Bruce Mardle wrote:I'm just reading the Wikipedia page on Phobos. It says, "Phobos orbits about 9,400 km (5,800 mi) from the center of Mars, or about 6,000 km (3,700 mi) from the Martian surface", whereas this APoD page says, "... Phobos orbits so close to Mars - about 5,800 kilometers above the surface...".
Which one is right?
"About 6,000 km" is virtually the same thing as "about 5,800 kilometers".
Bruce Mardle wrote:
I'm just reading the Wikipedia page on Phobos. It says, "Phobos orbits about 9,400 km (5,800 mi) from the center of Mars, or about 6,000 km (3,700 mi) from the Martian surface", whereas this APoD page says, "... Phobos orbits so close to Mars - about 5,800 kilometers above the surface...". Which one is right?
Phobos is in an elliptical orbit such that the height about the Martian equator varies from 5840 km to 6123 km.
~5800 km is the best "closest" approach number
whereas ~6000 km is the best average number.
neufer wrote:Phobos is in an elliptical orbit such that the height about the Martian equator varies from 5840 km to 6123 km.
~5800 km is the best "closest" approach number
whereas ~6000 km is the best average number.
If I may be a little pedantic here... all closed orbits are elliptical. The reason that Phobos's distance from the surface varies is because the eccentricity of its orbit is greater than zero.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
neufer wrote:
Phobos is in an elliptical orbit such that the height about the Martian equator varies from 5840 km to 6123 km.
~5800 km is the best "closest" approach number
whereas ~6000 km is the best average number.
If I may be a little pedantic here... all closed orbits are elliptical.
http://en.wikipedia.org/wiki/Argument wrote:
Elliptical arguments: <<Often an argument is invalid because there is a missing premise--the supply of which would render it valid. Speakers and writers will often leave out a strictly necessary premise in their reasonings if it is widely accepted and the writer does not wish to state the blindingly obvious. Example: All metals expand when heated, therefore iron will expand when heated. (Missing premise: iron is a metal). On the other hand, a seemingly valid argument may be found to lack a premise – a 'hidden assumption' – which if highlighted can show a fault in reasoning. Example: A witness reasoned: Nobody came out the front door except the milkman; therefore the murderer must have left by the back door. (Hidden assumption- the milkman was not the murderer).>>
Chris Peterson wrote:
The reason that Phobos's distance from the surface varies is because the eccentricity of its orbit is greater than zero.
Eccentric, a. [F. excentrique, formerly also spelled eccentrique, fr. LL. eccentros out of the center, eccentric, Gr. ; out of + center.]
4. Not coincident as to motive or end.
His own ends, which must needs be often eccentric to those of his master. Bacon.
5. Deviating from stated methods, usual practice, or established forms or laws; deviating from an appointed sphere or way; departing from the usual course; irregular; anomalous; odd; as, eccentric conduct. This brave and eccentric young man." Macaulay.
He shines eccentric, like a comet's blaze. . Savage
neufer wrote:Phobos is in an elliptical orbit such that the height about the Martian equator varies from 5840 km to 6123 km.
~5800 km is the best "closest" approach number
whereas ~6000 km is the best average number.
If I may be a little pedantic here... all closed orbits are elliptical. The reason that Phobos's distance from the surface varies is because the eccentricity of its orbit is greater than zero.
Ok, let's be pedantic. By definition, an ellipse has an eccentricity of between zero and one. If the eccentricity was zero, the orbit would be circular, parabolic if equal to one, and hyperbolic if greater than one. So, neufer's statement that Phobos's orbit is elliptical is sufficient. The eccentricity may determine the amount of variance, but to state that the orbit is elliptical is enough to say the distance varies.
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
bystander wrote:Ok, let's be pedantic. By definition, an ellipse has an eccentricity of between zero and one. If the eccentricity was zero, the orbit would be circular, parabolic if equal to one, and hyperbolic if greater than one. So, neufer's statement that Phobos's orbit is elliptical is sufficient. The eccentricity may determine the amount of variance, but to state that the orbit is elliptical is enough to say the distance varies.
No, simply stating the orbit is elliptical is not a sufficient answer. In orbital mechanics, all closed orbits are ellipses, with the eccentricity greater than or equal to zero, and less than one. A circular orbit is an elliptical orbit with e=0. The point here is that the distance of Phobos from the surface varies over its orbit- that is not a consequence of an elliptical orbit, but a consequence of an orbit with a non-zero eccentricity.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
Chris Peterson wrote: In orbital mechanics, all closed orbits are ellipses, with the eccentricity greater than or equal to zero, and less than one. A circular orbit is an elliptical orbit with e=0.
Well, they got it wrong. A circle is not an ellipse. An ellipse has two foci, a circle only one (the center). Might as well say a parabola is an ellipse (which obviously it is not), as that is the other limiting case.
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
Chris Peterson wrote:
In orbital mechanics, all closed orbits are ellipses, with the eccentricity greater than or equal to zero, and less than one. A circular orbit is an elliptical orbit with e=0.
Well, they got it wrong. A circle is not an ellipse. An ellipse has two foci, a circle only one (the center).
Might as well say a parabola is an ellipse (which obviously it is not), as that is the other limiting case.
http://en.wikipedia.org/wiki/Ellipse wrote:
<<An ellipse (from Greek ἔλλειψις elleipsis, a "falling short") is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis. An ellipse is also the locus of all points of the plane whose distances to two fixed points add to the same constant.
A line segment is a degenerate ellipse with semi-minor axis = 0 and eccentricity = 1, and with the focal points at the ends. Although the eccentricity is 1 this is not a parabola.>>
neufer wrote:Circles are special cases of ellipses
A line segment is a degenerate ellipse with semi-minor axis = 0 and eccentricity = 1>>
This is wrong, too. By definition, an ellipse is a conic section with an eccentricity between zero and one, non-inclusive ( 0 < e < 1 ).
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk. — Garrison Keillor
A circle is a Platonic section: the curve obtained
as the intersection of Plato with a plane.
bystander wrote:
neufer wrote:
Circles are special cases of ellipses
A line segment is a degenerate ellipse
with semi-minor axis = 0 and eccentricity = 1>>
This is wrong, too.
By definition, an ellipse is a conic section with an eccentricity between zero and one, non-inclusive ( 0 < e < 1 ).
http://en.wikipedia.org/wiki/Conic_section wrote:
<<In mathematics, a conic section (or just conic) is a curve obtained as the intersection of a cone (more precisely, a right circular conical surface) with a plane. Traditionally, the three types of conic section are the hyperbola, the parabola, and the ellipse.
The circle is a special case of the ellipse, and is of sufficient interest in its own right that it is sometimes called the fourth type of conic section.>>
Chris Peterson wrote: In orbital mechanics, all closed orbits are ellipses, with the eccentricity greater than or equal to zero, and less than one. A circular orbit is an elliptical orbit with e=0.
Well, they got it wrong. A circle is not an ellipse. An ellipse has two foci, a circle only one (the center). Might as well say a parabola is an ellipse (which obviously it is not), as that is the other limiting case.
They did not get it wrong. A circle is an ellipse. The equation used to describe a circle is exactly the same as that used for an ellipse- and in orbital dynamics, no special simplified equation is used for the special case of a circular orbit. Every orbit is treated as an ellipse mathematically, with a circular orbit simply having an eccentricity of zero.
It's simply a matter of convention if you want to call a circle its own thing, or call it a ellipse with zero eccentricity. The latter is the convention in orbital mechanics, because it makes the analysis simpler.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory https://www.cloudbait.com
In the 1950's as a result of It's unnatural-looking, elongated shape, the huge indentation carved out at one end had the USSR conclude that Phobos was a space station built by an advanced civilization inhabited on Mars. It's likeness reminiscent of a potato has a certain appearance that easily plays on the human imagination (think face on Mars) especially with the less than optimum telescopic capabilities of the era. Russia's romantic attachment with Phobos might help explain why their most recent probe to Mars and its vicinity had as its main objective a daring attempt to land on Phobos. it is a tragic loss for all mankind the adventure, knowledge to have been acquired never achieved as a result of the failure in Earth orbit of the Mars probe within hours of lift-off. The rage emanating from the Russian space agency as a result of its incredibly sorry record of near total failure of Russian probes to Mars had the agency go as far as to blame the most recent disaster of its Mars missions on nothing less than sabotage, a brazen accusation of egregious proportions by any means.
x00x wrote:In the 1950's as a result of It's unnatural-looking, elongated shape, the huge indentation carved out at one end had the USSR conclude that Phobos was a space station built by an advanced civilization inhabited on Mars. It's likeness reminiscent of a potato has a certain appearance that easily plays on the human imagination (think face on Mars) especially with the less than optimum telescopic capabilities of the era.
Not quite. The shape of Phobos was not known from direct observation until the Mariner probes (the Mariner 7 flyby in 1969 got a low-resolution silhouette, Mariner 9 showed it in detail in 1971). What did attract attention early (Iosif Shklovskii worked on this, IIRC) was the apparent orbital decay rate of Phobos, which would have required a very low density to match what was known of the Marian atmosphere. (It turned out that the rapid decay was an artifact of data with larger errors than believed, an all-too-frequent occurrence). This low calculated density led to speculation about Phobos being hollow, and therefore an artifact of the greatest interest.
Postby klaskinaragingwind » Fri Nov 02, 2012 5:52 pm
If you turn this picture counter clockwise about 60 degrees those "Egyptian Hieroglyphics" resolve into four stylized people: A man with a crown, a lady with a basket, and their two girls.... looks a lot like the old carvings of Sumarian kings noted in Steve Quayle's book Angel Wars.
Curious to say the least.
Also I would note that the "crater" around two o'clock position near the front looks a lot more like a dent in metal than a crater in rock. Low velocity impact perhaps and/or high metallic content in the rock would be my guess.
K
Tara_Li wrote:Looking at the full image - in the area around pixel location 1997,1652 - there's something curious, at least to my eye. Zoomed in, it's about 5 pixels (35 ish meters) across, and very sharply demarked. The obvious answer is that it's a small upthrust of some kind, or a large loose boulder - but to my eye, it *LOOKS* almost as though it's above the surface. It would be awesome to have caught a meteor coming in before it struck, or a bouncing rock from ejecta.
I also thought that it might be above the surface so I zoomed up.
It looks like a bright yellow and red crashed rocket body.
Does anyone know who it belongs to?
Harvey2 wrote:I also thought that it might be above the surface so I zoomed up.
It looks like a bright yellow and red crashed rocket body.
Does anyone know who it belongs to?
Drat! I thought I'd shovelled enough dust onto it to obscure it until I could get back with the puncture kit.
One thing I thought odd, given Phobos's presumed origin as a captured asteroid, was its similarity in reddish color to Mars. A number of other asteroids have been photographed, and appeared to be gray. Was this only due to monochrome film?
