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What's slowing down the two Voyagers?



 
 
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  #81  
Old July 8th 04, 01:55 PM
Sander Vesik
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In sci.space.policy Keith F. Lynch wrote:
Henry Spencer wrote:
Jonathan Silverlight wrote:
The discussion did veer that way (thanks to yours truly) but the
Voyagers aren't showing anomalous acceleration, AFAIK.


Rather, it's not practical to measure any. The Voyagers hold
attitude with thruster firings, and it's impractically difficult to
model those precisely enough to establish whether the Voyagers *are*
experiencing an anomalous acceleration similar to what was measured
with the Pioneers.


How hard would it be to hold attitude some other way? It's not like
the Voyagers have much else to do at this point. Might as well use
them to try to detect the anomalous acceleration.


As I understand it, it would be impossible.


I suspect it's something unique to the Pioneers. Maybe the helium
from the RTG decay preferentially leaked out in one direction?


--
Sander

+++ Out of cheese error +++
  #82  
Old July 8th 04, 11:56 PM
Tim Auton
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"Keith F. Lynch" wrote:
[unmodellled acceleration in the Pioneers]
I suspect it's something unique to the Pioneers. Maybe the helium
from the RTG decay preferentially leaked out in one direction?


Perhaps, but to convince the scientific community you'd have to come
up with a good model for it. There are plenty of papers which say it
*could* be this, that or the other.


Tim
--
My last .sig was rubbish too.
  #83  
Old July 9th 04, 05:36 AM
Henry Spencer
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In article ,
Keith F. Lynch wrote:
Rather, it's not practical to measure any. The Voyagers hold
attitude with thruster firings, and it's impractically difficult to
model those precisely enough...


How hard would it be to hold attitude some other way? It's not like
the Voyagers have much else to do at this point. Might as well use
them to try to detect the anomalous acceleration.


There really isn't much that can be done about it on the Voyagers.
They aren't equipped with reaction wheels, and aren't designed or
balanced for a stable spin.

I suspect it's something unique to the Pioneers. Maybe the helium
from the RTG decay preferentially leaked out in one direction?


If (dim) memory serves, that was actually considered, but there wasn't
enough thrust to be had that way. The one real uncertainty is how well
the photon thrust from the RTGs (and the reflection of same from the
spacecraft) has been modeled; there are ongoing arguments about that.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |
  #84  
Old July 10th 04, 02:45 AM
dave schneider
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(Henry Spencer) wrote in message ...
In article ,
Benign Vanilla wrote:
Even the biggest dish antennas are nearing the limits of picking up
Voyager's weak and distant signal...


I wonder why we would not launch some intermediate satellites to act as
repeaters. Seems they could be light, fast and cheap and extend the life of
these other missions.
What are the complexities?


Mostly, that the idea doesn't work very well. The problem is that a relay
satellite halfway to (say) Voyager 1 will be hearing a signal only four
times as strong as what Earth is hearing... and the antennas and receivers
on Earth are much more than four times as good as the ones on a cheap,
lightweight relay satellite.


At what point would it be worth it to add a long leg to the net of
earth-based dishes by placing a dish at, say, a Lagrangre point?

I don't think this would pay much in terms of making the effective
antenna any larger than building another eb dish, and synchronizing
things would probably be a bit fussy.

My take is that the advantage would be that this antenna would be
available for long stretches because diurnal pointing issues wouldn't
be involved. There would be occultations occuring slightly later or
slightly before the eb antennas experience them.

In fact, this sort of application would probably only happen if there
was another reason to populate that position and the dish could be
piggybacked on that for relatively low cost.

/dps
  #85  
Old July 10th 04, 08:00 AM
Mike Williams
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Wasn't it dave schneider who wrote:
(Henry Spencer) wrote in message news:Hzrzu9.318@spsystems.
net...
In article ,
Benign Vanilla wrote:
Even the biggest dish antennas are nearing the limits of picking up
Voyager's weak and distant signal...

I wonder why we would not launch some intermediate satellites to act as
repeaters. Seems they could be light, fast and cheap and extend the life of
these other missions.
What are the complexities?


Mostly, that the idea doesn't work very well. The problem is that a relay
satellite halfway to (say) Voyager 1 will be hearing a signal only four
times as strong as what Earth is hearing... and the antennas and receivers
on Earth are much more than four times as good as the ones on a cheap,
lightweight relay satellite.


At what point would it be worth it to add a long leg to the net of
earth-based dishes by placing a dish at, say, a Lagrangre point?

I don't think this would pay much in terms of making the effective
antenna any larger than building another eb dish, and synchronizing
things would probably be a bit fussy.

My take is that the advantage would be that this antenna would be
available for long stretches because diurnal pointing issues wouldn't
be involved. There would be occultations occuring slightly later or
slightly before the eb antennas experience them.

In fact, this sort of application would probably only happen if there
was another reason to populate that position and the dish could be
piggybacked on that for relatively low cost.


Well, yes. A modest sized dish at a Lagrange Point would be wonderful
for radio astronomy, because the resolution increases with the size of
the baseline. At present the synthetic apertures we can construct are
limited to the size of the Earth. Placing an antenna at a Lagrange point
increases the baseline by a factor of 30.

Synchronising the feed would be no harder that it is for long baseline
arrays on Earth. It might even be slightly easier because the linking
signals may be able to travel in a straight line rather than hopping
round the Earth.

It's not much use for tracking space probes, since you don't need high
resolution for that, you just need high gain. Gain is proportional to
the total surface area of the linked antennae. So you're much better off
building big dishes on the ground than modest sized antennae in space.

Having a continuous feed from a space probe might be a slight advantage,
but we've developed techniques, such as on-board data storage, to cope
with that. If you're piggybacking on a project that doesn't want to be
pointed at your probe all the time, then you lose the continuity anyway.


--
Mike Williams
Gentleman of Leisure
  #86  
Old July 10th 04, 03:49 PM
Henry Spencer
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In article ,
dave schneider wrote:
At what point would it be worth it to add a long leg to the net of
earth-based dishes by placing a dish at, say, a [Lagrange] point?
I don't think this would pay much in terms of making the effective
antenna any larger than building another eb dish...


Not at all, for spacecraft communications. For that sort of work, you
need a big dish not for angular resolution, but for collecting area.
Using multiple antennas far apart improves angular resolution, but does
nothing much for collecting area.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |
  #87  
Old July 12th 04, 06:38 PM
Jonathan Silverlight
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In message , Mike Williams
writes
Wasn't it dave schneider who wrote:
(Henry Spencer) wrote in message
news:Hzrzu9.318@spsystems.
net...
In article ,
Benign Vanilla wrote:
Even the biggest dish antennas are nearing the limits of picking up
Voyager's weak and distant signal...

I wonder why we would not launch some intermediate satellites to act as
repeaters. Seems they could be light, fast and cheap and extend the life of
these other missions.
What are the complexities?

Mostly, that the idea doesn't work very well. The problem is that a relay
satellite halfway to (say) Voyager 1 will be hearing a signal only four
times as strong as what Earth is hearing... and the antennas and receivers
on Earth are much more than four times as good as the ones on a cheap,
lightweight relay satellite.


At what point would it be worth it to add a long leg to the net of
earth-based dishes by placing a dish at, say, a Lagrangre point?

I don't think this would pay much in terms of making the effective
antenna any larger than building another eb dish, and synchronizing
things would probably be a bit fussy.

My take is that the advantage would be that this antenna would be
available for long stretches because diurnal pointing issues wouldn't
be involved. There would be occultations occuring slightly later or
slightly before the eb antennas experience them.

In fact, this sort of application would probably only happen if there
was another reason to populate that position and the dish could be
piggybacked on that for relatively low cost.


Well, yes. A modest sized dish at a Lagrange Point would be wonderful
for radio astronomy, because the resolution increases with the size of
the baseline. At present the synthetic apertures we can construct are
limited to the size of the Earth. Placing an antenna at a Lagrange point
increases the baseline by a factor of 30.


Presumably we're talking about the Earth-Moon Lagrange point. I know
dishes have been placed in Earth orbit for VLBI, but aren't you going to
have problems with the fact that your Lagrange dish is in some horrible
looping orbit around its nominal location? The O'Neill colony fans have
done work on this.
 




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