Three issues are discussed in the sci.astro post below: 1.) Oort cloud comets may have present radiogenic heating, 2.) this internal heat may make it possible to detect them in the infrared with the planned 100 meter telescopes, and 3.) construction of such telescopes can be done quickly and cheaply using liquid mirror telescopes with atmospheric shrouds reaching up to the mesosphere.



Bob Clark

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Newsgroups: sci.astro
From: "Robert Clark" <rgregorycl...@yahoo.com>
Date: 14 Jul 2005 12:50:31 -0700
Local: Thurs,Jul 14 2005 3:50 pm
Subject: Re: Detecting the Oort cloud.

Robert Clark wrote:
> Jonathan Silverlight wrote:
> > In message <1120918848.582339.148...@z14g2000cwz.googlegro ups.com>*,
> > Robert Clark <rgregorycl...@yahoo.com> writes
> > ...
> > > 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/scien...00000000000000

> 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/2004/0412...1206-7_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.


It *may* be that detecting them with the planned 100 meter
optical/infrared telescopes is feasible. David Jewitt co-discover of
the crystalline ice on Quaoar believes the internal heating creating
this form of ice is still active:

Crystalline Ice on Kuiper Belt Object (50000) Quaoar.
David Jewitt (University of Hawaii) and Jane Luu (Lincoln Laboratory,
MIT)
http://www.ifa.hawaii.edu/faculty/jewitt/quaoar.html

Then this is likely to be the case also with other Kuiper belt and
Oort cloud objects, Sedna for instance. Sedna was detected with just a
1.2 meter telescope at 90 AU. Then *if* internal heating can allow Oort
cloud objects to maintain a similar surface temperature despite being
100 times further away from the Sun, a 100 meter telescope could detect
similar sized objects at around 10,000 AU.
The temperature required for forming the crystalline ice is above 110
K. The researchers theorize internal heating allows it to form
subsurface then outgassing, cryovolcanism or impacts brings it to the
surface. If this is an ongoing process then given the trillions of Oort
cloud objects it may be we could observe this warmer ice during the
process of it coming to the surface at 110 K for some comets.

As to creating the 100 meter or larger optical telescope I suggest the
method of liquid mirror telescopes with the addition of an atmospheric
shroud to retain mesospheric pressures through a column above the
telescope:

Newsgroups: sci.astro, sci.physics, sci.engr.mech, sci.space.policy
From: "Robert Clark" <rgregorycl...@yahoo.com>
Date: 29 Apr 2005 09:55:15 -0700
Local: Fri,Apr 29 2005 12:55 pm
Subject: An atmospheric envelope for ground-based telescopes.
http://groups-beta.google.com/group/...8f86925357765e

Note that liquid mirror telescopes can cost 1/100th the cost of a
similarly sized solid mirror telescope. Then such a telescope itself
might only be $10 million rather than the $1 billion projected for a
usual 100 meter telescope. This would allow very many to be built at
different sites and latitudes, markedly increasing the total collecting
area for astronomical observations.
The method suggested for keeping the shroud erect was through
pressurized fluid providing thrust, but a more easy to implement method
may be electrohydrodynamic propulsion (EHD):

Newsgroups: sci.astro, sci.space.policy, sci.physics,
sci.electronics.design, sci.electronics.misc
From: "Robert Clark" <rgregorycl...@yahoo.com>
Date: 17 May 2005 12:53:31 -0700
Local: Tues,May 17 2005 3:53 pm
Subject: Long cables to power "ioncraft" to orbit?
http://groups-beta.google.com/group/...3c11b037e41226

There was debate on this thread about whether this method would work
for generating thrust at high velocity. However, the method is
well-established for operating at low speeds, which is all that would
be required for erecting and maintaining the shroud at altitude.
Two factors that need to be calculated are the mass of the shroud
required to keep a near vacuum from sea level up to approx. 100,000 ft.
over a 100 meter diameter, and the electrical energy cost for raising
this mass and keeping it aloft.
Note that the efficiency of this propulsion method as measured by the
ratio of weight lifted (grams) to electrical power required (watts) is
proportional to the air pressure, so more power would be required at
high altitudes. However, the reduced pressure means the required
thickness of the shroud could be reduced at high altitudes so less
weight would need to be supported at high altitudes and so less power.
Then the result may be equal amount of power is required even at high
altitudes.


Bob Clark
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Considering infrared detecton I think we can expect a first answer in the next two years since ESA is launching Herschel next year. The collecting area may be too small but maybe something turns up in the first high-tech far infrared survey from outside our atmosphere.

http://sci.esa.int/science-e/www/are...cfm?fareaid=16



edited for url

An extra-solar probe would have the best view. If you look at a distant mountaintop--you see haze. If you are on the mountain top--yourself-it seems clearer.