We will soon be able to resolve Mars microbes from orbit. ;-)
Robert Clark
"It took 20 years to increase the resolution by a factor or 10 over
Viking with the Mars Global Surveyor mission. But only 10 years to
increase the resolution over that of MGS by a factor of 10 with Mars
Reconnassance Orbiter.
Could we increase the resolution over MRO by another factor of 10 to,
gulp, 3 cm per pixel in only 5 years this time?"
Funny though, that rather off-the-cuff estimate of mine is close to
what is possible.
To resolve 3 cm in the optical from say a 300 km orbit would require a
6 meter mirror. The James Webb Space Telescope will have a 6.5 meter
mirror and is scheduled for launch in 2013. But it was originally
scheduled for launch in 2011:
James Webb Space Telescope.
http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
So going by this rate, it'll be 3mm/pixel 2.5 years after that, and
300 microns 1.25 years after that, and ...
Hmm, in less than a decade then we should be able to resolve microbes
from space.
Admittedly though, the JWST is a 4 billion dollar mission. Also it
uses a beryllium metal mirror for infrared astronomy only. The
beryllium makes the mirror lightweight but it is unclear if you can
achieve the much more stringent smoothness requirements at optical
wavelengths with a metal mirror.
As for the data storage and transmission of the large files for such
high resolution images, data storage capacity and costs are doubling
and halving each year, respectively:
Bye-bye hard drive, hello flash.
By Michael Kanellos
Staff Writer, CNET News.com
Published: January 4, 2006, 10:00 AM PST
"Currently, NAND chips double in memory density every year. The
cutting-edge 4-gigabit chips of 2005, for example, will soon be
dethroned by 8-gigabit chips. (Memory chips are measured in gigabits,
or Gb, but consumer electronics manufacturers talk about how many
gigabytes, or GB, are in their products. Eight gigabits make a
gigabyte, so one 8Gb chip is the equivalent of 1GB.)
"Another driving factor in the uptake of the technology is cost: NAND
drops in price about 35 to 45 percent a year, due in part--again--to
Moore's Law and in part to the fact that many companies are bringing on
new factories."
http://news.com.com/Bye-bye+hard+drive,+hello+flash/2100-1006_3-6005849.html
MRO uses the type of flash memory chips discussed here.
Also, interestingly NASA had planned a laser communication orbiter for
Mars for launch in 2010 before it was canceled:
Record Set for Space Laser Communication.
By Ker Than
Staff Writer
posted: 05 January 2006
02:11 pm ET
http://www.space.com/missionlaunches/060104_laser_comm.html
Mars Telecommunications Orbiter: Interplanetary Broadband.
By Bill Christensen
posted: 05 May 2005
06:41 am ET
http://www.space.com/businesstechnology/technology/technovel_marstelecom_050505.html
This would have allowed data transmission rates of a hundred times
greater than what is currently available.
It was the great cost overruns overruns that led to cancelling of the
Mars Telecommunications Orbiter, and great cost overruns also
threatened JWST as well.
That the costs for computer technology are dropping exponentially with
capacity increasing exponentially is no doubt fueled by the free market
in this sphere.
Conversely, that launch costs are staying static is no doubt because
the launches are controlled by large governments. When private
companies become the primary financer and purveyor of launches, the
launch costs will also drop dramatically.
Bob Clark
Sam Wormley
> To resolve 3 cm in the optical from say a 300 km orbit would require a
> 6 meter mirror.
What is the effect of the Martian atmosphere on the goal of 3 cm
resolution?
John Popelish
> On another space oriented forum I noted:
>
> "It took 20 years to increase the resolution by a factor or 10 over
> Viking with the Mars Global Surveyor mission. But only 10 years to
> increase the resolution over that of MGS by a factor of 10 with Mars
> Reconnassance Orbiter.
> Could we increase the resolution over MRO by another factor of 10 to,
> gulp, 3 cm per pixel in only 5 years this time?"
>
> Funny though, that rather off-the-cuff estimate of mine is close to
> what is possible.
> To resolve 3 cm in the optical from say a 300 km orbit would require a
> 6 meter mirror. The James Webb Space Telescope will have a 6.5 meter
> mirror and is scheduled for launch in 2013. But it was originally
> scheduled for launch in 2011:
>
> James Webb Space Telescope.
> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
>
> So going by this rate, it'll be 3mm/pixel 2.5 years after that, and
> 300 microns 1.25 years after that, and ...
> Hmm, in less than a decade then we should be able to resolve microbes
> from space.
I would rather the money be spent of building infrastructure
in orbit around Mars (the functional equivalent of cell
phone and GPS service, planet wide), and then drop hundreds
or thousands of small, self reliant rovers with lots of
different specialized sensors to comb the planet, get close
enough to actually find those microbes, if they exist, and
lots of other observations, and report back through the
overhead services. And this time, include on the rovers,
flea bots, that clean the lenses and solar cells of dust.
AustinMN
<snip>
> So going by this rate, it'll be 3mm/pixel 2.5 years after that, and
> 300 microns 1.25 years after that, and ...
> Hmm, in less than a decade then we should be able to resolve microbes
> from space.
So how long until Mars orbiters will be better able to resolve atomic
and sub-atomic scale images from orbit better than Earth-bound
instruments do it today? ;-)
Austin
dbo...@mindspring.com
The required smoothness is no problem. I have a Be mirror flat that
was polished to about 10 angstrom roughness.
El Guapo
news:fNWdncLVcpGBvTjY...@comcast.com...
OK, you had me until the flea bots.
Yousuf Khan
> On another space oriented forum I noted:
>
> "It took 20 years to increase the resolution by a factor or 10 over
> Viking with the Mars Global Surveyor mission. But only 10 years to
> increase the resolution over that of MGS by a factor of 10 with Mars
> Reconnassance Orbiter.
> Could we increase the resolution over MRO by another factor of 10 to,
> gulp, 3 cm per pixel in only 5 years this time?"
>
> Funny though, that rather off-the-cuff estimate of mine is close to
> what is possible.
> To resolve 3 cm in the optical from say a 300 km orbit would require a
> 6 meter mirror. The James Webb Space Telescope will have a 6.5 meter
> mirror and is scheduled for launch in 2013. But it was originally
> scheduled for launch in 2011:
>
> James Webb Space Telescope.
> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
Wow, from this it looks like it's going to be orbitting at the L2
Lagrangian Point, which would mean that it is stationary with respect to
the Earth and the Sun, and it will be behind the Earth on the night side
of the planet at all times, 1.5 million km's out. If it's behind the
Earth, wouldn't it automatically be shielded from the Sun's rays? Or is
the Earth's shadow not big enough out there?
Yousuf Khan
Sam Wormley
The earth's moon will perturb it position.
AustinMN
I can't answer your question, but from my understanding, being cold
(i.e. shielded from the sun) is a good thing when studying infrared.
Is it nuclear powered?
Austin
John Popelish
Do you think they are a worse idea than sitting and waiting
for a tornado to pass directly over the panel to suck the
dust off, like Spirit and Opportunity do? Why not make a
little version of one of these to keep the solar panels
spotless?
malibu
I had a 1 GB computer with 600 Gb of memory
in my hand yesterday. It weighs about 5 pounds.
It is the size of a modem.
It is our new video producer.
Cost us $200.
Sells for $750.
Ka-ching!
John
jacob navia
> Do you think they are a worse idea than sitting and waiting for a
> tornado to pass directly over the panel to suck the dust off, like
> Spirit and Opportunity do? Why not make a little version of one of
> these to keep the solar panels spotless?
>
> http://www.vcdiscounter.com/01robotic.html
Thos are designed for a thick atmosphere and not a thin one,
but it is true that it should work!
Sam Wormley
> I had a 1 GB computer with 600 Gb of memory
Perhaps you had have a 1 GHz computer with 600 GB of RAM in
your hand.
rick++
Life is likely to cause an atmosphere out of chemical equilibrium.
For example on Earth free oxygen is unstable without life and would
eventually combine into rocks.
Mars is slightly out equilibrium with the presence of methane
discovered a while back.
Phineas T Puddleduck
"AustinMN" <tacoo...@hotmail.com> wrote:
> I can't answer your question, but from my understanding, being cold
> (i.e. shielded from the sun) is a good thing when studying infrared.
> Is it nuclear powered?
>
> Austin
The colder the better. When you are looking for black body's emitting at
say 100K, then having a telescope at 100K is the equivalent of shining a
torch down your telescope and trying to see the moon l-)
--
Saucerhead lingo #2102 "However, since PTP is in reality NOT a budding
astrophysicist..." ... "Perhaps if we try distraction as a tactic people
will forget we cannot answer simple conflicting issues with our nonsense
theory"
--
Posted via a free Usenet account from http://www.teranews.com
Yousuf Khan
>> Wow, from this it looks like it's going to be orbitting at the L2
>> Lagrangian Point, which would mean that it is stationary with respect
>> to the Earth and the Sun, and it will be behind the Earth on the night
>> side of the planet at all times, 1.5 million km's out. If it's behind
>> the Earth, wouldn't it automatically be shielded from the Sun's rays?
>> Or is the Earth's shadow not big enough out there?
>>
>> Yousuf Khan
>
>
> The earth's moon will perturb it position.
Well, that's for sure, but I don't think it's going to be sitting
smack-dab in the centre of the Lagrangian point. I think it's actually
expected to "orbit" the point.
Also the Moon is 384 thousand km out, whereas the Lagrangian point is
1.5 million km out, so it's almost exactly 4 times further away than the
Moon. I don't think it's going to be a huge perturbation, and it will
likely be damped out by the orbiting motion.
Yousuf Khan
Sam Wormley
Here is what happens at L2 perturb by the moon...
http://map.gsfc.nasa.gov/about/aboutmap_pl.html
http://map.gsfc.nasa.gov/aboutmedia/990029.jpg
Yousuf Khan
> I can't answer your question, but from my understanding, being cold
> (i.e. shielded from the sun) is a good thing when studying infrared.
> Is it nuclear powered?
Actually, the webpage I linked to showed that it's got solar panels, and
in fact it's got something that no other space craft has ever had, a
huge sunshade which protects the telescope from the Sun. So obviously,
NASA thinks the Sun is still going to be visible from the L2 point. I
was just wondering out loud how big the Earth's shadow could be at that
distance?
Yousuf Khan
Sam Wormley
Another craft alread in the L2 position used a sunshade--continuously
shaded from the Sun, Earth, and Moon by the spacecraft to allow lower
thermal disturbances.
http://map.gsfc.nasa.gov/about/aboutmap_pl.html
Length of Earth's shadow (umbra) is about 1,400,000 km.
L2 is about 1,500,000 km.
Timberwoof
wrote:
http://en.wikipedia.org/wiki/Image:Lagrange_points.jpg
The math is easy: you need the size of the Sun, the distance to the
Earth, the size of the Earth, and the distance to L2. But it gets
complicated... The red arrows that point inward and the blue arrows that
point outward mean that the point is semistable. You can orbit around
the L2 (or L1 or L3) when the orbit is in a plane perpendicular to the
line between the Sun and Earth. The size of that orbit determines
whether the satellite sees the Sun.
(The Wikipedia article on Lagrange points is probably also worth
reading. I'm an expert because I read it. ;-)
--
Timberwoof <me at timberwoof dot com> http://www.timberwoof.com
It's easy to say a war is so important your neighbor should go fight it for you.
Henry Spencer
>Wow, from this it looks like it's going to be orbitting at the L2
>Lagrangian Point, which would mean that it is stationary with respect to
>the Earth and the Sun, and it will be behind the Earth on the night side
>of the planet at all times, 1.5 million km's out. If it's behind the
>Earth, wouldn't it automatically be shielded from the Sun's rays? Or is
>the Earth's shadow not big enough out there?
L2 is too far out for Earth to block the Sun completely. There is some
blockage, which helps make the L2 region a good place for infrared
telescopes, but solar power is still practical there if you oversize
the solar arrays somewhat.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | he...@spsystems.net
mme...@cars3.uchicago.edu
Actually, since the distance of L2 is about 0.01au and the Earth's
diameter is about 0.01 of this of the Sun, L2 is in the vicinity of
the distance where the Earth's shadow dwindles to nothing.
Mati Meron | "When you argue with a fool,
me...@cars.uchicago.edu | chances are he is doing just the same"
Matthew Ota
the instrument bus. The spacecraft in not nuclear powered.
Matthew Ota
Timberwoof
Hmmm. Do you think geometry might supply the answer? How big is the
craft's orbit about L2?
WillE1
"Yousuf Khan" <bbb...@yahoo.com> wrote in message
news:45a55f27$1...@news.bnb-lp.com...
Assuming objects sent to L2 don't bump into one another, here are a few more
L2 environmental concerns.
http://maxim.gsfc.nasa.gov/documents/Mission_Concept_Work/ISAL_January_2002_SST/SST_ISAL-1/Super_Star_Tracker/L2-natural-environment.pdf
Will E.
Robert Clark
> AustinMN wrote:
> > Robert Clark wrote:
> > <snip>
> > > So going by this rate, it'll be 3mm/pixel 2.5 years after that, and
> > > 300 microns 1.25 years after that, and ...
> > > Hmm, in less than a decade then we should be able to resolve microbes
> > > from space.
> The required smoothness is no problem. I have a Be mirror flat that
> was polished to about 10 angstrom roughness.
How wide is this flat? Could it be polished to this smoothness for a
parabolic mirror?
Bob Clark
Robert Clark
> ...
> To resolve 3 cm in the optical from say a 300 km orbit would require a
> 6 meter mirror. The James Webb Space Telescope will have a 6.5 meter
> mirror and is scheduled for launch in 2013. But it was originally
> scheduled for launch in 2011:
>
> James Webb Space Telescope.
> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
>
The mirror of the JWST weighs less than 400 kg out of the total weight
of 6000 kg.
It would be interesting to find out how much a craft with this size
mirror would weigh if it didn't have to carry the sunshield and
associated mass for keeping the craft at cryogenic temperatures.
Bob Clark
Steve Willner
In article <Uziph.300832$FQ1.29488@attbi_s71>,
Sam Wormley <swor...@mchsi.com> writes:
> Here is what happens at L2 perturb by the moon...
> http://map.gsfc.nasa.gov/about/aboutmap_pl.html
> http://map.gsfc.nasa.gov/aboutmedia/990029.jpg
That diagram is not about perturbations at L2. It shows how WMAP
took advantage of gravitational assists from the Moon to _reach_ L2.
JWST could do the same thing, but I don't believe it's part of the
present mission plan. (The lunar assist orbit was a very clever
invention of some folks at GSFC.)
As others have written, JWST will be in a "halo orbit" around L2 and
will be in continuous sunlight. It needs several layers of sun
shielding to keep the telescope cold. Many previous missions have
used similar techniques. There's a diagram of the JWST configuration
at http://jwst.gsfc.nasa.gov/observatory.html .
I haven't explored the rest of the web site, but there are more
details in some of the links. I expect most of the relevant
information is there somewhere.
--
Steve Willner Phone 617-495-7123 swil...@cfa.harvard.edu
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.)
Sam Wormley
> [irrelevant groups snipped]
>
> In article <Uziph.300832$FQ1.29488@attbi_s71>,
> Sam Wormley <swor...@mchsi.com> writes:
>> Here is what happens at L2 perturb by the moon...
>> http://map.gsfc.nasa.gov/about/aboutmap_pl.html
>> http://map.gsfc.nasa.gov/aboutmedia/990029.jpg
>
> That diagram is not about perturbations at L2. It shows how WMAP
> took advantage of gravitational assists from the Moon to _reach_ L2.
> JWST could do the same thing, but I don't believe it's part of the
> present mission plan. (The lunar assist orbit was a very clever
> invention of some folks at GSFC.)
Please note the continuing loop at the L2 position.
Robert Clark
> Robert Clark wrote:
>
> > To resolve 3 cm in the optical from say a 300 km orbit would require a
> > 6 meter mirror.
>
> resolution?
It is very likely less than for Earth's atmosphere which gives a best
resolution of .5 arcsec even from the best sites. A .5 arcsec
resolution at 300 km would correspond to a resolution of 11 cm on the
ground. In Mars thinner atmosphere, the results would probably be much
better than this.
However, I've seen some references that suggest the effects of air
scattering is much less pronounced looking down, as with spy
satellites, than when looking up, as with telescopes.
In any case adaptive optics could probably bring the seeing close to
the diffraction limit anyway.
Bob Clark
Sam Wormley
Thanks Bob.
Steve Willner
In article <2nzph.301886$FQ1.216029@attbi_s71>,
Sam Wormley <swor...@mchsi.com> writes:
> Please note the continuing loop at the L2 position.
Right. That's the halo orbit; nothing to do with lunar
perturbations.
Sam Wormley
>>>> http://map.gsfc.nasa.gov/about/aboutmap_pl.html
>
> In article <2nzph.301886$FQ1.216029@attbi_s71>,
> Sam Wormley <swor...@mchsi.com> writes:
>> Please note the continuing loop at the L2 position.
>
> Right. That's the halo orbit; nothing to do with lunar
> perturbations.
>
And the period of the halo orbit is not in sync with
the lunar period?
Steve Willner
Sam Wormley <swor...@mchsi.com> writes:
> And the period of the halo orbit is not in sync with
> the lunar period?
Correct. See http://www.stsci.edu/jwst/overview/design/orbit.html
Halo orbit period is about 6 months. I think it must depend on the
radius of the orbit around L2; presumably one could select a period
of one lunar month, though it would seem to be a bad idea. Another
paper that discusses the concepts is at
http://highorbits.jhuapl.edu/missions.htm , and the Wikipedia article
at http://en.wikipedia.org/wiki/Lagrangian_point looks pretty good,
though I haven't studied it in detail.
You can guess that lunar perturbations will be negligible just from
noticing how far L2 is from lunar orbit. Perturbations will be
roughly (1/81)*(D_moon/D_earth)^2, where 1/81 is the Moon/Earth mass
ratio and D_moon/D_earth is the radius of the Moon's orbit divided by
the distance from Earth to L2.
Sam Wormley
> In article <swPph.302917$FQ1.183562@attbi_s71>,
> Sam Wormley <swor...@mchsi.com> writes:
>> And the period of the halo orbit is not in sync with
>> the lunar period?
>
> Correct. See http://www.stsci.edu/jwst/overview/design/orbit.html
> Halo orbit period is about 6 months. I think it must depend on the
> radius of the orbit around L2; presumably one could select a period
> of one lunar month, though it would seem to be a bad idea. Another
> paper that discusses the concepts is at
> http://highorbits.jhuapl.edu/missions.htm , and the Wikipedia article
> at http://en.wikipedia.org/wiki/Lagrangian_point looks pretty good,
> though I haven't studied it in detail.
>
> You can guess that lunar perturbations will be negligible just from
> noticing how far L2 is from lunar orbit. Perturbations will be
> roughly (1/81)*(D_moon/D_earth)^2, where 1/81 is the Moon/Earth mass
> ratio and D_moon/D_earth is the radius of the Moon's orbit divided by
> the distance from Earth to L2.
>
Thank you, Steve.
-Sam
Keith F. Lynch
> Yousuf Khan <bbb...@yahoo.com> wrote:
>>> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
>> Wow, from this it looks like it's going to be orbitting at the
>> L2 Lagrangian Point, ...
> L2 is too far out for Earth to block the Sun completely. There
> is some blockage, which helps make the L2 region a good place for
> infrared telescopes, but solar power is still practical there if
> you oversize the solar arrays somewhat.
Presumably the solar panels will be flat, shiny, and perpendicular to
the direction to the sun -- which is also the direction to the earth.
Will it show up as a bright spark of light in the night sky, a
perpetual sun glint?
--
Keith F. Lynch - http://keithlynch.net/
Please see http://keithlynch.net/email.html before emailing me.
bbbl67
> Length of Earth's shadow (umbra) is about 1,400,000 km.
> L2 is about 1,500,000 km.
Ah, I see, thanks.
bbbl67
> Actually, since the distance of L2 is about 0.01au and the Earth's
> diameter is about 0.01 of this of the Sun, L2 is in the vicinity of
> the distance where the Earth's shadow dwindles to nothing.
I assume from that if the Earth were in orbit around a star like VY
Canis Majoris (2000x diameter of the Sun), we can forget about having a
much of a night-time, as the light from it is likely going to wrap
around the far side of the planet almost? :-)
Yousuf Khan
bbbl67
> Presumably the solar panels will be flat, shiny, and perpendicular to
> the direction to the sun -- which is also the direction to the earth.
> Will it show up as a bright spark of light in the night sky, a
> perpetual sun glint?
At 1.5 million miles away, I doubt it will even be seen without a
telescope.
Yousuf Khan
Oren Tirosh
> In article <45a55f27$1...@news.bnb-lp.com>, Yousuf Khan <bbb...@yahoo.com> wrote:
> >> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
> >Wow, from this it looks like it's going to be orbitting at the L2
> >Lagrangian Point, which would mean that it is stationary with respect to
> >the Earth and the Sun, and it will be behind the Earth on the night side
> >of the planet at all times, 1.5 million km's out. If it's behind the
> >Earth, wouldn't it automatically be shielded from the Sun's rays? Or is
> >the Earth's shadow not big enough out there?
>
> L2 is too far out for Earth to block the Sun completely. There is some
> blockage, which helps make the L2 region a good place for infrared
> telescopes, but solar power is still practical there if you oversize
> the solar arrays somewhat.
Does any planet have L2 within its cone of darkness? Jupiter perhaps?
Robert Clark
Actually you should check this calculation. I'm informed via email of
a mistake I made in calculating resolution on a different topic. I
didn't convert from degrees to radians.
Let me try that again. The formula for resolution is the Rayleigh
criterion:
Angular resolution.
http://en.wikipedia.org/wiki/Angular_resolution#Explanation
sin(θ) = 1.22λ/D
Now for the angle θ small and in *radians*, sin(θ) is approximately
equal to θ, and θ is approximately equal to the ratio of the distance
to be resolved divided by the distance to the object. A .5 arcsec angle
is .5/(60*60) = 1/7200 degrees. Converting to radians this is
(2π/360)/7200 radians. Then this is about the ratio of the resolvable
distance to the distance to the body. At 300,000 m then the resolvable
distance would be 300,000 times this or .73 m, 73 cm.
But note this is assuming the atmospheric dispersion puts the same
limit on resolution looking down as with looking up. As I said there is
some question about this. And in the case for Mars the atmospheric
dispersion limits should also be much less.
Bob Clark
mme...@cars3.uchicago.edu
>
> Now for the angle =CE=B8 small and in *radians*, sin(=CE=B8) is approximat=
>ely
>equal to =CE=B8, and =CE=B8 is approximately equal to the ratio of the dist=
>ance
>to be resolved divided by the distance to the object. A .5 arcsec angle
>(2=CF=80/360)/7200 radians. Then this is about the ratio of the resolvable
>distance to the distance to the body. At 300,000 m then the resolvable
>distance would be 300,000 times this or .73 m, 73 cm.
> But note this is assuming the atmospheric dispersion puts the same
>limit on resolution looking down as with looking up.
Where do you see ***anything*** having to do with atmospheric
dispersion in the above.
As I said there is
>some question about this. And in the case for Mars the atmospheric
>dispersion limits should also be much less.
Robert Clark
When I was giving the resolution a mirror about Mars could achieve I
was assuming it would give diffraction limited performance. Sam raised
the question about the additional limits imposed by atmospheric
scattering.
On Earth for telescopes looking up, this puts a limit on the
resolution a scope can achieve no matter what its size. The number I've
seen quoted was the best you could do would be no better than that of a
20" scope regardless of the telescope size(!)
Large ground based telescopes without methods to counteract this such
as adaptive optics can only detect fainter objects not achieve better
resolution.
However, I've seen mentioned this doesn't happen for a satellite
looking down to the surface. In that case resolution limits imposed by
the atmosphere wouldn't apply for satellites imaging the surface.
There was some discussion on this topic on this thread:
Spy satellites for astronomy.
http://groups.google.com/group/sci.astro/browse_frm/thread/a0ae7386d655847c/
Bob Clark
mme...@cars3.uchicago.edu
The sentence you wrote above, following the discussion of resolution
limits, says "this is assuming the atmospheric dispersion puts the same
limit on resolution looking down as with looking up.". Well, no, if
"this" refers to the text quoted then *this* is assuming nothing of the
sort. That's all.
zzbu...@netscape.net
Sam Wormley wrote:
> Robert Clark wrote:
>
> > To resolve 3 cm in the optical from say a 300 km orbit would require a
> > 6 meter mirror.
>
> What is the effect of the Martian atmosphere on the goal of 3 cm
> resolution?
None, since orbital resolution is not how
you resolve microbes.
The only thing sattelites are going to
determine is the dust concentration in
the Martian atmoshere.
Keith F. Lynch
I think you mean 1.5 million *kilometers*.
A specular reflector reflecting the sun is *much* brighter than a
white object. The sun's disk is about 9.3 milliradians wide in
Earth's sky, and has an apparent magnitude of -26.7. A specular
reflection of the sun, from a perfect reflector, will have the same
amount of light per unit solid angle.
Five magnitudes represent a factor of a hundred. An object with one
tenth the angular diameter will reflect one hundredth the light,
so will be five magnitudes dimmer. At 1.5 million kilometers, a
reflector 14,000 kilometers wide will appear as bright as the sun (if
it's aimed just right). Knock it down to 14 kilometers, and that
drops it by 15 magnitudes, to -11.73, still bright enough to read
by. Knock it down to 14 meters, and you're down to +3.3, still a
moderately bright star.
At L2, the Earth will block 84% of the sunlight, so make it +5.3. And
of course the solar panels aren't perfect reflectors, and probably
aren't 14 meters in diameter. But the final number is intriguingly
close to the limit of the normal unaided eye (+6.0). Even if it isn't
quite visible to the eye, it ought to be easily visible in a pair of
basic binoculars.
But a better question is why the successor to the 17-year-old Hubble
Space Telescope is intended to last only five years. And cannot be
reached for maintenance. And will be unable to do some things Hubble
could do.
bbbl67
> But a better question is why the successor to the 17-year-old Hubble
> Space Telescope is intended to last only five years. And cannot be
> reached for maintenance. And will be unable to do some things Hubble
> could do.
Well how long was Hubble originally supposed to last? I seem to recall
it's gone a little longer than its originally anticipated expiration
date. It's likely JW might go longer than its expiration date too.
Also there was some talk of outfitting Hubble with some of the new
robot arms that the space station has, so that it could do some
self-servicing. That was one of the proposals in lieu of sending a
space shuttle to service it. I wonder why they didn't outfit the JW
with some of these robot arms at the outset?
Also it will take 5 seconds for signals to reach back and forth from
Earth to the L2 point.
Yousuf Khan
Keith F. Lynch
> Well how long was Hubble originally supposed to last?
I don't know. Do you? I assumed it was intended to last indefinitely
with routine maintenance, just like observatories on the ground.
When the 200-inch Hale Telescope was completed in 1948, there were no
plans to shut it down in 1953, or even 1965. It is of course still in
operation, as are telescopes more than twice as old.
> Also there was some talk of outfitting Hubble with some of the new
> robot arms that the space station has, so that it could do some
> self-servicing. That was one of the proposals in lieu of sending a
> space shuttle to service it. I wonder why they didn't outfit the JW
> with some of these robot arms at the outset?
Because they are incapable of doing much? Human hands, even in
spacesuit gloves, are still incomparably more dextrous. Besides,
sometimes spare parts are needed, such as the replacement gyros and
batteries now needed at Hubble.
William Hamblen
> bbbl67 <yjk...@gmail.com> wrote:
>> Well how long was Hubble originally supposed to last?
>
> I don't know. Do you? I assumed it was intended to last indefinitely
> with routine maintenance, just like observatories on the ground.
That isn't a good assumptiom, because maintaining the Space
Telescope is far more difficult than maintaining a ground based
telescope. The original mission planning was for a service life
of 15 years.
Bud