Re: Detecting the Oort cloud.

Jonathan Silverlight wrote:
> In message <1120918848.582339.148300@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> Robert Clark <rgregoryclark@xxxxxxxxx> writes
> >Robert Clark wrote:
> >> Assuming Oort cloud comets have about the same reflectivity as known
> >> comets, could the planned 100 meter telescopes resolve them at the 1
> >> light-year distance of the Oort cloud?
> >> Current telescopes can not a disk for stars but detect them as point
> >> light sources. Could a 100 meter telescope detect an Oort comet as a
> >> point light source?
> >> Since there is theorized to be a cloud of comets there could a 100
> >> meter telescope detect them as a cloud of objects if not individually?
> >>
> >>
> >>
> >> Bob Clark
> >
> > Is the interferometry that can be done with radio telescopes dependent
> >on distance? That is, does being at longer distance make the accurate
> >combining of the different signals more difficult? I know there have
> >been successful experiments with one component of the array in orbit.
> >Could we place one component on the Moon?
> > Could we then detect Oort cloud comets at radio wavelengths?
> >
>
> Comets don't radiate at those frequencies - or indeed any others; they
> just reflect. Anyway, an interferometer isn't very sensitive; it just
> has much better resolution.
> An Earth/Moon interferometer has certainly been proposed, though - look
> at <http://www.astro.virginia.edu/~rjp0i/museum/future.html>, for
> instance.
> There was a proposal to use the Hubble Space Telescope to look for Oort
> clouds around novae. The idea was that the increase in radiation would
> evaporate the comets and the spectrum of the resulting water vapour and
> other gases would be detectable.


Yes, but comets do have a thermal emission which does extend into
radio wavelengths. This report on the planned uses of the Square
Kilometer Array suggests large Kuiper belt objects could be detected at
100 AU:

Kuiper Belt Objects.
"In spite of their large numbers, KBOs are faint, difficult sources,
and it will be especially troublesome at visible and infrared
wavelengths to find members of the outer Kuiper belt, beyond 50 AU. The

brightness of reflected sunlight drops off with distance d as d-4,
making objects in the outer belt 5 magnitudes fainter than comparable
inner belt objects. Similarly, the low temperature of the KBOs,
dropping from around 40 K in the inner belt to only 20 K in the outer
belt, means that the thermal emission does not start to dominate the
reflected sunlight until far-infrared wavelengths.
"By contrast, the SKA operating at 20 GHz will be almost ideally suited

to study KBOs. The linear scale which the SKA will resolve at a
distance of 40 AU is 200 km. After an 8 hour integration it should be
possible to achieve a 5 [sigma] detection on a 120 km object, as shown
in Table 3.1. Even at a distance of 100 AU, the smallest detectable
object would be 350 km in diameter, and there may be many of these
objects if their size distribution is similar to that which we have
measured on the inner edge of the belt. If self-gravitating clusters of

KBOs exist, capable of generating clumps of dust emission such as are
seen around other stars, the SKA may be the tool required to find
them."
http://www.ras.ucalgary.ca/SKA/science/node24.html#SECTION00413000000000000000

The recently discovered Kuiper belt object Sedna at around 90AU (it
has a highly eccentric orbit) and 1200 km across for instance would be
detected by the SKA. However, at best if you were detecting internal
heat the radiated energy would be smaller by a factor of 100^2 = 10,000
times at 10,000 AU than at 100 AU. So you would need a radio telescope
100 kilometers across rather than the 1 kilometer across equivalent
collecting area of the SKA. At worst if this thermal emission is only
solar generated, it would be smaller by a factor of 100^4 = 10,000,000
and you would need a telescope 10,000 km across.
That there is or has been internal radiogenic heating in comets has
been supported by some studies:

Radioactive Heating of Porous Comet Nuclei.
D. Prialnik and M. Podolak
Icarus
Volume 117, Issue 2 , October 1995, Pages 420-430
http://dx.doi.org/10.1006/icar.1995.1166

Some scientists have even argued the amount of this heating at least
early on in the Solar Systems history may have been enough to provide
comets with liquid water interiors.

Interestingly a report in Nature last year supports the idea of
radiogenic heating in deep Kuiper belt objects:

Chilly Quaoar had a warmer past.
Mark Peplow
Crystalline ice suggests remote object has radioactive interior.
http://www.nature.com/news/2004/041206/pf/041206-7_pf.html

It will be interesting to find out if Sedna which is even further out
than Quaoar also has this crystalline ice on its surface.

As to forming such ultra large radio telescopes I suggest creating
parabolic wire mesh surfaces under tension. This method may be
problematical for optical mirrors because of the accuracy at the
nanometer level required, but for radio telescopes which only require
accuracy at the millimeter or larger level it should be feasible. This
is especially true for radio telescopes using wire mesh rather than a
solid surface as they would be well suited for the parabolic shape to
be traced out by tensioned cables.
However, a single radio telescope 100 km across may not be workable
even with this method. But it may be the cost is reduced so much with
this method that single universities and research institutions could
afford to build their own 10km wide telescopes. Then there could be so
many of the 10km wide radio telescopes as to make up a 100 km wide
equivalent collecting area telescope.


Bob Clark


================================================
Newsgroups: sci.astro, sci.physics, sci.optics, sci.materials,
sci.engr.manufacturing
From: "Robert Clark" <rgregorycl...@xxxxxxxxx>
Date: 12 Mar 2005 14:58:12 -0800
Local: Sat, Mar 12 2005 2:58?pm
Subject: Telescope mirrors under tension.

....
Another possibility for a mirror under tension is given by this
surprising fact:

Parabolas and Bridges.
"If you hang a flexible chain loosely between two supports, the curve
formed by the chain looks like a parabola, but isn't. It is a catenary,

a more glamorous curve which can be represented algebraically by
hyperbolic functions [y = A (cosh kx - 1)]. In this case, the vertical
load on the chain is uniform with respect to arc length. A whirling
skipping rope is another example of a catenary.
"The load on a suspension bridge is (approximately) uniform with
respect to the horizontal distance. In this case, the curve is a
parabola ..."
http://www.du.edu/~jcalvert/math/parabola.htm

Shape of a suspension bridge cable.
http://aemes.mae.ufl.edu/~uhk/SUSPCAB.jpg

Hanging With Galileo.
"Take a flexible chain of uniform linear mass density. Suspend it from
the two ends. What is the curve formed by the chain? Galileo Galilei
said that it was a parabola, and perhaps you made the same guess. This
time Galileo was not correct. The curve is called a catenary. However,
it is easy to see how he could arrive at this answer through casual
observation and incomplete deduction.
....
"We can get back to the chain solution later. First consider this
extension. What about the curve formed by the cables of a suspension
bridge? Is it too a catenary? No, it is a parabola. So, what gives? How

can this be a parabola while the other one is not?"
http://whistleralley.com/hanging/hanging.htm

Then to form a parabolic surface you could have the suspension cables
arranged in concentric circles hanging from the mirror supporting a
weight. In this case gravity would be working to *form* the surface
shape.
A problem is that just with liquid mirrors you might need to keep the
mirror horizontal so it would have to be zenith-pointing. However, it
may be that by varying the cable lengths you could maintain the
parabolic shape.


Bob Clark
================================================

.

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