Easy Questions?

[beagle]

Plan B.

[Bob in a quantum-state-of-faith]

Plan B.

Robert L Forward proposed a solar sail with an external propulsion unit:  a giant sun-pumped laser gun.   Put the laser in close orbit around the sun, say between Mercury and Venus.  But, tilt it's orbit such that if the orbit were an 'equator' the North Pole points in the direction you wish to travel.  That way, the laser is never in the sun's shadow for your trip.    As we all remember, a laser is Light Amplification by Stimulated Emission of Radiation-- that is, you "pump" light-energy into the device-- why not use direct solar energy to power it?

As an added benefit, you can use the engine for communications as well as thrust:  modulate the beam in some fashion-- say switching it on/off, or if you're able, fluctuating it's frequency between two or more modules-- the more different frequencies, the more 'bits' you have in your communication beam.

How to use it for return communication?  That's just as easy:  the solar sail, to work, has to be reflective of the frequency(ies) of your laser beam.   For efficiency, your sail material has to be almost nebulous-- some form of molecular film or near-film, say 10 km in diameter.  Such a film cannot have massive cables, thus you'd have thousands, if not millions of think strands attaching the sail to the ship.   You place molecular actuators at the end of each strand, and make individual strands conductive--- adjoining strands complete the circuit, while maintaining tremendous redundancy-- a must, as the sail will get punctures along the way.   Anyway, these actuators will flex the sail a bit, changing it's "aiming" point slightly.

At as little as a few light-hours, this sail flexing will swing the return-bounce from the beam well off the receiver mounted at the laser-engine.   This swinging of the beam away-and-back represents one.  A steady beam over a given clock-cycle represents a zero.   The clock-cycle is determined by the beam itself:  as it switches on/off, or the frequency is modulated.

Thus, you have your return communication cycle, embedded within the data on the outgoing beam.  Naturally, the return signal takes twice as long as the outgoing message.

And yes...... I've given this much thought in the past.



Edit: Now, some are going to point out a potential problem:  the laser is continually moving with respect to the line-of-travel-- minute adjustments of the beam's tilt can easily compensate for that, keeping the ship on track.  But, you'll say-- what of the return beam?

By the time direct ship-to-earth beams become too weak to be useful, say about a year out-- the return beam will be spread out enough, that it's diameter would easily be larger than the platform's solar orbit-- such that it won't matter where the platform is in it's orbit.  And the deflection would make the return beam miss the orbit of Pluto or more-- such are the magnifying effects of such very long distances-- a few fractions of degree off-target, and at a light year, your beam goes wide by the time it gets back to earth's neighborhood.  If the beam is that wide, why not put the receiver on Earth?  You could-- but, once in a great while, the Earth would be in the sun's shadow, unless your journey were directly "north" or "south" with respect to the earth's solar orbit (equator).   A platform in orbit around the sun, but 60 degrees 'ahead' or 'aft' of the earth would solve that-- but why not put the receiver on the platform, which was deliberately put in an orbit such that the sun could not cast a shadow.

Careful, belt-and-suspenders people will suggest to place two (or more) receivers in various locations within the solar system, as a redundancy.  An excellent suggestion.

[Swatopluk]

Stanislaw Lem had similar ideas. In Fiasko the initial thrust is provided by a huge battery of lasers on a moon of Jupiter that is aimed at a reflective plate at the bottom of the interstellar space ship. Once the ship is fast enough it collects interstellar hydrogen and 'burns' it in the ship's own drive (whether this is technically feasible is debated among scholars).
Btw, Lem estimated that a spaceship could reach the Andromeda galaxy within the lifetime of the crew if the whole Earth moon could be used as fuel
---
I too had the idea of contra-rotating twin accelerators to compensate for angular momentum. My design would use several of those pairs forming a torus with fusion power plants forming a ring inside. The linear accelerators would run through the center.
If heavy ions made from neutral particles (e.g. from noble gases) are used the charge pair would be electrons that could be disposed of easily.
The whole point of using large (and heavy) accelerators is to make use of relativistic effects in order  to overcome the efficiency problems of ion drives.

All interstellar drives under serious discussion I know of are of the type 'low acceleration over very long times'. If the crew needs gravity, it will have to use the centrifuge effect.

[Swatopluk]

Travels directly 'north' or 'south' have the advantage of always having contact with all places in the solar system but the disadvantage that the initial orbital velocity cannot be used. That's also the reason why everyone tries to get the launching pads as near to the equator as possible. The Russians have to put more fuel into their rockets because the 'push' of the rotating Earth is weaker at their latitudes than in Florida or French Guayana.

[Sibling DavidH]

Speaking of Light Sails and lasers, has anyone read The Mote in God's Eye, by Larry Niven and Jerry Pournelle?  A good story, and it goes into this question in some detail.

[Bob in a quantum-state-of-faith]

Speaking of Light Sails and lasers, has anyone read The Mote in God's Eye, by Larry Niven and Jerry Pournelle?  A good story, and it goes into this question in some detail.

I had forgotten that aspect of the story-- perhaps it's time for a re-read. 

Have you read the follow-on stories?  Forgive me, but I cannot recall the exact titles, unfortunately.  'The Gripping Hand' was one, I think.

[Sibling DavidH]

Have you read the follow-on stories? 

No, I haven't seen them.  I rarely buy SF new, so I just have to keep my eyes open in the second-hand places.  Pournelle & Niven are high on my permanent look-out list.  Thanks for the tip!

[beagle]

The whole point of using large (and heavy) accelerators is to make use of relativistic effects in order  to overcome the efficiency problems of ion drives.

I think the problem is that to increase the mass of the expelled ions by just one Kg each second would take of the order of 10^17 Joules/S, i.e. the approximate energy output of 20 Million PWRs.

[Swatopluk]

Yeah, but if each ion weighs a kg, then the change in momentum would be enormous.

Actually, I do not believe that homo sapiens will ever travel to the stars bodily. Too many chances for species suicide or at least going back to an essentially pre-industrial age.

[Sibling Zono (anon1mat0)]

Not so easy question: why is krill and zooplankton attracted to cold waters more than warm waters (specially in the Pacific ocean)?

[Swatopluk]

Warm waters are usually poor both in oxygen and nutrients.
E.g. the seas around Galapagos are normally icy cold but teem with life. When El Nino hits, the seas warm up and marine iguanas and sealions starve in large numbers because there are neither fish nor algae left to feed on.

[Aggie]

Ayuh, gases dissolve better in cold water than warm - easily observed by opening a warm beer at the same time as a cold one.

I suppose this might hold for phytoplankton as well - less CO2 in the water might make conditions less favourable, but that's pure speculation.

[Opsa]

Fair enough, here's another question:

There's a current television ad which describes wind energy as being "renewable". Is this technically correct?

[Aggie]

Interesting question - technically it's solar energy and the sun isn't actually renewable. 

From the way I understand it, taking wind energy out of the atmosphere should reduce the total kinetic energy of the atmosphere (side question: could enough turbines theoretically reduce the occurrence of hurricanes?), but the energy originates primarily in the sun, AFAIK, and for our purposes is renewable.

Right now, wind turbines are manufactured, transported and constructed using primarily fossil fuels - until the cradle-to-grave mining and processing of raw materials, manufacturing of all related parts, transportation, construction, maintenance, disassembly and disposal / recycling of a renewable energy capture system is achieved using the energy produced by that system, it's not IMHO truly renewable energy production.  However, we aren't likely to achieve an electrically-based industrial sector until electricity becomes much cheaper than burning fossil fuels, which will depend on cheap capture of renewable energy. The introduction period of renewable energy generators should be seen as a ramp-up to a renewable energy economy instead of a direct and immediate off-set of fossil fuel use, IMHO.

[Sibling Zono (anon1mat0)]

Renewable possibly in the sense that winds are constantly 'renewed'  and will remain in such pattern as long as our primary (the sun) keeps shining. On the same token hydroelectric and solar depend on the sun and will be around for a long time. Geothermal depends on whatever makes the center of our planet hot (quite likely a natural fission reactor) and will remain until said reactions stop in a long while. In both cases we are talking millions or billions of years.

That is in contrast to fossil fuels of which there is a finite amount stored on the planet and which are likely to last for the next 100-300 years at the current rate of consumption. In a completely technical term fossil fuels can potentially renew themselves only that the process of renewal is extremely slow taking millions of years.

In general and simplified terms wind/hydro/solar/geothermal are practically unlimited, fossil fuels are not.