statistics of solar studies
delivered, they could return with samples or mined ores by folding their sails and dropping back down the gravity well. At some point in the return journey, if it were necessary, the sails could be again deployed as brakes to bring them into parking orbits ready for the next outward trip. Perhaps, by careful design utilising batteries topped up by solar photovoltaic power, they could even dispense with fuel and reaction mass altogether. Then again, they could make outward
E-sail specifi c acceleration as function of thrust for four tensile strengths. Dashed lines show tether wires thinner than 18m. The 10 MPa curve corresponds to current ultrasonically bonded aluminum tether. A system base mass of 10kg and a voltage of 30kV are assumed. Dotted horizontal line gives the level of solar gravity at 1AU for reference (propulsion systems reaching solar gravity acceleration are usually considered very good)
Image source Janhunan et al [6] 2010
propulsion would be outward only, but robots and materials in transit could move around the solar system for very little cost. One picture which comes to mind is
of a continuous (if leisurely) bucket chain of gossamer freighters. Spreading their fi laments they would take research or industrial equipment, or components of bigger robot entities, outwards under sail power. Judicious use of fl ypast gravitational drag could decelerate them to close velocity matching at their destination. Their loads
Fresh blew the wind...
Where the sun starts and stops is no more perfectly defi ned than the edge of Earth’s atmosphere. The Earth is, in fact, like the rest of the planetary system, well within the outer envelope of the heliosphere, a bubble blown within the ambient interstellar gasses and dusts and sustained by the outward pressure of fast and slow solar wind streams comprising ionised particles of variable composition. Through this bubble blows a so-called ‘breeze’ of interstellar particles that interact with the heliosphere in varying degree. The slow (less than about 500 kilometres per second) wind stream, strongest in the plane of the ecliptic, is the
most of the heliosphere and is strongest above and below the solar poles. The pattern is simplest in quiescent phases, less well defi ned at times of high activity.
The winds originate in magnetic reconnection as oppositely charged fi elds collide and release energy. MDI research using SUMER spectrometry and magnetograms from SOHO suggests[8]
one-way deep space trips; a quick scratchpad calculation suggests that (continuously accelerating up to the velocity of the solar wind itself) they would easily overhaul Voyager 1 despite its several decades of head start. Given the diffi culty of testing prototypes and the novel nature of envisaged construction materials, to mention only two obvious factors, computer-based modelling of the sail is a given. Such models, dealing as they do with environments and problems for which no experience exists, must necessarily be based on statistical rather than deterministic bases. Nevertheless, solar power could be the way that humankind eventually leaves its sun behind and reaches (if only by robotic proxy) Proxima Centauri – which seems a pleasant thought on which to fi nish.
References and Sources For a full list of the references and sources cited in this article, please visit
www.scientifi c-computing. com/features/
referencesfeb11.php
The results of oblateness measurements from a Solar Diameter Sextant fl ight are shown in a DADiSP worksheet
Up, up and away
One aspect of the sun’s suspected part in global warming is investigated using balloon fl ights generating data through automated statistical software. A variation of around 0.5 per cent in the solar constant* has been suggested as a signifi cant factor in long term climate change. Direct measurement data showed a rise of roughly 0.1 per cent increase across one solar activity cycle, but whether that represents an oscillation segment or part of a sustained increase over a much longer period has to be indirectly inferred. Correlation of reliable temperature records (maintained over the past century and a half) with solar luminosity would be a big step in that direction. While the necessary historical information on luminosity did not exist per se, oblateness observations during solar eclipses have bequeathed a body of data on variation in diameter. Solar modelling links diameter to luminosity, but the constants in that linkage needed to be established. Simultaneous direct experimental measurement of both variables could provide the evidence base for establishing those constants. Beginning with a conceptual design in the mid 1980s[7]
, the NASA/Yale
balloon borne Solar Disk Sextant project has produced diameter and oblateness data with great precision and in considerable volume across two decades.
that they originate
Solar Dynamics Observatory image of the Sun taken on 10 Jan 2011, in extreme ultraviolet light, showing a dark coronal hole Image source NASA 2011
more concentrated. It is the more insistent in its effects on the solar system on general and Earth in particular. The fast wind travels further, fi lls
16 SCIENTIFIC COMPUTING WORLD
in electromagnetic funnels within coronal holes (regions of low light intensity and open magnetic fi elds). Observations from JSA solar observatory Hinode place the source of the slow wind in Alfvén waves (created when convection,
sound waves or other dynamic processes shift or deform magnetic fi elds) which transfer energy from the solar surface up through the corona.
The wash of data from each balloon fl ight is
processed with numerous collateral factors used as quality control. Changes in the instrument itself are detected, correlations made with other known data, and so on. The multiple analytic, comparative and diagnostic strands necessary for all of this are handled in automated DADiSP worksheets. *Solar constant: the total radiant energy per unit time per unit area passing through the mean earth orbit, with an approximate value of 1.353 kilowatts per square metre per second.
www.scientific-computing.com
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