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iantresman
2006-Sep-12, 11:47 AM
Has any work been done on confirming redshift distances using parallel and taking satellite telescope measurements on opposite sides of the Earth's orbit (or other planet)?

Regards,
Ian Tresman

antoniseb
2006-Sep-12, 12:14 PM
Has any work been done on confirming redshift distances using parallel and taking satellite telescope measurements on opposite sides of the Earth's orbit (or other planet)?

Sometimes I am surprised by what you don't know. You're a pretty clever guy Ian, but in this case, the best measurements we've made using parallax from satellite telescope measurements so far was done with the ESA Hipparcos probe, and only gets good direct parallax measurements out to one or two hundred light years. The Gaia probe should be able to get 100 times better than that in ten years or so.

If we could get a Gaia-like mission sent on a very rapid trajectory out of the Solar System, with enough power to communicate high data rates to us over that distance, we could get direct parallax measurements to AGNs by the time the craft was a significant fraction of a light year distance from us.

iantresman
2006-Sep-12, 04:41 PM
Many thanks, easy when you know, and summarised on Wikipedia here (http://en.wikipedia.org/wiki/Gaia_probe)

Regards,
Ian Tresman

antoniseb
2006-Sep-12, 06:07 PM
Glad to help. The ESA site has even more detail information. BTW, the Wiki article suggests that Gaia will measure distance to quasars, but all it will do is measure their varying luminosity.

iantresman
2006-Sep-12, 11:06 PM
Glad to help. The ESA site has even more detail information. BTW, the Wiki article suggests that Gaia will measure distance to quasars, but all it will do is measure their varying luminosity.

Unless, of course, they are closer than expected.

Regards,
Ian Tresman

antoniseb
2006-Sep-12, 11:17 PM
Unless, of course, they are closer than expected.
Well, yes, but it would be surprising if any quasars are less than 10,000 light years from here.

RussT
2006-Sep-12, 11:28 PM
[Sometimes I am surprised by what you don't know.]

Which is why I many times hesitate to post anything like this in Q&A!

Just because we are uncertain of specifics in any particular realm does not means that anything else we are saying is lacking in understanding or potential applicability!

[(or other planet)?]

But this did bring to mind something I have never seen mentioned!

What if...we put a Chandra or Hete, or Hubble (or all three) in orbit around MARS, and instead of pointing it at the planet, pointed it out in space?

What would we be able to determine from this different perspective?
Would we be able to synch then to earth orbital satellites?
Could we use the much farther apart satellites to do more robust parallax measurements.
Could redshifts be more accurately determined?
ETC< ETC???

RussT
2006-Sep-12, 11:30 PM
Well, yes, but it would be surprising if any quasars are less than 10,000 light years from here.

I dion't think he is suggesting that they are in our galaxy, as once thought.

antoniseb
2006-Sep-13, 12:29 AM
Could we use the much farther apart satellites to do more robust parallax measurements.
Could redshifts be more accurately determined?

If we put Hipparcos around Mars, we'd have parallax to stars about 40% further away than we do now. If we put it around Jupiter, it would be five times further, but the data rate is probably higher than anything we could have managed from Jupiter when that probe was constructed, not to mention the several extra years of getting it there, and the 25 times larger solar panels required.

None of these would have helped determine the Red Shift of any object with any greater precision.

iantresman
2006-Sep-13, 02:08 PM
What if...we put a Chandra or Hete, or Hubble (or all three) in orbit around MARS, and instead of pointing it at the planet, pointed it out in space?

What would we be able to determine from this different perspective?

I was suggesting that we take imagery from opposite sides of the planet's orbit, providing the opportunity to triangulate with a much wider "base". Could we synchronise? That's the $64,00 question.

Regards,
Ian Tresman

antoniseb
2006-Sep-13, 02:27 PM
I was suggesting that we take imagery from opposite sides of the planet's orbit, providing the opportunity to triangulate with a much wider "base".

That is pretty much what Hipparcos did. The diameter of the Earth's orbit is the baseline for all interstellar parallax measurements we've made so far. So far as I know, diurnal parallax has only been used to measure the distance to the Moon, a few passing asteroids, and in failed attempts to find the distances to comets and the Sun.

RussT
2006-Sep-13, 09:30 PM
I saw a science channel program (Tuesdays are on the universe) that showed that they do the parallax measurements 6 months apart so that the earth is on opposite sides of the sun for as wide a distance as possible.

But somehow, I still think that numerous things could be learned from a multiple instrument satellite in orbit around Mars, pointing skyward!

And with Mars having less gravity, taking readings of numerous different kinds from the line of sight at the horizon of Mars could tell us some intersting things as well.

We have been able to get tons of info from the satellite(s) the Martians probes on the surface are reporting to, so there should be no problems with any of this.

Nereid
2006-Sep-14, 12:00 AM
I saw a science channel program (Tuesdays are on the universe) that showed that they do the parallax measurements 6 months apart so that the earth is on opposite sides of the sun for as wide a distance as possible.

But somehow, I still think that numerous things could be learned from a multiple instrument satellite in orbit around Mars, pointing skyward!

And with Mars having less gravity, taking readings of numerous different kinds from the line of sight at the horizon of Mars could tell us some intersting things as well.

We have been able to get tons of info from the satellite(s) the Martians probes on the surface are reporting to, so there should be no problems with any of this.Well, it'd be pretty expensive, and I'm not sure any such mission would produce any significantly new data, over what GAIA will give us.

Did you have anything in particular in mind?

Re parallax: the design of HIPPARCOS was quite ingenious - the way it collects data that can be reduced to give good estimates of both parallax and proper motion, from such a small platform, was good to start with. Then with it failing to reach its planned orbit, the way that the mission team recovered, and met essentially all of the original design goals was simply wonderful!

Even more noteworthy is that the raw data can now be re-analysed, using the enormous increase in computing power between then and now, to tease out quite tiny perturbations (e.g. hits by micrometeorites), and correct for them ... yielding a significant improvement in consistency and accuracy of the final data.

To me, this brings home the importance of good experimental/observational design ... there's a lot we can do, right 'here on Earth', for relatively modest investments, to advance the accuracy and sensitivity of observations to levels that surpass the wildest dreams of astronomers only a century or so ago.

RussT
2006-Sep-14, 12:36 AM
Well, it'd be pretty expensive, and I'm not sure any such mission would produce any significantly new data, over what GAIA will give us.

Did you have anything in particular in mind?

Re parallax: the design of HIPPARCOS was quite ingenious - the way it collects data that can be reduced to give good estimates of both parallax and proper motion, from such a small platform, was good to start with. Then with it failing to reach its planned orbit, the way that the mission team recovered, and met essentially all of the original design goals was simply wonderful!

Even more noteworthy is that the raw data can now be re-analysed, using the enormous increase in computing power between then and now, to tease out quite tiny perturbations (e.g. hits by micrometeorites), and correct for them ... yielding a significant improvement in consistency and accuracy of the final data.

To me, this brings home the importance of good experimental/observational design ... there's a lot we can do, right 'here on Earth', for relatively modest investments, to advance the accuracy and sensitivity of observations to levels that surpass the wildest dreams of astronomers only a century or so ago.

I would have to think, that all of the things we have accomplished in the last 20 years FAR surpasses our wildest expectations of 50 years ago and certainly of 75 or 100 years ago!

I just can't help but thinking that any view by whatever intrumentation from a different perspective 'could' be tremdously useful.

Can we turn any of the current orbitors of Mars around to face space?

Nereid
2006-Sep-14, 12:52 AM
[snip]

Can we turn any of the current orbitors of Mars around to face space?Sure ... just as Cassini, Galileo, the Voyagers, the Pioneers, ... all could, and did, image/detect stars. But to what end would you want them to 'face space'? Their cameras'/imagers' stability, angular resolution, PSF, flatness of image geometry, ... all are far inferior to the capabilities of the HST (or even, perhaps, Gallex, MOST, Spitzer, ...).

RussT
2006-Sep-14, 01:00 AM
Sure ... just as Cassini, Galileo, the Voyagers, the Pioneers, ... all could, and did, image/detect stars. But to what end would you want them to 'face space'? Their cameras'/imagers' stability, angular resolution, PSF, flatness of image geometry, ... all are far inferior to the capabilities of the HST (or even, perhaps, Gallex, MOST, Spitzer, ...).

I see, and obviously didn't have any idea this was the case!

Tim Thompson
2006-Sep-14, 01:32 AM
I am quite confident that it will never be possible to confirm cosmological distances using parallax in the solar system. The baselines available are just too small. There was a mission plan, many years ago, called the Thousand Astronomical Unit (TAU) (http://www.daviddarling.info/encyclopedia/T/TAU.html) mission. It was a brainchiled of Aden & Marjorie Meinel, when they were at JPL. That was back in the late 80's, and would have launched ~2005, but the mission was never approved, probably because it would have taken 50 years to reach its target distance. In any case, that baseline would have permitted parallax measurements of the distance to just about any star we could see, anywhere in the Milky Way. But it still falls far short of the baseline distance we would need to do parallax for nearby galaxies, let alone truly distant ones.

So one needs some other method to calibrate the extragalactic distance scale. Mostly standard candels are used, such as cepheid or RR-Lyra variables, and type-Ia supernovae. The whole point of the Hubble Key Project (http://www.ipac.caltech.edu/H0kp/) was to use cepheids to calibrate the distance scale as far as the HST could see them.

But there are ways to get geometric distances, with some help from the source. Herrnstein, et al., 1999 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999Natur.400..539H&amp;db_key=AST&amp;d ata_type=HTML&amp;format=&amp;high=4366fa465118278) used rotating water masers in NGC4258 (M106 (http://www.seds.org/messier/m/m106.html)) to measure the distance at 7.2 +/- 0.3 Mpc. That's about 23,000,000 light years, which is not as far as the HST Key Project can see, but it's a good start. There are other known maser host galaxies, but that's the only one I know with a geometric distance determination. And that's still a far cry from the galaxies at cosmological distances, like maybe 1,000,000,000 light years or more.

Nereid
2006-Sep-14, 01:35 AM
I see, and obviously didn't have any idea this was the case!A crude way to see this, qualitatively, is to ask how close your favourite space probe needs to be, to Mars/Jupiter/Saturn/Venus/whatever before the images its best camera can take become better than the best the HST can do.

It's a pretty eye-opening exercise (if you didn't already know).

RussT
2006-Sep-14, 07:11 AM
Thanks Guys and Gals, very informative

iantresman
2006-Sep-14, 10:15 AM
I am quite confident that it will never be possible to confirm cosmological distances using parallax in the solar system. The baselines available are just too small.

Would the rotation and/or movement of the Milky Way over time provide a sufficent baseline? I don't know whether it can be measured absolutely, but Wikipedia suggests that our Galaxy is moving at around 600km per second or 19 billion km per year (ie. 4.5 times the distance that Pluto lies from the Earth). Of course it would depend on how other galaxies are moving, but they can't always be moving in the same relative direction?

Regards,
Ian Tresman

Nereid
2006-Sep-14, 11:46 AM
As Tim Thompson has already pointed out, the likelihood of us measuring parallax distances on cosmological scales is slim indeed, in the lifetime of anyone alive today (though it might be fun to sketch, on the back of a small envelope, just what kinds of missions might do the trick).

There are, as Tim also pointed out, other ways to measure distance, than standard candles. One geometric method he didn't mention is gravitational lensing, especially of quasars, where you can use their variability. Microlensing may be a technique that could be used too, once telescopes like the ELT and OWL are built (though I can't imagine any telescope allocation committee approving a proposal to tie up the thousands of hours of dark time that may be necessary to get a decent result).

And then there's proper motion - could we extend the (water) maser VLBI technique, to quasar or GRB jets, for example?

antoniseb
2006-Sep-14, 11:55 AM
Would the rotation and/or movement of the Milky Way over time provide a sufficent baseline?

One difficulty with this method is that you cannot tell what apparent motion of an object is because of parallax, and what is because of the objects proper motion. We are not making a back and forth measurement unless you want to wait 100 million years or more. OK, we could do it in 25 million years using the up and down oscillations throught the plane of the galaxy. That is still uite a long time for an observing project.

Spaceman Spiff
2006-Sep-14, 03:06 PM
As Tim Thompson has already pointed out, the likelihood of us measuring parallax distances on cosmological scales is slim indeed, in the lifetime of anyone alive today (though it might be fun to sketch, on the back of a small envelope, just what kinds of missions might do the trick).

There are, as Tim also pointed out, other ways to measure distance, than standard candles. One geometric method he didn't mention is gravitational lensing, especially of quasars, where you can use their variability. Microlensing may be a technique that could be used too, once telescopes like the ELT and OWL are built (though I can't imagine any telescope allocation committee approving a proposal to tie up the thousands of hours of dark time that may be necessary to get a decent result).

And then there's proper motion - could we extend the (water) maser VLBI technique, to quasar or GRB jets, for example?

And here (http://arxiv.org/abs/astro-ph?papernum=0303445) is yet another potential means.

Nereid
2006-Sep-14, 09:12 PM
And here (http://arxiv.org/abs/astro-ph?papernum=0303445) is yet another potential means.Very cool! :clap: