Author Topic: Mission to the Gravitational Focus  (Read 21091 times)

Offline TakeOff

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Re: Mission to the Gravitational Focus
« Reply #40 on: 01/26/2015 11:34 AM »
I wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling.

Are you perhaps thinking of nuclear-electric propulsion?  A nuclear-thermal rocket cools itself by dumping heat into its exhaust.

Quote
The gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.

For a weak gravitational field like the sun's, the angle (in radians) by which a ray of electromagnetic radiation is deflected as it passes the sun is 4GM/(c2b), where G is the universal gravitational constant, M is the mass of the sun, c is the speed of light and b is the impact parameter, the distance of closest approach of the ray.  This is independent of frequency, so everything will focus at the same point.

Quote
But with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.
This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.

Why bother with a rotating tether?  Just go to the focus and look back at the sun with a coronograph (i.e., a telescope having a little dot in its center to block out the light of the sun itself).  Image the ring of light around the sun and then process the image to remove the massive distortion.

It seems to me that the real limiting factor with the gravitational-lens telescope is that you can look only at one very small part of the sky, namely the part that's opposite the sun.
Great answer! (Especially since you bring good news).

Combining the gravitational lens with a coronagraph and why not a diffraction disk(1) too, and of course an interferometer, would be a triple whammy with too much of everything, thank you. But I fear, I don't know I just fear, that there is a limit to the resolution achievable in telescopes, because space itself contains gas which scatters light. If the light is scattered before it hits the telescope, there's no telescope technology which can fix that. There might be a limit of resolution beyond which better telescopes don't help.

1) http://www.colorado.edu/news/releases/2015/01/23/new-space-telescope-concept-could-image-objects-far-higher-resolution

Now to the aiming issue:
If one aims the first FOCAL telescope towards the center of the Milky Way, isn't it dense enough that always something would be in sight? If we first aim it at Sagittarius A-star (the super massive black hole), or at the center of the Andromeda galaxy, but it then drifts off because of the galactic orbit of the Sun or something, wouldn't one even then just glide over to other (a bit less, but still) interesting random objects in that general direction? One would hardly miss everything. And even then, I suppose that at least a tiny spot of the microwave background would be very well mapped.

Would continuous propulsion perpendicular to the trajectory, once in the focus, with a nuclear electric engine, be enough to keep a chosen target in line with the precision required?

Is there a show stopper for this idea?
« Last Edit: 01/26/2015 12:06 PM by TakeOff »

Offline Stormbringer

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Re: Mission to the Gravitational Focus
« Reply #41 on: 01/26/2015 11:39 AM »
a couple of additional points.

(1)  If the thing was pointed in the direction of alpha centauri you would have three targets for the price of one. possibly more depending on whether any other interesting but more distant stars were in the general direction. But proxima, AC A and AC b are within spitting distance of each other. You could definitely hit those three without worrying about traversing along the 550 au orbital.

(2) if there were a probe sent to AC or whatever a relay station at 550 AU lensing would also amplify the telemetry, data and comms signals so that the transmitter size on the probe could be reduced to reasonable size.

http://www.centauri-dreams.org/?p=10123
« Last Edit: 01/26/2015 11:46 AM by Stormbringer »
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Offline TakeOff

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Re: Mission to the Gravitational Focus
« Reply #42 on: 01/26/2015 11:47 AM »
A FOCAL type mission would be the mission of the century. 2-3 decades preparation and then 5 or so decades travel time. Every decade a new mission should be sent to aim at another target: The galactic center, Andromeda, the most distant galaxy known, an interesting exoplanet, nearest Sun-twin star (to study its sunspot cycle and activity for our own protection) et c.

Along the way there would be several spin offs. Radio telescopy, interferometry in pocket launchable format, heliophysics both for the near flyby and for adjusting the signal for coronal distortions, spacecraft design for century lifetime with respect to energy supply, upgradability and self-repairs from radiation and micro impact damages. (Radio communication with Earth should be easy since it has a big radio telescope directed our way.)

And it is directly useful as a precursor for the first interstellar probes to be launched in the 22nd century. "Long term" is very popular if one asks people in general. FOCAL would provide a long term vision with step wise tangible progress along the way.

Offline TakeOff

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Re: Mission to the Gravitational Focus
« Reply #43 on: 01/26/2015 12:00 PM »
a couple of additional points.

(1)  If the thing was pointed in the direction of alpha centauri you would have three targets for the price of one. possibly more depending on whether any other interesting but more distant stars were in the general direction. But proxima, AC A and AC b are within spitting distance of each other. You could definitely hit those three without worrying about traversing along the 550 au orbital.
I don't do the math, but I'm not sure that is true. The gain is so enormous that even the angular distance between Centauri A and B might be huge for a solar gravitational lensing telescope. I think one would have to pick one of them and cannot turn enough to go from one to the other.

To catch more than one object with one such telescope, I think that objects which are naturally gravitationally lensed by galactic clusters, or with good timing by microlensing, could be candidates. Or just a dense mess like a galactic core. Or of course a transiting planet, then you either observe the star, or the planet, they do line up regularly.
« Last Edit: 01/26/2015 12:03 PM by TakeOff »

Offline Stormbringer

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Re: Mission to the Gravitational Focus
« Reply #44 on: 01/26/2015 12:29 PM »
I think AC a and AC b are within the distance from Sol to Uranus. you may have a point with Alpha Proxima even though they are .16 light years apart but it is harder to imagine AC a and AC b would necessitate anything more than a mere flick of an attitude thruster. I meant only that you would not have to wait for the scope to make a appreciable fraction of one orbit to reposition.
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Offline aceshigh

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Re: Mission to the Gravitational Focus
« Reply #45 on: 01/26/2015 02:11 PM »
problem is getting to 1000 AU from the Sun.

That's 0.015 light years away.

Even New Horizons would take some 100 years to get there (can anybody do the math?)

Offline Stormbringer

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Re: Mission to the Gravitational Focus
« Reply #46 on: 01/26/2015 02:41 PM »
If some of the near term prospects for faster transport work out this will not be a problem.

I consider NEP VASIMR short term. i consider the Slough Fusion rocket near term. Even M2P2.

if these do not happen it will only be because of political will and funding. It will not be a technology show stopper that prevents any of these from happening.
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Offline JasonAW3

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Re: Mission to the Gravitational Focus
« Reply #47 on: 01/26/2015 02:52 PM »
A good start might be to dive in close to the Sun and there blast off a high thrust engine to make maximum use of the Oberth effect in apohelium. Solar Probe Plus will have a top speed of 200 km/s. The dive would require the orbital speed of Earth to be canceled, which is done either by a flyby of Jupiter or like Solar Probe Plus, by several flybys of Venus.

I wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling. It would only need to work for hours or days. Could nuclear weapons be used to give a shove when the Oberth bonus is at its maximum?

For the multi-decadal journey from there on, nuclear electric ion propulsion would be great. The gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.

It is a mission which seems doable this century, and it is an interstellar mission of sorts. It could be used to reconnaitre a star before a truly interstellar mission is sent there, and also to communicate with such an interstellar probe using the same phenomenal magnification.

But with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.
This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.

(Old thread, but unfortunately interest seems to be too low to start another one on the same topic)

Well, since you want to dive close to the sun in the first place, why not a Nuclear Fusion drive?  you're going to wind up preheating the fuel to a fairly high temp, so pumping the fuel into the fusion chamber should not be an issue, simply open a valve.

The heat should be high enough that pushing it over the edge to fusion shouldn't require much energy, and with magnetic constrictionthe fuel will be under such high compression, it'll be close to fusion in the first place!
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Offline Nilof

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Re: Mission to the Gravitational Focus
« Reply #48 on: 01/27/2015 03:13 PM »
A good start might be to dive in close to the Sun and there blast off a high thrust engine to make maximum use of the Oberth effect in apohelium. Solar Probe Plus will have a top speed of 200 km/s. The dive would require the orbital speed of Earth to be canceled, which is done either by a flyby of Jupiter or like Solar Probe Plus, by several flybys of Venus.

I wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling. It would only need to work for hours or days. Could nuclear weapons be used to give a shove when the Oberth bonus is at its maximum?

For the multi-decadal journey from there on, nuclear electric ion propulsion would be great. The gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.

It is a mission which seems doable this century, and it is an interstellar mission of sorts. It could be used to reconnaitre a star before a truly interstellar mission is sent there, and also to communicate with such an interstellar probe using the same phenomenal magnification.

But with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.
This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.

(Old thread, but unfortunately interest seems to be too low to start another one on the same topic)

Well, since you want to dive close to the sun in the first place, why not a Nuclear Fusion drive?  you're going to wind up preheating the fuel to a fairly high temp, so pumping the fuel into the fusion chamber should not be an issue, simply open a valve.

The heat should be high enough that pushing it over the edge to fusion shouldn't require much energy, and with magnetic constrictionthe fuel will be under such high compression, it'll be close to fusion in the first place!

Any temperature you'll reach with sunlight alone will be many, many orders of magnitude below that required for fusion.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline aceshigh

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Re: Mission to the Gravitational Focus
« Reply #49 on: 01/27/2015 05:01 PM »
If some of the near term prospects for faster transport work out this will not be a problem.

I consider NEP VASIMR short term. i consider the Slough Fusion rocket near term. Even M2P2.

if these do not happen it will only be because of political will and funding. It will not be a technology show stopper that prevents any of these from happening.

what's your concept of "near term"? And what is the latest news about Slough's Fusion rocket?

Offline nadreck

Re: Mission to the Gravitational Focus
« Reply #50 on: 01/27/2015 05:24 PM »
My take on the ideal vehicle to get you to the focal point:

3.5 separate main propulsion systems:

Solar sail
Solar Thermal hydrogen high thrust near the sun
Nuclear Electric hydrogen ion (for braking which occurs for most of the flight)

So the solar sail gets deployed close as the probe 'rounds the bend at solar perihelion (5Giga meters say) and is spin stabilized, it is let go when the probe has to start braking (probably about 20 AU out from the sun but depends on Nuclear electric ion engine thrust to vehicle weight ratio and to the top speed of the probe).

For the highest impact I suggest that the solar sail includes central panel that is deformed later but has a point on the probe as its focus to heat hydrogen that cools the solar heat collector, if any MHD benefit can be added to the hydrogen that is heated this way by the ion engine, then go for that too. Plan the thrust to probe weight ratio to not exceed the acceleration to the sail and shrouds from the solar sail. As soon as the hydrogen budget for this is used up deform the reflector on the sail, continue to add thrust with the nuclear electric until it would run the craft into the sail or until when braking needs to start.


It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline Stormbringer

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Re: Mission to the Gravitational Focus
« Reply #51 on: 01/27/2015 06:16 PM »

what's your concept of "near term"? And what is the latest news about Slough's Fusion rocket?

i categorize as near term technologies that could work now if someone would throw all the components together in an integrated system or that appear to be approaching that state of TRL where it is rational to assume the current progress to that state will be uninterrupted by show stopping physics or technological implementation problems.

I do not take into account politics and funding.

VASIMR only needs a power source to work and those power sources are all available except fusion. it can be powered by batteries and solar panels. it can be powered by fission. NOW. Fusion in the near future. Available power is adequate. suitable Nuclear reactors are essentially available now from several companies including GE and Hitachi needing only to clear regulatory hurdles.

M2P2 is nearly as ready as VASIMR. I say this because all of the elements have been tested. working earthbound prototypes have been built. But a integrated space worthy system has not been considered to the degree VASIMR has if at all. you cannot find schematics of a flight article. but this should be fairly simple to do if someone decides to do it. thus M2P2 could be done in a short period of time anytime someone says the word.

Fusion appears to be viable within the decade. a number of private efforts and private-public partnerships have competing designs (which is a good thing.) several of these designs have data that suggests proper scaling increases will result in the critical break even which is just one step away from operational implementation. 

latest Slough information i could find:

http://www.helionenergy.com/?page_id=199
« Last Edit: 01/27/2015 07:43 PM by Stormbringer »
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Offline ChrisWilson68

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Re: Mission to the Gravitational Focus
« Reply #52 on: 02/03/2015 10:01 AM »
I wonder what high thrust engine would be suitable? Nuclear thermal has about 4 times higher exit velocity than chemical rockets (if I'm not mistaken). But radiative cooling is a serious problem near the Sun, nuclear thermal needs cooling.

Are you perhaps thinking of nuclear-electric propulsion?  A nuclear-thermal rocket cools itself by dumping heat into its exhaust.

Quote
The gravitational focus is an endless line so there's no need to slow down. At different distances, different radio frequencies are magnified the most, so going on outwards is a benefit.

For a weak gravitational field like the sun's, the angle (in radians) by which a ray of electromagnetic radiation is deflected as it passes the sun is 4GM/(c2b), where G is the universal gravitational constant, M is the mass of the sun, c is the speed of light and b is the impact parameter, the distance of closest approach of the ray.  This is independent of frequency, so everything will focus at the same point.

Quote
But with an aperture to focal length ratio of about 65000 the intensity will be terrible, made even worse by the fact that one can only use a ring shaped aperture and not the full disc.
This "ring issue" is addressed by using two dishes, one at each end of a rotating tether. That way the telescope would in effect be ring shaped.

Why bother with a rotating tether?  Just go to the focus and look back at the sun with a coronograph (i.e., a telescope having a little dot in its center to block out the light of the sun itself).  Image the ring of light around the sun and then process the image to remove the massive distortion.

It seems to me that the real limiting factor with the gravitational-lens telescope is that you can look only at one very small part of the sky, namely the part that's opposite the sun.
Great answer! (Especially since you bring good news).

But I fear, I don't know I just fear, that there is a limit to the resolution achievable in telescopes, because space itself contains gas which scatters light. If the light is scattered before it hits the telescope, there's no telescope technology which can fix that. There might be a limit of resolution beyond which better telescopes don't help.

Gas scattering light doesn't limit resolution, it simply diminishes brightness.  If we can see an object at all with ordinary telescopes, it has plenty of brightness to make an extremely high resolution image at the gravitational focus.

Online gosnold

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Re: Mission to the Gravitational Focus
« Reply #53 on: 02/03/2015 08:11 PM »
Gas and dust scattering is a problem for coronographs, because it introduces parasitic light that comes from the same direction as the desired signal. It may diminish the brightness of the target a bit, but it will mostly raise the noise floor, making it harder to detect the target.

Offline JasonAW3

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Re: Mission to the Gravitational Focus
« Reply #54 on: 07/03/2015 09:41 PM »
Assuming for a moment that an optical version of the VLA radio telescope could be constructed and launched to the Gravitational Focus area, would there be a sufficient improvement in optical acuity to justify such an array?

     I was considering a 10 kilometer diameter array of smaller optical telescopes all tethered to a central point/control and processing hub, using multiple tethers, say about ten legs, each having five smaller optical telescopes spaced evenly along each leg of the tethers, for a total of fifty smaller telescopes, plus a central aiming telescope at the hub.

     The whole array would be rotating at about .01 G of acceleration and would use reaction control wheels to both aim and steady the telescopes.  If required to rotate the whole array, the telescopes and tethers would be reeled in to the central hub after closing the optical ports, the master attitude control wheels would be used to turn the whole array to the desired attitude and the telescopes and tethers would be reeled out to their new stations.

      Essentially I am suggesting an optical synthetic aperture array that would give the rough equivalent to an optical telescope with a ten kilometer mirror.

just a thought...
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Offline Nilof

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Re: Mission to the Gravitational Focus
« Reply #55 on: 07/04/2015 01:29 AM »
One thing which would be very interesting (but very difficult) would be having a Neutrino detector out there. Gravity is the only force which is practical for bending the path of Neutrinos. Having an array of neutrino detectors at the lensing point distance could in theory allow for an imaging neutrino telescope. That could be a pretty big deal, since Neutrinos usually come from interesting sources, and completely ignore any interstellar gas or dust. It should be able to see a background from the big bang nucleosynthesis era and observe the effects of inflation much more directly.

Now, actually building something of the sort would be a massive undertaking and would likely involve highly capable robots building detectors using ice from local comets. It'd be limited to looking at small patches of sky and would mostly see noise unless each individual detector is made absolutely gigantic.
« Last Edit: 07/04/2015 01:32 AM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Stormbringer

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Re: Mission to the Gravitational Focus
« Reply #56 on: 07/04/2015 01:54 AM »
WRT the VLA idea just using the gravity lensing effect multiplies the power by millions and millions of times.  I do not see how the added complexity and points of failure for no certain benefit would be justifiable.

http://www.centauri-dreams.org/?p=785

http://www.centauri-dreams.org/?p=10123

(mind the paywall:  )

http://physicsworld.com/cws/article/print/2004/jun/10/gravitational-lensing-brings-extrasolar-planets-into-focus

http://www.icarusinterstellar.org/sun-gravitational-lens/

the HST if placed at the gravitational focus could resolve a mercury sized planet at 1000 light years distant (and theoretically at least at any distance) at the same resolution level and detail it could looking at mercury in our own solar system. 
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Offline Stormbringer

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Re: Mission to the Gravitational Focus
« Reply #57 on: 07/04/2015 02:12 AM »
the signal amplification and presumably the resolving power of a light would be multiplied by 10^8th power.

10^8 is 10 times itself 8 times.

that's a magnification factor of... ten times ten times ten times ten times ten times ten times ten times ten times ten.

thats 100,000,000.

thats 100 million times the original resolving power of the telescope or reception threshold or transmitting power of a radio communications system.

Would a VLA really be necessary?
« Last Edit: 07/04/2015 02:14 AM by Stormbringer »
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Offline Paul451

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Re: Mission to the Gravitational Focus
« Reply #58 on: 07/04/2015 03:35 AM »
WRT the VLA idea just using the gravity lensing effect multiplies the power by millions and millions of times.  I do not see how the added complexity and points of failure for no certain benefit would be justifiable.

The reason was in the links you included. Gravitational lens telescopes have the resolving power of the same telescope at that distance from the sun (ie, 550-1000AU). HST resolves Pluto to about 4 pixels at a mere 40AU. Therefore if you want anything more than a single pixel (actually a sub-pixel) image of an exo-planet, you need a large array.

Offline Stormbringer

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Re: Mission to the Gravitational Focus
« Reply #59 on: 07/04/2015 03:45 AM »
granted you could learn more from a VLA but even with one pixel if there are contrasting areas on the planet you can deduce such things as snow, ice caps, clouds, continents, oceans, major forests, major deserts maybe even city lights and forest fires. With four you can do more still. Of course you can do a lot more with better and better resolution but you don't need much to uncover a wealth of information. Some of the very impressive big seeming detail revealing pictures of distant stars began with just 8 (5?) pixels (in width) and end up looking like medium res pictures of our sun from good earth bound telescopes.

« Last Edit: 07/04/2015 04:07 AM by Stormbringer »
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