Lunar Observatories: Grand Plans vs. Clear Problems
http://www.space.com/scienceastronomy/061205_moon_clash.html
I'm delighted to see a professional scientist question the assumption
that there are any advantages to astronomy on the moon vs. astronomy in
space, and to even say that the former is more difficult than the
latter.
platform-a solid anchor", I question if the phrasing has any practical
meaning. Hubble seems to be doing just fine, despite the handicap of
lacking a "permanent platform".
Regards,
Mike Combs
From: spacesettlers@yahoogroups.com
[mailto:spacesettlers@yahoogroups.com] On Behalf Of Alex Michael Bonnici
Sent: Tuesday, December 05, 2006 1:50 PM
To: ACC-LIST; The Carl Sagan Club Online; Carl Sagan; Spacesettlers;
MajorMatt; Major Matt Mason Club; Tom Swift
Subject: [spacesettlers] Lunar Observatories: Grand Plans vs. Clear
Problems
Lunar Observatories: Grand Plans vs. Clear Problems
Humans will return to the Moon no later than 2020, paving the way for
treks to Mars and beyond. When liftoff happens, astronomers don't want
to be left in the dust.
http://www.space.com/scienceastronomy/061205_moon_clash.html
On 12/5/06, Combs, Mike wrote:
(...)
> When I see statements like "...the Moon also provides a permanent
> platform-a solid anchor", I question if the phrasing has any practical
> meaning.
(...)
interferometry of geographically large base is possible in the Moon,
but not in space. Such technique requires that two or more telescopes
stay at exactly the same distance within a very small (light
wavelength scale, I think) safety margin, and that is simply not
possible with free-floating space telescopes. And from what I remember
optical interferometry could potentially image (in visible light)
Earth-sized extrasolar planets in nearby stars...
From: spacesettlers@yahoogroups.com [mailto:spacesettlers@yahoogroups.com] On Behalf Of Lucio de Souza Coelho
> interferometry of geographically large base is possible in the
> Moon, but not in space.
Yeah, I know; I too have seen that same assertion, and in more than one place. Odd, then, that NASA is proceeding on an orbital mission which will indeed do optical interferometry in space:
http://planetquest.jpl.nasa.gov/technology/formation_flying.cfm
Noting this in one of my articles, I opined, "It could be that if one's primary concern is for staying within a fixed budget, as opposed to justifying lunar development desired for its own sake, more economical alternatives present themselves."
Regards,
Mike Combs
On 12/5/06, Combs, Mike wrote:
(...)
> Yeah, I know; I too have seen that same assertion, and in more than one place. Odd, then, that NASA is proceeding on an orbital mission which will indeed do optical interferometry in space:
>
> http://planetquest.jpl.nasa.gov/technology/formation_flying.cfm
(...)
atmospheric caracterization of Earth-sized planets, and for this it
will use a relativelly small baseline - separations in the range of 40
to 100 meters between telescopes, perfectly achievable in space. That
is far different from what could be achieved by telescopes based on
Luna with baselines of hundreds or even thousands of *kilometers*.
Such geographic-scale interferometric telescopes could in theory image
continents and even weather systems in extrasolar worlds. As far as I
understand it is simple physics, resolutions using interferometry get
better as you increase your baseline.
So if your goal is just to say that there is a pale blue dot around
such star, you really don't need the Moon. If your goal is gather more
information from the pale blue dot, perhaps you should consider the
lunar alternative.
On Dec 05, 2006, at 21:56 UTC, Lucio_de_Souza_Coelho wrote:
> atmospheric caracterization of Earth-sized planets, and for this it
> will use a relativelly small baseline - separations in the range of 40
> to 100 meters between telescopes, perfectly achievable in space. That
> is far different from what could be achieved by telescopes based on
> Luna with baselines of hundreds or even thousands of *kilometers*.
Why do you feel that similar (or larger) baselines couldn't be achieved
with space-based telescopes?
Best,
- Joe
Joe Strout -- joe@...
Yes, the resolution does get better with distance between telescopes
using interferometry. Placing the telescopes 1000km apart gives you
the resolving power of a telescope 1000km wide, but the light
gathering power is still only the sum total of each individual
telescope. Since the light we can see reflecting off the planet would
be decreasing according to the inverse square law, the images still
would be extremely poor compared to images of planets in our solar
system. A Lunar interferometer would UNQUESTIONABLY be able to easily
distinguish planets, moons, etc., The light coming from each object
would be very crisp, but so dim that there would be little contrast
between light and dark spots. Remember, the planets are only
radiating infrared light, not visible light. They reflect a tiny
amount of the visible light radiated by the nearby star. The data
will be useful for spectral analysis, but probably will not generate
very stunning photos. Still, telescopes have come a long way, and the
folks who design them are definitely up to the task of compensating
for the very dim light coming from these planets. The study of these
exoplanets looks to be one of the more exciting areas of science for
the next few years, and I thoroughly expect that a lot of our
theories about geology, climate, planetary science, and astrobiology
will have to be modified to match all these new observations we'll be
making over the next few decades.
On Dec 5, 2006, at 3:56 PM, Lucio de Souza Coelho wrote:
On 12/5/06, joe@... wrote:
(...)
> Why do you feel that similar (or larger) baselines couldn't be achieved
> with space-based telescopes?
(...)
orders of magnitude larger than the wavelength of light, therefore the
error in the relative positions of those space telescopes would render
any attempt of interferometry useless. If you place the scopes over
the surface of a geologically dead body like the Moon, though, you
guarantee that they will keep the same relative positions to each
other within an error margin in the scale of light wavelength.
Perhaps that will change in the future using some new, unforeseen
tecnology, but until then the Moon (or another similar body) seems the
option for high-resolution geographical-scale baseline optical
interferometry.
> From: Lucio de Souza Coelho
> Satellite geographic positioning systems (like GPS) have error margins
> orders of magnitude larger than the wavelength of light,
between. Or then again, they could do it that because line-of-sight
rangefinding is (as I understand it) the only way to build a vissible light
interferometry telescope. Good luck getting line of sight on the moon; it's
curved as I recall.
> From: Douglas May
> but the light gathering power is still only the
> sum total of each individual telescope.
Yes, but the light gathering capacity is equal to the sum total of each
individual telescope X time. Because the moon rotates there is a limit to
how long any theoretical lunar telescope can stare at one point in space. A
space based telescope has no such limitation.
And, um, isn't the moon's maximum baseline kinda limited to it's diameter?
John
On Dec 05, 2006, at 22:34 UTC, Lucio_de_Souza_Coelho wrote:
achieved
> > with space-based telescopes?
>
> Satellite geographic positioning systems (like GPS) have error margins
> orders of magnitude larger than the wavelength of light, therefore the
> error in the relative positions of those space telescopes would render
> any attempt of interferometry useless.
Well yes, if you intended to use GPS for your formation flying.
Obviously you wouldn't do that; the satellites would have direct
measurements of their spacing (most likely using interferometry, in
fact), and use that to control their position.
If you can do it for satellites 40 m apart, I see no reason why it
would be any harder for satellites 1000 km apart. The problem (and
solution) are exactly the same. Station-keeping is station-keeping, as
long as the satellites are within line-of-sight of each other.
Best,
- Joe
Joe Strout -- joe@...
On Dec 5, 2006, at 5:41 PM, ANTIcarrot wrote:
> each
> individual telescope X time. Because the moon rotates there is a
> limit to
> how long any theoretical lunar telescope can stare at one point in
> space. A
> space based telescope has no such limitation.
The time available for the exposure will be a huge limiting factor
for a lunar interferometer. Space based interferometers would not
have to worry about the rotation of the moon, but the rotation of the
target object would be an issue. I don't know how long of an exposure
time we would need, but if it's more than a few minutes (which it
will be), the rotation of the object planet begins to become a
factor. I would guess at that distance we would be looking at
extremely long exposures. The choice then becomes between a faint,
low-contrast image, or a higher contrast but smeared image. Again,
improvements in optics are making this situation better, and spectral
analysis will still be able to give us an idea of the makeup of the
planet. Obviously we are talking about the edge of technology here. I
have read before somewhere of a solar gravitational focal point
somewhere out beyond the Kuiper belt where telescopes would be
dramatically more effective. Yet one more reason to get the colonies
started.
doug
From: spacesettlers@yahoogroups.com [mailto:spacesettlers@yahoogroups.com] On Behalf Of Lucio de Souza Coelho
> image continents and even weather systems in extrasolar worlds.
You should read the following paper by Gerard O'Neill:
A High Resolution Orbiting Telescope, Science 160, No. 3830, 843 (May 1968).
In this paper he proposes a space telescope composed of many 1-meter mirrors. The mirrors use lasers rather than GPS for position-sensing for station-keeping (as others have pointed out). And as I recall the resolution was indeed sufficient to chart weather patterns on extra-solar planets.
Regards,
Mike Combs