Pioneer radiation pressure

How much radiation pressure is assumed to be acting in the outward
direction? I am wondering how the magnitude compares to the inward
anomaly.

Dear dearcilla:

"dearcilla" wrote in message
oups.com...
How much radiation pressure is assumed to be
acting in the outward direction? I am wondering
how the magnitude compares to the inward
anomaly.

Inward anomaly is very constant.

Outward radiation pressure falls off by 1/r^2.

David A. Smith

"N:dlzc D:aol T:com (dlzc)" N: dlzc1 D:cox wrote in
message news:zrRAg.23077$6w.12503@fed1read11...
Dear dearcilla:

"dearcilla" wrote in message
oups.com...
How much radiation pressure is assumed to be
acting in the outward direction? I am wondering
how the magnitude compares to the inward
anomaly.

Inward anomaly is very constant.

Outward radiation pressure falls off by 1/r^2.

To add a little detail:

The outward radiation pressure from the Sun
was comparable to the anomaly at about 13 AU
and much smaller over the range considered.

The telemetry radio beam pushing away from
Earth is 8W while the anomaly is equivalent
to the radiation pressure from a beam of 63W.

The total radiation from the RTGs is about 2kW
so if that was 1023W away from the Sun and
977W towards the Sun, it would explain the
anomaly. That means a difference in emissivity
of about 6.3% between the sides of the RTG but
nobody has any idea how such a large difference
could be produced.

George

Dear George Dishman:

"George Dishman" wrote in message
...

Thanks for the more complete answer.
....
The telemetry radio beam pushing away from
Earth is 8W while the anomaly is equivalent
to the radiation pressure from a beam of 63W.

.... and directed in the other direction.

The total radiation from the RTGs is about 2kW
so if that was 1023W away from the Sun and
977W towards the Sun, it would explain the
anomaly. That means a difference in emissivity
of about 6.3% between the sides of the RTG but
nobody has any idea how such a large difference
could be produced.

Assuming
- the dish is aimed Sunward, and
- the craft rotates more than once in a sampling period around
the central axis of the dish, and
- the RTG is off axis perhaps even in line-of-sight to the Sun,
then
The thrust could "simply" be the time integration of a thrust of
this amount of power, times the view factor of the craft from the
RTG (30% would be a little large, the rest lost to space), times
the cosine of the angle between the lines [RTG and the centroid
of the craft-area-visible-to-RTG] and central rotational (aka.
Sunward) axis. Somewhere in there, the RTG heated surface
radiates diffusely to space, actually providing the net thrust.
Seems like this might be too weak...

David A. Smith

"N:dlzc D:aol T:com (dlzc)" N: dlzc1 D:cox wrote in
message news:Ag3Bg.23485$6w.2477@fed1read11...
Dear George Dishman:

"George Dishman" wrote in message
...

Thanks for the more complete answer.
...
The telemetry radio beam pushing away from
Earth is 8W while the anomaly is equivalent
to the radiation pressure from a beam of 63W.

... and directed in the other direction.

Yes.

The total radiation from the RTGs is about 2kW
so if that was 1023W away from the Sun and
977W towards the Sun, it would explain the
anomaly. That means a difference in emissivity
of about 6.3% between the sides of the RTG but
nobody has any idea how such a large difference
could be produced.

Assuming
- the dish is aimed Sunward, and
- the craft rotates more than once in a sampling period around the central
axis of the dish, and

The spin rates were nominally 4.28 rpm for Pioneer 10
(shown as "4.8" on page 3) and 7.8 rpm for Pioneer 11
but varied as shown in Fig 11 and 12.

The sampling rate was generally once per minute.

- the RTG is off axis perhaps even in line-of-sight to the Sun,

The two RTGs were on booms about 3m out from the axis
perpendicular to the line to the Sun and behind the
dish.

then
The thrust could "simply" be the time integration of a thrust of this
amount of power, times the view factor of the craft from the RTG (30%
would be a little large, the rest lost to space), times the cosine of the
angle between the lines [RTG and the centroid of the
craft-area-visible-to-RTG] and central rotational (aka. Sunward) axis.
Somewhere in there, the RTG heated surface radiates diffusely to space,
actually providing the net thrust. Seems like this might be too weak...

See section VIII, B "RTG heat reflecting off the
spacecraft".

"The RTGs are located at the end of booms, and
rotate about the spacecraft in a plane that
contains the approximate base of the antenna.
From the closest axial center point of the RTGs,
the antenna is seen nearly 'edge on' (the
longitudinal angular width is 24.5). The total
solid angle subtended is 1-2% of 4pi steradians
[104]. Even though a more detailed calculation
yields a value of 1.5% [105], even taking the
higher bound of 2% means this proposal could
provide at most ~40 W. But there is more [106].

.. The largest opening angle of the fins is seen
only by the narrow-angle parts of the antenna's
outer edges. Ignoring these edge effects, only
~2.5% of the surface area of the RTGs is facing
the antenna. This is a factor 10 less than that
from integrating the directional intensity from
a hemisphe ... So, one has only 4W of directed
power."

George

Dear George Dishman:

"George Dishman" wrote in message
...

"N:dlzc D:aol T:com (dlzc)" N: dlzc1 D:cox
wrote in message news:Ag3Bg.23485$6w.2477@fed1read11...
Dear George Dishman:

"George Dishman" wrote in message
...

Thanks for the more complete answer.
...
The telemetry radio beam pushing away from
Earth is 8W while the anomaly is equivalent
to the radiation pressure from a beam of 63W.

... and directed in the other direction.

Yes.

The total radiation from the RTGs is about 2kW
so if that was 1023W away from the Sun and
977W towards the Sun, it would explain the
anomaly. That means a difference in emissivity
of about 6.3% between the sides of the RTG but
nobody has any idea how such a large difference
could be produced.

Assuming
- the dish is aimed Sunward, and
- the craft rotates more than once in a sampling
period around the central axis of the dish, and

The spin rates were nominally 4.28 rpm for Pioneer 10
(shown as "4.8" on page 3) and 7.8 rpm for Pioneer 11
but varied as shown in Fig 11 and 12.

The sampling rate was generally once per minute.

So more than enough to "average out" between velocity samplings.

- the RTG is off axis perhaps even in line-of-sight
to the Sun,

The two RTGs were on booms about 3m out from
the axis perpendicular to the line to the Sun and
behind the dish.

Couldn't tell the geometry very well from the picture I saw. But
really sure that even half the heat output could *not* be
directed Sunward.

then
The thrust could "simply" be the time integration
of a thrust of this amount of power, times the
view factor of the craft from the RTG (30% would be a little
large, the rest lost to space),
times the cosine of the angle between the lines
[RTG and the centroid of the craft-area-visible-
to-RTG] and central rotational (aka. Sunward)
axis. Somewhere in there, the RTG heated
surface radiates diffusely to space, actually
providing the net thrust. Seems like this might
be too weak...

See section VIII, B "RTG heat reflecting off the
spacecraft".

"The RTGs are located at the end of booms, and
rotate about the spacecraft in a plane that
contains the approximate base of the antenna.
From the closest axial center point of the RTGs,
the antenna is seen nearly 'edge on' (the
longitudinal angular width is 24.5). The total
solid angle subtended is 1-2% of 4pi steradians
[104]. Even though a more detailed calculation
yields a value of 1.5% [105], even taking the
higher bound of 2% means this proposal could
provide at most ~40 W. But there is more [106].

.. The largest opening angle of the fins is seen
only by the narrow-angle parts of the antenna's
outer edges. Ignoring these edge effects, only
~2.5% of the surface area of the RTGs is facing
the antenna. This is a factor 10 less than that
from integrating the directional intensity from
a hemisphe ... So, one has only 4W of directed
power."

It seems to me that they are factoring out the 1.5% of 2pi
steradians twice here. Inherent in that is that light is
directed uniformly over the full 2pi. Why then:

"only ~2.5% of the surface area of the RTGs is facing the
antenna. This is a factor 10 less than that from integrating the
directional intensity from a hemisphere"

This is double dipping in that particular well.

Where the correct (IMO) "further reduction" in available thrust
comes from is determining the Sunward component, versus the
reradiated heat that simply "sprays" radially from the Sunward
path.

It is clear that the RTG cannot provide sufficient thrust, given
all the facts.

A mystery it stays...

David A. Smith

"N:dlzc D:aol T:com (dlzc)" N: dlzc1 D:cox wrote in
message news:SM7Bg.23488$6w.20475@fed1read11...
Dear George Dishman:
....
The sampling rate was generally once per minute.

So more than enough to "average out" between velocity samplings.

Yes.

- the RTG is off axis perhaps even in line-of-sight
to the Sun,

The two RTGs were on booms about 3m out from
the axis perpendicular to the line to the Sun and
behind the dish.

Couldn't tell the geometry very well from the picture I saw. But really
sure that even half the heat output could *not* be directed Sunward.

to Sun - ) dish
|
| boom
|
[ ] RTG

The RTG is a cylinder with the axis along
the boom. There are six thin fins along the
surface which provide the majority of the
radiating surface. The end of the cylinder
and the vanes is quite small.

See section VIII, B "RTG heat reflecting off the
spacecraft".

"The RTGs are located at the end of booms, and
rotate about the spacecraft in a plane that
contains the approximate base of the antenna.
From the closest axial center point of the RTGs,
the antenna is seen nearly 'edge on' (the
longitudinal angular width is 24.5). The total
solid angle subtended is 1-2% of 4pi steradians
[104]. Even though a more detailed calculation
yields a value of 1.5% [105], even taking the
higher bound of 2% means this proposal could
provide at most ~40 W. But there is more [106].

.. The largest opening angle of the fins is seen
only by the narrow-angle parts of the antenna's
outer edges. Ignoring these edge effects, only
~2.5% of the surface area of the RTGs is facing
the antenna. This is a factor 10 less than that
from integrating the directional intensity from
a hemisphe ... So, one has only 4W of directed
power."

It seems to me that they are factoring out the 1.5% of 2pi steradians
twice here. Inherent in that is that light is directed uniformly over the
full 2pi. Why then:

"only ~2.5% of the surface area of the RTGs is facing the antenna. This is
a factor 10 less than that from integrating the directional intensity from
a hemisphere"

This is double dipping in that particular well.

Two different wells.

a) from the viewpoint of the RTG, the dish is seen
edge on and covers 1.5% of 4 pi steradians. If
the RTG radiated isotropically the dish would
intercept less than 2% of 2kW = 40W.

b) The radiation is not isotropic. Most of the heat
is radiated from the surface of the fins hence
perpendicular to the cylinder axis. From the
viewpoint of the dish only the end cap of the
cylinder and the ends of the fins can be seen
and they are 2.5% of the RTG radiating surface.

Where the correct (IMO) "further reduction" in available thrust comes from
is determining the Sunward component, versus the reradiated heat that
simply "sprays" radially from the Sunward path.

The RTG only sees the back of the dish so all the
radiated heat will be directed away from the Sun to
some extent. They didn't calculate the factor but
allowed a generous uncertainty to take care of it.

It is clear that the RTG cannot provide sufficient thrust, given all the
facts.

A mystery it stays...

Indeed.

George

Dear George Dishman:

"George Dishman" wrote in message
...

"N:dlzc D:aol T:com (dlzc)" N: dlzc1 D:cox
wrote in message news:SM7Bg.23488$6w.20475@fed1read11...
Dear George Dishman:
...
The sampling rate was generally once per minute.

So more than enough to "average out" between
velocity samplings.

Yes.

- the RTG is off axis perhaps even in line-of-sight
to the Sun,

The two RTGs were on booms about 3m out from
the axis perpendicular to the line to the Sun and
behind the dish.

Couldn't tell the geometry very well from the picture
I saw. But really sure that even half the heat output
could *not* be directed Sunward.

to Sun - ) dish
|
| boom
|
[ ] RTG

The RTG is a cylinder with the axis along
the boom. There are six thin fins along the
surface which provide the majority of the
radiating surface. The end of the cylinder
and the vanes is quite small.

Vanes being oriented largely radially from the axis of the boom
and containing it in their respective planes, no doubt.

See section VIII, B "RTG heat reflecting off the
spacecraft".

"The RTGs are located at the end of booms, and
rotate about the spacecraft in a plane that
contains the approximate base of the antenna.
From the closest axial center point of the RTGs,
the antenna is seen nearly 'edge on' (the
longitudinal angular width is 24.5). The total
solid angle subtended is 1-2% of 4pi steradians
[104]. Even though a more detailed calculation
yields a value of 1.5% [105], even taking the
higher bound of 2% means this proposal could
provide at most ~40 W. But there is more [106].

.. The largest opening angle of the fins is seen
only by the narrow-angle parts of the antenna's
outer edges. Ignoring these edge effects, only
~2.5% of the surface area of the RTGs is facing
the antenna. This is a factor 10 less than that
from integrating the directional intensity from
a hemisphe ... So, one has only 4W of directed
power."

It seems to me that they are factoring out the 1.5%
of 2pi steradians twice here. Inherent in that is that
light is directed uniformly over the full 2pi. Why then:

"only ~2.5% of the surface area of the RTGs is
facing the antenna. This is a factor 10 less than that
from integrating the directional intensity from a
hemisphere"

This is double dipping in that particular well.

Two different wells.

a) from the viewpoint of the RTG, the dish is seen
edge on and covers 1.5% of 4 pi steradians. If
the RTG radiated isotropically the dish would
intercept less than 2% of 2kW = 40W.

This treats the RTG as a sphere, or point source.

b) The radiation is not isotropic. Most of the heat
is radiated from the surface of the fins hence
perpendicular to the cylinder axis. From the
viewpoint of the dish only the end cap of the
cylinder and the ends of the fins can be seen
and they are 2.5% of the RTG radiating surface.

This is where the geometry of the RTG is handled, and its
departure from spherical. OK, not double dipping.

I wonder what insulation they provided between the RTG and the
boom? Not that I expect it to have a much different thermal
gradient than along a pan (stove/cooktop) handle... so insulation
could be important too.

The RTG is basically a thermopile, that takes the heat
differential between the radioactive source and the "environs",
and generates a DC voltage. Are the fins where this connection
to the environs occurs, I wonder? This "connection" will (also)
radiate heat...

Where the correct (IMO) "further reduction" in
available thrust comes from is determining the
Sunward component, versus the reradiated heat
that simply "sprays" radially from the Sunward path.

The RTG only sees the back of the dish so all the
radiated heat will be directed away from the Sun to
some extent. They didn't calculate the factor but
allowed a generous uncertainty to take care of it.

Being already at least an order of magnitude smaller than
necessary, further small gains aren't terribly important.

It is clear that the RTG cannot provide sufficient
thrust, given all the facts.

A mystery it stays...

Indeed.

Cool! Work for someone.

Happy Sunday, George.

David A. Smith

In article ,
"George Dishman" writes:
The telemetry radio beam pushing away from
Earth is 8W while the anomaly is equivalent
to the radiation pressure from a beam of 63W.

The total radiation from the RTGs is about 2kW...

Have you seen any pictures or diagrams of what Pioneer looks like
from the vantage point of one of the RTGs? That might give a hint of
where asymmetric radiation pressure could be coming from. I realize
the experts don't think this is the explanation, but it seems hard to
rule out.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Steve Willner wrote:

Have you seen any pictures or diagrams of what Pioneer looks
like from the vantage point of one of the RTGs? That might
give a hint of where asymmetric radiation pressure could be
coming from. I realize the experts don't think this is the
explanation, but it seems hard to rule out.

That's exactly what I think. I'm interested in this mainly
because George is, so I read his posts. If I were more fired
up about the subject I would get some 3-D drawing software,
the dimensions of Pioneer, and the detailed thermal gradients
and reflectivities of all the materials on the spacecraft, to
work out exactly what gets radiated where. The base of the
fins must be hotter than the edges; radiation from any point
goes outward in all directions, with some of it reflected and
some absorbed by an adjacent part; a thermal blanket may have
come loose at launch, putting it in a completely unexpected
location, etc., all makes for a problem too hard for me.

-- Jeff, in Minneapolis