Category: Planets

Urban Astronomy – Mars in October

October 19, 2020

The night sky is pretty much a mystery to most City Dwellers.  The glare of city light drowns out all but the brightest stars – and planets don’t do much better.  If you are interested, I can tell you where to look to see these far-off worlds.  If you were not interested, you would have stopped reading after the first sentence.  So, at this point, I know my audience.  😉

Urban Astronomy

    I used the moon as a pointer for Saturn and Jupiter last time.  On October 29, the moon will be over by Mars in the evening sky.  I include a sky map for that date at 20:00 (8 PM) – from Heavens-Above.com” – below. 

Skymap from Heavens-Above.com  

 The moon will be near full at the time.  Mars happens to be near its closest to the Earth right now, as well.  The sky map can be used by printing it out and holding it (or your phone, with the chart displayed) over your head.  You need to orient the chart with the sky, of course.  If you know where the North Star is, use that – or there are more than one compass apps for your phone to be downloaded for free.

    When this reporter was young (an era also referred to as the Cretaceous Period), it was thought that Mars must certainly harbor life – at the very least, plant life – since it showed seasonal changes that were attributed to vegetation.  After a fly-by passage of the space probe Mariner 4 past Mars, it became apparent that Mars more closely resembled the crater-covered surface of our own moon.  Measurements of the atmospheric pressure on the Red Planet also made it quite unlikely that life as we know it – could survive on Mars.  Subsequent orbiters and landers have all failed to produce evidence of life on Mars.  The seasonal changes are now known to be dust patterns.  While the seasonal ice caps at the poles do contain water ice, they also are composed of carbon dioxide ice frozen out from the mostly CO2 (and extremely low-pressure) atmosphere.  Despite what many ill-informed Mars enthusiasts think, a human would need – for a walk on Mars – a space suit just like those used by Moonwalkers of the 1960’s/70’s.  Still, Mars would be “A Nice Place to Visit”

   Mars is where the moon is and about as bright as it ever gets.  Ya’ can’t miss it!

   Notice that Jupiter and Saturn are still hanging around at 8 PM – both are not far from where they were previously.  You should be able to spot them unaided,  but use the skymap, if not.

   Mars and the moon will be up all night, not just at 8 PM, so don’t try to use that as an excuse! 😉

Urban Astronomy

 

October 17, 2020 Introducing a new category

Despite this reporter’s career as a Geophysicist, he was an Astronomer by education.  These fields of study overlap when the Astronomy is “planetary”, since Geophysics involves the study of a particular planet to be found under the feet of a standing Astronomer.

  Your Humble narrator also has the habit of swimming – the only form of exercise he can manage to “stick to”.    If one can manage to show up at the pool early in the morning, a single-use swimming lane is to be obtained.  Large swimmers are particularly disadvantaged when “sharing” lanes – and said narrator is large. 

    Reporters are often instructed – by Editors – to refer to themselves in the third person (as above).  Since I am my own Editor here, I am now putting an end to that.

      If I can snag a lane in the outdoor pool, I am able to swim alone and in relative darkness, but even so there is considerable urban lighting with which to contend – not to mention the glaring lamps in the sides of the pool.  Nevertheless, the Astronomer/Swimmer takes note when there are visible stars and planets available for viewing.  

   A few months back, it happened that there were four planets visible from the outdoor swimming pool.  My natural tendency is to point things like this out perfect strangers.  However, some of the people around at this time of morning know me – by sight at least.  And we are all eccentric individuals, or we would not be swimming at 5 AM.  Since having four “Eyes Only” planets in the sky at once is a fairly rare thing, I pointed these out.  The average reaction was one of surprise.  They had no idea of what was over their heads.

The oldest Astronomer joke in the world is to promise to point out one more than the number of planets actually visible.  So, I promised five and pointed out four.  The punch line of this ancient bit of comedy relies on the audience to ask where the fifth one might be. Then the Astronomer replies,  “You’re standing on it!”.   Of course, no one asked about that fifth planet and the joke went flat.  Don’t you people have any curiosity?  Yes, it is an old joke, but none of them ever heard it before – because it is so old.

   Now, the interesting part about that stale humor – if you think about it – is the point it makes.  Specifically, that all those small round lights that this Swimmer is pointing out are actually massive worlds – a lot like the one we are standing on.  Some of them are stunningly larger, some smaller than our own planet.  Some are closer to – or farther from – the Sun.  There are places where “the world” is profoundly different from the “world” we experience.  With those thoughts, the human imagination is expanded beyond the mundane existence of day-to-day life.

  The night sky is pretty much a mystery to most City Dwellers.  The glare of city light drowns out all but the brightest stars – and planets don’t do much better.  If you are interested, I can tell you where to look to see these far-off worlds.  If you were not interested, you would have stopped reading after the first paragraph.  So, at this point, I know my audience.  😉

Some description of how far apart objects in the sky is needed, and “degrees” are another unfamiliar subject for Urbanites.  Extend your thumb and put it at arm’s-length and close one eye.  Now your thumb covers one degree of sky.  For Apollo 13 fans – this is why Jim Lovell was making the same weird gestures.  He was, in fact measuring the sky in degrees.

  Most Sophisticated Urbanites are also unfamiliar with compass directions.   The moon shows up pretty well, however.  So, I will use the moon to “landmark” where you Urbanites should look to find points of interest in the sky.  Those will be planets mostly, since at least four of same are bright enough to be seen in the City Glare.  The moon moves quickly across the starry sky, so pay attention to the dates and times. This first installment is for the night of October 22nd at 20:00 (8PM). 

Skymap from Heavens-Above website:  https://heavens-above.com/SkyChart2.aspx This link is calibrated from my patio. If you are near me, it will work for you. If not, you can change the location to your own house, with one of the links at left of the homepage.

At that date and time the moon will be near Jupiter and Saturn in the South-Southwestern sky.  Jupiter and Saturn are approaching a conjunction which will peak in December when the two planets will be within a tenth of one degree of each other – in the Earth’s sky, only.  They will actual be separated by about five Astronomical Units.  And – an AU is the distance from the Earth to the Sun (about 93,000,000 miles).

   All of my description, the thumb-and-one-eye-closed bit and the skychart are just illustrations.  This is Urban Astronomy – just look where the moon is!  😉

Scenic Views in the Solar System

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This was posted before, over at the old WordPress site.  I have since updated the pictures with the latest images of Pluto and Charon.  Also, comparison to the view of the moon from Earth is included in each picture and a lot of reference lines were deleted.  January 25, 2019

Steve Campbell     December 2015

Introduction

The full moon, especially when it is near the horizon has always been a most impressive sight and there are few that compare…at least on Earth. The truth is that your index finger at arm’s length can cover the moon. You will need to close one eye for this trick, though. If you saw Tom Hanks in Apollo13, you know the drill. The full moon covers less than one half of one degree of arc. By the way, the idea that the moon is somehow magnified by being near the horizon is just an illusion.  If you measured the angle it “subtends”, you will come up with the same number when it is overhead or low in the sky. Call it a Jedi mind trick, if you like. Figure A is a simulation of the Moon, as seen from the Earth. So that we can make a fair comparison, the field of view here is 10 degrees across. I will keep that constant as we move around. The lines are just the plotted orbits of the moon and other planets in the background.

moonfromearthsolo

Figure A: A simulation of the Moon as seen from Earth. The field of view is 10 degrees wide. That scale will be continued in the following images – until it doesn’t work.  I’ll include the moon in those images as a reference.

In the neighborhood

Impressive as our full moon view may be there are other, more breath-taking vistas to be had, with a small matter of transportation to be solved. There is one that has actually been experienced by Apollo astronauts on eight missions – each with a crew of three – that reached the moon. So, that is exactly 24 people, right? Wrong, twenty one. John Young and Eugene Cernan both went to the moon twice and landed once. Jim Lovell went twice and alas, did not land either time. But all these guys saw the Earth from the moon, even if only 12 saw it from the surface. That sight is even more striking. Figure B is a simulation of the Earth as seen from the moon.

earth_from_moon_comp

Figure B: Simulation of the Earth as seen from the moon. Same scale

As awe-inspiring as this view may be, you can still hide this behind a single digit, but you will need to use your thumb. The Earth’s atmosphere is not depicted here. That would make it more interesting, especially during a lunar eclipse. At that time, the Earth would be illuminated only by artificial lights and the occasional burst of lightening. If it were a total lunar eclipse, the rim of Earth’s atmosphere would be aglow with the red hues of every sunset and every sunrise in the world. The lunar landscape would be bathed in a soft red glow. Not bad, eh?

The Outer Limits

Until recently it was assumed that the Earth-moon system was the closest to a double planet, but that ended in 1978 when Pluto’s moon Charon was discovered. Compared to the Earth/Moon system, Pluto and Charon are much closer to the same size and much closer together. So, you would think –and be correct – that Charon would appear much larger from Pluto than the Moon from Earth. Figure C is a simulation (same scale) of Charon as seen from Pluto.

charon_from_pluto

Figure C: Charon as seen from Pluto. Note that this is an actual image of Charon, from the New Horizon’s flyby.

As you can see clearly, Charon is a much more imposing figure than the Earth from our moon. To continue our arm’s-length-finger imagery: I am quoting a New Horizons researcher whose name escapes me when I say “It would take three fat fingers to cover up Charon.”

However, I would point out a few drawbacks to this one. First, the lack of color (as compared to Earth, at least) should subtract a few points. Second, Charon will not be seen as “full” except for twice in a Pluto year, when the plane of the Pluto/Charon orbit is aligned with the Sun/Pluto orbit. A Pluto year is about 248 Earth years. Third, the Sun is very dim out there by Pluto and the whole scene will be very obscure. Fourth, Charon is tidally locked, always turning the same face toward Pluto and Pluto does likewise to Charon. This means that Charon always appears in the same spot in the sky, unless you are in the other hemisphere, then you never see it anyway. And, finally that eclipse scenario we talked about with the Moon and Earth only happens around those times of “full Charon” (every ~124 years) and it has no atmosphere to make it more interesting.  The alert reader (that guy, again) will point out that Pluto must be much larger as seen from Charon than the other way around. Correct. Figure D is just that scenarion.

pluto_from_charon_comp

Figure D: A simulation of Pluto as seen from Charon. Again, the surface features are guesswork and not updated by the recent New Horizon probe’s imagery.  

You see that, in our 10 degree wide view, Pluto has filled it, at least vertically. Clearly, this is the most stunning view, yet. One full hand, at arm’s length might not even cover this up. As I note in the caption, this is not the real image of Pluto, which, as it turns out is much more interesting. However, most of the objections of the view of Charon apply here. It is very dark. Pluto will always be in the same spot in the sky, if it is in the sky, at all. Pluto will only be “full” once in 224 years and the eclipses will be about that rare as well. There will be a “rim of light” during a full eclipse, but it will be pale blue.

Now is where the real estate people would put the “hook” that draws you in to the thing they are really selling. As it turns out, there are much more amazing views available, much closer to home, far more dynamic, interesting and much better illuminated than those “outer limits” properties that you have been seeing!   Let’s talk about the Jupiter neighborhood.

There is this moon called Io (pronounce by purists with a short “I” and by everybody else, with a long “I”). It is the closest of the four main “Galilean” satellites of Jupiter. Io is a small moon of Jupiter that is still 37% larger than Earth’s moon.

It has a view of Jupiter that is un-paralleled by any other self-respecting Jovian moon (i.e., with a significant surface gravity). I have summoned up another simulation at our same scale of 10 degrees across., in Figure E.

jupiterfromio_10comp

Figure E: Simulation of Jupiter seen from Io. Clearly a change of scale is in order!

This is an order of magnitude greater than what we have looked at previously. We need to back off of this 10 degree wide view. Figure F is a view with 45 degrees as the width of field. To show how much we have “zoomed out” please see the inset at lower left that shows the moon as seen from Earth at the same scale.

jupiterfromio_25comp

Figure F: Jupiter (in a section of sky that is 25 degrees across) as seen from its closest large moon, Io.   Note the comparable view of the Earth’s moon – from Earth.

This is clearly the most spectacular view we have yet imagined. The arm’s-length comparison:  Tom Hanks would have to hold up a pizza pan that is 13* inches across to cover this sight. And here, we will have a view of Jupiter that goes from “full” as you see it here, through a half-phase to a crescent, to “new” and back in less than two days.  Add to that the fact that Jupiter rotates in about nine hours and the clouds are in constant motion and changing. And also, the colors (in comparison to our own moon) are vivid and diverse. When Io is eclipsed, passing behind Jupiter, there will also be lightening and auroras that should surpass anything seen on Earth from orbit.

*Ironic, that!  😉

Conclusion

I think that when it comes to spectacular scenic outlooks in the Solar System, we have found the ideal spot – Io. Remember to move to the side facing Jupiter and the view is always available.  But, before you decide you would like to live there, I should say that I failed to mention that it is a very active place, geologically speaking – far more so than Earth. Io is prone to sudden outbreaks of hot molten Sulfur volcanoes. It also orbits in a radiation belt around Jupiter. And to quote astrobio.net (7): “The radiation in Jupiter’s belts is a million times more intense than in Earth’s belts.”.

So, you see that there is this great view at Io, but there are a few details that complicate things.

Hasta Luego,

Steve

 References

  1. Solar System Simulations:   http://space.jpl.nasa.gov/
  2. Apollo Astronauts:  https://en.wikipedia.org/wiki/List_of_Apollo_astronauts#Apollo_astronauts_who_walked_on_the_Moon
  3. Charon: https://en.wikipedia.org/wiki/Charon_(moon)
  4. Planetary Statistics:  http://www.windows2universe.org/our_solar_system/planets_table.htm
  5. Moon comparisons: https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size
  6. More on Jovian moons:  https://en.wikipedia.org/wiki/Moons_of_Jupiter
  7. Jupiter Radiation Belts: http://www.astrobio.net/news-exclusive/hiding-from-jupiters-radiation/

Neptune – The Farthest Giant

 

 

Steve Campbell      March 2016

I managed to destroy this post unintentionally.  I fixed it and threw in a new Triton movie YouTube link. “Sail past Triton  “

Neptune was the first planet to be discovered by mathematical means.  After the discovery of Uranus and subsequent observations of the Seventh Planet, it was observed that its orbit was not meeting expectations of Kepler’s Laws.  It was determined that there must be another planet -farther away – that was influencing the orbit.  That planet was later discovered and quickly thereafter found to have a large satellite. (1)

Neptune is the farthest Giant Planet from the Sun and not surprisingly the last to be visited by a spacecraft.  In the old days, when your author was young, they called Jupiter, Saturn, Uranus and Neptune “Gas Giants”.  Now they reserve that title only for Jupiter and Saturn and call Uranus and Neptune “Ice Giants”.  As you may know they now call Pluto and Eris “Dwarf” Planets (I can’t agree with that).  As of today, Mercury, Venus, Earth and Mars are still “Terrestrial Planets”.  But soon, it seems to me, each planetary body will have its own unique category.

Quoting from my two-part series:

“One thing that might puzzle the average student might be why we had images of all the Outer planets by the 1970s and 80s and nothing but a dot or smudge for Pluto.  That all relates to what was called at the time “The Grand Tour”.  As it happened, there was an alignment of the outer planets in the 70’s and 80’s such that it would be possible to use gravity assisted orbital adjustments (“the slingshot effect”) to make it possible for a space probe to visit Jupiter, Saturn, Uranus and Neptune in one long and carefully managed trajectory.”

Again from Sneaking Up on Pluto:

“The Voyager probes (one of which actually made the complete “Grand Tour”) each had a main antenna that was capable of constant communications with the Earth.  This necessitated what is called a “scan platform” that held the instruments that need precise pointing, that moved independently of the antenna.  That is – in part – because data storage was actually on a ½ inch, 8 track magnetic tape with a total capacity of about ½ Megabyte and a top baud rate of 56 kilobits per second (2).  That’s what I said – “Stone Knives and Bear Skins!” – so, real-time transmission was required for image data.”    That configuration had proven troublesome on Voyager II at Saturn and data were lost. Voyager was – despite my demeaning reference – quite advanced at the time and its foundation of observations and imagery is still quite impressive.

 The image below depicts the identical Voyager 1 and Voyager 2 Spacecraft. (4) The dish antenna is 3.7 meters in diameter (12 feet, 2 inches) across.  The arm extending to the right contains the main experiments and the imaging “scan platform”.  The left arm holds the three radioisotope thermoelectric generators that power the probe.  The gold disk on the “body” is the famous “Golden Record” with messages and images of Earth for anyone “out there”.  This metal phonograph record with  data recorded in the grooves on the surface  (i.e., not a CD) was a pet project of Carl Sagan.  Carl has been inserted into the picture at the proper size to give it a sense of scale. 

This photo is from his brief and little-known “Fonzarelli” period.  Raise your thumbs, Carl!

voyager_nasawebcarlFigure A: The Voyager Spacecraft        NASA/NASA website

Neptune

About time we got around to the planet, I hear you thinking.  I have a table of planetary statistics (3) that serves as a good introduction for any planet.  You may expect to see this table in future posts.  Please see Figure B, below.

Figure B: Table of Planetary Statistics        NASA

You will notice that Neptune has 17 times the mass of the Earth and about 3.9 times the diameter.  That only works out that way because the mean density of Neptune is 30% that of Earth.  If it were as dense as the Earth, Neptune (of the same diameter) would have 60 times the Earth’s mass.   All four Giant Planets are low-density like that, but Neptune is the densest of them.  Likewise, all Giant Planets are fast spinning and Neptune is slower than most, rotating in 16 hours.  The Navy has adopted a 16 hour rotation of duties and sleep aboard out nuclear subs, so submariners would be right at home on Neptune.  Just a small tangent, I’ll get back on track, now.

Figure C: Neptune as seen by Voyager 2         NASA/JPL

The clouds were somewhat of a surprise after the Voyager’s views of Uranus – which was almost featureless.  The big dark spot (named rather predictably, the “Great Dark Spot”) was another surprise as were the winds (1500 mph) stirring these features around.  These are the fastest winds in the all the Planets (5) and unexpected out in the cold dark zones of the outer Solar System.

Neptune takes 164 years to orbit the Sun.  It’s a long wait for Summer, eh?  Neptune, like all Giant Planets (plus Mercury and Earth) does have a magnetic field and in fact, it is much stronger than the Earth’s.  That would seem to indicate that it has an iron inner core.  But it cannot be very large, or the overall density would be larger.  It is in fact estimated that the core part of Neptune at its center is about Earth-sized.  Most of what is above is water, ammonia and methane (CH4) ice (estimates vary for thickness).  That is considered to be its “mantle”. 

The atmosphere above that is hydrogen, helium and methane.  The white clouds you see vary in composition depending on pressure.  The higher clouds where pressures are about Earth-like (1 bar) are probably methane vapor.  Lower down and at higher pressures are clouds of ammonia, hydrogen sulfate and even water vapor, like the clouds on Earth.

How thick each of these layers might be is still open to interpretation and you can find many differing diagrams, most with no dimensions mentioned.  So, having looked at those, I will guess that the core is 4000 miles in Radius (about the same radius as Earth).

Later photos of Neptune by the Hubble Space Telescope have shown considerable changes in Neptune’s atmosphere, since Voyager.

Triton

Triton. The terrain is similar to what we now know Pluto to be like. The colors are not the same, however. That may be due to a difference in instrumentation between Voyager 2 and the New Horizon probe that looked the ninth Planet.

See this YouTube video for a “Sail past Triton  “

Triton is the largest moon of Neptune and by no coincidence, the first discovered (17 days after the discovery of Neptune, itself).  It is unique in several respects.  It is the only “large” satellite to orbit in a “retrograde” sense.  By large, I mean to say that it is near to the size of our own Moon.  By retrograde, I mean that it orbits in a direction opposite to the rotation of its planet.  That and the high inclination of Triton’s orbit seem to indicate that it was captured.  For reasons we won’t go into, it is easier for a moon to be captured in a retrograde orbit than otherwise.  Jupiter and Saturn have lots of former asteroids as moons, but they tend to be small and far away.  Triton is so close that it is being slowly pulled closer to Neptune and in several billion years will be shattered into a ring like Saturn’s.  You might expect a captured moon to be in an eccentric orbit that varies in distance from its planet, but Triton’s orbit is so close to exactly circular that the difference is not worth mentioning.  It stays at about 220,483 miles from Neptune which, coincidentally is about the same distance from the Earth to our own moon.  It orbits Neptune in 5.8 days and rotates in the same time.  That is to say, it keeps the same side toward Neptune, just as our Moon does to Earth.  Now, some of my readers are sharp enough to notice that our moon takes 28 days to orbit.  Why so different if the distances are near the same?  The difference, of course is that Neptune is 17 times as massive as the Earth, as I mentioned a few paragraphs ago.  This will be on the test! 😉   Figure D, below is a Voyager 2 image of Triton

Figure D: Triton         NASA/JPL

            All other large satellites orbit the same way and are therefore by definition, prograde.  Triton also looks quite distinctively different from most other planetary satellites, which tend to be rather uniform and crater covered (admittedly with many exceptions).  It has an atmosphere that, while very thin, has detectable clouds.  It shares the much modified and differentiated characteristics that we now know of on Pluto.  That tends to confirm the “capture” hypothesis.

            You may ask, “Just how does a passing object become “captured”? “.    There are several ways.  One would be for Triton to have collided with a smaller moon, as it passed near Neptune.  That might slow it just enough to wind up in an orbit.  As it would have collided with a prograde moon, that would be especially effective since that would almost double the velocity difference between the two and quadruple the energy delivered to the passing Triton.  That should have left a mighty crater on Triton.  While nonesuch was seen by Voyager, such a crater could have since been covered by the glacier-like deposits of Nitrogen ice (the part that looks like cantaloupe peel) that are visible in Figure D.  Likewise, that crater might have been in the darkened part of Triton, that was not visible when the Voyager went zooming by at the greatest velocity ever given to a man-made object (at the time)

A second possibility would be “gas drag” as Triton passed through the upper atmosphere.  That would seem unlikely, unless Neptune had a more extensive atmosphere at the time.  Since it may have been captured billions of years ago, that is entirely possible, but still just speculation.

            Another possibility was detailed in a paper by Craig Agnor (University of California, Santa Cruz) and Douglas Hamilton (university of Maryland) in 2006. (6)  First, I should explain that Pluto and all the other Smaller Planets out past Neptune have been designated as Kuiper Belt Objects (KBOs).  If Triton had been one of a co-orbiting binary pair of KBOs, it is possible that a pass near Neptune would have captured it, while at the same time, ejecting its companion to a more distant orbit of the Sun.  It all has to do with relative motion of the three bodies. This hypothesis has the virtue of not relying on chance collisions or hypothesized “greater atmospheres”.  This idea was made more believable by the discovery that many KBOs are indeed, binary.  Not the least of these is Pluto, who’s biggest satellite (Charon) is about one half its own size.  It has been estimated that 15% of KBOs may be binary in nature.  That a KBO could have come near to Neptune is not unlikely since Pluto itself comes nearer the Sun than Neptune as it was during the late years, last Century.  Having said that, I must also remark that Pluto is now in a resonance with Neptune that keeps the two safely apart.  I mean to say, that when Pluto comes nearer to the Sun, it is still very far from Neptune and always will be.

But, all in all, I think these guys are very near the mark with their hypothesis.

Conclusion

Neptune is another fascinating member of the Solar System and I learned a lot by researching to write this article.  I hope you find it interesting as well.  You may wonder why I do this.  Well, those of you who know me know that I suddenly have time on my hands.  It is a blessing…and a curse. ;-).  Also, I have always had a fascination with the Solar System that goes back to my days in Elementary School. 

You and I are truly fortunate to live in a time when these mysterious dots of light in the sky that were the Planets are now becoming known as Great Worlds, many that dwarf the Earth in size and complexity and others that are revealing the secrets of Nature that have been heretofore unknowable.

References

  1. Discovery: http://www.universetoday.com/21621/who-discovered-neptune/
  1. Voyager Data Rate: http://itknowledgeexchange.techtarget.com/storage-disaster-recovery/nasas-voyager-used-8-track-tape-to-go-into-space/
  1. Table of Planetary Statistics: http://nssdc.gsfc.nasa.gov/planetary/factsheet/planet_table_british.html
  1. Voyager details: https://en.wikipedia.org/wiki/Voyager_2#
  1. Winds: http://www.space.com/21157-uranus-neptune-winds-revealed.html
  1. Triton Capture: http://news.softpedia.com/news/How-Neptune-Captured-Triton-23313.shtml

A Nice Place to Visit, But…

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Where to Live, if Not Earth

Life is in fact a very rare phenomenon, despite the fact that it surrounds most of us (in no small part with the “rest of us”).  You might think it to be common, but that is because you are a part of the commonality of it all.  Life is not common – outside of our planet, where it is common.

Some points about the rarity:

  1. The Earth is in the “Goldilocks Zone” (GZ) which means that it is at the right distance to be “not too hot and not too cold” for life. This is a well-known and rather trite fact to quote, but nevertheless true.  Depending on who you ask, the planets Venus and Mars are also in the GZ but apparently lifeless, thus far.
  2. The Earth is thought to have its relatively fixed spin axis because of the stabilizing influence of the moon. Other planets, without such large satellites are known to have violent shifts in inclination making environments hostile to life.  Mars has been suspected of major axial shifts on a short (geological) time scale, for example. Such a large satellite (relative to the planet it orbits – its “primary”) is a very rare condition and the only other example in the Solar System is the Pluto / Charon double planet.  And that planet -while extremely spin-stable – is way to Heck and Gone outside the GZ.
  3. Neither Mars nor Venus has a magnetic field like that of Earth which protects life from massive Solar and Cosmic radiation. It turns out that little Mercury does have a global magnetic field, but it’s hot dry surface is covered by a pitiful excuse for an atmosphere with not much more than vanishing small amounts of hydrogen and helium.  And the bare, igneous rocks of Mercury are basking in the glaring sunlight that is more than four times as intense as that experienced by Earth.
  4. The stable, near circular orbits of the planets that are common in our solar system are not so common in detected planets around other stars (Exoplanets).
  5. We have studied the Solar System extensively and know at least that conditions for life are extremely rare and Life has not yet been shown to exist anywhere else but on our own Earth.
  6. It stands to reason that intelligent life is even more scarce.  I submit to you that it is so rare as to be vanishingly small.

 When Dinosaurs Ruled the Earth

When I was young (that was back in the Cretaceous Era when dinosaurs ruled the Earth), very little was known of what lay beyond that same Earth, even within our own Solar System.  It was thought that conditions for Life probably existed on Venus and almost certainly on Mars.

Seasonal changes on Mars that we now know to be dust patterns were taken as sure signs of life. We are now accustomed to sharp clear images of the planets thanks to space probes and extensive image processing of telescope photos.  What was available in the early Sixties was not much more than grainy, very low-resolution pictures.  As far as Mars was concerned, the expectation of life there was still active right up until the first fly-by images came from Mariner 4 on July 14, 1964.

Atlantis Region on Mars - Mariner 4
Actual Mariner 4 Image of Mars from 1964.  Note the linear artifacts and the extremely  low resolution.       Photo Credit: NASA

 Those images showed a moon-like, crater-covered surface and the atmosphere was confirmed to be a vanishingly thin layer of carbon dioxide. Those who had held out hope for life on Mars were bitterly disappointed.  I know because I was there and this was not at all what I had been led to expect.  I am not pointing fingers, you understand.  Virtually everybody – scientists included – expected to see something like the early images of desert areas of Earth from space – with, maybe, some cactus plants (roadrunners and coyotes optional).  But, these Doses of Reality really were indistinguishable from the airless, lifeless moon and a bitter disillusionment to all.  It happened that Mariner 4 did not see the more interesting parts of Mars that later restored some (diminished) hope for life when Mariner 9 images began to arrive.

Probes to Venus sent back even worse news. Cloud-covered Venus had been expected to be a very warm, rainy, damp or perhaps ocean-covered place.  All of those scenarios were explored in stories and novels, back in the Golden Age of Science Fiction.  If that “Golden Age” phrase puzzles you, go and read these authors:  Heinlein, Azimov, Clarke and Bradbury.

Earth observations had already begun to indicate harsh conditions. Measurements from probes revealed that planet to be a quite literal “Hell-hole” with a hot, thick, dry and “crushing” atmosphere that could melt lead.  Liquid water – long thought to be a Life “prerequisite” is just a busted myth on Venus.

400px-Venera9
Actual surface image from Venus by the Soviet Lander Venera in 1975.  The probe died not long after this. The vertical “artifacts” are data packets that occasionally interrupted  the image transmission.  Data storage in those days was measured in kilobytes and so things had to be done in “near-real-time”.

While Jupiter does possess a magnetic field, it is also surrounded by intense belts of radiation that are trapped in that field.  In this regard, I have found some quotes that throw doubt on the chances of hospitable environments on Jupiter’s moon Europa – long thought (after the Voyager probes, that is) to be a good candidate for life.

NASA_jupiter-moon-europa
Europa. The lack of craters and the linear features (we call ‘em “cracks” where I come from) suggest active Geology.  Some have suggested that the contrasting dark colors of said cracks suggest biology.  That may be grasping at straws, IMHO

Some of those doubts:

A Probe called Pioneer 10 was actually the first to encounter Jupiter and there was some concern about radiation levels:

“The level of radiation at Jupiter was ten times more powerful than Pioneer’s designers had predicted, leading to fears that the probe would not survive; however, with a few minor glitches, it managed to pass through the radiation belts, saved in large part by the fact that Jupiter’s magnetosphere had “wobbled” slightly upward at that point, moving away from the spacecraft. However, Pioneer 11 did lose most images of Io, as the radiation had caused its imaging photo polarimeter to receive a number of spurious commands. The subsequent and far more technologically advanced Voyager spacecraft had to be redesigned to cope with the massive radiation levels.”   (Magnetosphere of Jupiter, n.d.)

And then there was this, which hammered fifteen more nails into the Europa/Life Coffin::

“The radiation level at the surface of Europa is equivalent to a dose of about 5400 mSv (540 rem) per day,[40] an amount of radiation that would cause severe illness or death in human beings exposed for a single day.:[41]

  1. The Effects of Nuclear Weapons, Revised ed., US DOD 1962, pp. 592–593

You might argue that there is a “Goldilocks” Zone deep under Europa’s Ice in a Salty Sea of liquid water.  Probably more liquid water than on all of the Earth. The miles of ice and water might well protect from radiation.  The idea that life could exist there is reasonable, but completely unproven. But is that someplace you want to spend your retirement?

There was also a very thorough work on the radiation dose expected for Apollo astronauts while passing through the Van Allen Radiation Belts (VABR) on the way to moon landings.  It concluded that the dose of radiation was relatively tolerable – mostly because the men did not spend much time there.

So far, places where Earth-like conditions exist are:  Earth.

Because:

  1. Almost all “Environments” off-Earth are without significant atmospheres, with way too much atmosphere or under miles of ice, submerged in salty seas.
  2. Many “Environments” off-Earth also tend to be radioactive enough to kill you in a day or two.
  3. None of these so called “Environments” has actually been shown to harbor any kind of life.
  4. In every case, these are places that many adventurous people would like to visit. But, without bringing along a complete life support system with you – including when you are “out for a walk” – they are no place you could “live”.

And Then We Get to Titan

Titan is the largest moon of Saturn.  It is the third largest moon in the Solar System and larger than planets Mercury and Pluto.  And it is just lousy with Earth-like qualities.  It has a very dense atmosphere that exceeds the Earth’s sea-level pressure by about 50%.

Looking on the Brightside of Titan
Titan in visible light. Photo credit: NASA

This first image is pretty much what Voyager 1 saw during its pass by Saturn, arranged especially to look at Titan.  It has been known that Titan has an atmosphere since that fact was discovered by Gerald Kuiper (rhymes with “hyper”) in 1944.  It became obvious then that to see under this veil of clouds would require more than the cameras aboard the twin Voyager spacecraft.  Mostly for that reason was Voyager 2 was cleared (after the successful encounter of Voyager 1 with the satellite) to ignore Titan and continue on a trajectory that would take it to Uranus and Neptune.  Had Voyager 1 failed at Titan, Voyager 2 would have followed its brother out of the plane of the Solar System, never having had the opportunity to see the last two Giant planets Uranus and Neptune.  Titan got such priority because of its atmosphere which is unique for satellites of any planet.

TitanInfraredd
Titan in Infrared light.  The colors here are – of course – “made up” since you cannot (nor can I – so don’t feel bad) see in infrared light. Having said that…does this not remind you – a bit – of the Earth?  Photo credit: NASA

This second image is from the later Cassini probe that was sent to orbit Saturn and (knowing what Voyager could not see) included an infrared camera which – with some filtering -could see down to the Titanian surface.

While there have been many learned speculations that an exotic form of life might exist that “breathes” hydrogen and exhales methane and “eats” acetylene, none has as yet been detected.  “Conventional” Life-as-we-know-it (LAWKI) does these things with oxygen, carbon dioxide and glucose, respectively.

There are indeed Methanogens (i.e., Life that makes methane) on Earth but all of them use liquid water as their “solvent” and none use hydrocarbon liquids as would be the case for the imagined Titanian lifeforms.

I have found no references that indicate the radiation environment is a problem at Titan. This large moon of Saturn is expected to have a subsurface ocean as Europa is thought to have, with the same speculations of “conventional” lifeforms and the same problems to be expected.

Titan is covered with hydrocarbons.  These are compounds of hydrogen and carbon that would be called “petroleum” here on Earth.  Methane rains down out of the atmosphere and heavier hydrocarbons – solids at that temperature – cover much of the surface resembling sand dunes.  These “organic” compounds that have that name because they are -on Earth at least – generally made by lifeforms.  I have not read any speculations that Titan’s organics are life-generated.

Titan by virtue of its distance from the sun to Saturn (being 10X that for Earth) receives sunlight arriving there at one percent of what the Earth enjoys.  The cloud cover that Titan is notorious for reduces that to one tenth of one percent. Photosynthesis would seem very unlikely.  That contributes to a big problem with the idea of Life on Titan.  Namely, the temperature, which is so very cold (call it -200 F) that liquid water is decidedly out of the question.

There are, in fact “Great-Lake-size” bodies of liquid on the surface, but they are composed of mostly methane (CH4) – which on Earth is the main component of what’s called Natural Gas.  As mentioned earlier, methane is a minor component of Titan’s atmosphere, just as water vapor is in our own Earthly atmosphere and it precipitates to the surface like same.

That would seem to wrap it up for extra-terrestrial life.  Several possibilities for environments which show a potential for life, but no evidence that such life exists.

 Whatever lifeforms we see fit to deposit…

(from Star Trek II, The Wrath of Khan, in reference to the “Genesis Project”)

TitanSUrfaceProbe2
The surface of Titan, photographed by the Huygens
Lander on Jan 14, 2005. Photo Credit: NASA

Titan’s atmosphere is composed of mostly nitrogen with about 5% methane near ground level with a trace of free hydrogen.   While Earth’s atmosphere is similar, being 80% Nitrogen – the remainder consisting mostly of oxygen with a few trace gasses – that of Titan has no free oxygen.

Again, the ground-level air pressure on Titan exceeds that which you  are currently experiencing – even as you read.  This makes Titan is the only place where walking around outdoors without a space suit might be possible.

There are a few problems with that idea, though.

  • Titan is extremely cold and would require the Walker to bundle up in arctic gear. Heavy-duty arctic gear.
  • This Pedestrian would be breathing from an air tank – like a SCUBA diver. There is (again) no free oxygen in the atmosphere. Let’s say that you use a re-breather and don’t exhaust any oxygen into the atmosphere.  Probably best considering the methane.
  • That methane (known to drizzle out of the haze you see in the distance) might require a raincoat over all that arctic gear. I doubt you would want to be soaked with evaporating methane when you go inside – where you will definitely have oxygen around.
  • The ground where the Huygens probe landed was described as comparable to wet clay. I can’t vouch for the stability or traction achievable on such a surface. Water ice would not be a problem, since at that temperature it is indistinguishable from rock. But those hydrocarbons that cover the surface are much closer to their melting points and could pose a slip hazard.  Other places – who knows?
  • Don’t let the picture fool you, it will be dark. That photo was taken with a sensitive research instrument. Remember that Titan orbits the planet Saturn, which is ten times as far from the Sun as the Earth.  Only one percent of Earthly sunlight reaches Titan and only one tenth of that is able to penetrate the clouds.  A moon-lit night on Earth might be the very best level of illumination you could expect.
  • You would need artificial lighting, especially through the roughly eight days of darkness when the sun is below the local horizon.
  • That brings up electrical generation. Solar panels would  be useless in the dimly lit haze. While fierce winds were measured during the descent of Huygens through the atmosphere, surface winds are likely  to be intermittent and not particularly strong.  So, you won’t want to  depend on windmills for power.  The probe was powered by batteries, which died after about 90 minutes.  No, you will be taking your own power supply to Titan and it will be nuclear in nature – something like a radio-isotope generator.  Every probe that made it past Jupiter yet has used such “nukes” for power.

Conclusions

Indigenous Life in the Solar System – once thought so likely – has been shown to be non-existent as far as we know.  This is despite decades of intense research toward finding such life.

There is not much to offer for Human occupation “off-Earth” , either.  Unless you want to live life completely indoors, surrounded by radiation shielding or in a space suit (much like being “indoors”)  Titan is pretty much it.

Having found this one special second place in the Solar System where some semblance of normal human activity is indeed possible, the list of activities is rather short. You won’t be farming.  Astronomy is out, but if you happen to be on the side of Titan that faces Saturn, that ringed planet might be barely visible through the haze.  Hiking is good and there might be some spectacular landscapes (not at the Huygens landing zone, I’ll admit).  Night-vision goggles might be appropriate.

There is one aspect might make it all worthwhile.  Titan’s gravity is a bit less than that of our Moon.  About 1/6th of Earth.  That combined with the thick atmosphere should make human-powered flight possible.  Not just possible, but easy!  Even if all your cold-weather clothes, SCUBA gear and “wings” weigh as much as you do, you  would still have only one third of your weight on Earth.

There is a sub-culture of misinformed people who  think that Humanity’s only salvation will be to move to “another planet”.  These folks tend to be against things like fossil fuels and nuclear power.  Ironically, if Titan (about the best you can find) is your “other planet”, you will be surrounded by petroleum and using nukes for electricity – in bitter cold and perpetual (more-or-less) darkness.

But you could fly!

Ex Scientia, Veritas,

Steve

Bibly:

Star Trek II quote:   “Instead of a dead moon, a living, breathing planet, capable of sustaining whatever lifeforms we see fit to deposit on it.” – Dr. Carol Marcus

Van Allen Belt Radiation: http://www.braeunig.us/apollo/VABraddose.htm

Mars Axis instability:  https://www.nasa.gov/mission_pages/msl/multimedia/pia15095.html#.WmOspainHIU

Venus (Venera):  https://en.wikipedia.org/wiki/Venus

Exoplanets:  https://www.nasa.gov/kepler/discoveries

Titan:     https://images.nasa.gov/search-results?q=Titan%20surface&keywords=&page=2&media=image,video,audio&yearStart=1920&yearEnd=2018

Radioisotope generator:  https://www.jpl.nasa.gov/news/fact_sheets/radioisotope-power-systems.pdf

Uranus: The Seventh Planet

SteveTrucker2  Homepage   Portfolio

Uranus

(OK, Reader! Wipe that smile off your face and pay attention!)

This poor planet suffers that name that sounds like two words your Proctologist might put together when discussing your condition.  It is laughed at so often that it inspired an article by a man name DeCotis.  I cannot locate the original article but I emailed him the following message.  I hasten to point out that this text – once sent – has been augmented, improved, embellished and even illustrated over the years and especially just prior to the posting of this article.

Mr. DeCotis,

Heartiest congrats to Space Online, Billy Cox and yourself on a wonderful bit of writing about the planet Uranus. I myself have long considered the name of first trans-Saturnian planet to be a problem. This became a matter of importance when, in 1977, it was discovered that, like other Gas Giants, “Neptune-Minus-One” had rings. This was before Voyager 2 got to #7 and was accomplished by watching that planet pass in front of a star. This is called an “occultation”. Unexpectedly, the star dimmed several times before and after the planet covered it. Only rings could explain it since expecting that many satellites to be linecd up in that fashion was improbable in the extreme. 

Knowing me to be an Astronomy student, people would ask me, “Are there really rings around Uranus?”. I understood that as a very personal and offending question and I was tempted to demonstrate the (negative) answer visually, but I refrained.

 Actually, I explained to them about the occultation, just as in the first paragraph – being a thoroughgoing Astronomy nerd.

UranusLightCurveCrop
Figure 1:  The actual light curve
from the 1977 occultation that
detected Uranus’ rings.

There was a Science Fiction B-movie about Uranus which was euphemistically entitled “Journey  to the Seventh Planet” back in the sixties*. Even as a teenager (or especially as a teenager, I suppose) it didn’t take long to figure out what they were avoiding. There was a brief movement (no disgusting pun intended) to transfer the emphasis to the first syllable but you can see (well, hear) immediately that this is a non-starter (“Urine-us”). The name would still be in the bathroom humor department and would only prompt a new round of adolescent jokes.

    It was about then that I decided that “Joe” was a nice enough name. But in order to differentiate whether we were talking about Lewis, Dimaggio, Cool, College, Blow, Six Pack or the Planet, we’d need to specify “Joe the Planet” for every reference. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Joe the Planet, Neptune and Pluto. I’ll grant you it’s a bit awkward at first but it should put an end to the pubescent snickering in astronomy lectures and planetarium shows.
Sincerely,

Steve

 *I looked up “Journey to the Seventh Planet” on YouTube recently – it still stinks.

In college, I was assured by actual Astronomy Professors that this planet’s name is “your ah noose” (accent on “ah” and “noose” rhymes with moose).  The other pronunciations you may have heard are either erroneous or may be attributed to the aforementioned “bathroom humor”.

Now that we have the nomenclature issue dealt with, let’s have a look at the planet itself.

Discovery:

Quoting a NASA planet resource website{1]:

“The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel,although he originally thought it was either a comet or a star. It was two years later that the object was universally accepted as a new planet, in part because of observations by astronomer Johann Elert Bode.

William Herschel tried unsuccessfully to name his discovery Georgium Sidus after King George III. Instead the planet was named for Uranus, the Greek god of the sky, as suggested by Johann Bode.”

So, it’s “Bode’s ill” – so to speak.  Don’t blame poor Herschel for the double entendre.  Nor his sister Caroline who joined the musician turned astronomer and accomplished many discoveries of her own:

“Caroline assisted Herschel until his death.  She discovered eight comets. She also discovered several deep-sky objects and was the first woman to be given a paid scientific position…”

Early Observation

There was not much to see.  Even in the most powerful “backyard” telescopes – as late as the 1980’s Uranus was a small dim pale blueish green dot.  A “professional” telescope of that era would be required to resolve the largest satellite, Titania as a featureless point of light.  Even in those elaborate instruments, Uranus maintained its elusive nature.

“Even through large telescopes the planet often appears fuzzy and indistinct. Brightness variations are sometimes reported, the likely result of changes in the planet’s atmosphere.”  [2]

UranusTelescopeView

Figure 2:  Uranus through a large “backyard” telescope.

Below is that table of planetary statistics that readers may have seen before.

PlanetaryStatisticsTable 1: Statistics for the Planets

The seventh planet is 19 times the Earth’s distance from the Sun.

SimpleAUDiagram

Figure 3: Simple “Visual aid” to depict the distance of Uranus (big green dot) from the Sun (Yellow asterisk) as compared to that of the Earth (little blue dot).  Only the distances are to scale – not the sizes of the Sun and planets.

In size, it is 31, 763 miles in diameter (four and a half times that of Earth).  Like all the Giant Planets, it rotates quickly (once every 17 hours and 12 minutes) and it is much less dense than the “Rocky Planets” like Earth.

The atmosphere is hydrogen and helium with some methane.  Deeper down, there is a “mantle” of water, ammonia and methane ices above a rocky core.  You see in figure 3A that they have not labeled the thicknesses of these layers.  That is a sure sign that they don’t really have a clue what those numbers should be!

Internal-structure-of-UranusFigure 3A: Internal Structure of Uranus

A notable unique feature of Uranus is the orientation of its spin axis relative to the plane of its orbit (see “obliquity in orbit” in the table).  In the Uranian summer and winter the axis of rotation of the planet points almost directly at the Sun – resulting in one hemisphere in constant sunlight and the other in darkness.  This is thought to have been caused by Uranus’ collision with a large planetoid late in its formation.  The diagram below explains the situation.

UranusPhases

Figure 4:  Seasons of Uranus

Uranus was visited by a space probe only once.  It was the third stop on what was called at the time “The Grand Tour”.  As it happened, there was an alignment of the outer planets in the 70’s and 80’s such that it would be possible to use gravity assisted orbital adjustments (“the slingshot effect”) to make it possible for a space probe to visit Jupiter, Saturn, Uranus and Neptune in one long and carefully managed trajectory.  The Voyager 2 Spacecraft did exactly that and arrived in the area of Uranus in 1986.

The Voyager 2 Spacecraft        

The Voyager probes each had a main antenna that was capable of constant communications with the Earth.  This necessitated what is called a “scan platform” that held the instruments that need precise pointing and moved independently of the antenna.  The constant contact was needed because data storage was actually on a ½ inch, 8 track magnetic tape with a total capacity of about ½ Megabyte and a top baud rate of 56 kilobits per second (4).  That’s what I said – “Stone Knives and Bear Skins!” – so, real-time transmission was required for image data.”  Voyager was – despite my demeaning reference – quite advanced at the time and its imagery and other data are still quite impressive. They made the most of the technology at hand.

The image below depicts the identical Voyager 1 and Voyager 2 Spacecraft. (4) The dish antenna is 3.7 meters in diameter (12 feet, 2 inches) across.  The arm extending to the right contains the main experiments and the imaging “scan platform”.  The left arm holds the three radioisotope thermoelectric generators that provided the electric power out in the dark reaches where solar panels would be quite ineffective.  The gold disk on the “body” is the famous Record with messages and images of Earth for anyone “out there”.    Carl Sagan, whose enthusiasm for the search for extraterrestrial intelligence (SETI) was well-known had thought to perhaps include a plaque with a message engraved upon it as had been done with the Pioneer space probes.  This Record (an actual grooved phonograph Long Playing (LP) disc – only metal, not vinyl) was the idea of Frank Drake.  SETI Nerds will recognize Drake as the inventor of the “Drake Equation” which is a formula to calculate how many extraterrestrial civilizations there might be.  That’s Frank in the inset, with his equation.  I put him there to give scale to the picture.

VoyagerDrake

Figure 5: The Voyager Spacecraft        NASA/NASA website

UranusBlandVoyagerPhoto

Figure 6: A Voyager view of Uranus in 1986. 

Even the dedicated planetary scientists had to admit they were disappointed with the rather very bland appearance of the planet.  In trying to describe the feelings of the Voyager team about the mediocrity of it all, Planetary Scientist Heidi Hammel had this to say,  “…poor Uranus…poor Uranus!”.[6]

There had been observations from Earth of clouds in this atmosphere, so what’s the deal?  You will see in the diagram in figure 4 that the solstice – that point in the orbit where one hemisphere is constantly roasting in sunlight – was in 1986.  Just when Voyager happened along.  Later observations were made with (much improved) telescopes in the years surrounding the Equinox of 2007 (see Figure 4) – when most of Uranus has 8 ½ hours sun and 8 ½  hours darkness – “barbeque” mode, as they say.  Those images showed Uranus in its more “flamboyant” mood. Figure 7.

UranusEquinox

Figure 7:  Uranus near Equinox.  Note the rings (R) – now markedly evident when they are seen edge-on.

It is probably worth noting that the Voyager camera and those of the Hubble Space Telescope (HST) are almost certainly quite different in their ranges of wavelengths and sensitivities, so they are not directly comparable.  Nonetheless, we may expect more blandness from “poor Uranus” around the Summer Solstice in 2028.

I should mention that there is a lot more science involved than just the images recorded by Voyager and results from those found new and interesting features, as well. For example, the magnetic field detected is not centered on the planet core and its poles are near the rotational equator.  This was totally unexpected.

The Satellite that “Saved the Show”

One of the major aspects of interest in the Giant planets was the characteristics and history of their satellites.  With Jupiter and Saturn, the space probes entered and left the planetary systems obliquely across the orbits of the moons and could, with luck, come close to several of them for detailed examination.  In the case of Uranus, the moons’ orbits are like circles on the sky and are approached as if in target practice.  The “Grand Tour” scenario of hopping from one outer planet too the next required very specific trajectories past the planets along the way.  That, and the angle of the sun left only one chance of a close approach to a satellite and even that would see only the perpetually lit hemisphere of the smallest of the major moons – Miranda (Figure 7) It could not have been predicted that this would be by far the most interesting of all the moons and the feature we could all point to when asked by non-Nerds why all this expense and effort was spent to go look at a blue-green billiard ball – with no number on it.

Miranda

Figure 8:  Miranda

Miranda is the smallest (about 300 miles across) of the major satellites and the closest to the planet (roughly 81,000 miles).  It circles Uranus in 1.4 days and always shows the same face to the planet.  This is looking down at the South pole.  In the season when Voyager arrived, this was pretty much all that would have been illuminated.

And, it looks like it has been broken apart and then shoved back together!  Not surprisingly, that is one idea of how it came to look so.

“Scientists disagree about what processes are responsible for Miranda’s features. One possibility is that the moon may have been smashed apart in some colossal collision, and the pieces then haphazardly reassembled. Another, perhaps more likely, scenario is that the coronae are sites of large rocky or metallic meteorite strikes which partially melted the icy subsurface and resulted in episodic periods of slushy water rising to Miranda’s surface and refreezing.”[8]

Uranus has four larger satellites.  The biggest is Titania which is still less than half the diameter of the Earth’s moon.  As mentioned earlier they were not well surveyed in the fly-by, but a map of Titania’s surface appears in figure 9.

TitaniaCasma.

Figure 9:  A Map of Titania’s surface.  Again, only about half the surface was illuminated and this is the least boring part of that.

The larger satellite also has some interesting surface features.  I am reminded of my own varicose veins.

Conclusions

  1. The seventh planet turns out to be rather dull and featureless, but only for the Southern Summer. The Spring Equinox brought considerable atmospheric activity after Voyager but now detectable from the improved cameras of the Hubble Space Telescope and other modern observatories.
  2. The satellites of all the Giant Planets all turned out to be far more complex than was first imagined by Earthbound observers. Uranus is no exception.
  3. Uranus still has that unfortunate name (despite my “Joe” recommendation) but we can overlook that because we are all adults, here. Right?…Right?
  4. Update: In all that discussion of the unfortunate moniker, I neglected to mention a sports cheer, popular around the time of the ring discovery.  It goes like this:  “Up Jupiter! Up Saturn!…”
    (So much for Adulthood 😉 )

Hasta Luego,

Steve

[1] NASA Photos:

https://solarsystem.nasa.gov/planets/uranus/#!

[2] William Herschel:

https://www.space.com/17432-william-herschel.html

[3] Uranus Telescope view:

http://www.nakedeyeplanets.com/uranus-telescope.htm

[4} Voyager details:  https://en.wikipedia.org/wiki/Voyager_2#

[5] Table of Planetary Statistics:   http://nssdc.gsfc.nasa.gov/planetary/factsheet/planet_table_british.html

[6] The Farthest: Voyager in Space – Netflix

[7]  Uranus Planetary Factsheet:  https://nssdc.gsfc.nasa.gov/planetary/factsheet/uraniansatfact.html

[8] Miranda in Depth – NASA:  https://solarsystem.nasa.gov/planets/miranda/indepth

Sneaking Up on Pluto (Part 1)

SteveTrucker2  Homepage   Sign123_Lone

Discovery

From 1930, when it was discovered by High School Graduate Clyde Tombaugh until recently, Pluto remained a dot of light in a telescope. The way to find a planet is to see it move amongst the “fixed stars”. The further from the Sun the planet is, the slower it moves. In Figure A you will see the original “Discovery Images” of Pluto.

fig8-pluto-discovery

Figure A: The “Discovery” photos of Pluto.

Lest it seem too easy, Tombaugh spent 10 months photographing the majority of the sky and poring through pairs of images like those above. Computer generated “blink comparisons” are now common and you have probably seen examples. In 1930 two photos (glass plates with silver-based photo-emulsions) were put into a contraption with two optical paths that were alternated to the eyepiece by means of a moving mirror. He was probably looking at the original negatives, not prints. Not only was this system far from perfect, but there were also asteroids that exhibit the same behavior as the targeted planet. Those had to be tracked down and eliminated by arguments based on their apparent velocity or brightness or perhaps by looking them up in the records, if they were known. There was a similar moving pair of dots in these very images – they are cropped out here. Those moved a bit slower, which would indicate an even greater distance from the Sun, but were brighter, which would indicate a smaller distance. The apparent slowness could be caused by an asteroid in a place along its elliptical orbit where it was moving mostly toward or away from Earth. Since nobody called it a planet then, I assume it was eliminated for one of those reasons. There are some dots that appear in one photo and not in the other, you should be able to see at least 5 examples of that in Figure A. That may be due to a difference in atmospheric conditions between successive photos. That is confirmed by the fact that the stars in the January 23 photo are a bit bigger (which means brighter in star images on photographic plates).   Another thing might account for single appearances would be a meteor falling through the atmosphere in a direction nearly straight at the telescope. So, you see that Tombaugh’s task was far from simple. One annoyance he did not have to deal with was the vast number of spacecraft now in orbit around the Earth.

Pluto was named by a contest, which was won by an 11year-old girl named Venetia Burney, from Oxford, England. She purportedly received a Five Pound Note for her prize. That does not sound like much, but it would be the inflation-adjusted equivalent of about 250 dollars at today’s exchange rates. She had kept to the tradition of choosing names from Greek mythology. I will just quote (5) an abbreviated explanation of those to put this in context:

  • Mercury (Hermes) is the god of commerce, travel and thievery in Roman mythology…
  • Venus (Aphrodite) is the Roman goddess of love and beauty…
  • Earth…is the only planet whose English name does not derive from Greek/Roman mythology.
  • Mars (Ares) is the Roman god of War.
  • Jupiter was the King of the Gods in Roman mythology…
  • Saturn (Cronus) is the Roman god of agriculture…
  • Uranus is the ancient Roman deity of the Heavens…
  • Neptune (Poseidon), was the Roman god of the Sea…
  • Pluto (Hades) is the Roman god of the underworld…

Let me just note here for you conventional people – I refer to Pluto as a planet. I know they decided to make a new classification of “dwarf planet”. So, if you object to me calling Pluto a “planet” please remember that Earth is a “rocky planet” and Jupiter is a “gas giant planet”. But they are all planets, are they not?

Back to Venetia: As I remembered, she chose Pluto because the first two letters would honor Percival Lowell, which was the name of a notable Astronomer and of the Observatory where Tombaugh made the discovery. Some say that it was because Pluto is a dark and far-away place like the underworld, and that might be another reason. However, I found that there was an interview with the lady herself in 2006 (2) in which she says:

“Yes, I don’t quite know why I suggested it. I think it was on March the 14th, 1930 and I was having breakfast with my mother and my grandfather. And my grandfather read out at breakfast the great news and said he wondered what it would be called. And for some reason, I after a short pause, said, “Why not call it Pluto?” I did know, I was fairly familiar with Greek and Roman legends from various children’s books that I had read, and of course I did know about the solar system and the names the other planets have. And so I suppose I just thought that this was a name that hadn’t been used. And there it was.”

Perhaps the other reasons were why the judges chose her as the winner. The interview seems to be on solid ground, but watch out on the internet. I found one source that said Clyde himself named the planet and I have known that not to be so, since I was young (back in the Cretaceous, when dinosaurs ruled the Earth). In another case, when I searched “who named Pluto”, the first hit was “The boy who named Pluto”. Let’s be charitable and assume that was about the Disney cartoon dog. By the way, did you ever notice that Pluto was a dog and Goofy was a dog, but Goofy stood on two feet, wore clothes and talked, in vivid contrast to poor Pluto? Some Disney dogs are more equal than others, it seems.

Back to the planet, now.

A Better View – Just Barely

In Science Fiction, Pluto was usually described as a nearly featureless ball of rock covered by layers of frozen gasses. After being examined by the Hubble Space Telescope the public image of Pluto was enhanced to a resolution of several dots. Some assumptions were made about what happens between the pixels and the processed image in Figure B is the result. This would seem to indicate that the SciFi characterization is erroneous. We will see.

I should mention that in 1978, a moon of Pluto was discovered, called Charon:

“…a Greek mythological figure:[12] Charon (/ˈkɛərɒn/ or /ˈkɛərən/Greek Χάρων) is the ferryman of the dead…” (4)

It is a remarkable satellite, being the largest – relative to its planet – in the Solar System.

gold_pluto_300

Credits: NASA/JHUAPL/SwRI

Figure B: The interpreted version of the Hubble Space Telescope image of Pluto.

Sneaking Up – Quickly

The New Horizons probe was launched on January 19, 2006. It is a relatively small spacecraft by modern standards and it was launched on one of the most powerful rockets available today. Even so, its speed toward Pluto was not nearly enough to get it there in ”merely” ten years. So, it was launched on a carefully chosen trajectory that would take it past Jupiter. There, it was accelerated by Jupiter’s gravity and redirected on a path toward Pluto. This is not a free ride, though. Jupiter gave a boost to New Horizons, but lost the same amount of energy (and didn’t miss it at all) from its revolution about the Sun. This sort of thing happens with many asteroids and comets that pass near Jupiter. Some are slung outward and gain speed, others are slowed and fall into orbits that take them closer to the Sun (a few, to collide with the inner planets) – and Jupiter gains a little. A few are captured into orbits around Jupiter itself. One comet (Shoemaker-Levy) famously was torn into multiple pieces by the tidal forces involved in a “close-encounter”. Those fragments were captured into an elongated orbit. The orbit – at the low end – happened to intersect the planet. That is another fascinating story, but I digress. Those of you who know me are not surprised.

New Horizons went speeding on toward Pluto. It was now the fastest known object in the Solar System – natural or manmade. Although it will not be in the Solar System much longer and will join four other spacecraft that are on their way to the stars. In January of 2015, the resolution of the photos from New Horizons became better than the Hubble images. Yet, still they were not much to see. In fact, Figure C, below was taken in early April and is the first color rendering of Pluto and its big moon Charon.

nh-first-pluto-charon-color-image

Figure C:   Pluto and Charon April 9, 2015           Credits: NASA/JHUAPL/SwRI

Are you disappointed? I was, too. I had the January “better than Hubble” date on my calendar for about six years and this was the best they had in April. But I understood that they did not do all that complicated image processing that they applied to the Hubble picture. Why not?   Because, in the New Horizons case, they had only to wait a few months to see far better resolution, so why bother? With Hubble, it was all they could hope for years and they had to have something to write papers about, in the meantime. I have been to the Lunar and Planetary Science Conference, several times. Those guys have to publish or die. It is not like Geophysics where you can accomplish actual profits by your efforts. Planetary Scientists are sadly dependent on Academic and Government funding. I found their attitudes to be shockingly predatory toward one another as compared to the polite, collegial attitudes of Geophysicists to which I was accustomed. But, I digress again. If you think I get sidetracked easily, imagine being me trying to get through college.

The Pluto imaging situation did improve as time went by and I can share with you another image, this time from early July of ‘15. The images cover most of what can be imaged by New Horizons. Charon and Pluto always show the same face to each other in their orbits around a common center. They are “tidally locked” which is an erudite way of saying the same thing. Also, the plane of their common orbit is not in the same plane as their orbit around the sun. That means that there are dark areas on both bodies that will not be seen by New Horizons. There was a time when we could have seen all of both, but that was in about 1985. I know that because I saw a lecture by a NASA Scientist about the subject by Dr. Paul Schenk (3). The good Doctor is a very good presenter – near as good as your humble Narrator. I had invited my family to travel the hour down to the Clear Lake area with me to see this public lecture and my niece gave me a provisional acceptance. I advised her that the dress code would be “business casual” (based on my Geophysics experience). This illusion was shattered when Dr. Schenk showed up in jeans and a polo shirt.

Figure D shows what will be seen, in greater detail.

7-1-15_Pluto_Charon_color_hemispheres_annotated_JHUAPL_NASA_SWRI

Figure D: Pluto and Charon – July 1, 2015       Credits: NASA/JHUAPL/SwRI

These images reveal that the earlier much-processed image from the Hubble Telescope is valid in its depiction of Pluto as varied in color and brightness. I see that this will need to be a series of at least two parts. But I assure you, Astute Readers, that there is much more and better to come.

Conclusion

Pluto stopped being a dot or a smudge and became a planet, with five (count ‘em, five) satellites – one that is near half Pluto’s size. It has craters, as you would expect out of most planets, but it also has vast smooth plains and mountains unassociated with any craters. The dot is now a fascinating variegated world. All this will be discussed in Part Two of this series.

More:   7-1-15_Pluto_Charon_color_hemispheres_annotated_JHUAPL_NASA_SWRI  Sneaking Up on Pluto (Part 2) 

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References

  1. Clyde Tombaugh:    http://starchild.gsfc.nasa.gov/docs/StarChild/whos_who_level2/tombaugh.html
  2. Venetia Burney Interview:    http://www.nasa.gov/multimedia/podcasting/transcript_pluto_naming_podcast.html
  3. New Horizons Mission:  Cosmic Explorations – A Speaker Series, Lunar and Planetary Institute, September 3, 2015  NASA’s exploration of Ceres and Pluto: An Update, Dr. Paul Schenk
  4. Charon:    https://en.wikipedia.org/wiki/Charon_(moon)
  5. Planetary Names: http://greek-mythology-gods.com/planets.html

Sneaking Up on Pluto (Part 2)

SteveTrucker2  Homepage

December 2015

Introduction

I always tend think of the exploration of the Solar System as something everybody knows about and that is probably erroneous for two reasons. First, not everyone was fascinated by the subject at the time, as I was and second, lots of you individuals are not old enough to have been paying attention when it happened. I would assume that some of this is taught in school, but I really can’t say, since it was just beginning to happen when I was at S.P Waltrip High in Houston. Both Shelly Duval and Patrick Swayze had graduated before I got there, in case some fans of either want to know.

One thing that might puzzle the average student might be why we had images of all the Outer planets by the 1970s and 80s and nothing but a dot or smudge for Pluto. That all relates to what was called at the time “The Grand Tour”. As it happened, there was an alignment of the outer planets in the 70’s and 80’s such that it would be possible to use gravity assisted orbital adjustments (“the slingshot effect”) to make it possible for a space probe to visit Jupiter, Saturn, Uranus and Neptune in one long and carefully managed trajectory. That’s another interesting story and I would be happy to tell it later.

Unfortunately, Pluto was not properly aligned to be next in the series of these visits. Why not? One way it was explained to me was: Any trajectory plotted for a probe approaching Neptune to send the probe to Pluto would intersect Neptune itself. So, that is why Pluto remained an unvisited backwater of the Solar System until now.

            The alignment of the outer worlds by 2006 was scattered enough that only Jupiter could help send the craft to Pluto and then only in a certain window of time. Missing that window would lengthen the mission severely or delay the launch by about twelve years until Jupiter came by again. Fortunately, the launch came off well on the first try.

A considerable amount of data was collected in the Jupiter flyby (2). A lot of what was last seen by the Magellan Orbiter was updated and enhanced. Figure AA is a view of the Jovian moon Io which is far more volcanically active than the Earth. That is pronounced with a short “I” by all the Ivory Tower PhD’s and a long “I” by normal people. Major changes in its Geology (Don’t give me a hard time about that word!) were detected.

Many other moons and Jupiter itself were imaged and studied, but we are talking about Pluto, here.

NH_IO

Figure AA: A view of Jupiter’s moon Io as seen from New Horizons during its fly-by of JupiterCredits: NASA/JHUAPL/SwRI

New Horizons Spacecraft

            Figure A is the New Horizons Spacecraft. The main body of the spacecraft is about the size of a grand piano and the whole thing masses as much as a medium sized truck

NewHorizonProbe

Figure A: The New Horizons Spacecraft  Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute       

New Horizons (NH) incorporates all that has been learned over the years. The Voyager probes (one of which actually made the complete “Grand Tour”) each had a main antenna that was capable of constant communications with the Earth. This necessitated what is called a “scan platform” that held the instruments that need precise pointing, that moved independently of the antenna. That configuration had proven troublesome on one of the Voyager probes and data were lost. That is because data storage was actually on a ½ inch, 8 track magnetic tape with a total capacity of about ½ Megabyte and a top baud rate of 56 kilobits per second (3). That’s what I said – “stone knives and bear skins!” – so, real-time transmission was required for image data.

The newer probes including NH have fixed instruments that are pointed by turning the entire spacecraft. This in turn means that the probe cannot talk to the Earth and take instrument readings at the same time. What makes it all possible is an enormous memory capacity that is capable of high data rates. This was a luxury that earlier probes could not enjoy. The disagreeable result was that the probe was “radio silent” as it collected the bulk of the science data at Pluto. This is simply because it was busy pointing instrument and taking readings.

That large dish antenna labeled “REX” in the Figure was a lot smaller than they might like but it was also a trade-off to allow more instrumentation. That means that, while the data could be acquired in a big rush during the fly-by, the data return rate was dismally slow by comparison. So much so that the NH is still downloading data, months after the fly-by and will be doing so until November of 2016.

The other instruments are described here as quoted from the New Horizons web site (1):

“The New Horizons team selected instruments that not only would directly measure NASA’s items of interest, but also provide backup to other instruments on the spacecraft should one fail during the mission. The science payload includes seven instruments:

Ralph: Visible and infrared imager/spectrometer; provides color, composition and thermal maps.

Alice: Ultraviolet imaging spectrometer; analyzes composition and structure of Pluto’s atmosphere and looks for atmospheres around Charon and Kuiper Belt Objects (KBOs).

REX: (Radio Science EXperiment) Measures atmospheric composition and temperature; passive radiometer.

LORRI: (Long Range Reconnaissance Imager) telescopic camera; obtains encounter data at long distances, maps Pluto’s farside and provides high resolution geologic data. SWAP: (Solar Wind Around Pluto) Solar wind and plasma spectrometer; measures atmospheric “escape rate” and observes Pluto’s interaction with solar wind.

PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation) Energetic particle spectrometer; measures the composition and density of plasma (ions) escaping from Pluto’s atmosphere.

SDC: (Student Dust Counter) Built and operated by students; measures the space dust peppering New Horizons during its voyage across the solar system.”

The alert reader will note that the same antenna (REX) that returns data to the Earth is also listed as an instrument. It is used to measure the changes in an Earth-NH transmission as the signal is eclipsed by Pluto’s atmosphere and surface and the same situation was also measured at Charon, thus characterizing the atmosphere of Pluto and of Charon (if any).

A Better View – Like “Way!”

            If you are wondering why I have gone on so long about the discovery, naming and early characterization of Pluto, Astute Readers, I will now confess:   I wanted to convey – just a bit – that long-delayed anticipation that I felt – literally for years – in awaiting the results of the NH mission. Hence, the title of this article “Sneaking up on Pluto”. That said, I hasten to present a Portrait of the Happy Couple, Pluto and Charon. Please see figure B.

nh-pluto-charon-v2-10-1-15Figure B. Pluto and Charon           Credits: NASA/JHUAPL/SwRI

First, I must point out that this graphic is a composite. That is to say that while they are absolutely valid images of Pluto and Charon, they have been cut and pasted into this “Family Album”.

You will no doubt notice some very intriguing and unexpected features of both the planet and its satellite. Far from a near-featureless cratered ice-ball, it is obvious (by lack of craters in some regions) that Pluto has undergone recent changes. There are distinct regions of very different character and color. Charon has a great chasm that spans its diameter and crosses its equator.

The early much interpreted, computer generated images from the scant data received by the Hubble Telescope that indicated differentiated terrain are richly confirmed. The most pronounced feature is the large plain of ice that quickly became known as the “Heart” at this resolution much of it seems featureless and hence craterless. The standard procedure for dating terrain on solar system objects is to count the number of craters of different sizes. When the craters are many and varied, the terrain is obviously very old. When you see an area with no craters, then it is very new, relatively speaking. The Heart was later “officially” named “Tombaugh Regio” in appropriate honor of Pluto’s discoverer.

I should mention that I referred to Tombaugh as a High School Graduate previously, but I feel obliged to point out that he later earned a PhD. My reference to his educational status at the time of the discovery was no slight, but rather was my tribute to the idea that Excellence does not require certification. I have known and worked with many brilliant PhD’s. I have also known and worked with some who were so over-specialized as to be (in my humble opinion) rather shallow and uninteresting people, outside of their rather small zone of competence.

The smooth-looking part (on the left) of Tombaugh Regio is now called “Sputnik Planum”. As we will see in the next images, it is not nearly as featureless as it first appeared.

Pluto in Detail

            I will take a leap forward now to some of the most up-to-date images. Figure D is a close-up of the dark region near the Southwest of the Tombaugh Regio. It covers a confluence of three terrain types, the smooth, icy plains at top, the mountains (obviously not associates with craters) in the center and more “conventional” cratered landscape at the bottom.

nh-dark-areas-9-10-15Figure D: The Dark Area at the Southwest of Tombaugh Regio. Please note the three distinct terrains Credits: NASA/JHUAPL/SwRI

 The icy plains are now revealed to have distinct polygonal divisions. The ice in question is actually solid Nitrogen and Methane which, at the ambient temperature of about -230° C behave much like Earth-temperature water ice and flow slowly into valleys as they accumulate. The Mountains at the center of this image are quite clearly not related to craters and probably contain a large fraction of water ice which at Pluto temperatures is as hard and durable as rock. I will cite the good Doctor Shenck (4), again for this insight.

Pluto has been moving farther from the Sun since 1985 and you might expect that the atmosphere could be condensing out to be frozen on the surface. What did puzzle me was the contention that we have no evidence of Pluto’s atmosphere actually freezing out as it moves farther from the Sun. I asked Doctor Shenck if there might be some deposition of atmospheric gasses in the seasonal total-dark areas of the Southern hemisphere and if there might be some data (yet to be downloaded) from instruments that might answer that. He replied positively to both questions.

This image in Figure D seemed to me to give some merit to the idea that some atmospheric “fall out” may have already taken place. The crater at lower left in the image, quite clearly indicates that the ice there accumulated, did not flow from anywhere else, but must have condensed (been deposited) out of the atmosphere. Also, I mentioned that the ice appears to be flowing into valleys and I ask you, how can ice flow if there is not a new supply being deposited on the existing mass of ice? For the record, this image was not available when I spoke to Dr. Schenck.

Just during the writing of this article the answers came from this quote from a NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute Press Release:

“Key to understanding activity on Pluto is the role of the deep layer of solid nitrogen and other volatile ices that fill the left side of Pluto’s ‘heart’ — a vast, 620-mile (1,000-kilometre) -wide basin, informally named Sputnik Planum. New numerical models of thermal convection within this ice layer not only explain the numerous polygonal ice features seen on Sputnik Planum’s surface, but indicate this layer may be up to a few miles thick. Evaporation of this nitrogen and condensation on higher surrounding terrain leads to glacial flow back toward the basin; additional numerical models of nitrogen ice flow show how Pluto’s landscape has been and is still being transformed.”

Figure E is a higher resolution image from the edge of the Tombaugh Regio that shows much more texture to the icy plains and a much better look at the mountains.

color-swath-use-12-10-15_closeupFigure E: High resolution color image at the edge of Tombaugh Regio         Credits: NASA/JHUAPL/SwRI

Figure F is an even higher resolution of another region of the icy plains. Notice how the ridges that divide the segments are seemingly being covered up with what look like dunes to me. I don’t know what to think of those. But the aforementioned press release indicates that there is a Nitrogen/Methane cycle of evaporation and condensation that drives the glacier-like accumulations. It seems that these gasses play the role of water on Earth, that exists in solid, liquid and gaseous forms on the same planet. Now, there is no evidence (yet) of any liquids on Pluto and I suspect that the cycle is one of sublimation (solid to gaseous) and deposition (gas to solid).

ClosePlainsDunesFigure F: Higher resolution image of the icy plains Credits: NASA/JHUAPL/SwRI

Conclusion

Despite the previously mentioned low data rate, the data are accruing at an overwhelming rate and the unmitigated diversity and complexity of this information will no doubt keep Planetary Scientists employed for years to come. I really need to publish this before it gets even further obsolete. But one thing is clear. Pluto is far from the static, frozen, cratered, icy rock it was imagined to be. It is a dynamic and complex world and IMHO, deserves the designation of “Planet” without qualifiers.

Still, it is a tantalizing irritation that the New Horizons probe only provided a “snap shot” of the situation at Pluto and we can only find out what happens next by a similar, massive effort to launch another such probe. It is perhaps a comfort to remember that the technology of the New Horizons probe is about 15 years out-of-date now and the next such probe would be faster, better, cheaper and -especially- less massive. It is not unreasonable to imagine that a Pluto orbiter could be within the realm of possibility. Even if decades later, a new probe could see the changes and the longer the delay, the more obvious those might be.

 Sneaking up on Pluto Part 1

References

  1. New Horizons Spacecraft: http://www.nasa.gov/mission_pages/newhorizons/spacecraft/index.html
  2. Jupiter Flyby: https://en.wikipedia.org/wiki/New_Horizons#Jupiter_encounter    http://pluto.jhuapl.edu/Mission/The-Path-to-Pluto/Jupiter-Encounter.php
  3. Voyager Data Rate: http://itknowledgeexchange.techtarget.com/storage-disaster-recovery/nasas-voyager-used-8-track-tape-to-go-into-space/
  4. New Horizons Mission:Dawn and New Horizons MissionsSept. 3, 2015 Lunar and Planetary Institute, Lunar and Planetary Institute September 3, 2015   NASA’s Exploration of Ceres and Pluto: An Update, Dr. Paul Schenk
  5. Charon:https://en.wikipedia.org/wiki/Charon_(moon)