"George Dishman" writes:

"Craig Markwardt" wrote in message
...
"Jeff Root" writes:
On the other hand, I wonder if you noticed that Max
specified "to Neptune and *back*"?

...

OK, I didn't notice that, and it makes the job much, much harder. I'm
not even sure it could be done. An orbit with perihelion at earth and
aphelion at Neptune has a period of about 60 years. I'm not sure if
gravity assist could be used, and of course the re-capture near earth
would be a major task.

Max's idea is to confirm that the anomaly changes direction
on an inward trip so to minimise systematics the plan would
be to launch on a fast, near radial trajectory out to say
Neptune, and then use a slingshot to bring the craft back
towards the Sun.

That would be quite a difficult bit of maneuvering!

Re-capture would be unnecessary, all that's needed is a long
enough baseline to determine the anomaly on each leg. The
essential feature would be to have both Doppler and ranging
so that the two could be compared to eliminate or confirm
non-dynamic effects, i.e. things affecting the signal rather
than the craft.

That isn't too hard to envisage if all that is being flown is
a spin-stabilised transponder. The simplest craft might be
nothing more than a large corner reflector with just enough
thrusters to do a course correction and some autonomous
navigation facility, you wouldn't even need to communicate
with it.

Well begging to differ, but I think that having thrusters is probably
anathema to a sensitive anomaly measurement, especially autonomous
thrusters :-)

My previous point was that NASA gets a lot of competitive proposals
for missions, and it will be hard to get a "bare-bones" mission ranked
more highly than other missions, given the high fixed costs of each
planetary mission.


My own thoughts on a mission have been on the lines of flying
a craft capable of receiving millisecond pulsar signals from
a number of sources simultaneously (using a synthetic aperture
technique) and using them like GPS to work out the location
and highly accurate timing on-board. Add an ensemble of atomic
clocks and you can measure and map the solar gravitational
potential by comparing the clocks to the pulsars to determine
the gravitational frequency shift. The self-determined location
could be radioed back and compared with the range and integrated
Doppler. It's a bit more expensive than the simpler reflector
but mapping the potential well of the Solar System might have a
chance of being seen as a new observation that could attract
some funds.

The problem I see with that is getting the timing resolution
from the pulsars. Pulse durations are quite large even for the
fastest repetition rates but on the other hand you get continuous
reception 24h a day. A 'millisecond' pulsar gives on the order of
10^6 pulses per hour so I guess maybe three orders of magnitude
improvement in timing, or roughly microsecond accuracy. It needs
tens of nanoseconds accuracy to be comparable to ranging data
which achieves tens of metres.

I understand that the best ground based observations (with large
antennae) can achieve timing precisions of about 0.001 of a pulse
cycle. Unfortunately there are no "millisecond" pulsars with periods
of 1 millisecond or less. On the other hand, millisecond radio pulses
are typically quite sharp. On the third hand, any kind of
spin-stabilized spacecraft will be a pretty poor pulsar receiver,
since there would be no feasible way to use focussing optics that I
can see.

Craig