Author Topic: Mission to the Gravitational Focus  (Read 23343 times)

Offline rklaehn

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Re: Mission to the Gravitational Focus
« Reply #20 on: 12/11/2011 08:36 AM »
Getting out there quickly could be done with a solar sail. Once beyond the 550au point, the sail can double as your reflector.

You could get out there with a solar sail. But forget about using a solar sail as an optical reflector. A reflector for a telescope is a precision optical instrument, whereas a solar sail is just a very large area of reflecting foil.
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Offline rklaehn

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Re: Mission to the Gravitational Focus
« Reply #21 on: 12/11/2011 08:39 AM »
Trouble is that it looks in one specific direction, and steers very slowly. You have to move the probe around the sun at 500AU to change view.

Maybe it we, in a decade or so, have a really good candidate for an earth-like exoplanet, then it may be worth considering sending a probe to the specific point were it is in focus.

A big expensive mission, but much easier (and faster) than sending a probe to the actual planet.

The kepler mission also looks into one specific direction. This would still be good enough for a survey of extraterrestrial planets in a small, previously chosen area of the sky. And you could give the probe some tangential velocity using a jupiter flyby to slowly scan a strip of sky.
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Offline DLR

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Re: Mission to the Gravitational Focus
« Reply #22 on: 12/11/2011 10:02 AM »
NASA studied a thousand AU probe in the late 80's. With a 1MW reactor and ion drives, it would have covered 1000AU in fifty years.

Scale up the nuclear reactor to higher power levels, and you can still be faster.

Aren't the Russians planning a 12MW reactor? I suppose a space tug this powerful could carry a telescope to 550 - 1000 AU in a reasonable time frame?
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Offline Tass

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Re: Mission to the Gravitational Focus
« Reply #23 on: 12/11/2011 11:48 AM »
Trouble is that it looks in one specific direction, and steers very slowly. You have to move the probe around the sun at 500AU to change view.

Maybe it we, in a decade or so, have a really good candidate for an earth-like exoplanet, then it may be worth considering sending a probe to the specific point were it is in focus.

A big expensive mission, but much easier (and faster) than sending a probe to the actual planet.

The kepler mission also looks into one specific direction. This would still be good enough for a survey of extraterrestrial planets in a small, previously chosen area of the sky. And you could give the probe some tangential velocity using a jupiter flyby to slowly scan a strip of sky.

For a given definition of "specific", one which was not the one used here.

Kepplers field of view is comparatively huge. Even at 600ly the object is only some 60000-70000 times bigger than the detector (meaning a 200m detector just to cover an earth-like planet). Your field of view is going to be little more than a single planet, not even a full solar system, let alone interstellar distances to survey more than one star at once. The chance of two stars in the same galaxy lying lined up well enough to fit them both is negligible.

If you have a tangential velocity then the strip covered will be way too narrow to hit anything at all in this galaxy. It has to be specifically steered to look at a certain point of interest.

If this solar lens thing works as well as it could in theory then you could theoretically do a Kepler type survey of the stars in a distant galaxy, and I don't mean Andromeda here. This is truly insane magnification.

But with an aperture to focal length ratio of about 65000 the intensity will be terrible,...

The math for this works out reasonable.  If the surface is as bright as Earth, a day-time shot would require about 41 seconds of exposure time at ISO 100 (unless I did that wrong).  Assuming the scope is tracking the planetary motion, a push-broom could be used to track planetary rotation making that exposure time work for that level of resolving power.

Ah right. Regular telescopes have a hard time seeing planets because a planet is much smaller than the resolution, so it only contributes a little bit of the light at a pixel. Not so in this case. Although we we lose a lot of light by only having a ring to gather it, we can also do way better than ISO 100 here. Great. 

We should keep in mind that the theoretical capabilities being discussed here are from spherical cow land. Achieving them in the real world would present tremendous challenges. However, you could fall short by many orders of magnitude and still do a lot better than conventional telescopes.

Definitely. All of it.

Motion I believe should be counterable. It should also be possible to correct for the sun being oblate. I don't know whether unpredictable turbulences in the sun are going to alter its gravitational field enough to smear the picture. 

 
« Last Edit: 12/11/2011 11:59 AM by Tass »

Offline rklaehn

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Re: Mission to the Gravitational Focus
« Reply #24 on: 12/11/2011 12:05 PM »
NASA studied a thousand AU probe in the late 80's. With a 1MW reactor and ion drives, it would have covered 1000AU in fifty years.

Scale up the nuclear reactor to higher power levels, and you can still be faster.

Aren't the Russians planning a 12MW reactor? I suppose a space tug this powerful could carry a telescope to 550 - 1000 AU in a reasonable time frame?

I would like to see giant nuclear reactors just as much as any other space enthusiast. But the political reality is that anything that involves reactors is not going to happen, at least from the US and europe. The best you can hope for is a RTG or some other (stirling?) radioisotope generator to power the telescope at 500au.

Fortunately you don't need nuclear electric propulsion to get to 500au. A solar sail could be used, or some other form of solar propulsion like solar electric or mini magnetospheric plasma propulsion. The problem with the latter is that you can only create radial outward forces. But for getting to 500au this is not a problem.

For using solar sails, see for example
http://intrance.org/paper/200410_Vancouver_Dachwald_Sail.pdf
or
http://interstellar.jpl.nasa.gov/interstellar/ISP_Space2K_v4.pdf
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Offline rklaehn

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Re: Mission to the Gravitational Focus
« Reply #25 on: 12/11/2011 12:41 PM »
Trouble is that it looks in one specific direction, and steers very slowly. You have to move the probe around the sun at 500AU to change view.

Maybe it we, in a decade or so, have a really good candidate for an earth-like exoplanet, then it may be worth considering sending a probe to the specific point were it is in focus.

A big expensive mission, but much easier (and faster) than sending a probe to the actual planet.

The kepler mission also looks into one specific direction. This would still be good enough for a survey of extraterrestrial planets in a small, previously chosen area of the sky. And you could give the probe some tangential velocity using a jupiter flyby to slowly scan a strip of sky.

For a given definition of "specific", one which was not the one used here.

Kepplers field of view is comparatively huge. Even at 600ly the object is only some 60000-70000 times bigger than the detector (meaning a 200m detector just to cover an earth-like planet). Your field of view is going to be little more than a single planet, not even a full solar system, let alone interstellar distances to survey more than one star at once. The chance of two stars in the same galaxy lying lined up well enough to fit them both is negligible.

Ok. Thanks for the correction. So the magnification is much better than I expected. So what you really want is a bunch of small telescopes looking towards the sun on a rotating tether, to cover a large area.

If you get there using a large solar sail, you could have multiple small telescopes spread over the area of the (slowly rotating) sail. I am thinking of something like the IKAROS sail, but scaled up to a diameter of several km.

The size of the field of view in radians is just the diameter of the telescope divided by the distance from the sun, right? Kind of like a pinhole camera with the pinhole being in the center of the sun, and the screen being the collection area of the spacecraft? That is really some mindboggling magnification...

If this solar lens thing works as well as it could in theory then you could theoretically do a Kepler type survey of the stars in a distant galaxy, and I don't mean Andromeda here. This is truly insane magnification.

So make sure there are some distant galaxies in the field of view and just take a look. At the very least you should be able to validate or invalidate theories about the gravity of the sun.
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Offline clongton

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Re: Mission to the Gravitational Focus
« Reply #26 on: 12/11/2011 12:46 PM »
I would like to see giant nuclear reactors just as much as any other space enthusiast. But the political reality is that anything that involves reactors is not going to happen, at least from the US and Europe. The best you can hope for is a RTG or some other (stirling?) radioisotope generator to power the telescope at 500au.

The US and Europe are eventually going to have to get over their childish fear of nuclear power if they are ever going to do anything significant in space beyond the inner solar system. This fear reminds me of a child being unwilling to give up its bottle in order to drink its milk from a cup.

Getting out past Mars by any significant amount will make SEP impractical without arrays that are tens or hundreds of square kilometers in area and those would be completely unwieldy. Nuclear power is the only way forward, unless someone here thinks matter-antimatter is safer? Yes, nuclear is dangerous, but *anything* powerful is dangerous. So what? Learn to safely control the danger and get on with it. As a species, are we still bottle-fed infants?

Using a constantly thrusting nuclear powered ion engine would put this telescope on station in no time flat, relatively speaking. We could actually begin imaging exoplanets in our own lifetime. It's either do that or stick with our baby bottle and hope that our grandchildren remember to turn on the switch in 75 years or so.
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Offline rklaehn

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Re: Mission to the Gravitational Focus
« Reply #27 on: 12/11/2011 01:10 PM »
The US and Europe are eventually going to have to get over their childish fear of nuclear power if they are ever going to do anything significant in space beyond the inner solar system. This fear reminds me of a child being unwilling to give up its bottle in order to drink its milk from a cup.

I completely agree. But the paranoia regarding all things nuclear is so strong that it will be another decade or more before nuclear fission gets seriously considered for power generation, let alone space applications. Maybe the US and europe will wake up when china and india power their entire economies using lithium fluoride thorium breeders, but maybe it will be too late then.

Quote
Getting out past Mars by any significant amount will make SEP impractical without arrays that are tens or hundreds of square kilometers in area and those would be completely unwieldy. Nuclear power is the only way forward, unless someone here thinks matter-antimatter is safer? Yes, nuclear is dangerous, but *anything* powerful is dangerous. So what? Learn to safely control the danger and get on with it. As a species, are we still bottle-fed infants?

It seems so. But I think that it is perfectly possible to get out of the solar system at high speed using solar electric propulsion as long as you don't plan to brake. You can do all your thrusting while close to the sun. Usually you will drop in to below venus orbit for the majority of the thrusting.

Quote
Using a constantly thrusting nuclear powered ion engine would put this telescope on station in no time flat, relatively speaking. We could actually begin imaging exoplanets in our own lifetime. It's either do that or stick with our baby bottle and hope that our grandchildren remember to turn on the switch in 75 years or so.

I fear that it's the latter. My hope is that it will be possible to start colonizing the inner solar system with just solar power. People living on the moon, mars, phobos or on a near earth asteroid will not have any irrational fear of nuclear power, since they will have all kinds of real dangers to worry about...
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Offline Seer

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Re: Mission to the Gravitational Focus
« Reply #28 on: 12/11/2011 03:06 PM »
The magnification factor for visible light is 10^8. There is a scientis called Maccone
who has

written articles & books about the gravitational lens. For alpha centauri the theoretical maximum resolution is 3 meters with a 12m telescope. I imagine the practical maximum is 1 km if only due to pointing stability.

Offline hop

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Re: Mission to the Gravitational Focus
« Reply #29 on: 12/11/2011 05:09 PM »
I don't know whether unpredictable turbulences in the sun are going to alter its gravitational field enough to smear the picture. 
From what I gather, optical effects of the solar corona are also a big concern.

Claudio Maccone has done a great deal of work on this. You can find some presentations and papers google. He also wrote a book: http://www.amazon.com/Deep-Space-Flight-Communications-Gravitational/dp/3540729429 which is rather pricey ;)

I just noticed he gave a talk at the SETI institute, I haven't watched.

Offline Robotbeat

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Re: Mission to the Gravitational Focus
« Reply #30 on: 12/12/2011 04:28 AM »
The US and Europe are eventually going to have to get over their childish fear of nuclear power if they are ever going to do anything significant in space beyond the inner solar system. This fear reminds me of a child being unwilling to give up its bottle in order to drink its milk from a cup.

I completely agree. But the paranoia regarding all things nuclear is so strong that it will be another decade or more before nuclear fission gets seriously considered for power generation, let alone space applications. ...
Not a problem when discussing a probe out to 550 AU. It'll be a while before we're ready.

Still, we're going to need to research fission (or radioisotopic) fragment rockets eventually. They're quite powerful.
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Offline Tass

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Re: Mission to the Gravitational Focus
« Reply #31 on: 12/12/2011 05:35 AM »
The size of the field of view in radians is just the diameter of the telescope divided by the distance from the sun, right? Kind of like a pinhole camera with the pinhole being in the center of the sun, and the screen being the collection area of the spacecraft? That is really some mindboggling magnification...

That is the assumption I worked with. However according to Macone you can use of-center light and so expand your field of view at the expense of resolution and intensity. So watching multiple stars are maybe not completely out.

Edit: Here's an interesting slideshow. It lacks the talk obviously, but it is still informative.

Second edit, forgot the link to said slideshow: http://www.cesr.fr/~g-wave05/presentations/optics/Koechlin.pdf
« Last Edit: 12/13/2011 03:27 PM by Tass »

Offline DLR

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Re: Mission to the Gravitational Focus
« Reply #32 on: 12/13/2011 11:29 AM »
I think you people are overestimating public opposition against fission reactors.

There are plenty of fission power plants under construction right now. Most in Asia and Russia, but some in the EU too.

Russia is planning to design and launch a space-based fission reactor in the next decade.

Launching a radioisotope power generator (which is kind of routine) is much more hazardous, since those things are stuffed with Plutonium and other nasty transuranics. Not so a "virgin" reactor: just U-235 and U-238. 

Anyway, this is not the topic of this thread. I brought it up because nuclear electric propulsion is probably the most mature technology when it comes to high-performance propulsion. A solar fryby + solar sail propulsion is completely untested. Fission reactors and electric engines on the other hand have been flown before.
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Offline Robotbeat

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Re: Mission to the Gravitational Focus
« Reply #33 on: 12/13/2011 02:47 PM »
I think you people are overestimating public opposition against fission reactors.
I agree, in the long-term (which is necessarily what this thread is about).
Quote
Russia is planning to design and launch a space-based fission reactor in the next decade.
I would bet they've been planning to design and launch a space-based fission reactor since Sputnik (they have actually done that, mind you, but the performance hasn't been spectacular).

I should also point out that there has been an interplanetary solar sail already flown (being flown right now)... The Japanese IKAROS. You probably already knew that, but I'm just pointing that out. Granted, it only went in to about Venus orbit, but the fission reactors we've flown so far are nowhere even CLOSE to the performance necessary for a mission to 550AU.
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Re: Mission to the Gravitational Focus
« Reply #34 on: 01/19/2012 03:00 PM »
I've been giving this proposed mission a lot of thought.  Now, getting out to 550 AU may well be out of our reach for now.  So I was thinking about something more doable, but I don't have the math figures to see if it's even feasible.

We know that every sizable mass will produce a noticeable gravitational lensing effect.  As the sun's gravitational focus is well beyond our technological reach at present, what about Jupiter?  True, it won't produce anywhere near the lensing effect that the sun will.  But where would be its focus and how much of an improvement would that get us?

Offline Robotbeat

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Re: Mission to the Gravitational Focus
« Reply #35 on: 01/19/2012 03:32 PM »
I've been giving this proposed mission a lot of thought.  Now, getting out to 550 AU may well be out of our reach for now.  So I was thinking about something more doable, but I don't have the math figures to see if it's even feasible.

We know that every sizable mass will produce a noticeable gravitational lensing effect.  As the sun's gravitational focus is well beyond our technological reach at present, what about Jupiter?  True, it won't produce anywhere near the lensing effect that the sun will.  But where would be its focus and how much of an improvement would that get us?
Jupiter's gravitational focus would be even further away than the Sun's.
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Offline Andrew_W

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Re: Mission to the Gravitational Focus
« Reply #36 on: 01/19/2012 07:09 PM »
Jupiter's gravitational focus is at about 5000 AU:
http://www.physicsforums.com/showthread.php?t=101614
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Offline TakeOff

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Re: Mission to the Gravitational Focus
« Reply #37 on: 01/26/2015 09:48 AM »
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)

Offline Bynaus

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Re: Mission to the Gravitational Focus
« Reply #38 on: 01/26/2015 09:59 AM »
Jupiter's gravitational focus is at about 5000 AU:
http://www.physicsforums.com/showthread.php?t=101614

Lets hope we find a Jupiter-sized planet in the Oort cloud, then... :)

Offline Proponent

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Re: Mission to the Gravitational Focus
« Reply #39 on: 01/26/2015 10:49 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.
« Last Edit: 01/26/2015 10:52 AM by Proponent »

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