On Sat, 28 Aug 2004 07:01:38 GMT, Painius wrote:
Thanks, Ugo... i was aware that it had been proposed, but i did
not know that it had been dismissed. And how could it be so
readily dismissed? We still have no idea how many TNOs are
out there, KBOs and such. And astronomers are proposing that
there are quite a few.

Argh... This is the 3rd time I'm writing this reply. The first 2 times my
machine reset by an unlucky set of circumstances...

*groans*

Anyway, space beyond Neptune is one *very* big expanse and if you combined
the masses of all the TNOs, it still probably wouldn't be that large of a
mass. So their effects are negligible, especially if you consider all those
TNOs to be spread out evenly. Think of them as the asteroid belt beyond
Neptune...

We need to establish an upper limit to the TNO number (mass,
quantity, density, whatever) which would not give Neptune an
orbital anomaly. And then we can check if this number (or a
lesser density number) falls within a window that would have an
effect on the Pioneers during and after they pass through them.
Are you saying that this or something similar has been done?

I'm not saying that. I remember reading a paper about Pioneer 10 anomaly
where they dismissed that. Try googling for "pioneer anomalous
acceleration" for more info.

How will we know? The terminal shock effect, even with just
a relatively small density, might be likened to driving on the
highway at 70mph and coming up on a traffic jam where the
cars are moving at a much slower speed. Of course, here we
are talking about a solar wind traveling at a million mph and
rather abruptly slowing to about 200,000 mph. Wouldn't a
slowdown of the Pioneers and Voyagers be one possible
indication?

I can't stress hard enough how *rare* solar wind is, especially at those
distances. The only effects of reaching the termination shock would be
electrical/magnetical/radiowave effects. These would only be measured by
the instruments still operating aboard the Voyagers.

You mention below that the Voyagers would be insensitive to
such anomalies.

I didn't say that. I said the anomaly would be impossible to pick out from
the Doppler data due to noise induced by constant thruster firings. The
anomaly could very well be there also, but is simply drowned out by the
attitude control system. Similar difficulties arose from light pressure
effects much nearer the Sun with Galileo, Ulysses and Cassini.

So this could mean that both the Pioneers
and the Voyagers have reached the the solar wind's terminal
shock, couldn't it? The Pioneers could be reacting to it while
the Voyagers can't.

No. The instruments onboard would be picking the transition up. In any
case, the anomaly has been observed on Pioneer 10 since around 1990 (if
memory serves), and that's a looong way to traverse the termination shock
unnoticed, isn't it?

Perhaps Galileo measurements were made on route to Jupiter? In any case,
measuring this anomaly so close to the Sun is very difficult due to the
effects of solar radiation pressure which can be hard to model and take
into consideration. The anomaly with the Pioneers was observed once the
craft got far enough from the Sun for the light pressure to become
insignificant.

And yet, it *has* been measured... _curious_.

I didn't say the "anomaly" wasn't measured with Galileo. I'm saying that,
as far as I know, the data aren't very clear because of ligh pushing on the
spacecraft. One also has to take into consideration the energy radiated
away from the RTGs which also complicate things. In the end, the data on
the 3 probes inside Jupiter's orbit are inconclusive, AFAIK.
Again, I say that the only (?) reason the anomaly on Pioneers was detected
so far away from the Sun is due to light pressure becoming negligible
there.

The reason no anomalies were observed with the Voyagers is because none
*could* be measured. The two spacecraft are 3 axis stabilized, using
maneuvering thrusters to keep the antenna pointed at Earth. Constant
firing
of these thrusters changes the spacecraft's momentum all the time (and in
a
random fashion, too), thus drowning any anomaly out...

Might be interesting if they could devise an experiment to
check for the anomaly by temporarily disabling these
thrusters, hmm?

It certainly would be interesting, but I think it is impossible. To get
tracking and Doppler data on a Voyager, the spacecraft antenna must be
pointed more or less precisely at Earth. If you were to shut down attitude
control for a long enough time for the anomaly to kick in with a detectable
speed change, the spacecraft would already have drifted off Earth point and
would require that the antenna be positioned properly again for
measurement. And guess what? That's the job of attitude control thrusters.
They would need to be fired again to point to Earth again and would thus
mess up all the anomalous speed change (small as it was). That's how things
are now and that's why spin stabilized or reaction wheel stabilized
spacecraft are only candidates for these kinds of measurements.

For new data on this matter, we'll probably have to wait for the New
Horizons mission to Pluto and Kuiper belt...

--
The butler did it.