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Solar light on Pluto



 
 
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  #1  
Old January 8th 06, 12:04 PM posted to sci.space.science
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Default Solar light on Pluto

Folks

Reading through various articles about the upcoming launch of the New
Horizon probe I did not manage to locate any solid information about
the amount of energy & light available from the Sun on Pluto.

I remember having read something like "about the size and brightness
of Venus as seen from earth".

If it's true I find it really remarkable that an earth based
telescope would manage to "see" Pluto...


--alexT

  #2  
Old January 9th 06, 10:13 PM posted to sci.space.science
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Default Solar light on Pluto

Reading through various articles about the upcoming launch of the New
Horizon probe I did not manage to locate any solid information about
the amount of energy & light available from the Sun on Pluto.


The sun emits about 1400 watts per square meter at Earth's orbit.

Pluto is (roughly) 40 times as far from the Sun as Earth.

Per the inverse square law, Pluto intercepts 1/1600th as much sunlight
as Earth, per square meter.

Therefore, Pluto receives approximately 0.875 watts of sunlight per
square meter.

If you want more precise numbers, you can google up precise values for
Pluto's current distance from the sun and the exact quantity of
sunlight received in Earth orbit.

If it's true I find it really remarkable that an earth based
telescope would manage to "see" Pluto...


Pluto is an object 2274000 meters in diameter. As a quick
approximation, that means it presents a disk of some 4.06E12 square
meters, intercepting a total of 3.55E12 watts (3.55 terawatts) of
sunlight. Using Pluto's lowest albedo value of 0.49, Pluto reflects
about 1.74 terawatts of sunlight for terrestrial telescopes to spot.
That's quite a bit of light.

Of course, Pluto doesn't focus its light like a manmade spotlight but
rather reflects that light across a hemisphere, but even minor
quantities of light add up when you're illuminating objects 2000km in
diameter.

Mike Miller

  #3  
Old January 10th 06, 01:33 AM posted to sci.space.science
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Default Solar light on Pluto

AlexT wrote:
Folks

Reading through various articles about the upcoming launch of the New
Horizon probe I did not manage to locate any solid information about
the amount of energy & light available from the Sun on Pluto.

I remember having read something like "about the size and brightness
of Venus as seen from earth".

If it's true I find it really remarkable that an earth based
telescope would manage to "see" Pluto...


--alexT


The brightness of an object at a given distance requires knowledge of
the luminosity of the light-emitting object and that distance via:

b = L/(4pi)d^2

The luminosity of the Sun is 3.9 x 10^26 Watts and the average distance
to Pluto is 5.916 x 10^12 meters. This will tell you the watts/square
meter at Pluto. To compare it to what we get here, put in the Earth's
average distance of 1.5 x 10^11 meters or do ratios of the equation for
Pluto compared to here, eliminating the L/(4pi) term.

The ratio of brightness number can be used to determine the magnitude of
the Sun compared to its magnitude he

m(E)-m(P) = 2.5 log [(b(P)/b(E)]

m(E) = -26.8, so you can do the ratio of brightness of Pluto to Earth to
calculate the magnitude of the Sun at Pluto. Venus at its most
brilliant comes in about -4.4, so you can check your answer against that
marker to answer your question.
  #4  
Old January 10th 06, 01:12 PM posted to sci.space.science
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Default Solar light on Pluto

AlexT wrote:
Reading through various articles about the upcoming launch of the New
Horizon probe I did not manage to locate any solid information about
the amount of energy & light available from the Sun on Pluto.


At the top of the Earth's atmosphere, the "solar constant"
(= incident power-per-unit-area from the sun across all wavelengths)
is 1340 Watts/m^2.

Pluto's mean distance from the sun is 39.4 times that of the Earth.
[The orbit is rather elliptical, so this distance
changes as Pluto orbits the Sun. At its closest
(most recently in Sept 1989), Pluto's distance from
the Sun is "only" 29.6 AUs (where an AU, "astronomical
unit", is the Earth's mean distance from the Sun);
at its farthest (which will next happen in the year
2113) Pluto's distance from the Sun is 49.3 AUs.]

The solar power-per-unit-area falls as the inverse square of the
distance from the sun, so at a distance of 39.4 times that of the
sun, the incident solar power-per-unit-area at Pluto is 0.86 Watt/m^2.
Right now Pluto is only 16 years past its perihelion (the point in
its orbit where it's closest to the Sun), so if we use the perihelion
distance of 29.6 AUs, that gives a solar power-per-unit-area of
1.5 Watt/m^2.

For comparison, this latter power-per-unit-area is equal to that
of a 100 Watt light bulb at a distance of 2.3 meters. That's
a bit dim for office lighting, but still plenty to read by.

ciao,

--
-- "Jonathan Thornburg -- remove -animal to reply"
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam
  #5  
Old January 10th 06, 02:52 PM posted to sci.space.science
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Default Solar light on Pluto

AlexT wrote:
Folks


Reading through various articles about the upcoming launch of the New
Horizon probe I did not manage to locate any solid information about
the amount of energy & light available from the Sun on Pluto.


I remember having read something like "about the size and brightness
of Venus as seen from earth".


If it's true I find it really remarkable that an earth based
telescope would manage to "see" Pluto...



In 2012, for example, Pluto will be 32.2 times as far from the Sun
as we are, so sunlight will be 32.2^2 = 1037 times weaker than
we see it. That means it still delivers 1.3 W/meter^2. The sun
wojld still apear quite bright; that would make it apparent
magnitude -18.5, which is still 250 times as bright as our
view of a full Moon. That's plenty of light to see your way
around or get good images (and it's not all that much less light
than Voyager 2 had to deal with at Neptune and Triton).

Bill Keel
  #6  
Old January 11th 06, 03:09 PM posted to sci.space.science
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Default Solar light on Pluto

Simple calculation with round numbers:

- Pluto at 40 AU vs. Earth at 1 AU

- thus, Sun appears (1*1)/(40*40) = 1/1600 times as
bright from Pluto as from the Earth

- that corresponds to 2.5*log10(1/1600) = 8 magnitudes
fainter

- from Earth, the Sun's apparent magnitude is about -26

- so from Pluto, the Sun's apparent magnitude is about -18

For comparison, Venus appears roughly magnitude -4 as
seen from the Earth. Thus, Venus as seen from the Earth
is about 14 magnitudes fainter (= 400,000 times fainter)
than the Sun as seen from Pluto.

I leave it to you to do the comparison of apparent
angular sizes.

Michael Richmond

  #7  
Old January 12th 06, 10:18 PM posted to sci.space.science
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Default Solar light on Pluto

Thanks everyone for those detailed answers !



--alexT

  #8  
Old January 12th 06, 10:53 PM
[email protected] tjp314@pacbell.net is offline
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Default

Quote:
Originally Posted by AlexT
Folks

Reading through various articles about the upcoming launch of the New
Horizon probe I did not manage to locate any solid information about
the amount of energy & light available from the Sun on Pluto.

I remember having read something like "about the size and brightness
of Venus as seen from earth".

If it's true I find it really remarkable that an earth based
telescope would manage to "see" Pluto...


--alexT

I seem to remember an article in S&T many years ago about the brightness and UV exposure at various distances from the sun. The article said that the sun from Pluto would be about 250 times brighter than the full moon from Earth.

If it were the angular size of Venus, it would be much brighter, as it is a light source, whereas Venus is a reflector.

-Tim.
 




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