Download Neptune`s Unusual Rings The structure of Neptune`s rings was not

Neptune’s Unusual Rings
The structure of Neptune's rings was not well known, unlike the Uranus rings which
had been discovered from Earth in 1977 by Dr. James Elliot, presently of MIT, and
accurately mapped by Voyager during the January 1986 flyby. According to Dr. Philip
Nicholson of Cornell, based upon relatively recent analyses of stellar occultation data by
himself, Dr. William Hubbard of the University of Arizona, and other observers, Neptune
appeared to be surrounded by at least three near-circular rings, ranging in distance from
41,000 to 67,000 km from the center of Neptune. In each ring, however, material did not
appear to provide complete azimuthal fill, thereby giving rise to the term "partial ring
arcs".
These ring arcs were fortunately very narrow, less than 20 km in radial width, and
the random chance of a spacecraft collision was well under one percent. Even if, perversely,
the Voyager navigators tried to hit one of these arcs, the chances would not exceed ten
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percent. The real concern was associated with a diffuse sheet of unseen particles that might
span a vast region, thousands of kilometers in radial width. If the source of these diffuse
particles was the outermost arc, Voyager could find safe passage beyond this arc, as drag
from the slower co-rotating plasma would cause the unseen particles to experience orbital
decay and migrate inwards towards Neptune.
The solution was obvious. We should make sure that Voyager crosses the equatorial
plane of Neptune just outside the 67,000-km ring arc region. Unfortunately, from a
modeler's point of view, Neptune is so far away that we cannot be sure of the exact location
of this ring, and must allow an uncertainty of 4,000 km in its exact distance from the center
of Neptune. In order for Voyager to cross the ring plane 71,000 km out, the Neptune
closest approach distance must not be less than 28,950 km from the center of Neptune,
pushing the Triton flyby distance farther out to 38,500 km.
But what if there were more distant ring material sources, such as one or more small
satellites, out beyond the most distant arc currently detected? After all, at impact speeds of
27 km/sec (60,000 mph), particles as small as 100 microns (the size of fine sugar grains)
could still inflict spacecraft damage. At this point, risk calculations became very involved,
requiring numerous assumptions about the dependencies of particles number densities and
spacecraft vulnerable areas upon particle size.
Neil Divine and MPO's Rex Ridenoure made these complex calculations, with resulting
particle hit probabilities ranging from nil to almost certainty. All in all, however, the project
believed the region beyond the outermost ring arc to be safe, but even a small element of
uncertainty was sure to raise the suspense level as the world followed the famous robot
during its high-speed dash through the Neptune system. If long-exposure images during
approach to Neptune should reveal potentially dangerous ring material regions, project
officials could ask the navigators to adjust the aiming point for a safer passage.
Neptune's Unknown Radiation
It is not difficult to form a mental image of risk concerns that would arise from
spacecraft passage through atmospheric or ring particle regions. But trapped radiation,
largely in the form of high-energy electrons, is more esoteric for the risk estimators to
handle. No direct or indirect measurements had ever been made of Neptune trapped
radiation. However, the actual detection of energetic particles at Jupiter, Saturn, and
Uranus strongly suggested that Neptune was also surrounded by a vast region of similar
particles trapped within a rotating magnetosphere.
As energetic electrons race about at near-light speeds, one could count the number of
particles per second that cross a given area. That number is referred to as "flux". If one
counts the total number of particles that have crossed a given area at the end of, say, an
hour, that number is referred to as "fluence". Flux and fluence affect spacecraft
components in different ways. High flux tends to cause temporary disturbance but
generally not permanent damage, whereas high fluence, akin to total radiation dose, can
lead to permanent loss of function.
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In, January of 1987, Divine and Project Scientist Edward Stone coordinated a meeting
(held at UCLA) of planetary trapped radiation experts, with the objective of developing an
upper limit model for electrons having energy levels near and in excess of 3 million
electron volts. At these levels, spacecraft damage could be possible. As Voyager 1 had
survived (albeit with some problems) Jupiter's intense radiation exposure of 53 million
electrons per square cm per sec (peak flux) and nearly two trillion electrons per square cm
(total fluence), it made sense to express the Neptune radiation model values as a fraction of
the actual values experienced at Jupiter.
The results were interesting and caused the risk takers to reflect carefully before
giving a green light for a close Neptune flyby. For the Neptune upper limit radiation model,
a close Voyager-2 flyby over the north polar region of Neptune would result in a peak flux
over half that encountered by Voyager 1 at Jupiter, but the total fluence would be thirty
times smaller. Also, the time period near peak flux would last only an hour, rather than the
several hours of high flux endured in passing through the vast Jovian magnetosphere.
To be on the safe side, however, the project planed to design the Neptune
near-encounter sequence with a few precautionary features. At Jupiter, the high radiation
flux led to increased background noise for some of the scientific sensors and, in a few cases,
extended periods of degraded performance. By the control of instrument gain settings and
other special on-board commands, the possibility of permanent degradation could be
minimized. Though probably the result of fluence at Jupiter and therefore not a serious
concern at Neptune, timing errors in Voyager's electronic master clock, such as those
experienced at Jupiter, could be corrected by special commands sequenced during the
Neptune flyby.
Going For The Gold
Living up to Voyager's past achievements at Jupiter, Saturn, and Uranus would be a
tall challenge for the remote Neptune encounter.
Earth-based CCD images suggested some Neptune
features, but the frigid atmosphere might only offer
scant detail, as the cloud patterns could be obscured
by a high altitude haze. As if to compensate the
devoted Voyager team, however, the atmosphere of
strange Triton should be sufficiently thin to provide
clear views of its alien surface, unlike the hidden
surface of Saturn's large moon Titan. Furthermore,
the Neptune partial ring arcs, believed to exist by
ground-based astronomers, would be tantalizing
targets for Voyager's sensors.
Several meetings were held by the Voyager
SSG to weigh science issues and select the best
trajectory for the only visit to the Neptune realm
Voyager-2 trajectory past Neptune,
during this century. Consistently, the preferred
viewed from side, NASA/JPL
attributes were very close encounters with both
Neptune and Triton, each followed by occultations of the sun and Earth as seen from the
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spacecraft. As frosting on the cake, it would also be super if the chosen flight path could
cause one or more stars to be seen to pass behind Neptune's ring arcs at available observing
times during the near-encounter flyby.
The close body encounters would provide the best photographic and optical sensor
resolutions, the best opportunity to measure gravitational and magnetic field properties,
and the best chance to detect energetic particles and plasmas. Passage of Voyager's radio
beam through an atmosphere or through ring material, as the media altered the signal
strength and phase, would allow the Radio Science Team to deduce key properties of these
media.
Occultation of the sun by an atmosphere would allow the Ultraviolet Spectrometer
Team to measure the presence of many different elements and compounds that could be
detected when they absorbed energy at very specific wavelengths out of the full energy
spectrum from the sunlight passing through the atmosphere. Finally, occultation of a star
by planetary rings, much like the flickering rays from a lantern passing behind a picket
fence, would allow the Photopolarimeter Team to learn much about the detailed structure
of the rings.
At first blush, it might seem that the best aiming point was obvious. Just fly as close
as safety permits, presumably just outside the outermost ring arc region. This was nearly
the case, but had to be tempered by at least two other considerations. Navigation errors
were more troublesome for the closer flybys, due to the
greater error amplification and closer proximity of various
targets. Also, several key events would occur very rapidly,
leaving insufficient time for the sequence designers to
schedule all of the desired observations.
It was decided to examine a range of trajectories with
Triton flyby distances from 20,000 to 70,000 km.
Arguments favoring the closer distances were better Triton
imaging resolution, better chance of obtaining both sun and
Computer lookback at Neptune,
Earth occultation by Triton (where the diverging occultation
Triton, and rings, NASA/JPL
zones still had an acceptable amount of overlap), and
sufficient radio signal bending by the Neptune atmosphere to obtain measurements below
the methane cloud deck. Arguments favoring the more distant flybys were better event
spacing for sequence design, with lower limb-tracking rates and reduced error sensitivities
for the limb-pointing demands during the Earth-occultation period by Neptune and its
atmosphere.
After a great deal of analysis and debate among the SSG members, using inputs from
my own team members, two recommendations were made. The Imaging Science Team
requested a 25,000-km Triton flyby, and the Radio Science Team requested a 55,000-km
Triton flyby (to reduce the Neptune-related limb pointing errors). Edward Stone, leader of
the SSG, decided upon a compromise Triton flyby distance of 40,000 km, both favoring the
chance for Triton dual-occultation experiments and ensuring that Voyager would pass
outside of the outermost known ring arc region during its inbound crossing of Neptune's
equatorial plane. The Voyager Project had chosen a new aiming point. Though 30,000 km
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farther out than the 10,000-km Triton flyby recommended in September of 1985, the new
aiming point was, ironically, close to the Triton flyby distance selected several years earlier
in August of 1980.
The 40,000-km Triton flyby was not only an exciting trajectory from a scientific point
of view, but also appeared to achieve the best balance between mission return and mission
risk. However, should the project acquire any new knowledge about the Neptune
environment that might suggest a different aiming point, Voyager carried sufficient
propellant reserves to have permitted a significant alteration of the trajectory as late as a
few weeks before the Neptune encounter.
In the absence of any new data about the Neptune environment, the navigators still
had to constantly assess the value of different navigation
strategies in meeting the mission and science objectives. For
example, their existing estimates for the Triton diameter were
very uncertain, ranging from as little as 2,000 km to as much
as 6,000 km. During approach to the Neptune kingdom, the
Voyager images would reveal the true size of Triton, and the
navigators might need to make slight trajectory adjustments
to preserve the greatest mission return.
Recalling August 10th, we were barreling in on the blue
giant at high speed, steadily increasing every second, a fact
that the JPL PIO people felt they had to constantly announce
Neptune as seen from Triton,
painting by (to be supplied)
in the form of huge distance numbers. At 9 am, Voyager 2
had traveled [2,742,542,050 miles] from Earth, and was now
[3,211,000 miles] from Neptune, etc. In the time it took to recite the range to a friend, it
had already changed by 50 miles. Triton picture resolution was now about 100 miles,
approaching the “Schiaparelli stage” where observers could speculate wildly about what
they were seeing. Were the dark areas huge seas and the light areas high plains? Or how
about rock fields and mountains for the dark areas and frozen ice landscapes for the lighter
regions? Other scientists were trying to detect the rings and small new moons by
scrutinizing long-exposure pictures of black space, trying to pick out true discoveries vs
useless “foo foo,” a word coined during the Saturn encounter to describe assorted
blemishes and dirty pixels.
In the next two weeks, the world would join the Voyager team for its final planetary
swan song, culminating in a close dive over Neptune’s north polar crown and dashing past
Triton a few hours later. Uranus had been disappointing with its featureless atmosphere,
but we had already seen a huge spot on bluish Neptune several weeks earlier, at about 22º
south latitude. This being the same latitude as the location of Jupiter’s Great Red Spot and
as scientists like calling a spade a spade, we had begun calling it the Great Dark Spot (GDS),
with its distinctly dark blue color. Many pondered whether gas giants have a proclivity for
forming Earth-sized storms at this latitude. Further south was a white streaky spot dubbed
Scooter, and further south yet a Small Dark Spot at about 55º south latitude. The three
spots moved around the planet at different rates due to the varying wind speeds with
latitude. The GDS was undoubtedly a huge storm feature which rotated opposite to that of
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cyclones, creating a pressure ridge forcing methane to condense into clouds one of which,
called the White Companion, hovered just above the GDS.
This was all pretty exciting, but most of all we wanted a close look at Triton, the large
retrograde moon suspected of having a frail atmosphere above its icy and still mysterious
surface. JPL was abuzz with the media, many in temporary trailers, others at press desks in
von Karman, and others mingling with project staff and imaging team members for tidbits.
Most hoped to learn of either problems or discoveries. “If it bleeds, it leads.” Or, “If it’s new,
it due.” For all but the best of the science writers, many got story facts wrong, but they
hated to let interviewees ever read their drafts to catch mistakes. So we rocket and
planetary scientists always cringed when we caught the final version on the newstand, as
we were usually quoted by the writers. It was always a relief to wipe our brows when the
words had no major errors. It was an interesting “trade-off,” whether to make the news and
impress your friends –- or whether to make the news and appear as an idiot, the result of a
scientifically illiterate or hasty writer. This fact has always caused me concern. Most people
believe what they read in the press to be true. From my own personal experience, however,
and even allowing for the technical nature of the space exploration experience, the error
rate is much higher than the public realizes.
The Neptune and Triton encounters offered the promise of an exciting finale for the
epic visit of Voyager to the outermost of the Jovian gas giant planets. As it turned out, the
world was elated with the discoveries, particularly the amazing world Triton, with active
geysers on its ancient surface. One of the most interesting discoveries from Voyager 2 was
finding The Great Dark Spot and the Scooter. These two features revealed much about
Neptune's erratic and dynamic atmospheric activity, as the wind speeds around these two
areas reach up to 2,000 kilometers an hour (1,200 miles per hour). Even more bizarre, the
Great Dark Spot imaged by Voyager 2 has recently disappeared! A New Dark Spot has
formed which scientists speculate will not last long either.
Amazing Triton
A global color mosaic of Triton was taken in 1989 by
Voyager 2 during its flyby of the Neptune system. Color was
synthesized by combining high-resolution images taken
through orange, violet,
and ultraviolet filters;
these
images
were
displayed as red, green,
and blue images and
combined to create this
color version. With a
radius of 1,350 (839
mi), about 22% smaller
than
Earth's
moon,
Triton is by far the
Lookback at Neptune and
Triton crescents, NASA/JPL
largest
satellite
of
Neptune.
It
is
one
of
Amazing Triton, still active, NASA/JPL
only three objects in the Solar System known to
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have a nitrogen-dominated atmosphere (the others are Earth and Saturn's giant moon,
Titan). Triton has the coldest surface known anywhere in the Solar System (38 K, about 391 degrees Fahrenheit); it is so cold that most of Triton's nitrogen is condensed as frost,
making it the only satellite in the Solar System known to have a surface made mainly of
nitrogen ice. The pinkish deposits constitute a vast south polar cap believed to contain
methane ice, which would have reacted under sunlight to form pink or red compounds.
The dark streaks overlying these pink ices are believed to be an icy and perhaps
carbonaceous dust deposited from huge geyser-like plumes, some of which were found to
be active during the Voyager 2 flyby. The bluish-green band visible in this image extends
all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost
deposits. The greenish areas include what is called the cantaloupe terrain, whose origin is
unknown, and a set of 'cryovolcanic' landscapes apparently produced by icy-cold liquids
(now frozen) erupting from Triton's interior.
Triton is the only large satellite in the solar system to circle a planet in a retrograde
direction -- in a direction opposite to the rotation of the planet. It also has a density of
about 2.066 grams per cubic centimeter (the density of water is 1.0 gram per cubic
centimeter). This means Triton contains more rock in its interior than do the icy satellites
of Saturn and Uranus. The relatively high density and the retrograde orbit has led some
scientists to suggest that Triton may have been captured by Neptune as it traveled through
space several billion years ago. If that is the case, tidal heating could have melted Triton in
its originally eccentric orbit, and the satellite might even have been liquid for as long as one
billion years after its capture by Neptune. Triton is scarred by enormous cracks. Voyager 2
images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles
several kilometers into the atmosphere.
Solar System Portrait
Carl Sagan had lobbied the Voyager project management to try to capture a picture
of Earth, looking back as Voyager 1 left the solar system. On Valentine's Day in 1990, the
Voyager flight team commanded Voyager 1 to aim its cameras at a string of small colored
dots clustered just to the right of the constellation Orion. The spacecraft was then 32
degrees above the ecliptic and
nearly 6 billion kilometers (3.7
billion miles) from the Sun. It took
39 wide-angle views and 21
narrow-angle images. The narrowangle
camera
took
three
consecutive
images
through
colored filters of seven of the nine
Solar system portrait by Voyager 1, NASA/JPL
planets.
This
enabled
image
processors at JPL to construct the colored portraits of the planets. The Image Processing
Laboratory then pasted together the wide-angle images into the mosaic shown. Voyager had
produced the first portrait ever of our Sun and planets together. Carl Sagan made much of
the “pale blue dot” which was all that could be seen of our world.
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The Silver-Threaded Quilt
Some events have remained in my soul for many years. In August of 1989, I had been
asked to present Voyager’s results and status at the Educator's Conference to be held in
Caltech’s Beckman Auditorium for over 600 science teachers from across the nation. I had
barely made it in time from JPL to Caltech by the scheduled time of the lecture. A host was
adlibbing to buy time. When he spotted me entering the rear of the auditorium, he
announced, “Charley is here.” At that moment, the entire audience got to their feet and
cheered as I walked down the carpeted aisle and
climbed the stage to the podium. I could not hold
back the tears as I struggled to find my first few
words. At that moment, all of the years of hard
work had been repaid a thousand times. The talk
went well, but unbeknown to me at the time, a 5th
grade teach from Libby, Montana, had been
inspired more than I could have imagined. Her
name was Diane Johnson and she later told me
that my presentation had "affected her life as a
science teacher.”
Silver-threaded quilt, Libby 5 th grade class
She was married at the time, but
subsequently passed away from cancer. But not before I had visited her in Libby and had
become close friends with her and her husband Mike. Several months after the conference,
Johnson and her 5th grade class decided to create a special space quilt for me as a gift for
inspiring them to do well in math and science. School funds had been tight, but Johnson
won a grant of $450 from the Libby Rotary Club and used the money to purchase a new
sewing machine, quilt materials, and assorted model rocket supplies. As the new sewing
machine seemed to only run backwards, they exchanged it for one that ran the correct way.
Work on the quilt began in Sept 1990, and finished a few months later. Special silver
threads were used in portions of the quilt to connect each patch made by a different
student. It was entirely the handiwork of the children, with only advice from Johnson.
The class also managed to buy 22 model rockets and even a special camera to
photograph them at the time of parachute deployment after rocket burnout. Johnson was
an avid wilderness backpacker, often taking many children along because of her love for
them. She was a wonderful teacher and person. After her death, her husband Mike took
many years to recover from her loss, with my last contact from him in Africa trying to
improve the conditions at a small native village. As a final note, Johnson said that it was the
quilt-makers wishes that I use the quilt to "snuggle in to be warm and comfortable" . . . and
not to hang it in a museum.”
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Finding the Heliopause
The Voyager Interstellar Mission (VIM) seeks to extend NASA’s exploration of the
solar system beyond the neighborhood of the outer planets to the electromagnetic limits of
the Sun's sphere of influence. An earlier
discussion of the Oort Cloud helps to
understand the vast extent of our sun’s
gravity field. But there is also a region of
electromagnetic waves and particles that
the Voyagers can sample, as the outer
reaches of the Oort Cloud are too far for
even the valiant Voyagers. The VIM will be
searching for the heliopause boundary, the
outer limits of the Sun's magnetic field and
outward
flow
of
the
solar
wind.
Penetration of the heliopause boundary
between the solar wind and the interstellar
medium will allow measurements to be
made of the interstellar fields, particles
The Heliopause and extended solar regions, NASA
and waves unaffected by the solar wind.
The VIM is an extension of the Voyager primary mission that was completed in 1989 with
the close flyby of Neptune and Triton by the Voyager-2 spacecraft.
At the start of the VIM, the two Voyager spacecraft had been in flight for over 12
years having been launched in 1977. Voyager 1 was at a distance of approximately 40 AU
from the Sun, and Voyager 2 was at a distance of approximately 31 AU. Both Voyagers are
escaping from the solar system at speeds in excess of 3 AU per year. The VIM consists of
three distinct phases: the termination shock, heliosheath exploration, and interstellar
exploration phases. The two Voyager spacecraft began the VIM operating in an environment
controlled by the Sun's magnetic field with the plasma particles being dominated by those
contained in the expanding supersonic solar wind. This is the environment of the
termination shock phase. At some distance from the Sun, the supersonic solar wind will be
held back from further expansion by the interstellar wind. The first feature encountered by
a spacecraft as a result of this interstellar wind/solar wind interaction will be the
termination shock where the solar wind slows from supersonic to subsonic speed and large
changes in plasma flow direction and magnetic field orientation occur.
Author Jay Gallentine has a wonderfully simple water faucet analogy for the
termination shock. Just turn on your kitchen faucet at full force with a flat plate in the
bottom of the sink. The water will hit the surface, then move out rapidly from the impact
site in a smooth flow. After a few inches, however, it will slow down and become turbulent
at an easily seen boundary – a miniature version of the termination shock.
Passage through the termination shock ended the termination shock phase and began
the heliosheath exploration phase. Voyager 1 crossed the termination shock at 94 AU in
December 2004 and Voyager 2 crossed at 84 AU in August 2007. Since passage through the
termination shock, the spacecraft have been operating in the heliosheath environment
which is still dominated by the Sun's magnetic field and particles contained in the solar
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wind. The heliosheath exploration phase ends with passage through the heliopause which is
the outer extent of the Sun's magnetic field and solar wind. The thickness of the heliosheath
is uncertain and could be tens of AU thick taking several years to traverse. Passage through
the heliopause begins the interstellar exploration phase with the spacecraft operating in an
interstellar wind dominated environment. This interstellar exploration is the ultimate goal
of the Voyager Interstellar Mission.
To the Far Stars
Voyager 1 is escaping the solar system at a speed of about 3.6 AU per year, 35
degrees out of the ecliptic plane to the north, in the general direction of the Solar Apex (the
direction of the Sun's motion
relative to nearby stars). Voyager
2 is also escaping the solar
system at a speed of about 3.3 AU
per year, 48 degrees out of the
ecliptic plane to the south. Both
Voyagers are headed towards the
outer boundary of the solar
system
in
search
of
the
heliopause, the region where the
Sun's influence wanes and the
beginning of interstellar space
can be sensed. The heliopause
has never been reached by any
spacecraft; the Voyagers may be
Future flyby of other stars. NASA/JPL
the first to pass through this
region, which is thought to exist
somewhere from 8 to 14 billion miles from the Sun. This is where the million-mile-per-hour
solar winds slows to about 250,000 mph -- the first indication that the wind is nearing the
heliopause. The Voyagers should cross the heliopause 10 to 20 years after reaching the
termination shock.
The Voyagers have enough electrical power and attitude control fuel to operate at
least until 2020. By that time, Voyager 1 will be 12.4 billion miles (19.9 billion KM) from
the Sun and Voyager 2 will be 10.5 billion miles (16.9 billion KM) away. Eventually, the
Voyagers will pass other stars. In about 40,000 years, Voyager 1 will pass within 1.6 light
years (9.3 trillion miles) of AC+79 3888, a star in the constellation of Ophiuchus, while
Voyager 2 will pass a similar distance from the star Ross 245 in the constellation of
Andromeda. In 296,000 years, Voyager 2 will pass 4.3 light years (25 trillion miles) from
Sirius, the “dog star” in Canis major and also the brightest star in the sky. The Voyagers are
destined to eternally wander the Milky Way. These are not very close stellar passages, as
our own sun’s nearest star is Alpha Centauri at a distance of about 4.6 light years. But it is
still fun and inspirational to imagine that we have built such fine machines for distant
travel to the far stars.
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In 1977, a committee chaired by Carl Sagan decided to select a large number of
images and sounds from Earth to be encoded on a gold-coated copper phonograph record,
the latter medium chosen in recognition of the 100th anniversary of the invention of the
phonograph record by Thomas Edison. Sagan’s committee chose 116 images and a variety
of natural sounds, such as those made by surf, wind, thunder, and even animal sounds,
including the songs of birds and whales. They also added
musical selections from different cultures and eras (including
Beethoven, Mozart, Stravinsky, and even Chuck Berry),
spoken greetings in fifty-five languages, and printed
messages from President Jimmy Carter and United Nations
Secretary-General Kurt Waldheim. In Sagan’s excellent book
Murmurs of Earth, you can learn much about the record
development and content selection process.
The collection of images on the record includes many
photographs and diagrams. Many images are of scientific
interest,
showing
mathemat
ical
and
physical
quantities
John Casani preparing to place
, the solar
the golden record aboard
system
Voyager 1, NASA/JPL
and
its
planets, DNA, and human anatomy and
reproduction.
Images
were
also
included of humanity, animals, insects,
plants and landscapes. The human
images show a broad range of cultures,
including
food,
architecture,
and
The record cover guide for use, Wikimedia Commons
humans
going
about
their
daily
activities. Many pictures are annotated with indications of time and distance scales.
A portion of President Carter’s remarks included:
“We are attempting to survive our time so that we may live into yours. We hope someday, having
solved the problems we face, to join a community of galactic civilizations. This record represents our
hope, our determination, and our good will in a vast and awesome universe.”
While Waldheim’s remarks included:
“We step out of the solar system into the universe seeking only peace and friendship, to teach if we
are called upon, to be taught if we are fortunate. We know that our planet and its inhabitants are but
a small part of the immense universe that surrounds us, and it is with humility and hope that we
take this step.”
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Reflections upon the Voyager Years
How does one reflect upon these incredible
Voyager years? Two robotic spacecraft revealed
enough new information about 4 planets, five
dozen moons, and myriad rings and radiation
fields to fill many astronomy textbooks for future
generations. I was personally so very fortunate to
have been part of this unique experience. It took
the nexus of three conditions – the planetary
alignments to permit an affordable Grand Tour,
the readiness of the U.S. spacecraft technology,
and my being in the right place and time – to
spawn a great adventure for this grateful Voyager Project and Science Leads, National
Earthling. The Voyager team members formed Geographic image in JPL Operations Center
lasting friendships bonded by a rich variety of
technical skills, the highest ethical standards, humor when often called for, devotion to a
common goal, and of course the thrill of new discoveries felt by all true explorers.
Many of us paid different prices in terms of lost time with family and friends, missed
vacations when we were younger and more fit for travel,
and the chance to develop skills in other areas such as art
and music. However, if given the chance to go back in time
and give up the Voyager years, most of us would likely
decline. No doubt,
in hindsight, we
might make other
adjustments in our
lives, but then so
would
most
people.
As
you
would
expect,
many people from
all
fields
have
The Gas Giants, artist Don Davis
reflected upon the
significance of the epic journeys of the two
Voyager spacecraft. I even included many
reflective quotes in Chapter 14 of the Voyager
Neptune Travel Guide that the reader may wish to
peruse. But I was especially touched by one
paragraph in the wonderful book by sociologist,
Some of Voyager’s many awards
aviation fan, and friend David Swift, a professor at
the University of Hawaii. In his book Voyager Tales, written over a period of many years
and published in 1997, Swift includes this quote by Neil Armstrong:
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Voyager is a modern Odysseus, which has uncovered
the unimagined intricacy and awesome spectacle
of the giant planets of our home solar system.
When asked for a publishable quote back in 1989 about the Voyager experience, I chose to
say:
Even in hindsight, I would change not one whit
of the Voyager experience.
Dreams and sweat carried it off, but most of all its legacy
makes us all Earth travelers among the stars.
Discoveries Summarized
Between the two probes, our knowledge of the four giant planets, their satellites, and
their rings has become immense. Jupiter has complicated atmospheric dynamics, lightning
and aurorae. Three new satellites were discovered. Two of the major surprises were that
Jupiter has rings and that Io has active sulfurous volcanoes, with major effects on the
Jovian magnetosphere.
When the two probes reached Saturn, they discovered over 1000 ringlets and 7
satellites, including the predicted shepherd satellites that keep the rings stable. Braids,
kinks, and spokes were both unexpected and difficult to explain. The atmospheric weather
was tame compared with Jupiter: massive jet streams with
minimal variance (a 33-year great white spot/band cycle is
known). Titan's atmosphere was smoggy, enshrouded in a
thick haze of hydrocarbons in a nitrogen-rich atmosphere.
The finding led scientists to predict there could be seas of
liquid methane and ethane on Titan's surface (later
verified by measurements from the Cassini orbiter).
Mimas' appearance was startling: one massive impact
crater gave it the Death Star appearance. Photographs also
revealed the half-young, half-old terrain of Enceladus that
pointed to some kind of geological activity (later
determined by Cassini to be tiger stripe fissures with jets of
water vapor and organic particles).
VOYAGER 2, thanks to heroic engineering and
The author enjoys the golden
record and Voyager memories,
programming efforts, continued the mission to Uranus and
NASA/JPL
Neptune. Uranus itself was highly monochromatic in
appearance. One oddity was that its magnetic axis was found to be highly skewed from the
already completely skewed rotational axis, giving Uranus a peculiar magnetosphere. Icy
channels were found on Ariel, and Miranda was a bizarre patchwork of different terrains.
Ten satellites and one more ring were discovered.
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In contrast to Uranus, Neptune was found to have rather active weather, including
numerous cloud features. The ring arcs turned out to be bright patches on one ring. Two
other rings and six other satellites were discovered. Neptune's magnetic axis was also
skewed. Triton had a cantaloupe appearance and active geysers.
The two VOYAGERs are expected to last until at least 2020. The absence of
unexplained perturbations in their navigational data gives negative evidence about
possible planets beyond Pluto. Their next major scientific discovery should be the location
of the heliopause. Low-frequency radio emissions believed to originate at the heliopause
have been detected by both VOYAGERs.
The End of Chapter IV
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