Craig Markwardt wrote:
"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!

Yep, I don't think he has got as far as considering how a
two way trip would be done.

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 :-)

Oh sure, I'm only thinking of a course correction good enough to
get the craft targetting the Neptune well enough for the slingshot
to be possble. Small corrections at either end with maybe 90%
in coast mode would be the idea, but I don't think such a mission
would have any value anyway, it's not a serious idea.

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.

I entirely agree, that was what I was saying to Max.

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.

The difference would be that ground based antennas have
limited time when the pulsar is above the horizon and of
course atmospheric effects and domestic RFI play a part,
but I wouldn't have thought much better than that could be
achieved in space.

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.

My thoughts were based on using a large near-spherical
structure like a C60 molecule with each facet being a fine
mesh ground plane with a small dipole and using a multi-
channel synthetic aperture technique to create a capture
area close to the diameter of the sphere. The craft wouldn't
need to spin, just be aware of its orientation and adjust the
phase of the antenna feeds accordingly. In fact the phase
could also be used to measure the orientation.

The same technique in reverse could be used to beam-form
the telemetry signal back to Earth, though choosing the
operating frequency would become a compromise.

Typically pulsars seem to need tens of metres of dish
diameter for good reception so facets of the order of a metre
or two would be needed giving frequencies in the 100MHz
to 1GHz range, maybe 408MHz?

http://zuserver2.star.ucl.ac.uk/~apo.../ap971214.html

http://www.georgedishman.f2s.com/astronomy/408MHz.png

The fun engineering challenge is to launch the craft "flat-pack"
and have the sphere deploy in space ;-)

George