Enthalpy

http://www.scienceforums.net/topic/76627-solar-thermal-rocket/page-2#entry818683 missions around each equatorial moon of Jupiter or Saturn, heavy mission to Europa, elliptic mission in Uranus or Neptune orbit. Proponents of solar sails claim they can do it as well. I've read no proposal with an ion engine to orbit each moon of a giant planet, within a few years, ending with one ton.

If you say "the particle is the wave" it makes many things easier. Though, this wave is not a 19th century styled one, because some properties like an electron's charge don't split - hence the usefulness of the particle idea. Beware most people only say "the wave defines particle's probability density" but I've seen no advantage to this more common expression up to now.

The worth of a huge amount of platinum is the money buyers are willing to pay. The price per kg would obviously drop if big amounts were available, and no platinum user would give 18T$ to get it, whatever the amount. Though, metals like gold or platinum have intrinsic advantages that let them prefer over copper for instance, when the use justifies the cost. Resistance to oxidation is the primary aspect to make electric contacts, jewels or coins. Huge amounts of presently precious metal would open them more uses. In some regions of the Inca's empire, bronze was made with gold instead of copper because of better availability. So even if they were as widely available as copper, presently precious metals would still be valuable, probably more than copper. I've seen no prospective figures by the projects of asteroid mining - which is reasonable since we ignore what asteroids may contain nor how to exploit them - but the goal isn't to grasp 18T$ of metal. Obtaining metal exceeding the mission costs would be a goal hard enough, and such a sum (G$) is obtainable from the buyers of precious metals. Well, the solar thermal engines makes the endeavour more reasonable, and I suggest elsewhere a sunlight-pumped laser and a hydrogen gun to analyze asteroid candidates.

Hayabusa is one, Jpl made an other. Previously, many Earth-orbiting satellites have been using ion thrusters to keep their attitude and orbital position. The next trend is an ion engine to circularize the transfer orbit to geosynchronous - which makes rocket resigners, especially for Ariane 6, wonder whether the launcher shall target the Gso or Gto (as if a good design for the Esc-B couldn't do both!). Ion engines are strongly limited by the electric power available on board, which they squander limitless. Solar panels limit them to a faint thrust hence lengthy operations when any possible, a nuclear reactor has other serious drawbacks - among others that it needs a huge cold sink. I consider the solar thermal engine a better compromise between thrust and efficiency when propulsion is needed.

A data relay would need a huge receiveing antenna. Imagine it half-way to Voyager: it would need half the diameter of the antennas we use on Earth. This diameter is usually not done on space probes, and anyway, building bigger antennas on Earth would do the job for cheaper and would serve more purposes. The Square Kilometer Array currently built for radioastronomy woud be fantastic for space probes as well. I can't tell whether both uses (if possible at the same time) are technically compatible, nor under what conditions (improved capabilities?) radioastronomers would be tempted. http://en.wikipedia.org/wiki/Square_Kilometre_Array One may also ask what Voyager can bring to us, and how expensive a relay would be. At the cost of sending a probe, I'd prefer a new one orbiting Uranus or Europa instead of a relay for Voyager. Leaving the Solar system presently is possible and is done. For instance New Horizons does it. http://en.wikipedia.org/wiki/New_Horizons The launcher put it directly on a Sun escape trajectory, but Jupiter was used to gain speed.

Our propulsion technology has not improved much since Von Braun or Korolev. They achieved 3km/s (kerosene) and 4.2km/s (hydrogen) ejection speed, we have 3.4km/s and 4.7km/s. This permits 500t rockets to send 1t probes with 10km/s relative to Earth. This speed is insufficient to move easily in our solar system. Earth's speed around the Sun is 30km/s for instance. For every single travel, we have to find tricks and use about every help available, just to reach other bodies. Presently we can send probes to our Moon, to Mars and Venus on a direct path and reach low circular orbits there, preferably by aerobraking there. That's all. For Mercury we need assistance from Venus that takes years, for Jupiter and Saturn we better get help from Venus and can only reach very elliptic orbits at the target, for Uranus and Neptune humans have only achieved flybies up to now (though elliptic orbits around these planets would be accessible with patience). Ulysses' mission over the Solar poles was possible thanks to a Jupiter flyby only. 67P is not in the ecliptic plane. Reaching its orbit is very costly, much more that Mars, and needs help, typically through gravitational assistance. If you remember that one Earth orbit takes a year, one asteroid orbit for instance 4 years, it's clear that several planetary flybies bring you to 10 years. Sometimes shorter paths would be feasible, but indirect ones permit much heavier hence capable probes, which scientists favour. ---------- Can propulsion improve? Chemical rockets, no. We use already the best sensible propellants, which may improve by 0.2km/s - not sensible ones bring only 0.5km/s more anyway. Then you have the solar thermal engine at 12km/s ejection speed, which opens many possibilities and eases many missions, but imposes strong constraints: small thrust, sunlight. I still check what it can do and how. It has never flown nor has been prototyped, to my knowledge. Ion engines improve the ejection speed further but need much power, so their constraints are even worse. They exist, find wider use at satellites around Earth, and made one nice mission to asteroids. Solar sails have not been built in the size and mass they need. These would bring the necessary performance, but only near the Sun (which permits some remote missions too). They should receive much more attention. For instance the Ulysses mission would have been faster, far better and lighter with a solar sail.

Static electric and magnetic fields don't propagate and stay local, decreasing quickly over distance. In that case people introduce "virtual" photons that have a negative energy, so that their wavevector is imaginary and lets the amplitude decrease over distance instead of oscillate. Though virtual particles fit nicely to make theories more homogenous, I have seen no case in electromagnetism where the notion is fertile. It brings often no result at all, or when it does, it's the same as a computation of the fields but less direct.

Specialized, yes: for music instruments. Musicians use drumsticks or hammers of varied hardness so percussion instruments give a more or less hard sound. I other areas, I dont' know.

Accelerometers can't be attached to single molecules, but the µm-sized accelerometers and pressure sensors we build are sensitive to individual molecule shocks. At moderate gas pressure we (I did 30 years ago) observe the global noise created by the many molecules, at low pressure we observe individual shocks through the signal given by the sensor. That's why I say "not an assumption any more, but observation". And that's why I write it's a model, not a theory as for a century ago. I wrote "find a newer book" because those textbooks that describe evidence gathered a century ago give the wrong impression that no other proofs exist. Meanwhile we have far stronger observations. Books would better give these instead of the old indirect historic evidence: that would be more useful to readers. I did not write "useless", because I used it for my sensors. You did misinterpret.

Hi lil'mizzfishkilla, welcome here! A torque is a force (the tension of your line) multiplied by a distance (the current radius of the spinning spool) - or sometimes less if the force is not perpendicular to the radius and to the rotation axis. So if the spool goes empty and the radius decreases, the same line tension makes a smaller torque, or conversely the same torque makes a bigger line tension. So if the brake acts on the spool hence produces a torque rather than a tension, the empty spool results already in a higher line tension. Increasing then the torque even more breaks the line.

The solar thermal engine lets also this undertaking look better. From one Atlas V 551 launch (18.8t on a 400km orbit), or H-IIB or Ariane 5, it lands 6.7t on a small asteroid, and can bring cargo back. http://www.scienceforums.net/topic/76627-solar-thermal-rocket/page-2#entry818683 The solar thermal engine can expel vapour instead, from ice found at the asteroid, and bring a part of an icy asteroid back http://www.scienceforums.net/topic/76627-solar-thermal-rocket/page-2#entry757663

Human hairs have more value than feathers. They serve to make wigs, lock prolongations... Using them instead of feathers would be a bit surprising. For a chemist yes, but religion is not science. Many faithful people believe that life is more than a bunch of molecules. And even without religion, is everyone easy with the idea that astronauts drink recycled urine?

Club, yes, great idea! Some clubs organize vacations around a science theme. Wikipedia, sure, but it's not exactly meant to learn. Science kits! Chemistry, biology, optics, electronics, electromagnetism...

Every part of the massive objects attracts other massive objects. If the pbject is not a sphere, the resulting direction attraction needs not be towards its center of mass. In addition to the nice example proposed by Timo, a very concrete case is Earth itself, which is a bit flattened. One consequence is that satellites on a nearly polar low orbit go more parallel to the north-south direction when they're nearer to the equatorial bulge, so that their orbital plane is not fixed but drifts slowly. This is used for "sun-synchronous" orbits, for instance with 800km altitude and 98° inclination. Then, the orbital plane drifts by 1 turn every year, so that its direction is constant versus the Sun. The satellite passes a a constant hour every day (said a bit quickly), for instance 10h and 22h, at local solar time. Spot, Ers and many more satellite families use these orbits.

Hello you all! A bizarre idea, maybe it would fit in the "speculations" section better than in astronomy... Could a gamma ray that arrives near the horizon of a black hole produce a particle pair? To my rudimentary understanding, the electric field of a heavy nucleus separates the particle and antiparticle created there by a gamma ray. Hawking radiation as well separates them, but without the initial gamma ray, just through the strong curvature of the gravitation field there - that is, in my naive and probably wrong representation, the virtual particle falling gains more energy than the rising one thanks to the curvature, and if this gain is big enough, exceeding the particles mass, they can become real, and one can escape the black hole if its kinetic energy suffices. So would an impinging gamma ray help the process, for charged particles? Since some energy comes from the black hole (for the unseeded Hawking radiation, it would be all the energy), a gamma of less than 1022keV could create an electron-positron pair. This means that, after gamma absorption and reemission, the gammas might serve as a catalyst, or at least, that the rate of pair creation can exceed forecasts that base on the population of gammas above 1022keV, if less energy suffices. As some black holes already have gamma rays originating in the accretion ring, this might produce more particles than unseeded Hawking radiation alone, especially for lighter particles like electrons and positrons. Did I read few years ago that an excess was observed? A special case would be a supernova creating a black hole, since the new hole is rather light, and photons are abundent. Thank you! Marc Schaefer, aka Enthalpy

I feel fruitless to discuss 10m glass blocks, because I never found glass of important thickness - probably impossible to cool within a reasonable time - and glass gets quickly opaque: through 0.3 width or length you don't see anything.

That's a strong argument. Thanks!

Are you really sure that the Brout-Englert-Higgs boson explains the mass of all particles?

Materials' properties for shocks aren't well known. Metals and ceramics are known for shocks at small speed that stay below the proof stress. Plastics and elastomers aren't accurately known. Inelastic flow isn't accurately known, and depends much on each material sample. And then you have the shape and size of the objects. For real cases, it needs a software to evaluate the sound propagation is the parts and then in the air.

The random motion of molecules is a daily observational evidence. We see it on any accelerometer or pressure sensor for instance. The kinetic model of gas doesn't have to be accepted any more. It was a theory a century ago. Find a more recent book maybe? Or one that gives more recent evidence instead of the historical deductions?

The rocket engine pushes on the gas it ejects, that's what accelerates the gas. Conversely, the ejected gas pushes on the rocket engine. You can also observe that a rocket engine has a hole at the bottom. There, the gas pressure can't compensate the effect it has on the top of the chamber. The result is a net force forward. Why both concepts would lead to the same computed force, the equivalent but easier way is to evaluate the momentum acquired by the expelled gas.

Diesel engines need more than 10:1 compression ratio to work. People who pointed that out need no explanation about the compression ratio. No nitromethane in a Toyota Corolla, especially if the user cares about mileage. The fuel from the station doesn't contain any - and benzole is just a good fuel. If the fuel of too low octane ignites early you destroy the engine, then the mileage is unimportant. And the proper octane won't let unburnt fuel at the exhaust - any effect of unburnt fuel observable on the mileage would bar the car from the roads. So for a sound car, the proper octane burns all the fuel, a lower octane won't improve. Thanks! Wider choice there than here (95 and 98, no low octane).

Hi Scilight, welcome here! Yes, gears usually serve to trade speed against torque. http://en.wikipedia.org/wiki/Gear The product of angular speed (then in radians per second) and torque gives a power, which the gear transmits (minus 1-3% losses in reasonably good cases) from one shaft to the other, so what is gained on torque or speed is lost on speed or torque.

Still away from QM+gravity unification and from QCD, sorry for that... Electric insulation by vacuum isn't well understood nor modelled. It would be very important since many devices use it presently: breakers and safety components in high-voltage technology - but even phenomenological models are not satisfactory because they still ignore what is important in the breakdown. The state of the art is still to observe the effect of rugosity, cleanliness, atomic number, electron work function, and seek correlations - figure that. As a result, no-one can tell which distance is needed for what voltage - possibly the distance isn't the important thing. This research needs people with open mind and broad scientific culture who won't jump immediately on Fowler-Nordheim, because cosmic rays, radioactivity, atomic diffusion, trace impurities or idontknowwhat must be more important and still hasn't been found. It must also be done at a laboratory in order to check quickly any theory. I know all books and courses describe QM+gravity unification as the ultimate goal, but many thousand people have pursued it for decades, making one's success less likely - and it wouldn't even change people's live before half a century. As opposed, there are many topics that need theories and where success is more probable; the segregation of elements and isotopes (if true!) would change much of astronomy within few years; galling and vacuum insulation would quickly change the knowledge of engineers and the daily life objects they design. There would be more topics like that. In his country or an other one, since research is international today.

Models of metals or solids used for electron emission are sometimes more evolved than a potential well or an electron work function, because the anisotropy of the crystal has an important influence. The work function if often measured for one or several specified crystal orientations with important differences. It still matters for polycrystalline materials since the emission zone is often tiny - even if the intended emission zone is broad, several 100meV difference in the work function mean that the emission occurs at the favoured orientations. Then, the bands in semiconductors are not isotropic. In non-stresssed GaAs they are but in Si, Ge and many more the conduction "band" is a mean of several valleys where the conduction electrons at minimum energy have a strong (and oriented) wave vector. For bulk conduction, all valleys sum to the equivalent of isotropic conduction, but for electrons flowing through a surface, where the wavevector's components perpendicular and parallel to the surface count separately they don't. This is known and modelled for heterojunctions which are produced and observed accurately, less so for electron emission. Valence bands would have been simpler but are bad electron emitters. Metals, with their complicated bands, make it more difficult; free surfaces as well, because they're always dirty. Electron emitters had been prototyped where avalanche (did they try heterojunctions as well?) injected hot electrons (kinetic energy well above the conduction band minimum) in a zone near the emitting surface. The hot electrons missed less energy to reach the vacuum level. These devices did show electron emission from a cold material but they didn't generalize. Usual emission models like Fowler-Nordheim don't use these refinements and are known to be rough approximations, where the dependance on T and V is good to keep but the fixed coefficient is seriously inaccurate. Rugosity makes it worse. Usually it's unavoidable, since polishing generally replaces few big hills by many smaller ones that concentrate the field just as well. Since electron emission is so sensitive to the field right at the surface, the rugosity is paramount but it's difficult to characterize. Insulation by vacuum is presently very important to technology but poorly understood. Usual models concentrate on electron emission, especially field emission, but their inaccuracy suggests that this is not the only important effect.