The article at:
http://www.spacedaily.com/reports/Thinking_Big_About_Space_Telescopes…
mentions the possibility of launching a Space Telescope with a
mirror of up to 8+ meters diameter using the Ares V rocket.
Are there any practical limits to how big a Space Telescope can be?
On Jun 28, 12:41?am, dumpst…@hotmail.com wrote:
> The article at:
> http://www.spacedaily.com/reports/Thinking_Big_About_Space_Telescopes…
> mentions the possibility of launching a Space Telescope with a
> mirror of up to 8+ meters diameter using the Ares V rocket.
> Are there any practical limits to how big a Space Telescope can be?
what you do is have many segments mounted on a solid surface like the
moon, the mirror doesnt need to be one gigantic piece.
resolution goes way up.
On Jun 28, 4:57 am, "hall…@aol.com" <hall…@aol.com> wrote:
> On Jun 28, 12:41?am, dumpst…@hotmail.com wrote:
> > The article at:
> >http://www.spacedaily.com/reports/Thinking_Big_About_Space_Telescopes…
> > mentions the possibility of launching a Space Telescope with a
> > mirror of up to 8+ meters diameter using the Ares V rocket.
> > Are there any practical limits to how big a Space Telescope can be?
> what you do is have many segments mounted on a solid surface like the
> moon, the mirror doesnt need to be one gigantic piece.
> resolution goes way up.
Unless your moon based mirror(s) and all that’s related is rad-hard
and otherwise physically robust, as well as thermally inert is why
you’ll likely have to do without utilizing our moon. Our moon’s L1 or
of it’s L2 is on the other hand offering terrific locations for the
station-keeping of such optics.
A "Clarke Station" of accommodating such fine optics, along with a
sufficient solar shade could easily accommodate 100 meters of combined
diameter as nicely station-keeping at our moon’s L1.
However, it would be best if the moon itself was providing the solar
shade, as fully doable once that pesky moon is relocated to Earth’s
L1.
–
Brad Guth
In article <1183005681.361672.184…@m37g2000prh.googlegroups.com>,
<dumpst…@hotmail.com> wrote:
>mentions the possibility of launching a Space Telescope with a
>mirror of up to 8+ meters diameter using the Ares V rocket.
>Are there any practical limits to how big a Space Telescope can be?
Nothing that we’re anywhere close to reaching. Note that mirrors made up
of multiple segments are already in use in large telescopes on Earth, and
JWST (nominally Hubble’s successor) is being built with one as well, so
there is no need to limit mirror size to launcher diameter. Indeed,
touting this as an application for Ares V is kind of silly; somebody’s
clutching at straws. The original JWST had an 8m mirror with an existing
launcher, although it ended up shrinking to 6.5m due to budget overruns.
The OWL project was talking seriously about a 100m mirror (on Earth) with
3000+ segments (even the *secondary* was 25m across and segmented!),
although I believe they’ve had to back off somewhat due to financial
constraints (pity). Of course, you need a pretty serious support
structure behind the mirror for something like that… but it would be
a lot easier in orbit.
—
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | he…@spsystems.net
On Jun 28, 7:45 am, BradGuth <bradg…@gmail.com> wrote:
- Hide quoted text — Show quoted text -
> On Jun 28, 4:57 am, "hall…@aol.com" <hall…@aol.com> wrote:
> > On Jun 28, 12:41?am, dumpst…@hotmail.com wrote:
> > > The article at:
> > >http://www.spacedaily.com/reports/Thinking_Big_About_Space_Telescopes…
> > > mentions the possibility of launching a Space Telescope with a
> > > mirror of up to 8+ meters diameter using the Ares V rocket.
> > > Are there any practical limits to how big a Space Telescope can be?
> > what you do is have many segments mounted on a solid surface like the
> > moon, the mirror doesnt need to be one gigantic piece.
> > resolution goes way up.
> Unless your moon based mirror(s) and all that’s related is rad-hard
> and otherwise physically robust, as well as thermally inert is why
> you’ll likely have to do without utilizing our moon. Our moon’s L1 or
> of it’s L2 is on the other hand offering terrific locations for the
> station-keeping of such optics.
> A "Clarke Station" of accommodating such fine optics, along with a
> sufficient solar shade could easily accommodate 100 meters of combined
> diameter as nicely station-keeping at our moon’s L1.
> However, it would be best if the moon itself was providing the solar
> shade, as fully doable once that pesky moon is relocated to Earth’s
> L1.
> –
> Brad Guth
Silly me, forgetting about the Usenet banishment policy, of their
swarm intelligence of such superior denial to none other than our
resident LLPOF warlord(GW Bush).
No wonder the use of our moon’s L1 remains as so taboo/nondisclosure
rated. Perhaps it’s because of the moon’s gamma and hard-Xray dosage
that’ll remain as rather unfriendly to our frail DNA, or perhaps it’s
because we’d have that cm/pixel resolution of the moon’s surface w/o
anything supporting of those silly NASA/Apollo missions to look at.
–
"whoever controls the past, controls the future" / George Orwell
–
Brad Guth
In article <JKDr6J….@spsystems.net>,
he…@spsystems.net (Henry Spencer) wrote:
> In article <1183005681.361672.184…@m37g2000prh.googlegroups.com>,
> <dumpst…@hotmail.com> wrote:
> >mentions the possibility of launching a Space Telescope with a
> >mirror of up to 8+ meters diameter using the Ares V rocket.
> >Are there any practical limits to how big a Space Telescope can be?
> Nothing that we’re anywhere close to reaching.
I’m not so sure about that — I believe we’re reaching limits in
nomenclature. We already have (at least as conceptual designs) the OWL
(Overwhelmingly Large Telescope) and the ELT (Extremely Large
Telescope). What’s left? Some ideas:
RLT (Ridiculously Large Telescope)
LLT (Liducrously Large Telescope)
FHT (Friggin’ Huge Telescope)
BFT (left as an exercise to the reader)
Sooner or later, we will run out of hyperbole.
Best,
– Joe
On 28 Jun, 12:57, "hall…@aol.com" <hall…@aol.com> wrote:
> On Jun 28, 12:41?am, dumpst…@hotmail.com wrote:
> > The article at:
> >http://www.spacedaily.com/reports/Thinking_Big_About_Space_Telescopes…
> > mentions the possibility of launching a Space Telescope with a
> > mirror of up to 8+ meters diameter using the Ares V rocket.
> > Are there any practical limits to how big a Space Telescope can be?
> what you do is have many segments mounted on a solid surface like the
> moon, the mirror doesnt need to be one gigantic piece.
> resolution goes way up.
Your scheme is far better that one large rocket, but not as good as it
could be. My scheme involves physical optics.
Let us have a telescope with incomplete fragments. Let it be (say) a
km across. The fragments are small ultra stable spacecraft working
very like LISA. LISA too has to operate within a fraction of a
wavelength. These fragments rotate to give the full resolution.
The best place is not LEO or even the Lagrange point. Best place is in
direct orbit round the Sun. Henry Spencer mentions "serious support
structure". You need that on Earth, not necessarily in space. LISA
after all does not have supports.
– Ian Parker
Ian Parker <ianpark…@gmail.com> writes:
>Your scheme is far better that one large rocket, but not as good as it
>could be. My scheme involves physical optics.
>Let us have a telescope with incomplete fragments. Let it be (say) a
>km across. The fragments are small ultra stable spacecraft working
>very like LISA. LISA too has to operate within a fraction of a
>wavelength. These fragments rotate to give the full resolution.
LISA does not keep the distances constant – they vary by many, many
kilometers. It instead measures distances very carefully, then subtracts
the known orbital effects. Actually holding the position is not
practical – it would take far more fuel than they have, and the larger
thrusters needed would have too much mechanical noise.
Lou Scheffer
In article <1183132453.217238.151…@c77g2000hse.googlegroups.com>,
Ian Parker <ianpark…@gmail.com> wrote:
>Let us have a telescope with incomplete fragments. Let it be (say) a
>km across. The fragments are small ultra stable spacecraft working
>very like LISA…
Remember that interferometers using multiple small mirrors work well only
on bright objects, because you need enough photons per second to form
detectable interference fringes. A general-purpose big telescope needs
lots of collecting area, too, and that means a big mirror, or several
big mirrors.
>The best place is not LEO or even the Lagrange point. Best place is in
>direct orbit round the Sun.
If you want to do really long-arm interferometers, yes, probably. But
long-arm designs have their problems for imaging work, and near-term
multi-mirror space telescopes are probably going to want relatively short
separations, in which case Earth’s L2 is a fine location.
>Henry Spencer mentions "serious support
>structure". You need that on Earth, not necessarily in space. LISA
>after all does not have supports.
LISA doesn’t yet exist, and is quite a challenging project. And as others
have already noted, LISA’s requirements are somewhat different from those
of an imaging interferometer.
As above, a general-purpose space telescope will want big mirrors, and
those almost certainly will be assembled from segments and will need, yes,
a support structure. As I mentioned, it won’t be nearly as heavy as it
has to be on Earth, but it will still be there, to provide rigidity to the
relatively flimsy segmented mirror.
Whether a multi-mirror interferometric telescope will want separations
long enough to prefer formation flying over physical structure is not
clear. Someday, probably, but possibly not for the first ones.
—
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | he…@spsystems.net
On 29 Jun, 23:50, h…@spsystems.net (Henry Spencer) wrote:
> In article <1183132453.217238.151…@c77g2000hse.googlegroups.com>,
> If you want to do really long-arm interferometers, yes, probably. But
> long-arm designs have their problems for imaging work, and near-term
> multi-mirror space telescopes are probably going to want relatively short
> separations, in which case Earth’s L2 is a fine location.
True for short separations. Near term telescopes are indeed going to
have short separations – see below
> >Henry Spencer mentions "serious support
> >structure". You need that on Earth, not necessarily in space. LISA
> >after all does not have supports.
> LISA doesn’t yet exist, and is quite a challenging project. And as others
> have already noted, LISA’s requirements are somewhat different from those
> of an imaging interferometer.
Yes it is indeed a challenging project. You are right when you say the
requirements are different for an interferometer. Essentially LISA
needs a known separation/rate of separation. An interferometer needs
to be fixed.
There is the aspects of gravitational waves General Relativity …
There is also, and I think you have hinted at this the "technology
platform" aspect.
There are a number of effects that can be used for long term fine
control. There is the pressure of solar radiation, there are magnetic
fields as well as ion drive. This can have an extremely high specific
impulse (10000km/s + exhaust velocity) if extremely low thrusts are
needed.
It will take a long time to get a telescope of 1km+ aperture – agreed.
That is however no reason why feasibility sudies should not start. I
think as high specfic impulse – low thrust ion drive is part of the
equation. Ion drives essentially manoever to a wavelength. I feel this
is going to be the future. Can it be done? Yes.
– Ian Parker
On 29 Jun, 19:48, Louis Scheffer <l…@cadence.com> wrote:
> Ian Parker <ianpark…@gmail.com> writes:
> >Your scheme is far better that one large rocket, but not as good as it
> >could be. My scheme involves physical optics.
> >Let us have a telescope with incomplete fragments. Let it be (say) a
> >km across. The fragments are small ultra stable spacecraft working
> >very like LISA. LISA too has to operate within a fraction of a
> >wavelength. These fragments rotate to give the full resolution.
> LISA does not keep the distances constant – they vary by many, many
> kilometers. It instead measures distances very carefully, then subtracts
> the known orbital effects. Actually holding the position is not
> practical – it would take far more fuel than they have, and the larger
> thrusters needed would have too much mechanical noise.
How does it measure the distances? The laser wavelength would need to
be an order of magnitude smaller than the telescope wavelength?
I can see this concept being great for radio telescopes – say three
5km diameter telescopes in a solar orbit at 10 AU, linked by visible
spectrum lasers. What would a 3 billion km baseline allow?
On 30 Jun, 11:10, Alex Terrell <alexterr…@yahoo.com> wrote:
- Hide quoted text — Show quoted text -
> On 29 Jun, 19:48, Louis Scheffer <l…@cadence.com> wrote:> Ian Parker <ianpark…@gmail.com> writes:
> > >Your scheme is far better that one large rocket, but not as good as it
> > >could be. My scheme involves physical optics.
> > >Let us have a telescope with incomplete fragments. Let it be (say) a
> > >km across. The fragments are small ultra stable spacecraft working
> > >very like LISA. LISA too has to operate within a fraction of a
> > >wavelength. These fragments rotate to give the full resolution.
> > LISA does not keep the distances constant – they vary by many, many
> > kilometers. It instead measures distances very carefully, then subtracts
> > the known orbital effects. Actually holding the position is not
> > practical – it would take far more fuel than they have, and the larger
> > thrusters needed would have too much mechanical noise.
> How does it measure the distances? The laser wavelength would need to
> be an order of magnitude smaller than the telescope wavelength?
> I can see this concept being great for radio telescopes – say three
> 5km diameter telescopes in a solar orbit at 10 AU, linked by visible
> spectrum lasers. What would a 3 billion km baseline allow?
LISA is measuring to within a very small fraction of a wavelength. You
can quite easily measure to within 1/20 of a fringe. LISA is however
not interested in absolute distance. It wants to measure the changes
when a gravitational wave passes.
Question – You don’t know how many wavelengths there are. You know a
fraction of a wavelength – not a wavelength.
Answer.
1) You count. There are a number of examples of this in engineering
workshops on Earth.
2) You change the wavelength of your laser slightly.
3) You measure to different points. If you have mirrors 1….N you can
have N(N-1)/2 measurements. Thus the exact number of wavelengths can
be deduced.
What would 3 billion km allow? This is in fact the approximate size of
a galactic black hole – I am now talking LISA again! The distance you
can measure depends on the stability of you laser. Perfectly stable –
infinite.
How stable can you get things. Well the best clocks (hydrogen masers)
are 10^16. Lift the clock 10cm and General Relativity comes in. 3
billion km represents about 10^19 at 1 micron. The LISA distances are
about the maximum limit of current technology.
BTW – People sometimes ask – do you need to measure time that
accurately. What do you do with a picrosecond? A – You look at
Galactic black holes!
– Ian Parker
- Hide quoted text — Show quoted text -
Ian Parker wrote:
> On 29 Jun, 23:50, h…@spsystems.net (Henry Spencer) wrote:
>> In article <1183132453.217238.151…@c77g2000hse.googlegroups.com>,
>> If you want to do really long-arm interferometers, yes, probably. But
>> long-arm designs have their problems for imaging work, and near-term
>> multi-mirror space telescopes are probably going to want relatively short
>> separations, in which case Earth’s L2 is a fine location.
> True for short separations. Near term telescopes are indeed going to
> have short separations – see below
>> >Henry Spencer mentions "serious support
>> >structure". You need that on Earth, not necessarily in space. LISA
>> >after all does not have supports.
>> LISA doesn’t yet exist, and is quite a challenging project. And as
>> others have already noted, LISA’s requirements are somewhat different
>> from those of an imaging interferometer.
> Yes it is indeed a challenging project. You are right when you say the
> requirements are different for an interferometer. Essentially LISA
> needs a known separation/rate of separation. An interferometer needs
> to be fixed.
> There is the aspects of gravitational waves General Relativity …
> There is also, and I think you have hinted at this the "technology
> platform" aspect.
> There are a number of effects that can be used for long term fine
> control. There is the pressure of solar radiation, there are magnetic
> fields as well as ion drive. This can have an extremely high specific
> impulse (10000km/s + exhaust velocity) if extremely low thrusts are
> needed.
> It will take a long time to get a telescope of 1km+ aperture – agreed.
> That is however no reason why feasibility sudies should not start. I
> think as high specfic impulse – low thrust ion drive is part of the
> equation. Ion drives essentially manoever to a wavelength. I feel this
> is going to be the future. Can it be done? Yes.
I would think that "big" mirrors on a large base interferometer space based
telescope is probably what it is going to take to actually see and explore
the planets around other stars. We’re beginning to get quite a long list
of "big" planets around other stars, maybe it’ll happen sooner than later.
—
Craig Fink
Courtesy E-Mail Welcome @ WeBeG…@GMail.Com
Ian Parker <ianpark…@gmail.com> wrote:
:
:Ion drives essentially manoever to a wavelength.
:
I’ll stop ice skating in hell and come take a look when they get that
working.
1) How do they know where they are to within a wavelength?
2) Since they will have to be under constant thrust to maintain that
(if they can figure out where they are), what happens when they run
out of fuel (about next Tuesday)?
3) How much can they see when they’re constantly filling the space
around themselves with ionized gas trying to stay on station?
If this is the future, then the future is never.
–
"Some people get lost in thought because it’s such unfamiliar
territory."
–G. Behn
Joe Strout <j…@strout.net> wrote in
news:joe-F4D66C.09483729062007@comcast.dca.giganews.com:
- Hide quoted text — Show quoted text -
> In article <JKDr6J….@spsystems.net>,
> he…@spsystems.net (Henry Spencer) wrote:
>> In article
>> <1183005681.361672.184…@m37g2000prh.googlegroups.com>,
>> <dumpst…@hotmail.com> wrote:
>> >mentions the possibility of launching a Space Telescope
>> >with a mirror of up to 8+ meters diameter using the Ares V
>> >rocket. Are there any practical limits to how big a Space
>> >Telescope can be?
>> Nothing that we’re anywhere close to reaching.
> I’m not so sure about that — I believe we’re reaching limits
> in nomenclature. We already have (at least as conceptual
> designs) the OWL (Overwhelmingly Large Telescope) and the ELT
> (Extremely Large Telescope). What’s left? Some ideas:
> RLT (Ridiculously Large Telescope)
> LLT (Liducrously Large Telescope)
> FHT (Friggin’ Huge Telescope)
> BFT (left as an exercise to the reader)
> Sooner or later, we will run out of hyperbole.
Well, I don’t know what acronym you’d want for this one but I
once read an article on future telescope designs that ended with
a ring of extremely large telescopes orbiting out beyond Pluto.
Each would transmit their piece of an image back to Earth to be
combined. The author (I recall it somewhat sadly because I lost
the book several years ago) said the resulting image would
resolve flags flying on a planet in the Andromeda galaxy.
- Hide quoted text — Show quoted text -
> Best,
> – Joe
On 30 Jun, 20:11, Fred J. McCall <fmcc…@earthlink.net> wrote:
> Ian Parker <ianpark…@gmail.com> wrote:
> :
> :Ion drives essentially manoever to a wavelength.
> :
> I’ll stop ice skating in hell and come take a look when they get that
> working.
> 1) How do they know where they are to within a wavelength?
> 2) Since they will have to be under constant thrust to maintain that
> (if they can figure out where they are), what happens when they run
> out of fuel (about next Tuesday)?
That is the question of sprcific impulse. This is not revolutionary
you know. The earth is not quite spherical and at GEO there are
gradients. Very gradual – true but they still need thust. One on the
early uses for ion propulsion is in fact geostationary station
keeping. You need a tiny thrust but over a very long time period.
Basic Physics tells us that momentum and energy are conserved. If our
velocity is 10,000km/sec we need 5w to give us a micronewton. That is
all a mirror fragment would ever need.
> 3) How much can they see when they’re constantly filling the space
> around themselves with ionized gas trying to stay on station?
There is very little ionised gas. It is moving at 10,000km/s and soon
clears. As I said the same thing is true for geostationary station
keeping.
> If this is the future, then the future is never.
ESA is already busy. Its interest though is primerally geostationary.
– Ian Parker
On Jun 27, 9:41 pm, dumpst…@hotmail.com wrote:
> The article at:
> http://www.spacedaily.com/reports/Thinking_Big_About_Space_Telescopes…
> mentions the possibility of launching a Space Telescope with a
> mirror of up to 8+ meters diameter using the Ares V rocket.
> Are there any practical limits to how big a Space Telescope can be?
Within the moon’s L1 could be those primary mirrors (as many and as
big as you’d like), and potentially of a secondary mirror as directly
planted upon the moon itself (roughly 58,000 km away) would obviously
make such things optical downright interesting, especially within
earthshine or otherwise lunar nighttime conditions.
Obviously a radar imaging alternative as based upon using the moon’s
L1 wouldn’t matter if it’s day or nighttime on the moon. But then,
what would we possibly do with having such a one meter/pixel
resolution of Saturn?
–
Brad Guth
Ian Parker <ianpark…@gmail.com> wrote:
:On 30 Jun, 20:11, Fred J. McCall <fmcc…@earthlink.net> wrote:
:> Ian Parker <ianpark…@gmail.com> wrote:
:>
:> :
:> :Ion drives essentially manoever to a wavelength.
:> :
:>
:> I’ll stop ice skating in hell and come take a look when they get that
:> working.
:>
:> 1) How do they know where they are to within a wavelength?
:>
:> 2) Since they will have to be under constant thrust to maintain that
:> (if they can figure out where they are), what happens when they run
:> out of fuel (about next Tuesday)?
:>
:
:That is the question of sprcific impulse. This is not revolutionary
:you know. The earth is not quite spherical and at GEO there are
:gradients. Very gradual – true but they still need thust. One on the
:early uses for ion propulsion is in fact geostationary station
:keeping. You need a tiny thrust but over a very long time period.
:
And to stay within fractions of a wavelength of each other you only
need a specific impulse of infinity, which would be just a bit
revolutionary.
:
:Basic Physics tells us that momentum and energy are conserved. If our
:velocity is 10,000km/sec we need 5w to give us a micronewton. That is
:all a mirror fragment would ever need.
:
Poppycock!
:
:>
:> 3) How much can they see when they’re constantly filling the space
:> around themselves with ionized gas trying to stay on station?
:>
:
:There is very little ionised gas. It is moving at 10,000km/s and soon
:clears. As I said the same thing is true for geostationary station
:keeping.
:
Only an infinite order of magnitude easier than what you’re
suggesting.
:
:>
:> If this is the future, then the future is never.
:>
:
:ESA is already busy. Its interest though is primerally geostationary.
:
In other words, even the Europeans aren’t crazy enough to think they
can do what you’re talking about.
–
"Some people get lost in thought because it’s such unfamiliar
territory."
–G. Behn
On 1 Jul, 18:39, Fred J. McCall <fmcc…@earthlink.net> wrote:
> Ian Parker <ianpark…@gmail.com> wrote:
> :
> :That is the question of sprcific impulse. This is not revolutionary
> :you know. The earth is not quite spherical and at GEO there are
> :gradients. Very gradual – true but they still need thust. One on the
> :early uses for ion propulsion is in fact geostationary station
> :keeping. You need a tiny thrust but over a very long time period.
> :
> And to stay within fractions of a wavelength of each other you only
> need a specific impulse of infinity, which would be just a bit
> revolutionary.
You don’t, you just need a method of calculating where you are (in
fact to a quarter wavelength). You do need very sensitive control and
this is what ion propulsion gives you.
> :
> :Basic Physics tells us that momentum and energy are conserved. If our
> :velocity is 10,000km/sec we need 5w to give us a micronewton. That is
> :all a mirror fragment would ever need.
> :
> Poppycock!
Energy = v^2/2. Let m kg be accelerated each second.
Power needed = m*(1e7)^2/2
Momentum transferred = mv = m*1e7
Therefore power for 1N = (1e7)^2/(2e7) = 5e6 watts simplifing.
or 5w per micronewton. OK a bit more (say 7-8 watts) may be needed
because of inefficiencies, 5w though represents the energy balance.
- Hide quoted text — Show quoted text -
> :
> :>
> :> 3) How much can they see when they’re constantly filling the space
> :> around themselves with ionized gas trying to stay on station?
> :>
> :
> :There is very little ionised gas. It is moving at 10,000km/s and soon
> :clears. As I said the same thing is true for geostationary station
> :keeping.
> :
> Only an infinite order of magnitude easier than what you’re
> suggesting.
> :
> :>
> :> If this is the future, then the future is never.
> :>
> :
> :ESA is already busy. Its interest though is primerally geostationary.
> :
> In other words, even the Europeans aren’t crazy enough to think they
> can do what you’re talking about.
No, it is an incremetal development. GEO ion drive, LISA are necessary
steps but the Physics is sound, unlike the Moon.
– Ian Parker
Ian Parker wrote:
> You don’t, you just need a method of calculating where you are (in
> fact to a quarter wavelength). You do need very sensitive control and
> this is what ion propulsion gives you.
A physical link between the primary mirror satellites. The physical link
being very high frequency laser light, giving relative position. Probably
four primary mirror satellites to get relative 3-D position, velocity,…
On 2 Jul, 12:23, Craig Fink <WeBeG…@GMail.Com> wrote:
> Ian Parker wrote:
> > You don’t, you just need a method of calculating where you are (in
> > fact to a quarter wavelength). You do need very sensitive control and
> > this is what ion propulsion gives you.
> A physical link between the primary mirror satellites. The physical link
> being very high frequency laser light, giving relative position. Probably
> four primary mirror satellites to get relative 3-D position, velocity,…
This could in fact be done with the fragments themselves.
- Ian Parker
Ian Parker wrote:
> On 2 Jul, 12:23, Craig Fink <WeBeG…@GMail.Com> wrote:
>> Ian Parker wrote:
>> > You don’t, you just need a method of calculating where you are (in
>> > fact to a quarter wavelength). You do need very sensitive control and
>> > this is what ion propulsion gives you.
>> A physical link between the primary mirror satellites. The physical link
>> being very high frequency laser light, giving relative position. Probably
>> four primary mirror satellites to get relative 3-D position, velocity,…
> This could in fact be done with the fragments themselves.
I’m not sure, aren’t they just 1-D in the direction of the object. To cancel
a primary star’s light to view the planets which are in the other two
dimensions additional information would be required.
Ian Parker <ianpark…@gmail.com> wrote:
:On 1 Jul, 18:39, Fred J. McCall <fmcc…@earthlink.net> wrote:
:> Ian Parker <ianpark…@gmail.com> wrote:
:>
:> :
:> :That is the question of sprcific impulse. This is not revolutionary
:> :you know. The earth is not quite spherical and at GEO there are
:> :gradients. Very gradual – true but they still need thust. One on the
:> :early uses for ion propulsion is in fact geostationary station
:> :keeping. You need a tiny thrust but over a very long time period.
:> :
:>
:> And to stay within fractions of a wavelength of each other you only
:> need a specific impulse of infinity, which would be just a bit
:> revolutionary.
:
:You don’t, you just need a method of calculating where you are (in
:fact to a quarter wavelength). You do need very sensitive control and
:this is what ion propulsion gives you.
:
1) No, it isn’t.
2) To do what you’re talking about would require thrusters firing all
the time. Hence my comment regarding "a specific impulse of
infinity".
:
ower needed = m*(1e7)^2/2
r 5w per micronewton. OK a bit more (say 7-8 watts) may be needed
:>
:> :
:> :Basic Physics tells us that momentum and energy are conserved. If our
:> :velocity is 10,000km/sec we need 5w to give us a micronewton. That is
:> :all a mirror fragment would ever need.
:> :
:>
:> Poppycock!
:>
:
:Energy = v^2/2. Let m kg be accelerated each second.
:
:
:Momentum transferred = mv = m*1e7
:
:Therefore power for 1N = (1e7)^2/(2e7) = 5e6 watts simplifing.
:
:because of inefficiencies, 5w though represents the energy balance.
:
It figures you prove the wrong part. Now show us how "that is all a
mirror fragment would ever need".
I repeat – poppycock!
:
:>
:> :
:> :>
:> :> 3) How much can they see when they’re constantly filling the space
:> :> around themselves with ionized gas trying to stay on station?
:> :>
:> :
:> :There is very little ionised gas. It is moving at 10,000km/s and soon
:> :clears. As I said the same thing is true for geostationary station
:> :keeping.
:> :
:>
:> Only an infinite order of magnitude easier than what you’re
:> suggesting.
:>
:> :
:> :>
:> :> If this is the future, then the future is never.
:> :>
:> :
:> :ESA is already busy. Its interest though is primerally geostationary.
:> :
:>
:> In other words, even the Europeans aren’t crazy enough to think they
:> can do what you’re talking about.
:>
:No, it is an incremetal development. GEO ion drive, LISA are necessary
:steps but the Physics is sound, unlike the Moon.
:
So you think that putting however many thousands of bits out in
various orbits and maintaining their positions to within some small
fraction of a wavelength (presumably while pointing them at things) is
"sound" but that somehow getting to the Moon is unattainable?
Your disconnect from reality is much worse than I’d thought!
–
"Some people get lost in thought because it’s such unfamiliar
territory."
–G. Behn
On 2 Jul, 12:41, Craig Fink <WeBeG…@GMail.Com> wrote:
> > This could in fact be done with the fragments themselves.
> I’m not sure, aren’t they just 1-D in the direction of the object. To cancel
> a primary star’s light to view the planets which are in the other two
> dimensions additional information would be required.
No you can have a laser going from a fragment to a number of others.
Basically each fragment has 3 degrees of freedom, so to fix positions
you need 3N-3 equations. Where does the -3 come from? Well we are not
interested in where the mirror is absolutely we are only interested in
it relatively.
We would like an overspecified set of equations. This will give us
greater accuracy and give us some idea of the accuracy of our
measurements. The accuracy in an overspecified set of equations is, of
course, the residual.
We need a residual. We don’t want to keep firing our ion thusters back
and forth when we don’t have to. In fact if I choose 1000km/s and 10
microN I will be using 1g every 3 years.
– Ian Parker
Fred J. McCall <fmcc…@earthlink.net> writes:
>Ian Parker <ianpark…@gmail.com> wrote:
>:Ion drives essentially manoever to a wavelength.
>I’ll stop ice skating in hell and come take a look when they get that
>working.
The basic idea is sound, though it’s not been done with ion thrusters.
It worked very well on Gravity probe B.
>1) How do they know where they are to within a wavelength?
There is a free-floating ball in a cryogenic chamber within the satellite.
They maneuver the satellite to keep the ball centered. Then you know the
satellite is on a pure free-fall trajectory to within a few nm. In this
case (GP-B or LISA) the experiments are designed so pure-freefall
trajectories will work. This may be possible with interferometers, too,
using variable optical delay lines.
If not, you can use interferometry between the crafts to establish
relative position (all that is needed) within a fraction of a wavelength.
>2) Since they will have to be under constant thrust to maintain that
>(if they can figure out where they are), what happens when they run
>out of fuel (about next Tuesday)?
The thrust values needed are extremely small. GP-B used cold gas
thrusters using evaporated liquid helium, about the least efficient
thrusters imaginable, and still worked for over a year. (They used
this since they had to vent the helium anyway, not for efficiency).
With halfway efficient thrusters, decades of operation are possible.
>3) How much can they see when they’re constantly filling the space
>around themselves with ionized gas trying to stay on station?
Either neutralize the gas (you want to do this anyway) or use a
similar technique that does not need ionization (LISA’s plan, I
think). Note that LISA will be doing exactly this kind of
maneuvering, while making super high precision optical measurements,
and this does not appear to be a big concern.
>If this is the future, then the future is never.
No, this is the past. It’s already been done.
Lou Scheffer