and trailing edge, the poles of the LDEF orbit (north and south), and radially inward (towards the Earth) and outward (towards space). The 13.1 second time resolution provided by the IDE electronics, combined with the high sensitivity of the MOS detectors and large collecting area (approximately 1 sq.m) of the experiment, conclusively showed that the small particle environment at the LDEF altitude of 480 km was highly time-variable, with particle fluxes spanning over four orders of magnitude. A large number of the 15,000 impacts recorded by IDE occurred in groups, which were of two types - the spikes, single, isolated events of high intensity and the multiple orbit event sequences (MOES), which were series of events separated in time by integer multiples of the LDEF orbital period. Even though the spikes were generally more intense, the MOES could be quite long-lived, some lasting for many days. A previous paper by Cooke et al. attributed the MOES to impacts by man-made debris particles in orbits intersecting that of LDEF. The 20 day longevity of one of these events - termed the May Swarm - led to the suggestion that the debris particles must be con- stantly replenished by their source, as the orbits of micron sized particles will rapidly decay under the influence of radiation pressure and other non-gravitational forces, entering Earth's atmosphere after only a few revolutions. However, the date of onset of the May Swarm (May 22) and the long duration of this event may indicate a possible correlation with the annual Arietid meteor shower, which peaks around June 8. As this seemed to hold the promise of a less artificial explanation than a satellite or rocket body continuously dribbling debris, it was decided to take a fresh look at parts of the IDE data set in an attempt to detect meteor showers within the impact record.

Cooke, William J.↗

NASA NTRS1995

Intermittency and extended self-similarity in the solar wind turbulence

Using the satellite measurements of the velocity field in the interplanetary plasma. we present some analysis which show the presence of a self-similar intermittent state in the solar wind turbulence. We used the so called Extended Self-Similarity hypothesis, which is well visible in the solar wind turbulence, showing convincing evidences for the presence of universal anomalous scaling laws. Through the Extended Self-Similarity we are able to calculate the scaling exponents of the velocity structure functions with very small uncertainties, and we show that these scaling exponents are in very good agreement with the multifractal models describing intermittency in magnetohydrodynamic flows.

Bruno, R.↗

NASA NTRS1987

Birkeland currents - Recent contributions from satellite magnetic field measurements

The important role that field-aligned Birkeland currents play in solar terrestrial coupling processes has been known from satellite observations for twenty years. Many satellite have been launched since the last Solar-Terrestrial Symposium with instruments capable of investigating the characteristics of Birkeland currents and their relationship to visible and UV auroral forms, particle precipitation, and convection flow patterns. An important result that has emerged is the close relationship between large-scale Birkeland currents and the orientation of the interplanetary magnetic field. These observations emphasize the fact that Birkeland currents are involved in a wide range of space plasma processes which range from small-scale ionospheric instabilities and irregularities to global scale energy coupling processes between the magnetosphere and solar wind.

Potemra, T. A.↗

NASA NTRS1995

Some features of the small-scale solar wind fluctuations

We have investigated small-scale variations of the solar wind ion flux measured with Faraday cups onboard the Prognoz-8 satellite. These measurements have a high time resolution of 1.24 seconds for intervals with a duration of several hours and as high as 0.02 seconds for some periods of about 1 hour duration. The main goal of this work is the determination of the quantitative features of fast ion flux fluctuations using mainly spectral analysis but also other methods. We also identify their association with interplanetary plasma parameters. Particularly, it is shown that the slope of the power spectra in the frequency range from 1E-4 to 6E-2 Hz is close to the classical Kolmogorov (-5/3) law. We also discuss some intervals with a very high level of the relative amplitude of flux fluctuations (10-20 percent) which were observed near the Earth's bow shock in the foreshock region. The use of the wavelet method for the long time series allows us to study the temporal evolution of power spectra.

Zastenker, G.↗

NASA NTRS2013

Curation of Microscopic Astromaterials by NASA: "Gathering Dust Since 1981"

Employing the philosophy that "Small is Beautiful", NASA has been collecting and curating microscopic astromaterials since 1981. These active collections now include interplanetary dust collected in Earth's stratosphere by U-2, ER-2 and WB-57F aircraft (the Cosmic Dust Program - our motto is "Gathering dust since 1981"), comet Wild-2 coma dust (the Stardust Mission), modern interstellar dust (also the Stardust Mission), asteroid Itokawa regolith dust (the Hayabusa Mission - joint curation with JAXA-ISAS), and interplanetary dust impact features on recovered portions of the following spacecraft: Skylab, the Solar Maximum Satellite, the Palapa Satellite, the Long Duration Exposure Facility (LDEF), the MIR Space Station, the International Space Station, and the Hubble Space Telescope (all in the Space Exposed Hardware Laboratory).

Frank, D. R.↗

NASA NTRS1992

Modelling the near-Earth space environment using LDEF data

Near-Earth space is a dynamic environment, that is currently not well understood. In an effort to better characterize the near-Earth space environment, this study compares the results of actual impact crater measurement data and the Space Environment (SPENV) Program developed in-house at POD, to theoretical models established by Kessler (NASA TM-100471, 1987) and Cour-Palais (NASA SP-8013, 1969). With the continuing escalation of debris there will exist a definite hazard to unmanned satellites as well as manned operations. Since the smaller non-trackable debris has the highest impact rate, it is clearly necessary to establish the true debris environment for all particle sizes. Proper comprehension of the near-Earth space environment and its origin will permit improvement in spacecraft design and mission planning, thereby reducing potential disasters and extreme costs. Results of this study directly relate to the survivability of future spacecraft and satellites that are to travel through and/or reside in low Earth orbit (LEO). More specifically, these data are being used to: (1) characterize the effects of the LEO micrometeoroid an debris environment on satellite designs and components; (2) update the current theoretical micrometeoroid and debris models for LEO; (3) help assess the survivability of spacecraft and satellites that must travel through or reside in LEO, and the probability of their collision with already resident debris; and (4) help define and evaluate future debris mitigation and disposal methods. Combined model predictions match relatively well with the LDEF data for impact craters larger than approximately 0.05 cm, diameter; however, for smaller impact craters, the combined predictions diverge and do not reflect the sporadic clouds identified by the Interplanetary Dust Experiment (IDE) aboard LDEF. The divergences cannot currently be explained by the authors or model developers. The mean flux of small craters (approximately 0.05 cm diameter) is overpredicted by Kessler and underpredicted by Cour-Palais. This divergence may be a result of beta-meteoroid fluxes, elliptical orbits or a combination of the two. The results of this study illustrate the definite need for more intensive study of the near-Earth space environment, particularly the small particle regime, as it is the most degrading to spacecraft in LEO.

Atkinson, Dale R.↗

NASA NTRS2022

Evolution of Biological Satellites: From Low Earth Orbit to NASA’s BioSentinel Deep Space Mission

NASA has set its sights on human exploration in deep space with the Artemis missions, with an ambitious plan to put astronauts back on the Moon and to eventually land human missions on Mars. Such missions will require significant countermeasures, likely both technological and biomedical, to protect biology from chronic radiation exposure. Small satellites like CubeSats can inform these countermeasures by querying relevant space environments with model organisms over relevant durations. NASA has launched five biological CubeSats into low Earth orbit (LEO) from GeneSat in 2006 to EcAMSat in 2017. Each one of these missions increased our understanding of the effects of spaceflight, while refining technologies and imparting valuable lessons to the next generation of CubeSats. The Artemis I rocket will carry ten CubeSats, each of them with its own objective. One in particular, BioSentinel, will conduct the first study of the biological response to interplanetary space radiation beyond LEO since Apollo 17. Once it reaches its heliocentric orbit – after a short lunar fly-by – BioSentinel will measure the DNA damage response to ambient radiation in a model organism, the budding yeast Saccharomyces cerevisiae, which will be compared to information provided by an onboard radiation sensor and to data obtained in LEO (on ISS) and on Earth. Once in interplanetary space, fluidic cards containing desiccated yeast cells will be activated by growth medium addition at different time points throughout the mission. Growth and metabolic activity will be tracked continuously via optical density. BioSentinel is a complete, autonomous spacecraft capable of conducting experiments in deep space. Its 4U BioSensor payload is a fully automated and adaptable platform that can perform biological measurements with a range of microorganisms in multiple space environments, including the ISS, free flyers, and other platforms like the Lunar Gateway and lander vehicles.

Sergio R Santa Maria↗

NASA NTRS2011

Dynamical Model for the Zodiacal Cloud and Sporadic Meteors

The solar system is dusty, and would become dustier over time as asteroids collide and comets disintegrate, except that small debris particles in interplanetary space do not last long. They can be ejected from the solar system by Jupiter, thermally destroyed near the Sun, or physically disrupted by collisions. Also, some are swept by the Earth (and other planets), producing meteors. Here we develop a dynamical model for the solar system meteoroids and use it to explain meteor radar observations. We find that the Jupiter Family Comets (JFCs) are the main source of the prominent concentrations of meteors arriving to the Earth from the helion and antihelion directions. To match the radiant and orbit distributions, as measured by the Canadian Meteor Orbit Radar (CMOR) and Advanced Meteor Orbit Radar (AMOR), our model implies that comets, and JFCs in particular, must frequently disintegrate when reaching orbits with low perihelion distance. Also, the collisional lifetimes of millimeter particles may be longer (approx. > 10(exp 5) yr at 1 AU) than postulated in the standard collisional models (approx 10(exp 4) yr at 1 AU), perhaps because these chondrule-sized meteoroids are stronger than thought before. Using observations of the Infrared Astronomical Satellite (IRAS) to calibrate the model, we find that the total cross section and mass of small meteoroids in the inner solar system are (1.7-3.5) 10(exp 11) sq km and approx. 4 10(exp 19) g, respectively, in a good agreement with previous studies. The mass input required to keep the Zodiacal Cloud (ZC) in a steady state is estimated to be approx. 10(exp 4)-10(exp 5) kg/s. The input is up to approx 10 times larger than found previously, mainly because particles released closer to the Sun have shorter collisional lifetimes, and need to be supplied at a faster rate. The total mass accreted by the Earth in particles between diameters D = 5 micron and 1 cm is found to be approx 15,000 tons/yr (factor of 2 uncertainty), which is a large share of the accretion flux measured by the Long Term Duration Facility (LDEF). Majority of JFC particles plunge into the upper atmosphere at <15 km/s speeds, should survive the atmospheric entry, and can produce micrometeorite falls. This could explain the compositional similarity of samples collected in the Antarctic ice and stratosphere, and those brought from comet Wild 2 by the Stardust spacecraft. Meteor radars such as CMOR and AMOR see only a fraction of the accretion flux (approx 1- 10% and approx 10-50%, respectively), because small particles impacting at low speeds produce ionization levels that are below these radars detection capabilities.

Nesvorny, David↗

NASA NTRS1980

The magnetic field of Saturn - Further studies of the Pioneer 11 observations

Analysis of magnetic field observations by the Goddard Space Flight Center high-field flux gate magnetometer on the Pioneer 11 spacecraft during Saturn encounter yields estimates of the planetary field. The field is mainly dipolar but rather weaker than expected, with a moment equal to 0.20 G cubic Saturn radii or 4.3 x 10 to the 28th G cu cm, opposite in polarity to earth's. Surprisingly, the field appears to be axially symmetric but with a small (0.04 Saturn radii) offset to the north so that N (S) polar field intensities are 0.6 (0.4) G, respectively. The deduced polar offset appears not to be an artifact of the limited spatial extent of the observations or the presence of fields of external origin. The average stand-off distance of the magnetopause is expected to be 20 Saturn radii, i.e., at the orbit of Titan, so that this largest of solar system satellites is immersed not only in the Saturnian magnetosphere but also at times in its magnetosheath and sometimes even in the interplanetary medium.

Acuna, M. H.↗

NASA NTRS2017

Deployable Propulsion and Power Systems for Solar System Exploration

NASA is developing thin-film based, deployable propulsion, power and communication systems for small spacecraft that could provide a revolutionary new capability allowing small spacecraft exploration of the solar system. The Near Earth Asteroid (NEA) Scout reconnaissance mission will demonstrate solar sail propulsion on a 6U CubeSat interplanetary spacecraft and lay the groundwork for their future use in deep space science and exploration missions. Solar sails use sunlight to propel vehicles through space by reflecting solar photons from a large, mirror-like sail made of a lightweight, highly reflective material. This continuous photon pressure provides propellantless thrust, allowing for very high delta V maneuvers on long-duration, deep space exploration. Since reflected light produces thrust, solar sails require no onboard propellant. The Lightweight Integrated Solar Array and Transceiver (LISA-T) is a launch stowed, orbit deployed array on which thin-film photovoltaic and antenna elements are embedded. Inherently, small satellites are limited in surface area, volume, and mass allocation; driving competition between power, communications, and GN&C (guidance navigation and control) subsystems. This restricts payload capability and limits the value of these low-cost satellites. LISA-T is addressing this issue, deploying large-area arrays from a reduced volume and mass envelope - greatly enhancing power generation and communications capabilities of small spacecraft. The NEA Scout mission, funded by NASA's Advanced Exploration Systems Program and managed by NASA MSFC, will use the solar sail as its primary propulsion system, allowing it to survey and image one or more NEA's of interest for possible future human exploration. NEA Scout uses a 6U cubesat (to be provided by NASA's Jet Propulsion Laboratory), an 86 sq m solar sail and will weigh less than 12 kilograms. NEA Scout will be launched on the first flight of the Space Launch System in 2018. Similar in concept to the NEA Scout solar sail, the LISA-T array is designed to fit into a very small volume and provide abundant power and omnidirectional communications in just about any deployment configuration. The technology is being proposed for flight validation as early as 2019 in a low earth orbit demonstration using a 3U cubesat, of which less than 1U will be devoted to the LISA-T power and propulsion system. By leveraging recent advancements in thin films, photovoltaics and miniaturized electronics, new mission-level capabilities will be enabled aboard lower-cost small spacecraft instead of their more expensive, traditional counterparts, enabling a new generation of frequent, inexpensive deep space missions.

Johnson, Les↗

NASA NTRS1997

Boundary Layer Formation in the Magnetotail: Geotail Observations and Comparisons with a Global MHD Simulation

We present Geotail plasma and field observations from the middle magnetotail near X(sub GSE) = -46 R(sub E) for the time period 1400 to 1800 UT on December 14, 1994. During that period, the Wind satellite monitored the solar wind plasma and interplanetary magnetic field (IMF) upstream of the bow shock. The IMF was northward and the plasma parameters near average. Geotail observed slow tailward flows and a northward field. The plasma and field parameters indicate that Geotail is either in the plasma sheet or in a boundary layer. We used the Wind solar wind plasma and IMF data as input for a global simulation of that time interval. Comparison of the simulation results with the observational data show very good overall agreement of the magnitudes of the plasma and field parameters. In particular, the simulation reproduces the slow tailward flows and northward field found at Geotail. Small scale temporal, variations are less well reproduced. The simulation shows the formation of a broad boundary layer (which we call tail flank boundary layer, TFBL) that consists of closed flux which is formed by magnetic magnetic reconnection of IMF and lobe field lines. The simulation results indicate that Geotail is located very close to the TFBL and may have entered the TFBL proper. We show that the TFBL plays an important role in energy transport from the solar wind into the magnetosphere during northward IMF conditions.

Raeder, J.↗

NASA NTRSDate not supplied

BioSentinel: NASA’s First Deep Space Biological Mission

Since Apollo 17 in 1972, NASA has sent no humans or other biological organisms outside of Earth’s protective magnetosphere. NASA’s current Artemis program plans to put astronauts back on the Moon and eventually land human missions on Mars. One of the major challenges to long-duration crewed travel and habitation in deep space is an in-depth understanding of the biological effects of space radiation, often convoluted by the impact of reduced gravity. Such missions will require significant countermeasures, likely both technological and biomedical, to protect organisms from chronic radiation exposure. Small satellite missions like CubeSats can inform these countermeasures by investigating model organisms in relevant space environments. The BioSentinel mission is comprised of four segments developed at NASA Ames Research Center: a 6U CubeSat (1U = 10-cm cube), an ISS payload launched in December 2021 and two ground units, one for the mission’s CubeSat and one for the ISS payload. The last three segments have been operational since January 2022 and serve as experimental controls. BioSentinel’s 6U CubeSat is planned to launch as a secondary payload on the Artemis-1 rocket. It will be deployed on a lunar fly-by trajectory and into a heliocentric orbit. BioSentinel will be the first interplanetary satellite to study the biological response to space radiation outside Low Earth Orbit (LEO) in almost 50 years. BioSentinel is a complete, autonomous spacecraft capable of conducting experiments in deep space. Its 4U BioSensor payload is a fully automated and adaptable platform that can perform biological measurements with a range of microorganisms in multiple space environments, including the ISS, free flyers, and other platforms like the Lunar Gateway and lander vehicles. Once it reaches its orbit, BioSentinel’s CubeSat will measure the DNA damage response to ambient radiation in a model organism, the budding yeast Saccharomyces cerevisiae, which will be compared to information provided by an onboard radiation sensor and to data obtained in LEO (on ISS) and on Earth. Once in interplanetary space, fluidic cards containing desiccated yeast will be activated by growth medium addition at different time points throughout the mission. Growth and metabolic activity will be tracked continuously via optical measurements. This paper describes BioSentinel’s objectives, science, data management, and preliminary results from the ISS and ISS ground control segments.

BioSentinel↗

NASA NTRSDate not supplied

BioSentinel: NASA’s First Deep Space Biological Mission

BioSentinel↗

NASA NTRS2014

Lunar Flashlight: Mapping Lunar Surface Volatiles Using a Cubesat

Water ice and other volatiles may be located in the Moon's polar regions, with sufficient quantities for in situ extraction and utilization by future human and robotic missions. Evidence from orbiting spacecraft and the LCROSS impactor suggests the presence of surface and/or nearsurface volatiles, including water ice. These deposits are of interest to human exploration to understand their potential for use by astronauts. Understanding the composition, quantity, distribution, and form of water/H species and other volatiles associated with lunar cold traps is identified as a NASA Strategic Knowledge Gap (SKG) for Human Exploration. These polar volatile deposits could also reveal important information about the delivery of water to the Earth- Moon system, so are of scientific interest. The scientific exploration of the lunar polar regions was one of the key recommendations of the Planetary Science Decadal Survey. In order to address NASA's SKGs, the Advanced Exploration Systems (AES) program selected three lowcost 6-U CubeSat missions for launch as secondary payloads on the first test flight (EM1) of the Space Launch System (SLS) scheduled for 2017. The Lunar Flashlight mission was selected as one of these missions, specifically to address the SKG associated with lunar volatiles. Development of the Lunar Flashlight CubeSat concept leverages JPL's Interplanetary Nano- Spacecraft Pathfinder In Relevant Environment (INSPIRE) mission, MSFC's intimate knowledge of the Space Launch System and EM-1 mission, small business development of solar sail and electric propulsion hardware, and JPL experience with specialized miniature sensors. The goal of Lunar Flashlight is to determine the presence or absence of exposed water ice and its physical state, and map its concentration at the kilometer scale within the permanently shadowed regions of the lunar south pole. After being ejected in cislunar space by SLS, Lunar Flashlight deploys its solar panels and solar sail and maneuvers into a low-energy transfer to lunar orbit. The solar sail and attitude control system work to bring the satellite into an elliptical polar orbit spiraling down to a perilune of 30-10 km above the south pole for data collection. Lunar Flashlight uses its solar sail to shine reflected sunlight into permanently shadowed regions, measuring surface albedo with a four-filter point spectrometer at 1.1, 1.5 1.9, and 2.0 microns. Water ice will be distinguished from dry regolith from these measurements in two ways: 1) spatial variations in absolute reflectance (water ice is much brighter in the continuum channels), and 2) reflectance ratios between absorption and continuum channels. Derived reflectance and reflectance ratios will be mapped onto the lunar surface in order to distinguish the composition of the PSRs from that of the sunlit terrain. Lunar Flashlight enables a low-cost path to in-situ resource utilization (ISRU) by identifying operationally useful deposits (if there are any), which is a game-changing capability for expanded human exploration.

Cohen, B. A.↗

NASA NTRS2023

Application of Fountain Code to High-Rate Delay Tolerant Networks

Space communication poses several unique challenges that are not always present in typical terrestrial communications. Currently, communication with satellites is based on point-to-point links, and development of an interplanetary internet is an active research area. Delay Tolerant Networking (DTN) has been proposed as a way to mitigate the long delays and disruptions found in deep space. A specialized version of DTN, called High-rate Delay Tolerant Networking (HDTN), has been developed by NASA to support a variety of missions requiring store-and-forward capability. However, there are still several features that are desired for HDTN including data fragmentation, multicast, and anycast. This project proposes the application of fountain code in HDTN as a means of fragmenting, distributing, and reassembling data (in the form of bundles) across multiple nodes (i.e. satellites) to any number of receivers (i.e. ground stations). Fountain code is shown to be a promising encoding method for use with the HDTN protocol suite due to its short runtimes, small encoded file sizes, and loss tolerance.

Noah Douglass↗

NASA NTRS1972

Properties of nearby interstellar hydrogen deduced from Lyman-alpha sky background measurements

For a sufficiently rapid relative motion of the solar system and the nearby interstellar gas, neutral atoms may be expected to penetrate the heliosphere before becoming ionized. Recent satellite measurements of the Lyman alpha emission above the geocorona indicate such an interstellar wind of neutral hydrogen emerging from the direction of Sagittarius and reaching to within a few astronomical units of the sun. A detailed model of the scattering of solar Lyman alpha from the spatial distribution of neutral hydrogen in interplanetary space is presented. This asymmetric distribution is established by solar wind and solar ultraviolet ionization processes along the trajectories of the incoming hydrogen atoms. The values of the interstellar density, the relative velocity, and the gas temperature are adjusted to agree with the Lyman alpha measurements. The results may be interpreted in terms of two models, the cold model and the hot model of the interstellar gas, depending on whether galactic Lyman alpha emission is present at its maximum allowable value or negligibly small.

Thomas, G. E.↗

NASA NTRS2005

The Interstellar Conspiracy

If we were designing a human-carrying starship that could be launched in the not-too-distant future, it would almost certainly not use a warp drive to instantaneously bounce around the universe, as is done in Isaac Asimov's classic Foundation series or in episodes of Star Trek or Star Wars. Sadly, those starships that seem to be within technological reach could not even travel at high relativistic speeds, as does the interstellar ramjet in Poul Anderson's Tau Zero. Warp-speeds seem to be well outside the realm of currently understood physical law; proton-fusing ramjets may never be technologically feasible. Perhaps fortunately in our terrorist-plagued world, the economics of antimatter may never be attractive for large-scale starship propulsion. But interstellar travel will be possible within a few centuries, although it will certainly not be as fast as we might prefer. If humans learn how to hibernate, perhaps we will sleep our way to the stars, as do the crew in A. E. van Vogt's Far Centaurus. However, as discussed in a landmark paper in The Journal of the British Interplanetary Society, the most feasible approach to transporting a small human population to the planets (if any) of Alpha Centauri is the worldship. Such craft have often been featured in science fiction. See for example Arthur C. Clarke's Rendezvous with Rama, and Robert A. Heinlein's Orphans of the Sky. Worldships are essentially mobile versions of the O Neill free-space habitats. Constructed mostly from lunar and/or asteroidal materials, these solar-powered, multi-kilometer-dimension structures could house 10,000 to 100,000 humans in Earth-approximating environments. Artificial gravity would be provided by habitat rotation, and cosmic ray shielding would be provided by passive methods, such as habitat atmosphere and mass shielding, or magnetic fields. A late 21st century space-habitat venture might support itself economically by constructing large solar-powered satellites to beam energy back to Earth.

Johnson, Les↗