Space Elevator
Threads - Space Lifts and
Satellites, First Reference - Space Elevator
On 14/8/2003,
Margaret Ruwoldt posted:
Having
just finished "Rockets", I've been rereading "The Science of Discworld
2: The Globe", which refers to the possibility of building a 'space
lift' that would allow easy (non rocket powered) departure from the
Earth's surface at the equator to a satellite, and thence the rest of
the solar system.
And two
questions occurred:
1.
Geosynchronous and geostationary: what's the difference?
2. If we did
manage to build a space lift, how far above the Earth's surface would
the satellite be? The Astronomy Picture of the Day site last month
asserted that "Geostationary orbits are very high up -- over five times
the radius of the Earth -- and possible only because the satellite
orbital period is exactly one day. It is usually cheaper to place a
satellite in low Earth orbit, around 500 kilometers, just high enough
to avoid the effect of Earth's atmosphere." See the nifty animation on
this page:
http://antwrp.gsfc.nasa.gov/apod/ap030714.html
These may be
dumb questions: I plead temporary (though literal) blondeness.
(But I'm still curious about the answers.)
Rob
Garaghty replied:
>
2. If we did manage to build a space lift, how far
> above the Earth's surface would the satellite be?
The
same as current geostationary satellites; about 30,000+km. Arthur
C Clarke's "The Fountains of Paradise" is all about building such a
lift.
The
"cable" in the book was made from a theoretical monofilament of carbon
grown in space which gave the required strength. Presumably
Clarke and the scientists work he based the book on calculated that no
currently known fibres or materials were
sufficiently
strong to support the weight of the cable. The space end of the
cable was counterweighted with a small asteroid; having sufficient mass
to stabilise the system in orbit.
It's
theoretically possible to build such a thing. The technology
required as far as I know is way beyond what we currently possess.
The
cable in the book came to the ground somwhere in south east asia I
think. It has to be somewhere on the equator.
You
can't have a geostationary satellite in low earth orbit (which is the
level that the Space Shuttle can reach). I don't have sufficient
grasp of the mathematics to explain it off the top of my head.
As
far as I know, there's no difference between geostationary and
geosynchronous orbits, but I'm happy to be illuminated otherwise!
Margaret Ruwoldt
answered:
>The
same as current geostationary satellites; about
>30,000+km. Arthur C Clarke's "The Fountains of
>Paradise" is all about building such a lift.
Thanks, Rob,
good to know I hadn't misunderstood the distance. Shall now return to
simply boggling at the enormous distance involved.
>You
can't have a geostationary satellite in low earth
>orbit (which is the level that the Space Shuttle can
>reach). I don't have sufficient grasp of the
>mathematics to explain it off the top of my head.
Me neither.
Presumably low orbits decay faster; something to do with Earth's
gravitational pull?
OTOH, here's a
facsimile version of Clarke's article:
http://lakdiva.org/clarke/1945ww/
The summary
seems to suggest that the huge distance is required to ensure the
satellite has an orbital period equal to Earth's sidereal day, ie 23
hours 56 minutes. Shall have to read the full article over the weekend,
and see whether I'm understanding that correctly.
Garry-Peter
Dalrymple replied:
2.
The thing to keep in mind is 'centre of gravity' of the structure,
swing a mass on a rope around your head and a wingless bug can walk out
along the string to a place where it is 'flying' faster than it could
fly with wings. The rope is held taut by the centripetal
force of the tethered mass.
At
it simplest, a skylift is formed by lowering a string down from a
satellite. Lowering the string would cause the satellite to ride
up as the string went down, conserving the momentum of the centre of
mass of the string-satellite system.
Winding
up the string with something from the ground would lower the orbit of
the centre of gravity of the string-satellite system.
Some
space lifts posit a 'stick' rotating end over end that touches the
upper atmosphere. I.e. it's centre is in orbit and the two ends
are more so and less so, with forces of tension balanced out around a
centre of mass.
The
Bean Stalk, a strand fixed to the ground, requires a balancing mass way
beyond geostationary orbit so that the strand is pulled up by tension
caused by the higher mass being held back by the stationary grounded
end.
A
third sort of skylift is a rotating loop of stuff, whose centre of
gravity would be in geostationary orbit, you clip on and ride it to
orbit
while someone winds it up by 'pushing' the nearly ground touching part
of the loop. Clipping on and riding it would lower the
loop, pushing it would make it rise.
Paul Williams,
replying to Nick, wrote:
>
Somehow i think i'd rather be broken down into a radio wave and
transported
> somewhere then an 'elevator' that goes into space
There is some
very interesting physics involved with the idea of 'Star Trek' style
'transporters'.
The heat/energy
involved with disassociating the atoms of a human being, - (then
collating this into information* - which could then be sent somehow) -
would make the 'transporter room' uncomfortably hot :-)
(It has been
said that Dr. McCoy was very wise to be wary of this technology...)
Best to take the
'elevator' methinks...
Jamie
Downs added:
here
is a very interesting website describing all the physics involved for a
space elevator...
www.liftport.com
Rob Geraghty
added:
>
Thanks, Rob, good to know I hadn't
> misunderstood the distance. Shall
> now return to simply boggling at
> the enormous distance involved.
Me too. I
had a look at the space elevator site which someone else posted.
Interesting that someone is (apparently) seriously proposing building
one. I can't imagine how they justify their claims that it's
possible to do so in under 20 years. Considering how pathetically
small the amount of material is that's been lifted into space by 20
years worth of space shuttle flights, these people think you could lift
the carbon nanotubes required to make a tape that would support its own
weight and that of a carrier over a distance of not 30,000km but
100,000km? The site has some hype, but it's very short on facts.
It would be
incredibly exciting if it could be done in that sort of time frame, but
given how little has been achieved in terms of lifting material into
permanent orbits so far, I find it unlikely.
I'm not
belittling the efforts of NASA and the Russian space programs. If
we spent the money currently being wasted on military spending on space
research, we'd probably have built a space elevator years ago.
>
Me neither. Presumably low orbits decay faster;
> something to do with Earth's gravitational pull?
When you have
something in orbit, essentially the curve of the earth is falling away
at the same rate as the object is falling toward the gravitational
attraction of the earth. It's in "free fall"; it just keeps
falling. As you move to higher orbits from the earth there comes
a point where the orbiting object and the rotating surface of the earth
are synchronous, so the object stays over exactly the same place as the
earth turns (geo-synchronous).
As you said, I
think it has to have an orbital period which is the same as the
sidereal period of the earth.
Margaret
Ruwoldt posted:
The
HighLift System company in Seattle, Washington, USA is developing the
Space Elevator, for real, as we speak:
Quote:
"Constructing
a vertical railroad stretching into space is no longer wistful fantasy
carried in science fiction novels. Just ask the folks at HighLift
Systems in Seattle, Washington. Selling the idea of a space elevator,
however, takes a lot of ground floor shoe leather and handshakes.
"For
the last few months, officials at HighLift Systems have been talking it
up with an alphabet soup of government agencies, like NASA, the Defence
Advanced Research Projects Agency (DARPA), the Federal Aviation
Administration (FAA), as well as the National Reconnaissance Office
(NRO)."
end
quote, from "Space Elevator Upstarts Settle Down To Business", By
Leonard David, Snr Space Writer, Space.com, 20.11.2002, at:
http://www.space.com/businesstechnology/technology/space_elevator_021120.html
The
entire article makes intriguing reading. it seems to be
genuine. So start saying your pennies, drachmas, denarii,
centimes, whatever for the trip in a couple of decades.
Developments
flowing from "Bucky balls", long chain Carbon 60 seems to be answer.
Nick wrote:
Or we could not
have the wars and the space lift and put all those funds toward feeding
the malnourished, reducing pollution and developing environmentally
friendly solutions and developing vaccines/cures to diseases and
dibelating conditions, and once all that's been done then we can build
a space lift and use it for whatever purposes it is proposed to be used
for.
But regarding a
space lift, wouldn't one just be a sitting duck for a metor shower or
something, i mean one little chunk of flying rock and u'd think it'd
cause some problems, let along a big flying rock, or any other piece of
space junk for that matter. Wouldn't we want to develop some proper
sheilding or something first.
Rob
Geraghty replied:
>
Or we could not have the wars and the space
> lift and put all those funds toward feeding
> the malnourished, reducing pollution and
> developing environmentally friendly
> solutions and developing vaccines/cures to
> diseases and dibelating conditions, and
> once all that's been done
The
things you've just described are not ever going to go away. Human
nature being what it is, wars are unlikely to go away either. But
we can try to reduce things like poverty, pollution, and
disease. we *should* try! Someone posted a link today to an
article in The Guardian which claimed that we have to somehow reduce
our energy consumption to one tenth of the current level in order to
survive in the long term. I'm not sure what the basis of that
claim was.
If
we reduce consumption I'd also wonder where the "wealth" was going to
come from to pay for eliminating poverty, pollution and disease.
It's a catch-22.
>
then we can build a space lift and use it
> for whatever purposes it is proposed to be
> used for.
One
of the things proposed is to use it to take the materials into orbit to
build power satellites. The idea is to have solar power
generators in orbit. The proposals I have seen transmitted the
power back to earth using microwaves (not at a frequency that boils
water like your oven ;).
On
the other hand, I'd wonder how much could be achieved using the same
amount of money needed to build the space lift instead to find more
efficient solar cells to generate power at the home, more efficient
lights, more efficient airconditioning, more
efficient
building designs, pollution free cars and trucks. As the people
of the American North East just discovered, so much of our "modern"
societies depends on cheap, freely available electricity as well as
liquid fuels.
>
But regarding a space lift, wouldn't one
> just be a sitting duck for a metor
> shower or something, i mean one little
> chunk of flying rock and u'd think
> it'd cause some problems
As
someone else recently pointed out to me, "space is big". :)
Seriously though, you're right. Damage to the "cable" or "ribbon"
as it's currently being described could be disastrous. The
chances of a strike that would cause significant damage would probably
be very small, but it's a consideration.
>
Wouldn't we want to develop some proper
> sheilding or something first.
Unfortunately
we haven't figured out the sort of physics required to make Star Trek
style shields. Let alone the means to power them. If we
had, space elevators might be made redundant by something like an
anti-gravity drive. Perhaps if we understood how gravity works to
attract mass, we could reverse the process. But all those sorts
of ideas really are science fiction at the moment. The reality is
that a "space elevator" cable or ribbon would be at the mercy of the
elements. Remember too you're talking about something which might
be as long as 100,000km. That's 100 times the distance between
Sydney and Brisbane,
just
to put it in perspective. Imagine making a cable that long.
By the way, am I hallucinating - if the "cable car" climbing the cable
of this space elevator travelled at 100kmh, it would take 1000 hours to
travel the 100,000km length of the cable. Does anyone want to sit
in a cablecar for 42 days?
Donald Lang
added:
This still seems
to be a wonderful idea whose time is not yet. The first problem is of course
materials. There are about half a dozen items that enter the
calculations. If you have a string hanging down from space you will need to know its
density, rpo, in kilograms per cubic metre, and its breaking stress in
Newtons per square metre. Your project engineer will insist that you
specify a safety factor. I suspect that because of public liability this will
have to be at least ten, so that your string is never subjected to as much
as one tenth of the stress that will break it. You need not specify the shape
of your string, but success will involve having its cross sectional area,
A square metres, varying correctly with the height above the ground.
Pause and
consider a segment, distant r metres from the centre of the Earth.
Of course r >
R, the radius of the Earth.
Sneaking in some
calculus, the segment is given length dr, and the mass then comes out
as rho*A*dr. There is a force of gravity pulling toward the centre of the Earth.
At the surface that would be
g * rho * A * dr.
A bit further
out it becomes
g * rho * A * dr
* (R/r)2 in accordance with Newton's inverse square law.
There is also a
force required to keep it circling around the Earth.
That magnitude is
rho * A * dr *
(omega2) * r, where
omega is in radians per second so that the string, staying
straight up and down, sweeps round the Earth once every sidereal day.
In passing,
those two forces balance out at the distance for a geostationary orbit. A rough
calculation makes that distance about 7 times the radius R of the Earth.
In some well
worn phrases /It is obvious that/It becomes obvious that/Students of
calculus will notice that/etc/the forces acting on that segment contribute to
the tension in the string, and the amount they contribute depends on
the area at that point. The area has to be in proportion to the
tension. The combination is the standard set up that produces compound
interest, or exponential, growth.
If the string
has area Ao where it is touching down you arrive at an equation.
A = Ao
e{(f*rho/S) * (r-R) * [(g * R/r - (omega)^2 * (R + r)/2]}
Now that I have
scared a lot of people away, (Sorry about that!) the string of symbols inside the
curly brackets make up three distinct and important components.
Firstly, f can
be manipulated, but not too much. I can even think of an argument to make it a
lot bigger as you get further from the surface of the Earth. If you break
the thread a hundred kilometres above the Earth that
will make a lot
of mayhem. If you break it several thousand kilometres up that is a real
disaster on a much larger scale, and amuch larger recovery time. I will settle
for claiming that f, the safety factor, must be at least 10.
In the same
simple bracket we have rho, the density of your string. As a minimum I would use
the density of water, a tonne (=103kgm) per cubic metre. You want it as
small as possible, but a number of strong materials
are five to ten
times as heavy.
The third factor
in the simple bracket, on the bottom, is the breaking strain of your
string. In 'common' materials, piano wire comes out pretty
high, like about
2 * 10 9 Newtons/metre2.
Putting those
together by taking the best features of different materials, the thing in the
simple bracket comes out as a number in the denominator
around 2 * 10
5.
The square
bracket on the right starts at 10 m/s/s at ground level and drops, not too
steeply. The numerical magnitude is still greater than 1 when you get to the
geosynchronous distance. At a distance above the Earth of one Earth radius, the
magnitude is still greater than 4.
That leaves us
with the middle also simple bracket, the distance up. So we sample at one Earth
radius up. That gives us ~6*106m.
Putting it all
together we discover that at one Earth radius up, we have
A = Ao
* e 120
Which is not
something to contemplate happily. The hopeful thought is that if you could get a
material about as dense as water with well over 100 times the tensile strength
of piano wire, and cheap enough so that you could make a very long cable
from it, all we would have to do is lift it into geosynchronous orbit,
and lower bits of it to make up our long string.
If you are now
sitting comfortably it might be a good idea to sit up straight again. We
are going to need someone who will explain how to manage tidal effects. As we
lower the end that is going to reach the surface of the Earth, the other
bodies in the neighbourhood will be pulling it in their preferred directions,
and the forces will vary along the deployed length of the string. I have
not done my homework but there could be some interesting
resonant
frequencies stimulated. (Back to the Tacomo Narrows Bridge anyone?)
So there is my
take. With current materials we have not a hope. We need materials that are
effectively a hundred times better in their properties, and pretty cheap too.
When we have
them, the dynamics of construction will still be hairy.
Sorry about the
length and the delay since the topic made its most recent appearance. Self
indulgence it is, but I can claim it is relevant to the purposes of the list.
Margaret
Ruwoldt replied:
DEE
calculated:
>Firstly,
f can be manipulated, but not too much. I can even think of an
>argument to make it a lot bigger as you get further from the
surface of the
>Earth. If you break the thread a hundred kilometres above the Earth
that
>will make a lot of mayhem. If you break it several thousand
kilometres up
>that is a real disaster on a much larger scale, and amuch
larger recovery
>time. I will settle for claiming that f, the safety factor, must be
at least
>10.
[grins]
I'm trying to imagine the
<<<<TWANNNNNGGGGG!!>>>> sound...
..not
to mention the splat(!) when the Earth end finally arrives. Yeesh. Not
a nice eventuality to contemplate :-(
>We
are going to need someone who will explain how to manage
>tidal effects. As we lower the end that is going to reach the
surface of the
>Earth, the other bodies in the neighbourhood will be pulling it in
their
>preferred directions, and the forces will vary along the deployed
length of
>the string. I have not done my homework but there could be some
interesting
>resonant frequencies stimulated.
Which
attractive bodies are you thinking of?
I
was also wondering about the already-extant constellation of low-orbit
satellites and other debris: presumably there'd have to be some method
of protecting the space lift's tether/string/passengers from accidental
collision.
Thanks
to DEE for the maths, and to everyone else who's contributed to this
thread. I've learned a few things and found lots more to ponder :-)
On 29/9/2005, this topic was revisited when Gray Dalrymple posted:
In today's SMH, bottom right of page 8, is an article about a tethered to the Earth satellite, a space elevator.
The article credits James Langton, The Telegraph, London.
AC Clarke proposed the orbiting satellite in 1945, can anyone suggest the first reference to a tethered satellite space elevator?
****************************************
Sydney Morning Herald,
Science fiction may take reality cheque
By James Langton
September 29, 2005
Space may still be the final frontier, but getting
there could soon be almost as simple as stepping into the office lift
at the start of the day.
The race is on to build the
first "space elevator' - long dismissed as science fiction - to carry
people and materials into orbit along a cable thousands of kilometres
long.
In a significant step, US aviation regulators have
given permission for the opening trials of a prototype, while a
competition to begin next month follows the $US10 million ($13 million)
X Prize, which led to the first privately developed craft leaving the
Earth's atmosphere, briefly, last year.
Supporters of the
elevator concept promise a future in space that is cheap and
accessible, and contrast it to NASA's announcement last week that it
will be relying on 40-year-old technology from the Apollo program for
its plan to return to the moon by 2018.
The elevator
would open up the possibility of tourists visiting a sky hotel 35,400
kilometres above Earth, with a view previously enjoyed only by
astronauts.
Russian scientists first envisaged a fixed link to space, and the idea
was popularised by the British science fiction writer and visionary,
Arthur C. Clarke, in his 1978 novel The Fountains of Paradise.
The
theory behind the space elevator is deceptively simple. With a base
station on Earth and an orbiting satellite, solar-powered "climbers",
each carrying up to 20 tonnes, would crawl up a single cable into space
over several days.
The cable would be held up by the
rotation of a 600-tonne satellite counter-weight, much like a heavy
object at the end of a spinning rope.
Kevin McKern replied:
The
concept of the space elevator first appeared in 1895 when a Russian
scientist Konstantin Tsiolkovsky was inspired by the Eiffel Tower in
Paris to consider a tower that reached all the way into space. He
imagined placing a "celestial castle" at the end of a spindle-shaped
cable, with the "castle" orbiting Earth in a geosynchronous orbit (i.e.
the castle would remain over the same spot on Earth's surface). The
tower would be built from the ground up to an altitude of 35,790
kilometers above mean sea level (geostationary orbit). Comments from
Nikola Tesla suggest that he may have also conceived such a tower.
Tsiolkovsky's notes were sent behind the Iron Curtain after his death.
Tsiolkovsky's
tower would be able to launch objects into orbit without a rocket.
Since the elevator would attain orbital velocity as it rode up the
cable, an object released at the tower's top would also have the
orbital velocity necessary to remain in geosynchronous orbit.
Building
from the ground up, however, proved an impossible task; there was no
material in existence with enough compressive strength to support its
own weight under such conditions. It took until 1957 for another
Russian scientist, Yuri N. Artsutanov, to conceive of a more feasible
scheme for building a space tower. Artsutanov suggested using a
geosynchronous satellite as the base from which to construct the tower.
By using a counterweight, a cable would be lowered from geosynchronous
orbit to the surface of Earth while the counterweight was extended from
the satellite away from Earth, keeping the center of gravity of the
cable motionless relative to Earth. Artsutanov published his idea in
the Sunday supplement of Komsomolskaya Pravda in 1960. He also proposed
tapering the cable thickness so that the tension in the cable was
constant-this gives a thin cable at
ground level, thickening up towards GEO.[7]
http://en.wikipedia.org/wiki/Space_elevator#History
Tony Hyland answered:
Arthur Clarke used the idea of a space elevator in his novel 'The fountains of paradise', first published in 1978. Clever lad though he is, I'm not sure that he actually invented this idea. The novel is set in a fictionalised version of Sri Lanka, conveniently shifted several degrees of latitude so that the space elevator can be built from the equator. Haven't read it since about 20 years ago, so can't recall any more detail.
Google reveals much more, including a company called Liftport http://www.liftport.com/ , which is seriously working on a space elevator.
Their FAQ mentions that the concept was first suggested in 1895 by Konstantin Tsiolkovsky.
Space.com also has a good article on space elevators:
http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html
That's a good start.
Tony Hyland (who actually included a space elevator in a children's non-fiction book on possible future technologies)
Wilson Roberto Afonso added:
Henk van der GAAST, wrote:
Tower of Babel?
Oh its probably lost in its translation all ready!
I thought about that, too. I fully expect religious groups to oppose the
construction of a space elevator based on the Tower of Babel myth, by
the way.
By the way, did anyone read the Red/Green/Blue Mars series? There's a
very scary (if, apparently, not very realistic) description of a space
elevator failure there... (early in the second book or late in the
first, I think)
Gary Dalrymple wrote:
Ah!
That raises an interesting theological thought.
Mohammed's body in a coffin after death is said to have been lifted up towards heaven, not to heaven, as in spite of extreme holiness it was still a 'corrupt' human body, but too holy to remain on earth, so it is where, in orbit?
Paul Williams responded:
This may be of interest:
"...However,
a space elevator would not be free-floating, it would be attached to
the ground. This means that not only would it be subjected to
the pull of gravity on its own mass, but on the entire cable as well,
which would weigh tens or even hundreds of thousands of tons. Unless
constant thrust was provided, the elevator would come crashing down.
Not
only that, but to keep it in step with the rotation of the Earth, one
would need to provide not only the thrust to accelerate the elevator to
the proper velocity, but
the entire length of the cable as well. This would almost undoubtably
cause the cable to lag behind, and bring the elevator closer to the
Earth. So, the cable would be in an arc shape instead of a line, and
even more would be needed to keep the elevator at a proper height..."
http://www.madsci.org/posts/archives/sep2000/968029181.Ph.r.html
Morris Grey commented:
Has any one checked to see what odds the bookies are giving on the possibilities of a working space elevator?
There are better odds on someone proving the existence of God. Who, if asked for His advice on the subject would say , 'It can't be done besides I prefer to keep my privacy right here in Heaven, well at least until the Howard Government legislates it away.'
Wilson Roberto Afonso replying to Paul Williams:
"...However,
a space elevator would not be free-floating, it would be attached to
the ground. This means that not only would it be subjected to the
pull
of gravity on its own mass, but on the entire cable as well, which
would weigh tens or even hundreds of thousands of tons.
Unlessconstant thrust was provided, the elevator would come crashing
down."
This
doesn't make a lot of sense. Free-floating or attached to the ground,
the effect of gravity is the same. That's why there is a counterweight
to keep the centre of gravity at the geostationary point. The elevator
will be in orbit, even if touching the ground.
Robert Moonan replying to the same comment:
Analogous to graded index optic fibre cable, the actual weight of cable wrt. gravity would be graded wrt. altitude, so the effect may not be as obtuse as you seem to be predicting.
Derek Williamson added:
The cable weight is also countered by the force due to circular motion as the earth and cable spin
Paul Williams responded:
Since I sent this,
http://www.madsci.org/posts/archives/sep2000/968029181.Ph.r.html
I've done *a little* homework and no longer believe that the Space Elevator is physically impossible.
It just appears to be extremely improbable.
I'm neither an engineer nor a physicist, so please correct any misunderstandings:
Regarding a (possible) physically perfect ribbon (*perfectium*):
100,000 km of *perfectium* ribbon, with a projected width of 2.44 metres, gives us a surface area of 244,000 metres or 244 sq km. This surface area needs to be protected from damaging atmospheric and 'biospheric' effects.
Lightning, it has been claimed, is not a worry. Storms we can discount, as it has been said that we would avoid them by moving the base of the Space Elevator away from them.
Planes can be steered clear - in the best of all possible worlds (terrorists must 'tow the line' on this.)
Birds could be scared away - we may hope - but there may well be a way.
As we leave the atmosphere we have extremes of solar weather and lots of space junk.
The space junk could be avoided.
Solar weather - I don't know. Say that it is not a worry.
But added to this:
The consensus appears to be that approximately 40,000 tonnes of meteorites (mainly dust) falls to Earth every year - this figure is pretty rubbery. These micro-meteors are not drifting slowly, they are travelling at relative speeds which could and probably would (I think), inflect some level of damage to the *perfectium* ribbon.
So, we would need some sort of self-repair mechanism for the minor impacts.
This doesn't appear to be impossible in principle.
Major impacts would be catastrophic - but hopefully rare.
It appears to me, in regards to the posited perfectium ribbon, that this is, basically, a lot of fun to play around with. As a likely future, it appears to be more than doubtful.
Securing a suitable asteroid and the rest would perhaps be manageable if and when we have cheap and portable fusion power technology.
But what then is the use of the Space Elevator?
Why am I reminded of the nuclear 'Plowshare' program...
The best 'bang for buck' will be if we can, one day (somehow) produce antimatter cheaply - don't hold your breath though...:-)
Ray Stephens commented:
I
neither believe nor disbelieve in the plausibility of a space elevator,
but I do suspect we'll come up with a better method to defy Earth
gravity than that by the time interstellar travel becomes commonplace.
If only because I doubt it would ever get the funding.
Methinks
the idea will fall through the cracks of the history of technology,
like the magnetic rail launch or Jules Verne's cannon. :)
Paul Williams added:
And now for the Space Fountain:
http://en.wikipedia.org/wiki/Space_fountain
Interstellar travel will almost certainly require antimatter to acheive reasonable time frames.
Using antimatter and an imaginary 100% efficient rocket engine, at an acceleration of 1g, it would only take 3.6 years (ship time) to reach Proxima Centauri (4.3 light years away) The fuel needed would be 10kg.
If we wanted to stop there, the fuel needed would be 38kg with a ship's time of approximately 7.2 years.
So round trip, about 15 years (we'd want to have a bit of a look around) and about 80kg of fuel.
Not too bad really.
Reference:
http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html
The Star Trek writers got it right when they posited antimatter as their energy source.
Getting a cheap source of antimatter may be problematic.
I think that, if we survive the next few hundred years, we may create self-replicating Von Neuman probes - which Phyisict Lawrence Krauss calculates would only take about 10 million years to spread throughout our galaxy. (Michio Kaku once calculated as little as half a million years)
Regarding possible civilisations with an outward urge.
We may ask where are their probes?
Paul Davies once ventured that they may be hiding on our moon - shades of 200: A Space Odyssey...
Best to stop now lest I get too carried away.
