http://www.planetary.org/news/2005/pioneer_anomaly_faq.html

The planetary society may be checking it out...

Some background reading...

http://arxiv.org/find/astro-ph/1/ti:+pioneer/0/1/0/all/0/1

Won't New Horizons be spin stabilized when not in encounter mode?

Quote from alan stern:



in the new horizons thread:
http://www.unmannedspaceflight.com/index.p...topic=675&st=20

so some more things to look forward to

Could not the Pioneer tapes be saved on a more modern support, before scraping the computers which can read it? So far as we know there may be still valuable data to infer from them, especially with the affair of the "pioneer anomaly".

Much was already said about this anomaly:

-the gravitaion law working differently at great distance...
-effect of cosmological dark matter surrounding the sun...


or simpler:
-systematic mistakes somewhere into the measurement chain.


Perhaps the NASA and many scientists prefer the second set of explanations. But so long as the first set cannot be completely ruled out, the question remains open.

I looked at some of those things, but still am not quite clear on what this anomoly is.
And what's up with saving this? I think I missed out on this stuff.

It looks like the Pioneers have long stopped transmitting, but the data hasn't been analyzed yet, and now NASA wants to destroy the computers that can read the tapes? Is that the gist of it?

that's what I understand yes...

The anomaly is basically that the pioneer spacecraft (I think it was seen in Ulyssus and Cassini as well) accelerate slightly different than what one would expect from our understanding of gravity.

Not Cassini but Galileo. Cassini is 3-axis stabilized (thrusters). Galileo was spin stabilized. The error introduced by the use of thrusters basically washes out any hope of seeing any other miniscule anomalous effects.

As far as I understand from readings in the press, the Pioneer probes (ant others) would slow down slighty more than expected from only the Sun gravity. This led to some interesting but far reached speculations.

Some asked if the gravity law was not exactly 1/R2.

The most consistent speculation is that, like the galaxy shows an excess (sometimes ten times) of invisible mass, called dark matter, the solar system may have some too (although much less, a fraction of a %). From close to the Sun, we feel only the Sun's attraction, but further in space we feel the Sun's more the dark matter's attraction. This could be explained if the dark matter is made of low energy subatomic particles; many would orbit the galaxy, with speeds in the 200km/s range. Some would orbit the Sun with still smaller speeds in the km/s range.


But the measurement of the anomaly is based on many far-reached and difficult estimates, such as the toss of sun's light on the space probe, or gaz leak from the reservoirs. A slight error or false assumption, and the result would be very different. This is why the Pioneer anomaly did not upset the science community. A precise measurement of this anomaly would perhaps do.

So presumably the New Horizons vehicle won't be stable enough, for long enough, either...

Painstaking and careful analysis is a more exacting description.

http://arxiv.org/find/astro-ph/1/au:+Turys...S/0/1/0/all/0/1

http://arxiv.org/abs/gr-qc/0104064

http://arxiv.org/abs/gr-qc/9906112

Anderson, Nieto & Turyshev's paper- rebuttal - papers with the astrophysical community spans more than a decade now, and signatures of the anomalies have been found everywhere they have found the time and money to look for them.

The community should be giving this type of research highest priority -

Nowhere is it written in stone gravity behaves exactly as was predicted and observed a more than a century ago. If the observational data indicates otherwise, why are we so certain the status que is the status correct?

The highest priority? I think finding all substantial Earth Crossing asteroids would be more important. Basic exploration of the Solar System should be more important. Looking for extra-solar planets should be more important. Galactic census missions should be more important. Studies of Sgr A* and the things orbiting it should be more important. Studies of distant Type 1a Supernova should be more important. etc.

The ESA has considered launching a probe specifically to measure this effect with greater precision, but it didn't get funded.

Perhaps you could tell us how you would probe this phenomenon, and what the various outcomes would tell us? Then we can talk about priorities.

Two things:

1) I would take the mother-and-hen approach suggested by Nieto. Pioneer 10 & 11 demonstrated the virtues of simple, spin stabilized, thermally balanced probes. I would add a lot of health monitoring gear, but otherwise the probes should be very simple. They should have transmitters capable of using both phase-locked loop and ultra-stable oscillator ranging frequency control, multiple bands.

There should be at least two different types of chicks - one designed with a great deal of solar drag, the second fairly streamlined. Placed into identical trajectories at the same time, this would allow differentiation of unknown forces from solar wind effects. Perhaps even better would be ‘umbrella chicks’ that could trade-off between solar sail and coast modes, thus keeping the clutch together.

The mother should hover close enough to calibrate and study emissivity and radiation, but distant enough not to disturb momentum.

2) If one assumes the Pioneer anomalies are real and not artifacts, there should be supportive evidence, and many fundamental implications.

The solar wind may not be the only force moving objects away from the center of the solar system. Anderson had to add a linear component to the solar wind to model the acceleration of Galileo and Ulysses: http://arxiv.org/PS_cache/gr-qc/pdf/0104/0104064.pdf p21:



(my bold)

Throw a spring constant into elliptical orbits, as Anderson did to model Ulysses, and all hell breaks loose. Fly-bys and orbits don’t return correct planetary masses, long distance navigation breaks down, and gravity anomalies crop up everywhere.

Such a force is consistent with known observational facts. Figuring out if it is real should be given highest priority.

It seems to me that a simple means of checking this notion would be to track long-period comets (many of them). No spacecraft required

Too wide of error bars - Comets are always outgassing, changing in mass, color, orientation. We are looking at accelerations of less than 60 ppm.

Also, Radar tracking has proven to be less accurate than physical modeling suggest, as we learned from the Mars Climate orbiter and Polar Lander...perhaps Anderson's unmodeled force is the culprit.

Climate orbiter was not radar tracked, it was transponder tracked. You get precise distance, but have to model angular position on the sky from indirect data. One way to get precise relative position in the sky is differential interferometry, where one vehicle, say a mars orbiter, is a reference source, and the approaching vehicle's position relative to the reference can be measured.

The problem with Climate orbiter was not with the tracking data, it was modelling the trajectory based on that data when bogus information <incorrect spacecraft peturbation values from the momentum wheel offloading with thruster firing) was included in the solution. The nav team had recognized and been bothered the entire flight by considerably (several times?) larger trajectory calculation variations than normal, but not been able to find a cause of the error with available time and resource.

Polar Lander wiped out due to either 1.) a programming error in the descent software that was almost guaranteed to eat their lunch. or 2.) any of considerable number of design defects that added risk to the landing but weren't "smoking gun" errors.

The Pioneer anomaly is just that.. it's a departure of observations from very precise, but known imperfect calculations. We don't know if there's a physical something based on "deep" important physics, or if our engineering models of non-gravitational forces on the spacecraft are subtly wrong.

It will cost at least a few hundred million dollars to fly a mission that will convincingly and unambiguously prove an "important" physics cause is either present or not, and *PRECISELY* measure that effect if it exists as a function of distance from the sun. *** NOT TRIVIAL ***.

There are a lot of other research proposals and proposed space missions with equal or greater chances of testing and getting an important answer to "deep" physics questions that would cost the same or less than a Pioneer Anomaly mission. The best strategy is to pick the strongest tests with the greatest likelyhood of surprise, for the least amount of money and do or fly those..... and to pick tests that test fundamentally different possible suprises in deep physics. Things like dark matter detection experiments, gravity wave observatories, Dark energy investigations have far more chance per doller of real surprises than a pioneer anomaly mission. We need to keep it as an open question with the possibility of a mission, but right now, it seems a high risk, probably low yield mission for the money.

The problem with a 'dark matter' mission is that the putative dark matter is entirely hypothetical, and may not (I would say probably doesn't) exist. Stanley Milgrom's MOND theory or Jakob Bekenstein's TeVeS theory seems to me to explain the observations in question much more easily than any dark matter theory does. There are also indications that the effects ascribed to 'dark energy' may also be a natural result of a full MOND or TeVeS theory.

http://xxx.lanl.gov/abs/astro-ph/0504130

On the other hand, the so-called 'Pioneer Anomaly' appears to be genuine. The alternative explanations (solar wind, asymmetrical thermal radiation, etc.) appear to be several orders of magnitude too small to adequately explain the observed acceleration. I personally would say that a proper investigation of the 'Pioneer Anomaly' would be at least as important as Gravity Probe B, and could be mounted at a reasonable cost too. The spacecraft itself would be relatively inexpensive; the biggest cost would be the launch vehicle.

Here are links to papers discussing possible approaches to a mission:

http://xxx.lanl.gov/abs/astro-ph/0504634
http://xxx.lanl.gov/abs/astro-ph/0409373
http://xxx.lanl.gov/abs/gr-qc/0409117

Bill

Bill:

Your links, er, don't!

Bob Shaw

That's strange; I just clicked on them and they sent me to the reports that I had referenced...

If they still don't work, I suppose that you can just go to

http://xxx.lanl.gov/find/astro-ph

and do a search for 'Pioneer Anomaly'. Or else just type out the links on your address bar.

Bill

Bill:

I promise, linky no worky.

________________________________________________________

Access Denied
Sadly, you do not currently appear to have permission to access http://xxx.lanl.gov/find/astro-ph

If you believe this determination to be in error, see http://xxx.lanl.gov/denied.html for additional information.

_________________________________________________________

Bob Shaw

Okay, I'll try posting the files directly to the board. Hope this works.

The first file is the 'dark energy' in TeVaS paper. The other three are the 'Pioneer Anomaly' papers.

Bill

I am at least as interested in the flight path eccentricities of Odysseus, Galileo and Pioneer 6 as the Pioneer 10 and 11. While the Pioneer probes indicate an acceleration towards the sun, an unsolicited acceleration away from the sun had to be used to model the paths of Odysseus and Galileo, and one of a greater magnitude: ~1x10^-8m/s^2.

Likewise, I mentioned the navigational problems associated with the Polar Lander and Climate orbiter, not because of the failure modes, but because of the difficulty navigators had both predicting and tracking the probes – with or without a unit conversion error.

After the Climate Orbiter failed to achieve a non-intersecting orbit, two teams of navigators worked on the flight path of the Polar Lander. Each time they tried to model and predict solar wind effects, they were frustrated – they could not correlate the small force corrections due to the solar wind with the path of the probe.

On both missions, NASA tried to use triangulation as well as the Doppler ranging data, and triangulation yielded surprisingly unsatisfactory results.

Please allow me enough latitude to use a hypothetical to demonstrate why I think we must track down the exact causes of these small force and/or navigational errors. Assume the forces are real, and assume they are caused by a gravitational equivalent to a change in the permeability-of-free-space that is a function of mass. This would mean that a probe moving towards the sun would be slowed as more energy is stored in a stronger ‘mass field’ nearer the sun.

The Newtonian equation for velocity then becomes a function of total proximal mass, not just the mass of the object in motion (Ek=Mv^2/2f(x) where f(x) @ 1 AU = 1, and becomes larger as the object approaches the sun, leaving more energy in the potential energy pool (Remember, this is ALL HYPOTHETICAL and the force changes are very small and tightly constrained.)

Look what would happen: As a probe approaches the sun, less potential energy is 'released' as kinetic. The velocity increases at a slightly slower rate than Newtonian predictions. Leaving close proximity to the Sun, the probe would require slightly less energy to achieve a greater acceleration, returning the probe to the predicted path. This is EXACTLY what the residuals look like in the Pioneer 6 pass near the limb of the sun, peaking (slowing) at closest proximity to the sun. Likewise, the 1/r ‘spring constant’ Anderson used to model the Solar wind effects upon Odysseus and Galileo follow this model.

There is more: If more potential energy is stored in a more massive environment, probes to Venus are proportionally slowed, while probes to Mars would experience a slight acceleration, achieve a slightly different orbit. This would cause us to overestimate the mass of Venue, and underestimate the mass of Mars. When we interpret the orbital gravimetric data, the smaller accelerations near the mountain peaks on Venus would then appear as negative gravity anomalies, and likewise, valley floors would appear as positive anomalies. This is what we observe.

On Mars, the situation is exactly opposite: The increase in the transfer to kinetic energy would cause us to underestimate the mass of Mars from orbiters, so the mountain peaks would be interpreted as positive gravity anomalies while valley floors would appear negative. This is also precisely what we observe.

There is more.

We now have gravity maps of Mars from distances varying from 300 to 800 km, but the 800 km data cannot be reconciled with the 300 km maps. The 300km data showing greater anomalies. The moment of inertia for Mars appears to be different if ranging data to the surface probes (Pathfinder and Viking) is used than the inertial moment necessary to explain the orbital gravity anomalies.

All of the Martian probes have landed at higher velocities than expected and, entered at higher attitudes. All descent trajectory models have required a thinner-than-expected upper atmosphere, and high surface winds.

I can go on and on, but I think that you get my point: It would not take a major change in solar dynamics to produce surprising errors. I have been arguing with Jason and Bruce that the rocks, the craters, the strata, and the Doppler descent data from Huygens could be better modeled with less shear wind and more mass.

Fortunately, missions are already in progress that can disprove this hypothesis: Messenger will pass close enough to the sun that the ‘limb effect’ observed by Pioneer 6 could be repeated. MRO will provide gravity maps at 150 km – if this hypothesis is true, MRO will map greater gravity anomalies than prior orbiters that cannot be fit with harmonic extrapolations. MCO will also provide us with a good average atmospheric gradient, one that will be steeper than expected if the planet is more massive.

Finally, careful mapping of the effects of Saturn’s moons on Cassini should reveal ‘unmodelable drag’ forces and even greater gravity anomalies than Galileo found on Ganymede.

I don’t expect any degree of agreement with this assessment, but I hope I have peaked your interest in the manifold scientific data returning from the robotic planetary missions; and there just might be more to learn than mission planners dreamed, a truly revolutionary prospective of the cosmos, one just as foreign to scientific thinking today as the Ptolemic model.

Hmmm... well, the effect must be very, very minor (at least in the local solar-system neighborhood), or else the planets wouldn't orbit in such a way as to generally validate the inverse-square law of gravitation.

It *does* occur to me that the inverse-square law relates to the "classic" three physical dimensions, and cosmologists are always saying that as many as 19 physical dimensions *must* exist. As far as we can tell (since we cannot directly measure anything outside of the three dimensions that are apparent to us), gravitation doesn't propogate along any of these other physical dimensions. Perhaps this is an indication that it *does* and the effect we are seeing is actually a relation between some other physical dimension(s) and the three we can perceive? This would mean that the inverse-square law could be maintained; we're just applying that law to a dimension that is not obviously connected to the three we can see.

I think it's time to start contemplating how these extra dimensions that cosmologists believe must exist inter-relate with the Universe as we observe it. Rather than assuming that these dimensions simply collapsed and vanished as energy levels decreased shortly after the Big Bang, maybe they still exist and interact with such things as gravitation... However, I think it's too early to say that this effect happens near massive bodies, and it's definitely too early to start making mass itself a variable factor, relative to its distance from other masses.

-the other Doug

Hi The Messenger,

We've interacted a few times on another forum. This is the clearest statement of what you've been trying to get to that I've seen. Thanks. I will now be trying to keep an eye on this.

Thanks to recent posters this thread took an interesting turn, that we could summarize that a mission specifically dedicated to the Pionneer anomaly would be potentially very interesting, but that it is not likely to fly one day.

It could become more likely if it is send with other equipments, for instance for the study of solar wind effect, interplanetary magnetic fields, etc. The overal design of such a probe could allow to measure the "Pioneer effect" with much more accuracy, or at least to prove/disprove its existence.

How could such a probe work?
Basically a test mass, a raw piece of metal, should freely navigate into the solar system, on a trajectory fleeing the sun, while being protected of any spurious accelerations: solar wind, electric/magnetic effecs, outgassing, position control, etc.

To achieve this, it would be completelly enclosed into a metal casing, while having no physical contact/interaction with it. The casing would use thrusters to lock itself on a fixed position relative to the test mass.

So the overall thing navigates as if it was in really complete vacuum, without solar wind, outgassing, etc. and it can provide accurate informations of pure gravitationnal nature, eventually different of the 1/r2 law, or accounting for unknown bodies. The info on the trajectory corrections achieved by the casing would on its side provide very accurate data on solar wind. This makes this probe more interesting and more likely to fly than just a Pioneer anomaly test probe.

The only spurious gravitationnal effect on the test mass would be... the gravitationnal field of the probe itself. Thus the test mass should be placed right at the center of mass of the probe. An error on this would produce a permanent offset that we could not distinguish from true effects. If preleminary calculations show this is a problem, we can use a better overal design: the probe is formed of three parts, linked with cables: at the centre the protective casing, and at the extremities the radio transmitters, thrusters, and any other payload useful for science (and also useful to make this mission more likely). The whole thing rotates on an axis which is perpendicular to the sun direction, right around the test mass.
This design will allow to know preciselly the centre of mass and to adjust it. But above all, any permanent offset will be cancelled, as it will pull at times toward the Sun, at times opposite to the Sun. So we can really maintain the test mass free of spurious gravitationnal effects from the probe itself.

Such a probe would be relatively light wheight, so that it could be launched on an interstellar trajectory directly from the surface of the Earth, without using gravitationnal assistance. So it could bring results after only a few years. Otherwise we can use Jupiter' assistance.


Will this design lead one day to a real experiment?

A rather interesting prospective explanation of the Pioneer anomaly could come from a completelly different field, from the exploration of the cosmic background radiation at 3°K by COBE and WMAP.

These two probes found results which strinkingly match the predictions of the standard cosmologic inflation theory in the very early stages of the universe, excep for some points. Here is a paper on this:
Cosmic Symphony (I did not read it, I read the french publication in the science review "Pour la Science", french edition of the Scientific American).

Among the possible explanations on these discrepancies was evoked the possibility of a cloud of matter (dust or neutral gas) near the solar system or in orbit around it. Such a cloud would be essentially of a very low density, and thus very transparent at any wavelengh. It would emit/absorb only radiations matching its black body temperature, which is, for a free body in far space, at equilibrium with the cosmic background! This would explain that such a cloud was never detected before: only a precise measurement of the cosmic background could allow for its discovery.

If such a cloud exists, its very low density multiplied by its huge dimentions would lead to a sizeable mass, more than a planet, and even in the order of a star mass.

This would perfectly explain the Pioneer anomaly, and even the variations found on this effect (at times toward the Sun, at times opposite) according if the probe goes toward the cloud or in another direction.

Eventually a Pioneer effect probe becomes more interesting.

some people seem to agree and applied for funding from ESA's cosmic vision 2015-2025:

http://sci.esa.int/science-e/www/object/in...fobjectid=35202

Richard's proposed test of the Pioneer Anomaly is clever, and controls several variables. Unfortunately it also has the potential of introducing another new one: Shielding the test mass from the 'unknown force', especially if it is electromagnetic in nature: In this case if the results were nil, the Pioneer Anomaly could be constrained to a possible emf - but what emf?

The cloud solution is also interesting, but the two Pioneer probes were heading for opposite sides of the solar system and produced the same relative error. (Two identical clouds?) Also, Nieto and other researchers have been able to all but eliminate the Oort cloud and Kuiper belt objects as likely candidates for the PA, and this evaluation seems to apply to rouge clouds as well. (They also constrain the potential for local Dark Matter & Energy.

http://lanl.arxiv.org/abs/astro-ph/0506281

Antoniseb has started a thread to discuss second WMAP release, specifically in the context of the 'local' anomalies, and I will post a response there. As you know from my postings on this thread, I am of the opinion many quirky observations are interelated.

http://www.unmannedspaceflight.com/index.php?showforum=44

There is much to learn.

Messenger,

my idea was intended to detect a gravitationnal effect (or gravitationnal-like). We could modify the shielding, for instance using a transparent shield to allow for electromagnetic effects. But in this case, we shall not know if the effect is gravitationnal or electromagnetic...
Or we can make several experiments, several test mass... but the probe has only one mass center. I do not come with more "clever" designs, unless to use several concentric shields around the test mass: one metallic, shielding the test mass from EM effects, and locked on the test mass without physical contact. And around, a second shielding, avoiding solar wind but transparent to electromagnetic effects, and locked on the first shield. This may allow for a separate measurement of both three effects, gravitationnal, EM and wind effect. But this fairly complicates the design, and does not make sure that the result would be better. Especially the intermediary shield will need some thrust, and thus it will "pollute" the results of the outer shield.

So I think it will be better to keep with my first simple design, perhaps adding it sensitive electromagnetic instruments. If gravitationnal effects are ruled out, we can still check for electromagnetic effects, but they offer much less potential for interesting discoveries. (but they cost much less to search)

Thank you also Messenger for your contribution on the Oort clouds and similar. We know litle things about the Kuyper belt and Oort cloud. We usually assume they have a syymmetrical structure (a disk for the Kuyper belt, a sphere for the Oort cloud). At least they would have gained such a symmetry after rotating around the Sun for 4 billion years. If they have such symmetrical structures, they cannot have gravitationnal effects in the inner solar sytem (a hollow spherical structure has no gravitationnal effect inside).

But many things may cause such clouds to have transcient or permanent "lumps" in it:
-part of the mass is in the form of massive bodies, such as the one recently detected which is larger than Pluto. (Many trajectory calculations should be remade accounting with it). It could even exist very dark and cold massive gaseous planets very far from the Sun.
-the solar wind shockwave with the interstellar wind concentrates mass
-a spherical cloud is in orbit around the Sun
-an interstellar cloud is currently at close vicinnity with the solar system (such clouds can have mass ranging from a giant star mass to Earth mass)
-a large wandering planet is currently at close vicinnity with the solar system


A last there is one difficulty with my probe: it will only tell us what there is in one direction. If we obtain enough precision, we could try to model the effect with an 1/R2 gravitationnal field, and find the culprit. But this will take ten years, and if the probe flight path is not in a proper direction, it will not work. So ideally we should send three probes in three perpendicular directions to definitively find (or rule out) any gravitationnal effects. In practice we could send one, wait two or three years to see the results, and take the decision to send another one in a proper direction.

Interesting idea, and pretty much identical to the concept for the LISA gravitational wave mission, which would reduce the costs of such a mission considerably if the systems could be reused.

Of cource that would rely on LISA ever geiing off the ground, I wrote a undergraduate report about LISA and that was a long time ago! (~9 years) I can't remember what the planned launch date was back then but I don't think it was more than ten years, today, still ten years (2015)!

James

The LISA observatory project will use three test masses, each with its own shield, to detect gravitationnal waves.

The purpose of the LISA and of a Pioneer effect probe is different: the LISA will have to detect much weaker effects, using laser interferometry, while a Pioneer effect probe can be accurate enough with radio location. The LISA would not be bothered with a permament offset in the results, as it is meant to detect only transcient or periodic effects. But such a permanent offset would mess up a Pionner effect probe, as it studies signals which are constant over years.

For this reason I proposed my rotative shield design to cancel any permanent offset. This cannot be used by LISA. So they are developping a very accurate tracking system: 10 nanometres! Figure that this is better than welding the shield to the test mass with steel rods. Eventually this technology may be better than my rotating design to cancel even permanent offsets. So re-using the LISA technology may save most of the development costs for a Pioneer effect probe.

All of these approaches use assumptions we should not be making. If there is a variation in the permeabiliy of space to mass necessary to explain the phenomena I outlined above, there is almost certainly a corresponding gradient in the speed of light.

This can be demonstrated with the Galileo paradox:

Put Galileo on the Dark side of Mercury, rolling his balls to measure the gravitational constant. Because of the nearby mass of the sun, the balls will roll slower, which would cause Galileo, not knowing about GR, to underestimate the G constant. EXCEPT Galileo's clock is also ticking slower, so the value would be very close to correct.

Watching the experiment from an Earth frame of reference, the balls would appear to roll slower, and a GR space curvature correction must be used to explain the phenomena.

Three things: 1) The same observations can be interpreted as a time or space dilation, depending upon the frame of reference. 2) Performing the experiment on Mercury, Galileo could be completely oblivious to the need for GR to explain the results from Earth. 3) It is relatively simple to transfer both frames of reference to a single coordinate system where the absolute pathlength through a given volume of space varies as a function of mass.

Magueijo eluded to this transformation in Faster Than the Speed of Light, where he found it difficult to prove his theory required a new physical concept; and not just a transformation of GR into a completely compatible coordinate system, where pathlengths and the speed of light vary, not time and space curvature. (I am of the opinion that this mathematic transformation provides a better conceptual bases for GR phenomenon.)

So any attempts to measure unknown or poorly characterized forces must also address an untested assumption in measurement theory: the Speed of light is an absolute constant that is not mass dependent; or more exactly: Current GR perameters correctly compensate for mass-dependant effects upon light.

Again, existing solar probes have tightly constrained any deviations from established GR constraints: Bertotti has used Cassini to constrain unexpected GR variance to a factor of 2.3x10^-5 near the Earth's orbit. Perhaps the best solar constraint on the speed of light is the Pioneer Anomally itself - 8x10^-9m/s^2, but this is only beyond the obital distance of Saturn. (Notice that since we use the two-way speed of light to determine the position of the Pioneer probes, the acceleration of the probes could be away from, rather than towards the sun, as long as it is of the same magnitude as any change in the speed of light.)

Bertotti, B., Iess, L., Tortora, P., “A test of general relativity using radio links with the Cassini spacecraft,” Nature 425, 374-376 (2003).

http://lanl.arxiv.org/PS_cache/gr-qc/pdf/0411/0411082.pdf

Again, much of the science needed to nail down these possible discrepancies can be extracted from the current generation of probes, but only if the experimentors are aware of the unbridled parameters and the need for additional onstraints.

Edited to add:

One more question about LISA - unless and until the current LIGO generation of gravity antenna detect ANY gravitational phenomena, should we be vesting in another experiment? IAOTO the waves do exist, but we may be searching with the wrong kind of antenna.

Heading off topic but...



Well LISA will be serching in a completely different frequency band. A band which should include waves from binary neutron stars which pretty much must exists given current observations (and at a known amplitude), unlike LIGO which can only detect much more exotic and theoretical objects and mergers. So yes I do think it's worth investing in, even given the non-detections at LIGO.

Damn! I'll say we need LISA, yesterday, not too many years from now. Any chance of bumping LISA ahead of PLANCK? The CMB has a local contamination issue that needs to be resolved to reathenticate, if possible, the accuracy of the WMAP results.

But A drag-free triangulated laser ranged probe orbiting the Sun will also provide constraints upon Pioneer-like acceleration anomalies if they effect lasar ranging.

Thanks jamescanvin for the image and the info it contains.

What would be fine is if somebody have the log-log plot, amplitude versus frequency, with expected domain for each gravitationnal wave source, and the expected sensitivity of each instrument.

What I heard (to check) is that the LIGO gravitational wave observatory is curently reaching its full sensitivity, but it still detected nothing (the only thing it could detect, neutron stars spiraling, would happen only once a year in average).

Yes, that was what I was looking for yesterday, but couldn't while rushing round.
Couldn't have been looking very hard as a quick search this morning and, ta-dar!



I think that curve is for Advanced LIGO , Standard LIGO is about one order of magintude less sensitive.



Yes, I don't think we need to start rethinking gravitational wave theory just yet, it's not too surprising that nothing has been detected by LIGO so far. Lets wait for Advanced LIGO first (2009)

James

Thanks very much James it is exactly what I wished!!! This graphics tells us exactly what we can expect or not!!






Yes, no need yet to rethink the gravitationnal wave theory, as LIGO today is only able to detect rare events, mainly neutron stars and black hole coalescence, and only the stronger.

Black holes coalescence is, I think, something very well understood (in the context of General realtivity. But even without relativity we can expect that star-sized masses spiraling a high speed will produce strong gravitationnal effects.). Neutron star coalescence and super nova core collapse are slightly less understood (especially SN core coalescence may be highly disordered and unsymmetrical) but the theory is still reliable. So it is expectable that we detect some events before 2009, and only some years after this date, if we detect nothing, the gravitationnal wave theory is at risk.

Once again, I'm asking a question that I probably ought to just Google up for myself, but it does go along with the thread...

One of the experiments in the Apollo 17 ALSEP was the Lunar Surface Gravimeter. As I recall, it was designed to detect gravity waves. (It failed because it was balanced in 1G and was entirely out of balance, and hence useless, in 1/6G.)

Does anyone know what types of waves the LSG was designed to detect? Would it have been more in the LISA range or the LIGO range?

I guess I'm wondering what kinds of things we might have been gathering data on for more than 30 years if the instrument had just been designed properly...

-the other Doug

I'd have to check, but I think the Apollo 17 instrument's name included the term "Tidal". They were looking at freequencies below those the seismometer would detect, at least in part... looking for whole-moon "ringing" frequencies, like the ringing of the whole earth after a Richter 8+ quake.

The instrument failed because it was balanced in 1 G with the aid of a loading mass or spring which was unloaded on the moon. The problem was an arithmetic booboo in the calculation of the design for lunar gravity.. the instrument had a "bias" range that was adjustable for a range of lunar gravities, or really more accurately, for a range of instrument sensitivities... The adjustable range of the instrument was such that actual lunar gravity (very well known) was outside the adjustment range. This is similar to the focus failure on Deep Impact's hi rez camera.. The camera was "focussed" from pre-launch out of focus conditions by heating the carbon-composite truss to drive out water vapor in vaccuum, intending to slow down and stop when the instrument approached and achieved perfect focus..... it never got there due to a ground base calibration problem. The actual focus point was outside of the adjustment range.

Oooh, dear! Don't let a certain scientist-astronaut know, this may have wasted some of that precious EVA time during which rocks could have been examined. I can just imagine a series of unexplained murders, with the victims attended to with a balance spring tied between a gnomon and a lunar rake...

I think that, even if such an instrument was properly designed, it had far from enough sensitivity to detect expected gravitationnal waves. Gravimeters are very sensitive indeed, they can detect such "low" masses as mountains, and even less (remember the historical Cavendish experiment which measured the effect of a some kilograms mass). But this is very far from enough to detect gravitationnal waves, which are many orders of magnitude under this level of sensitivity. Otherwise it would not be necessary to build such complicated experiments as LIGO, it would be enough to send a gravimeter in the ISS.

Perhaps the most powerfull recent gravitationnal event was the supernova in 1978, but who knows what happens in the gravitationnal field.

When the gravimeter was proposed, selected and designed, the PI (Weber, I think) was claiming or about to claim possible detections of grativational waves with ultrasensative suspended cylinders. The consensus then and now was that plausible sources of waves in the frequency range that the sensors could detect were many orders of magnitude too small to detect (at least at distances out where there was any chance of an event occurring). But theory in 1970 was much less precise than now, too. By observing natural tidal oscillations of the moon and very low frequency seismic signals, they hoped to have a valuable experimet regardless of whether the "blue sky" gravity wave search was a bust or not.

Yes I remember that there was hopes to find gravitationnal waves with large aluminium cylinders which may resonate and amplify the wave signal. The first experiments to find gravitationnal waves started with such detectors, but they never produced anything. But the search was open...

Oh, that particular scientist-astronaut was well aware of the problem -- for one thing, when the PI found his instrument wouldn't uncage, he *insisted* that this particular scientist-astronaut must have deployed it improperly, must not have leveled it right. So Houston told him to go back and re-level the experiment -- three times. When told it would not uncage, Schmitt even kicked it, hard, and then re-leveled it again. It still did not uncage.

Schmitt was, indeed, *quite* angry that such a screw-up had cost precious lunar surface EVA time.

From what I understand, though, even with the main beam caged, the LSG actexd as a fair one-axis seismometer...

-the other Doug

Not in the experiment title, no -- Apollo 17 carried two gravimeters, the Lunar Surface Gravimeter (LSG) and the Lunar Portable Gravimeter (LPG). The tidal reference may have been in the detailed description of the LSG, but it was not part of the instrument's name.

The LPG was the same type of instrument used by oil companies to find salt domes underneath otherwise flat land -- oil and gas are often entrained in salt domes. It detected negative anomalies on the slopes of the massifs and positive anomalies on the valley floor, indicating just how much more massive the basaltic valley fill is when compared to the massifs. IIRC, the anomalies were on the order of 10 to 30 milligals.

So, the LSG was an ultra-sensitive seismometer that hoped to use the entire mass of the Moon to detect gravity waves? Interesting... even if we now think that gravity waves would have been undetectable with such an instrument.

-the other Doug

other Doug:

Was the PI's body ever found?

Bob Shaw

"From what I understand, though, even with the main beam caged, the LSG actexd as a fair one-axis seismometer...
-the other Doug"

As far as I know there were no science results whatever published from the instrument in that mode. I think the sensativity was far too low for any signal other than astronauts stomping by.

The failure to observe any evidence of SN1987A by any gravity wave detectors is not a good omen. True, the GW spectrum of a supernova is up-in-the-air, but an explosion of that magnitude, that close, should have created enough broad spectrum transients we should have found something, especially since the timing of the event is well known.

I wouldn't pin my life's savings on Advanced LIGO - which seems to be progressing slightly ahead of schedule. Every generation of gravity wave detectors from Weber's work in the '70's on, have been built with the expectation that a GW event was just one pixal below the horizon.

Advanced LIGO:
http://www.ligo.caltech.edu/docs/G/G050364-00.pdf


http://arxiv.org/PS_cache/astro-ph/pdf/0507/0507254.pdf

FWI worth, there is still a great deal of contraversy in the supernova community about just what the gamma rays, and expansion RINGs associated with 1987A mean. John Middleditch amoung others, is convinced both the rings and rays reveal a binary event, and he argues most supernovae involve binary systems. This grates against SN Ia theory, but his arguments, (including the 'double humped' light curves observed in many SN Ia spectra.) are strong.

http://arxiv.org/abs/astro-ph/0310671

http://arxiv.org/abs/astro-ph/0311484

Very interesting article, although arduous to read. To summarize, the purpose of LIGO is to detect the cosmic background gravitationnal noise caused by very early cosmological events, far before the electromagnetic background. Today LIGO has not yet achieved this goal, but it made only short runs of data sampling which were rather aimed at improving sensitivity and eliminating instrument noise. With a long run at expected maximum sensitivity they expect to detect the level of gravitationnal waves predicted by the most recent theories of inflation. If they really achieve this design sensitivity and still not fing a gravitationnal background noise, the theories of inflation are at risk. Still only some years of work to let us know...





This is often like this in difficult scientifical achievements. Look for instance at the tokamac, the quantum computer, superconduction at ambient temperature... This is also due to the fact that the first researchers were really optimistic. Today evaluations of gravitationnal waves are, alas, much more pessimistic, and if they were know in 1970 Weber would not have started his experiment. Weber simply did what was best possible to do at his epoch, knowing what we knew.




Please remember that the curious set of three non-coplanar rings around SN1987A were already here before the explosion. They were discovered after, with close examination of the place, but such rings are more likely of the planetary nebula family. It was said at this epoch that there will be new hubbub here when the expanding fireball would reach the first ring, 20 years later (2007). Also we are still to detect the predicted blinking of the central object indicating the presence of a pulsar.

It has been hoped that cosmic sources of gravitational radiation (as opposed to supernovae and massive object collissions) are stronger than predicted, or that there's unpredicted sources that have relatively frequent events strong enough for the first generation LIGO detection systems, but basically nobody expects it. Detectors have gotten much better than Weber's original cylinders, but my impression is that expectations of predicted source strength-frequency combinations have been such that no detector so far has been expected to detect anything by the general gravitational wave community. LIGI, I believe, works at much too high a frequency for detecting big bang related radiation and the like.

As far as SN1987A goes, we're detecting the blast wave interacting with the inner edge of a lumpy ring which is progressively lighting up as the blast expands. There is still no trace of either a black hole or neutron star in the supernova remenant inside the ring, or of energy being emitted from one. Some models in the past have suggested that in some cases there may be nothing left, but those I think are in disrepute, so the non-observation of a massive object is "A Puzzlement"