SOLAR SAILING ----------- John Pazmino NYSkies Astronomy Inc email@example.com www.nyskies.org 2010 July 24
Introduction ---------- On 2010 July 20-22 NYC Technical College (City Tech) hosted the 2nd International Symposium on Solar Sailing. Delegates from at least 14 countries convened at the college to review and preview the use of sails for space travel. The conference was a bit expensive for casual attendance but it offered a free public lecture/panel on Wednesday 21 July. It featured several delegates giving short summaries of their work. The lecture opened at 18h EDST and ran thru a little before 20h. The highlight speaker was Dr Kawaguchi of Japan. He discussed and showed his IKAROS spaceprobe, the first ever true solar sail experiment. It's on its way to Venus and then heliocentric orbit and already set out its sail. A week or so before the lecture it recorded positive acceleration of the sail due to incident sunlight. After the talks was a panel of all of the speakers to field questions from the floor and a general dialog about solar sailing..
City Tech ------- City Tech is a component of the City University of New York but is its curriculum is mostly for vocational and technical fields. In its own circle, City Tech is well respected and turns out career-worthy graduates. It is on Jay Street, a main north-south street in Boro Hall and Cadman Plaza, Brooklyn. It's reached by buses and subways working these two districts. I arrived at the college by the IND 6th Av line at its Jay Sy-Boro Hall station. From there it was a 200 meter walk north straight on Jay St. It mainly offers two-year degrees. With care in choosing courses a student obtains an easy transfer to a four-year college for the bachelor's degree. The college works closely with Polytechnic University, now part of New York University, a blocks south nearer to Boro Hall. It has career paths with several of the companies in MetroTech, a corporate campus also near Boro Hall. City Tech is deceiving small. It has a couple halls, newly built for it or renovated in older buildings. It has no campus in the normal sense, which is a common feature in many other City University units. Yet it enrolls some 15,000 students!
Registration ---------- For some peculiar reason the lecture required sign up by getting a ticket from Brown Paper Ticket, a website selling theater tickets. The various notices for the lecture gave confusing instructions for registering. Detective work and flat-out inquiries got most attendees to the right place. There was no cost for the solar sail talk but you had to step thru all the registration items, with the price entered as '$ 0'. There is no actual ticket issued.. You were placed on a list to be checked off upon entry. I printed out the completion page. just in case there was a dispute. Fellow NYSkier Julius Chini, of New York Institute of Technology, also wanted to attend. With his computer acting up, he asked me to cut a ticket for him. I did and then delivered a printout to his college office a day before the symposium. After going thru all of this fuss, there was NO ONE AT ALL collecting tickets or checking lists at the lecture hall! Any one could walk in off of the street with no challenge. In fact, in the general public notice there was no mention of sign up for the meeting.
Lecture hall ---------- I arrived at about 17:45, reaching the street at Jay-St station. After entering the College I asked for the 'solar sail talk'. I was pointed thru certain doors to the Atrium Theater. A little ways inside I came to a large arena, lighted by skylight and ringed by sofas. The sofas had 'reserved' signs on them. At one side was a decorated table with front-facing seating. This must be the 'atrium' theater? I sat in a sofa for a couple minutes. No one else came along and no crew attended the place. An inquiry at a nearby office got me to the proper room. This was a regular auditorium having some 100 seats and nothing at all associated with 'atrium'. The room was pleasantly cool and remained so for the entire meeting. The lectern was lighted by spotlamps. Their range cones spilled partly onto the projection screen but did not overly obscure pictures shown on it. Area lighting at first was dismally dim. I could not read handouts or sign an attendance sheet in the low light. Only at the end, during the panel, were ceiling lighting turned on. A few audience were in seats, a few chatted around the walls, crew was fiddling with the AV gear. I sat with Prof Chini. I noticed no one else from local spacefaring groups. At 18:00 the meeting was opened by Louis Friedman, Planetary Society. The Society cosponsored the conference and has a sail project in design stage, Cosmos-1. Other speakers were Junichiro Kawaguchi, Japan space agency; Malcolm Mcdonals, University of Strathclyde, UK; Les Johnson, NASA Marshall SFC; and Roman Kezerashvili, City Tech. There were barely 30 in the audience! The attendees spread out, like repellent electric charges, thruout the seats, leaving large holes between them. I was a bit surprised, being that this lecture was part of the overall conference. Be as it may, this number remained stable thruout the entire procedings, with occasional exchange of people as some left early and others came late. Later in the week I learned there was a bus tour of the City for the delegates concurrent with the symposium. I agree that most delegates wanted this chance to see the City, a unique feature of the meeting, and catch up on the symposium from colleagues later.
Solar sailing ----------- I give here a blended summary of the talks with some comments from other associates regarding solar sails. A solar sail is a large thin light-weight sheet attached to a spacecraft. It collects sunlight, which imports its momentum to the sail and craft. The pressure comes from the photons as an effect of electromagnetic radiation. By altering the area, reflectance, angle of the sail, the direction and strength of the photon thrust can be adjusted to manoeuver the spacecraft. It replaces rockets or jets, saving the mass, complexity, safety hazard, finite supply of fuel. As you may imagine, the pressure of sunlight is minuscule. That's why it is common to associate solar sails with humongous sizes, slow accelerations, long duration flights. Yet such sails are practical for other simpler spacefaring functions, as illustrated in this meeting and elsewhere. The sail and its rigging are folded up delicately inside the spacecraft and unfurled when it gets beyond the atmosphere. Because it must face the Sun a solar sail can have embedded or attached solar cells for electric power. If the craft returns to Earth, the sail can be discarded for destructive decay in the atmosphere.
Radiation ------- Electromagnetic radiation, within which light is merely one range of wavelength, conveys energy from source to target. This is the energy that activates the target such as a radio, signal, television, telephone. Remote control by radio waves was first demonstrated in 1898 in New York by Tesla. He manipulated a motorized boat in a tub of water by working a radio console at one side. Communication by radio was also demonstrated by Telsa and by Marconi in 1899 to inaugurate the electromagnetic industry on Earth. The amount of energy, the field strength, is usually very weak. When the wave hits a surface it imparts its energy to it, usually as heat, agitation of the molecules in the surface. Some of this energy is captured internally by the surface, as an antenna, to drive the electronic device. An object standing in sunlight gets amazingly hot for this reason. Solar heating is now commonly used in winter, displacing fuel-burning heaters, by shining thru sunward-facing windows. In the transfer of energy from the wave to the target, a pressure is applied. The amount of this pressure varies with the reflectance of the target thru a 2:1 swing. A fully reflective target, a mirror, receives twice the pressure as a fully absorbing target. This is why an object of varied colors suffers different heating from part to part. Parts of light color are (a bit) cooler than those of dark color. This is why in tropical or torrid countries houses tend to be painted white or pastel colors.
Pressure ------ The pressure produced by impingent radiation is surprisingly simple:
(pressure) = (irradiation)/(lightspeed), absorbing target
(pressure) = (2)*(irradiation)/(lightspeed), reflecting target
Since no real target is totally reflecting or absorbing, the realized pressure is somewhere between these extremes. Most solar sails are very reflective, like by metal coatings, to maximize the pressure. This tactic also lessens somewhat the required area and mass to be mounted on the spacecraft. By applying dimensional analysis, the formula is:
[newton/meter2] = [watt/maters2]/[meter/second] = [joule/second.meter2]/[meter/second] = [joule/meter2]/[meter], see below = [newton.meter/meter2]/[meter] = [newton/meter2]
The line 'see below' is a diversion. In engineering, pressure is often associated with energy. When a high-pressure tank bursts, it releases high energy suddenly. The [joule/meter2]/[meter] is collapsed to [joule/meter3], an energy density. A tank under high pressure is also a tank laden with dense energy. If this is released suddenly, in a blowout, the consequences are similar to a chemical bomb. Applying numbers to the pressure formula, based on the insolation from sunlight at the Earth, we obtain:
(pressure) = (2)*(irradiation)/(lightspeed), reflecting target = (2)*(1.40 kW/m2)/(3e5 km/s) (9.33e-6 N/m2) (9.33 uN/m2)
The 'u' is the ASCII substitute for Greek mu, the symbol for 'micro-'. This 9.33 micronewton/meter2 IS TINY. Compared to Earth's atmosphere pressure at sealevel, it's about 9e-11 atmospheres. I believe this a LESS than the pressure of the evanescent atmosphere on the Moon!
Solar system ---------- The above calcs are for a sail at Earth distance from the Sun. Since irradiation from the Sun follows the inverse-square rule, so does the realized pressure on the sail. For a given distance from the Sun in AU, where 1AU is the mean Earth-Sun distance, we have:
(pressure at x AU) = (9.33 uN/m2)*(1/x)^2
For a sail in orbit of Mercury, 0.37AU from Sum, the pressure is
(pressure at 0.37AU) = (9.33 uN/m2)*(1/x)^2 = (9.33 uN/m2)*(1/(0.37))^2 = (9.33 uN/m2)*(2.70)^2 = (9.33 uN/m2)*(7.30) = (68.15 uN/m2)
This is still a very small thrust. For targets farther away than Earth, the pressure becomes vanishingly small and likely not all that useful for spacecraft operation. A sail near Saturn, about 10AU from Sun, receives a pressure of only about 0.093 uN/m2.
Spectrum ------ For a good first approximation, the Sun is a blackbody radiator running at 6,000K. At this temperature most of the emitted energy is within the optical or visual band of the spectrum. Little emits in the infrared or ultraviolet. While the maximum wavelength of emission is around 550nm, with the energy output falling off toward the longer and shorter wavelengths, for solar sail work, we allow the full spectrum to fall on the sail. There is no attempt to filter or other wise modify the spectrum of the sunlight impinging on the sail. The total energy received from the Sun at 1AU is quite 1,400 watt/meter2, a figure that varies only mildly with the cycle of sunspots. It also is stable secularly because the Sun is a main sequence star, generating its output by the hydrogen-burning process. This will last for the next several billion years. Note well that this value is substantially greater than that used for solar power on Earth. Our air diverts about 20% of the sunlight to make the blue sky and heat the air itself. This is what drives weather circulation around the world. What's left as direct beams from the solar disc is at most 1,120 watt/meter2. And this is for a Sun in the zenith shining on horizontal plates. It also neglects removal of energy by cloud and water vapor. For practical purposes, 1,000 watt/meter2 is the insolation for terrestrial power from the Sun.
Frontal area ---------- The area in the above and all other sail calculations is the area faced toward the Sun. This is independent of the angle or texture of relief or shape of the sail. It's the cross section of the sunlight beam that completely fills the sail to its perimeter. Altering this area alters the thrust, the 'area' part of the force formula. There are several ways to do this, only one of which was ever employed. This method is the tilt or angle method. the entire sail as a single unit (the usual construction) is turned more or less toward the Sun. The frontal area is the entire faceon area times the cosine of the angle. Zero degree is faceon; 90, edgeon. This cites the angle between the incident sunlight and the normal to the surface. Some authors go by the plane with zero degree as edgeon and 90 as faceon. This flips the trig function to the sine. Read the author's explanation carefully! An other way, so far only proposed, is to make the sail in pleats or folds. Spreading them out exposes more area while collapsing them reduces the area. An other idea is to warp the sail to curl it into a smaller area or flatten it to the maximum area. Yet an other is to make the sail with flaps, vanes, louvers. Opening or closing these varies the frontal area.
Sail geometry ----------- The size and shape of a solar sail is strongly based on the means of putting it out from its stowage. The cheapest and simplest way is by spinning the spacecraft and letting centrifugal force pull out the sail. Springs in the rigging pop open the sheet and tension it into final shape. The details are crucial, Every component must work just right else the sail is deployed incompletely. It may then be useless for the mission. In other experiments with tethers or daughter modules, failures are almost expected. The wire jams or breaks, the module is oriented wrongly, some function doesn't work, and so on. Things work very differently in space than on earth. A mechanism that does its thing perfectly on a lab bench can go terribly wrong in outer space. Most sail proposals, and that of IKAROS, are square, consisting of four triangular sheets meeting at the central fuselage. Spars or struts of strong low-mass material spring out along the sides of the triangles, the diagonal of the square, to hold the sheets in place. This design is often a rigid one, where there is no intent to reshape the sheets to change area or angle. These factors are altered by moving the entire spacecraft. Adding shaping capability means a much more complex system with more things to go wrong. An other common plan is a disc supported by a peripheral ring that springs open, like a folding beach hat. Struts attach the sail to the craft at the center of the ring. A third is the blade, petal, or paddle sail. These look like the usual array of solar electric panels, rectangles on spars outward from the fuselage. These are mounted on rotating bases for area and nagle adjustment. A pretty, but not yet practical, scheme is the daisy of many blades in a circular pattern around the fuselage. Each is on a dirigible base for angle and area adjustment but together they form a close fitting round frontal area. The antagonism is to have a large light-weight sail with as fool- proof a deploying and mounting mechanism as possible. Only with advanced robotics or a human EVA can a scrambled sail be fixed.
Reflectance --------- The standard sails call for a reflective surface to get the most pressure, as explained above. That's why metallic membranes are so often mentioned. An other reason is that these sheets can be made extremely thin, mere microns, to reduce the sail mass. Such a sail has a fixed reflectance with no means of altering it. The only credible, and elegant!, reflectance-changing mechanism is that on IKAROS. It uses LCDs of variable darkness according as signal is applied to them.
Durability -------- Like anything else in longterm space setting, a solar sail can suffer degradation from the ambient conditions. Radiation of certain wavelengths, meteoroids, cosmic and solar wind, space junk can injure the sail. There's so far no long-endurance experience with sails, but that with other spaceprobes systems indicate that the danger of fatal damage may be overstated. Yes, a large rock or debris can crash thru the sail and rip it out of service. But such episodes are rare and does not demand fortifying a sail against them. The effort would be too costly in mass and reliability. Deterioration by sputtering, punctures, spalling of the coating will take longer than the mission lifespan. The greatest hazard is the manipulation of the sail. The mount with motors, tethers, gears can fail just as easily and spontaneously as any other spacecraft device. When that happens the sail no longer can guide the craft. Its trajectory is then biased by the now fixed attitude of the sail or the sail has to be jettisoned as space junk.
Solar wind -------- Besides electromagnetic radiation, the Sun pours out a continuous wind or rain of protons and electrons. Layfolk hear of this 'solar wind' because it louses up electric power, telcomms, data links, signal and control, timing, navigation. In fact, any thing that works thru electromagnetism can be screwed up by impact of the solar wind. Happily Earth's own magnetic field shields us from the constant bombardment of these particles. This shield is lacking on, as example, the Moon. When the particles enter our magnetic field they are diverted along the field lines of force away from the ground. In extreme cases, during a solar storm, the particles do penetrate to the lower atmosphere to cause, among other effects, the aurorae polares. Can't the solar sail be propelled by the solar wind? In principle, yes. In reality, no. The pressure, the sum of the impacts of each proton and electron, is 1/1000 of the radiation pressure, about 9 nanonewtons/meter2. For solar sail work, we ignore the contribution of the solar rain. For other aspects of satellite design, we MUST mind the solar wind. The plasma stream can -- and repeatedly does! -- zap onboard electronics. The SOHO probe, specially hardened against solar wind, is decked once in a while by a strong blast of wind and is out of action for hours or days. The spring 2010 incident with the Galaxy-11 telcomms satellite may have been caused by a solar wind zapping.
Other light source ---------------- One futuerist plan is to aim Earth or Moon based lasers at a solar sail to augment sunlight. The sunlight and laser attack the sail from different angles, allowing greater flexibility to govern the spaceprobe's trajectory. This is not yet feasible but laser apparatus is improving rapidly, mostly from military work. It will take an ordinary military-grade laser to put a spot of light equal to or stronger than sunlight on a distant solar sail. The laser beam is collimated and weakens with a fractional power of distance. It should be possible to operate a solar sail in the outer solar system where sunlight is too weak. The aim may be as accurate as for narrow-beam radio transmission. Pulsing or varying its strength offers ability to finely adjust the craft's flight path. The laser wavelength can be optimized for the reflective properties of the sail material. The use of lasers brings up the possibility of waypoints with stations like asteroids or planetary moons. When a solar sail craft comes nearby, it can call on the laser to give it a trajectory correction. With such facilities, a solar sail is no longer stricta mente 'solar' but a photon sail propelled by artificial illumination. So far no serious proposal for a laser-augmented sail project is in sight. In theory IKAROS can receive a military laser beam but japan didn't ay anything about that. Planetary Society's Cosmos-1 is pura mente a passive sunlight experiment.
So what? ------ Believe or not, a form of solar sailing was in use for many years in space exploration! Spaceprobes sport solar electric power plates, that catch sunlight to make electric for the craft. The usual arrangement is a set of blades or panels attached radially from the fuselage. They make electric by facing toward the Sun for maximum frontal area. This requires that the probe change orientation or that the panels be on dirigible mounts. The radiation pressure on the solar blades is enough to be measured and it must be accounted for in keeping the craft on its trajectory. In this case the solar pressure is compensated by thrusters as a nuisance force. On the other hand, it can be an extra force to exploit in running the mission. A recent example is the MESSENGER probe on its way to Mercury. MESSENGER made several swingbys of Earth, Venus, and Mercury. In each case its trajectory had to be adjusted delicately to aim at the sweet spot for the swingby so the probe leaves the planet on a correct path for the next phase of its travel. MESSENGER has jets for manoeuvering it but in a couple instances, when it was close to the Sun and receiving an increased insolation, the solar panels were used as sails. With abundant sunlight, the precise facing of the panels for electric wasn't critical. The panels were angled to create the right direction and strength of sunlight thrust to adjust the trajectory. In other cases the fuselage of the probe was the sail. Being painted or covered with material of various reflectances, turning the craft this way or that alters the sunlight pressure between the two extremes. This extra pressure can adjust the orbit to save use of onboard fuel-burning thruster. What is new now is that there's a spacecraft on its way to Venus with a dedicated solar sail. What's more, so far as at mid July 2010 in time for the City Tech symposium, the effing thing works as planned. I describe this project below.
Modifying orbits -------------- The practical way to employ a solar sail is to modify the trajectory of a spaceprobe in freefall under gravity. The main motion is imparted by the rocket that placed the craft in orbit around the Sun. The sail does not and can not do this. The sail isn't opened until after the craft is on its way to the target by rocket propulsion. Take the case of a solar orbit near Earth for geophysical studies. The probe is in the orbit and then opens up its sail. If the sail is turned edgeon to the Sun, so it looks like a line from the Sun's eye, there is no pressure and the sail has no effect on the probe's path If the sail is broadside to the Sun, to expose its full area, the maximum force is delivered to the craft. The craft is pushed outward to an orbit farther from the Sun. This is like aiming a thruster at the Sun with reaction radially away from it. In reality, this force opposes the inward pull of solar gravity, leaving on the craft a weakened radial force to let it migrate to a higher orbit. The diagram here shows the two extreme orientations. The spacecraft 'O' is running left to right while sunlight flows upward across it. The sail '-, |' is edgeon for the no-thrust running and faceon for the maximum thrust.
/|\ | | O | | -----+----- | +O ---------------> | | craft trajectory max thrust | | | no thrust flow of sunlight
A more plausible orientation is shown below. The craft is running left to right while sunlight flows upward across it. The frontal area is less than full so the total force is less than the maximum. In addition, the force is split into two components, one normal to the sail to deliver the thrust and one in the surface that does nothing. The amount of the normal component is a cosine function of the angle between the sail and sunlight. Here authors differ. Some treat the angle between the sunlight and normal of the sail, after standard math practice. Others measure from the plane itself. Both are correct but you must read the author's explanation carefully. Be wary! Some authors aren't clear which they are using and their formula can be misapplied elsewhere. In the left scene below the sail faces partially rearward. The force on the sail is a push, relative to the probe's motion. Just like by a chemical jet the craft speeds up and migrates to a higher, farther, orbit. To slow down the craft and let it settle to a closer orbit, the sail is angled forward like in the right scene. The push now opposes the craft's motion like a retrojet.
\ / \ O /|\ O / + | + ----------------> \ | / craft trajectory \ | / | thrust speeds craft | thrust slows craft | flux of sunlight
This satisfies the objection sometimes raised that solar sails can only move a spaceship away from the Sun and not toward it. The flaw is that the craft is always falling thru the Sun's gravity field and will return if slowed down by any means, sail, rocket, whatever. The direction and strength of the sail force is quite flexible, limited only by the mechanics of the rigging. The more freedom to adjust the sail, the more complex is the rigging and more ways there are for mishaps. The mission planner must balance the two factors. On the other hand, the sail can help with orbit alterations that other wise require major rocket costs, like change of plane. Angling the sail 'down' or 'up' relative to the axes of the craft can induce a swerve out of the orbit plane. It takes time, weeks to months, but it may be more effective than an onboard rocket.
IKAROS ---- There were many proposals for solar sail projects since the 1970s, when they were recovered from calcs by Lebedev of Russia in 1901[!]. They were included in space travel fiction since at least the 1960s but there was no actual successful sail project fielded in space until 2010. Planetary Society tried twice to send a small sail into Earth orbit in the mid 2000s. Both failed due to launch calamities. A third attempt is in the works, described by Friedman at this lecture. It, Cosmos-1, is seeking to be a piggyback payload aboard some upcoming opportune launch to put it at least 800km elevation. This gets it above the bulk of atmosphere where air drag will not spoil the sail's function. The idea is to use the sail to raise the probe's orbit to some higher level. It wasn't clear if this test is really needed now in light of the wonderful test made by IKAROS only a week before the symposium. There few of these in the pipeline in the next few years. The majority of launches are to low Earth orbit of only a couple hundred kilometer elevation or to geostationary orbit. During the dwell time Society is refining the design and construction of the probe. Dr Kawaguchi stole the lecture with his pictures and videos of the IKAROS mission. For some of us, it was watching history in the making, a genuine placement of a functioning solar sail in space and its operation to modify a spacecraft's flight path. IKAROS is a Japan project to test solar sails on a real mission. It was a secondary package along with a conventional probe to visit Venus. It launched in May 2010 and entered interplanetary trajectory in June. The sail, 200m2 in a square shape, is the main thrusting unit on IKAROS. The sail was packed up on the outside of the fuselage and deployed by spin ejection. Two daughter craft, released before the sail was opened, are cameras to monitor and document the sail. Kawaguchi showed their views of the sail opening up slowly and deliberately and perfectly to its full extent. Dr Kawaguchi was as proud as a peacock to note that on July 9 IKAROS realized a significant acceleration of speed from the sail! The thrust was only 1.1mN, precisa mente the amount expected for the frontal area and solar distance on that date. The resulting acceleration was detected by Doppler shift of the radio comms from the probe. While this is for only the line-of-sight component, the 3D acceleration vector had exactly this component as actually measured. IKAROS will fly by Venus in December 2010 and then stay in heliocentric orbit for the next three years. The mission plays with sail operations until then. A corollary idea, mentioned at this lecture by Dr Kezerashvili, is to wander close to the Sun, in its strongest gravity field, to test Einstein curvature of spacetime. The sail-propelled craft can make the delicate course adjustments by which to assay the Einstein effects. IKAROS itself isn't up to this level of experiment bt it's a viable purpose for a future mission.
Special features -------------- The IKAROS sail has two interesting features that space fans so far didn't catch on to. One is the builtin solar electric cells. They are a square band about halfway out from the center of the sail. They power the craft since they have to, with the sail, face the Sun. This feature was proposed for other projects but never actualized until now. One problem was developing solar cells thin and light enough to incorporate into the sail. The other is the way the sail changes reflectance. In addition to shifting the angle of incidence of sunlight, there is a band along the outer edge of the sail of liquid crystal cells. These, like those on an electronic digital display, are light or dark according as signal is applied to them. By darkening certain ones of the cells, the sail along this band, loses reflectance and weakens thrust. The pattern of darkening is govermed by the signal profile, just like the numbers on an 7-segment display. This is truly an innovative method, one that is simple, durable, cheap, energy efficient and dead effective.
Possible missions --------------- Already solar sails, whether the dedicated one on IKAROS or the make-do one on MESSENGER, are showing the feasibility and practicality of sails for space missions. They abate the need for fuel-burning rockets for moifiying trajectories. This is a task certain to be done on just about any long-distance long-duration flight. No one seriously believes a single ballistic impulse from Earth will put a spacecraft exactly on target. With the tiny thrust, what kind of payload can you field? Over the last decade, particularly since the 21st century opened, robotics and sensing technology made immense advances. A module the size of a computer system box, considered small in the 1990s, can be fitted into a chicken egg today. It uses far less electric, is vastly lighter in mass, yet performs the same function as well as or, plausibly, better than the old unit. Already the US Army uses remote sensors the size of dice. A soldier throws a bunch into a hideout. They relay enemy voice, radio, and computer signal to a safe base. Larger modules, lizard size, crawl into enemy land and get GPS fixes and operating specs of vehicles and machinery. This trend will continue down to the molecular level where already crude experiments are underway to cobble molecules into nano-nano machines. This is best illustrated in medical fields, where microscopic 'machines' can circulate thru your body to keep tabs on various functions and relay readings to a medical response facility. It is entirely within prospect that a spaceprobe the capability of Cassini -- with a Huygens-like probe to land on Enceladus -- will be carried to its nose cone in a shoulder bag. The Einstein test probe described above need not be larger or heavier than a bagel. Such miniaturized lo-mass lo-energy probes are the market for solar sails.
Belbruno flight ------------- A sail because it offers only a low-energy propulsion is a perfect companion for a Belbruno trajectory. In many cases so far the Belbruno path was a rescue operation when conventional rockets failed. The slow crawl of Hayabusa from asteroid Itokawa is one example of a Belbruno path in a rescue situation. The probe's rocket sprang a leak near the asteroid, losing almost all of its fuel. Only a residual amount remained, enough to send the probe back to Earth, a four-year journey!, where it delivered up its soil sample in June 2010. The first project with a planned Belbruno path was SMART-1, taking some 18 months to reach the Moon, rather than the brute-force three days. It was a successful flight, costing a small fraction of a regular one with a rocket onboard. MESSENGER is also on a Belbruno path, also by design, to Mercury, a planet only a few months away by direct heavy-thrusting rocket. It's taking five years to get to Mercury. On the way, it did swingbys of Mercury, too fast to attempt an orbit, but close enough for some good science data collecting. It also got trajectory correction by flying by earth and Venus. To supply the tiny pushes needed to ride the subtle wrinkles in the gravity fields of the solar system, a solar sail may be a viable mechanism. It can be steered far more precisely and delicately than jets and consumes no fuel that must otherwise be carried on the ship. There is an other interesting idea, best for an inner solar system flight. Ion motors have low thrust, like sails, and are powered by electric generated from sunlight. If the electric plates are part of a sail the two can work in concert to navigate the spaceprobe. No combined sail-ion ship is flying yet, but since both methods are now proved ones, having an ion motor with the sail can be helpful if the craft is near a target's shadow. Inspecting an asteroid or moon is a possibility. When the Sun is blocked in shadow, the ion motor kicks in. The combo gives a longer lasting ion fuel supply for more ambitious manoeuvers. An interesting prospect is for a solar sail to thrust a threat asteroid away from Earth. If the threat is recognized long enough in advance, like for Apophis, a sail can be mounted on the asteroid to apply a continuous thrust, of appropriate strength and direction, to alter the orbit away from Earth. The deflection need be only a hundred thousand kilometers at Earth, say a third the Earth-Moon distance.
Agitated agitation ---------------- Proposing to whoosk across the solar system or to other stars by sails does have a giggle factor. One prime reason is the exaggerated promotion by space fans and futurists. You see the lavish articles and animations of an oceanliner-size spaceship coupled to a Texas-size sail zipping by the planets. These fables are issued by some advocates who otherwise are respectable authors. They may parrot what some other author says without lifting the bonnet. 'Dr So-&-So designed a spaceship that eliminates the need for fuel! It runs on free and ever-lasting sunlight!' The artwork is equally ridiculous, like a pasteup from a country club, gazebo, and planetarium. These writings are often put out to excite the public to space exploration. 'Wow! I'll tell Congress to raise my taxes so I can fly around space in that spaceship!' If you believe that, I know LOTS of places where you can kiss your money good-bye. Not only in internal litterature, like spacefaring newsletters, but also at public exhibits I see such promotions. 'This is what our Vida Futura County Space Society is pushing for in the US space program.' Will the public reale mente give credence to a booth with posters of people lounging before a spaceship window with its solar sail billowing outside? There are several reasons for such nonsensical writings and shows. I see one as the lack of good science. Oh, the writer may be educated but he doesn't apply his learning skills to space agitation. I see gaffes like 'There's no weight in outer space, so we can neglect the mass of the sail', 'With no friction, all the sail's acceleration goes into the spaceship', 'Passengers can hibernate for ten thousand years while the ship accelerates to 9/10 speed of light', 'Sunlight is a uniform flow over the whole solar system'. One factor often missed out is the Sun's gravity. On a sail there are always TWO forces, the outward radiation pressure and the inward gravity. Neglecting the gravity leads to erroneous notions of a sail's performance. Concepts like pressure, acceleration, photon, energy, radiation, mass, force are routinely scrambled in some space litterature. A clear understanding of them is absolutely crucial for any proper agitation for space programs. The continued use of oldstyle measures -- and not progressing to metrics -- adds to the loose thinking. The end result is not a mass public uprising for space exploration but a distaste to pay for what appears to be a rocket hobbyist's playground. Need less to say, this public reaction drags down, like a solar sail in atmosphere, the whole space promotion effort.
Conclusion -------- This was a great lecture/panel. Only at the end, during the panel when the speakers sat on the edge of the stage (there was no seating on the stage) were there some flaky thoughts. These were handily offset by sensible questions. The diversion of an asteroid was in the questions. So was one about the optimum shape of a sail. These were mixed with those about interstellar human flight of only a few years duration due to the fantastic speeds built up by sunlight. We left the auditorium after Q&A. A table for space travel books was almost completely bypassed for poor placement, on the outside of the sweep of traffic leaving the auditorium. From there we drifted into the torrid evening air and headed home.