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The Voyager Spacecraft
Quite a long time ago two spacecraft called Voyager 1 and Voyager 2, and
weighing one ton each were launched on interplanetary expeditions. These two spacecraft were actually
launched way back in 1977 – 20th August (Voyager 2) and 5th September 1977
(Voyager 1) to be precise – and
between them they visited and photographed the outer planets (Jupiter, Saturn,
Uranus and Neptune) in incredible detail. At that time of my life I was living
in England, and I well remember the excellent BBC documentaries, "Encounter
with Uranus" and "Encounter with Neptune", which showed some of
the incredible images sent back from the far reaches of the solar system.
In the first twelve years of their lives, both spacecraft produced a wealth of
discoveries about the four gas giants, Jupiter, Saturn, Uranus and Neptune, and
their 48 moons.
Among their discoveries they revealed
that Jupiter's atmosphere has dozens of huge storms, that the hazy atmosphere of
Saturn's moon, Titan may hold the secrets of the origin of life, that Miranda, a
small moon of Uranus, has a jumble of old and new surfaces, and that Neptune's
moon Triton has active geysers.
As a result of their last planetary encounters, both these spacecraft were
ejected at great speed out of the plane of the solar system, Voyager 1 heading
"upwards" at an angle of 35 degrees to the ecliptic plane, and Voyager
2 heading "downwards" with respect to Earth’s orientation at an
angle of 48 degrees to the ecliptic. (The ecliptic is the horizontal plane of
the solar system and is basically the path which all the planets and the moon
follow as they move across the sky). The distances our two spacecraft have
now traveled is pretty significant. Voyager 1 currently (August 2007) is
the farthest human-made object, at a distance from the sun of about 15.5 billion
kilometers (9.7 billion miles or 104AU). Voyager 2 is about 12.5 billion
kilometers (7.8 billion miles or 84AU) from the sun. This means that they are
about half a light day away from us! Hard to believe,
isn’t it, that something traveling at 3.6 AU per year (17 Km per second) since
1977 can still only be 14 light hours away from us. Remember that the nearest
star is approximately 4 light YEARS away.
Power Source
Both craft are powered by what is known as RTG’s, which stands for
Radioisotope Thermoelectric Generators. These are devices powered by the decay
of Plutonium, and at launch they generated 470 watts of 30 volt electrical
power. Due to the natural decay of the fuel source (which is how they work in
the first place), the power levels have been falling, and at the beginning of
1997 had fallen to about 335 watts for both spacecraft. However, this level of
power generation is still sufficient to run most of the on-board instruments
until perhaps the year 2020,
and so what had started as an interplanetary mission was converted by NASA to
the Voyager Interstellar Mission (VIM) in 1989.
Mission Objectives
The VIM objectives are to study the Termination Shock Boundary, the Heliosheath
through to the Heliopause, and finally the Interstellar Phase. So what exactly
do these terms mean? Well, I think we all know that the sun produces a
continuous stream of energetic particles, which we call the solar wind. This
plasma flow travels past the Earth and other planets at supersonic speeds as it
heads outwards from the sun. I think we also know that the sun and planets are
moving through space at a fair old speed, and the two Voyagers are racing ahead
of the sun in it's passage through space. Because of this
"forward" motion, the solar system creates a bow wave in the direction
of motion, at the point where the plasma stream comes into contact with the
interstellar "winds". This bow wave has the effect of slowing down the
solar wind particles, and a point is therefore reached where this slowing
becomes sufficient to reduce their speed to subsonic levels. This is the first
point for which the spacecraft will be looking, called the Termination Shock
Boundary. The exact location of this boundary is not known, but most current
estimates put it at between 82 and 93AU from the sun. So you can see that if
these calculations are correct, the boundary will be reached sometime between
the years 2003 and 2006. The next stage of the
mission will be when the spacecraft reach the limit of effect of the solar wind.
This will be the point beyond which they are subject only to the effects of the
interstellar winds, and is known as the Heliopause. Recent estimates are
that it will take Voyager 1 between 7 and 21 years to reach the Heliopause.
Until the Heliopause is
reached, the spacecraft are operating inside what is know as the Heliosheath,
which is the general area under the direct influence of the sun and the solar
wind. It is not known how extensive the Heliosheath is, but it could be
tens of astronomical units thick, taking several years for the spacecraft to
traverse. Once the Heliopause is reached, the spacecraft will enter the
true interstellar environment, and will be the first objects to completely
escape the influence of the sun. We have to hope that the craft and their
power supplies can endure until that point is reached.
How Long Will They Last?
So how long will the power last for the spacecraft systems? Well, as I said
earlier the power generated will gradually decrease as the fuel decays, and
plans are in place for gradually shutting down the various spacecraft systems to
eke out as far as possible the power which is available. First to be turned off
were the ultra-violet observation systems in year 2000, but now that this has
been done, the other instruments can be kept operating for several years. These
are the magnetic field instruments, the low energy charged particle
investigations, cosmic ray and plasma wave measurements. In about the year 2011,
gyro operations will be terminated. This will end the capability to rotate the
spacecraft, which could compromise our ability to maintain communication and
retrieve the data. Around the same time they will turn off the digital tape
recorder, which further compromises data playback and retrieval, but scientists
are confident they can maintain contact, and insist this is a necessary
compromise to maximise on the useful life for the two craft. Finally,
around the year 2018, power sharing between instruments will be initiated.
However, JPL plans to be able to continue to collect meaningful data at least
through 2020, after which point there will be insufficient power to run any of
the instruments, and our little messengers will finally be dead, nearly half a
century since they were launched.
Long after they fall silent, the
Voyager twins will keep speeding away from our solar system, each carrying a
disk of recorded images from Earth. Included are greetings from many Earth
languages, images of life on our planet and Man's achievements. Long after
our sun has swelled to become a red giant star, probably destroying the Earth in
the process, the Voyager craft will still be moving among the stars.
Perhaps long after mankind itself has disappeared from the cosmos they will
still be wandering. If they are ever found by another intelligence in the
farthest distant future, I wonder what they will make of the images of the
creatures who made it so long ago and so very far away.
More Data
Just a few more facts about the Voyagers before we leave them to their fate.
Each mission cost $865 million and a total of 11,000 people were involved with
the program through the encounter with Neptune. They carry with them special
time capsules, intended to communicate a story of our world to
extra-terrestrials. The Voyager message is carried on a 12-inch gold-plated
copper disk containing sounds and images which portray the diversity of life and
culture on Earth. The contents of the record were selected for NASA by a
committee chaired by Carl Sagan and it contains 115 images and a variety of
natural sounds, such as those made by surf, wind and thunder, birds, whales, and
other animals, musical selections from different cultures and eras, spoken
greetings from Earth-people in fifty-five languages, and printed messages from
President Carter and the then United Nations Secretary General, Kurt Waldheim.
Each record is encased in a protective aluminum jacket, together with a
cartridge and a needle. Instructions, in symbolic language, explain the origin
of the spacecraft and indicate how the record is to be played. The 115 images
are encoded in analog form. The remainder of the record is in audio, designed to
be played at 16-2/3 revolutions per second. It will be forty thousand years
before the spacecraft have any chance of making a close approach to any other
planetary system. As Carl Sagan noted, "The spacecraft will be encountered
and the record played only if there are advanced space faring civilizations in
interstellar space. But the launching of this bottle into the cosmic ocean says
something very hopeful about life on this planet."
Let’s hope he’s right!
LATEST NEWS (24th May 2006)
A trio of surprise from Voyager 1 has
revealed intriguing new information about our solar system's final
frontier. The surprises come as the long-lived spacecraft approaches the
edge of our solar system, called the heliopause, where the sun's influence ends
and the solar wind smashes into the thin gas between the stars. There is
more yet to be learned as Voyager begins the final leg of its race to the edge
of interstellar space, as it is expected to pass beyond the heliopause into
interstellar space in eight to 10 years, with Voyager 2 expected to follow about
five years later.
Voyager 1 has already passed the termination shock, where the
million-mile-per-hour solar wind abruptly slows and becomes denser and hotter as
it presses against interstellar gas. It was expected the wind beyond the shock
would slow to a few hundred thousand miles per hour. But the Voyager scientists
were surprised to find that the speed was much less, and at times the wind
appeared to be flowing back inward toward the sun. Scientists say this
could mean that the outward pressure of wind was decreasing as the sun entered
the less active phase of its 11-year cycle of sunspot activity.
Another surprise: the direction of the interplanetary magnetic field in the
outer solar system varied more slowly beyond the termination shock. As the
sun rotates every 26 days, the direction of the field alternates every 13
days. That field is carried out by the solar wind, with the alternating
directions forming a pattern of zebra stripes moving outward past the
spacecraft. One could imagine a zebra with giant "magnetic stripes"
running past the spacecraft and Voyager 1 "observing" an alternating
stripe every 13 days. After the shock, the "zebra" with its
stripe pattern was moving at nearly the same speed as Voyager, so that it took
more than 100 days for the stripe to pass the spacecraft and for the magnetic
field to switch directions.
Perhaps the most puzzling surprise is what Voyager 1 did not find at the
shock. It had been predicted that interstellar ions would bounce back and forth
across the shock, slowly gaining energy with each bounce to become high speed
cosmic rays. Because of this, scientists expected those cosmic ray ions would
become most intense at the shock. However, the intensity did not reach a maximum
at the shock, but has been steadily increasing as Voyager 1 has been moving
farther beyond the shock. This means that the source of those cosmic rays is in
a region of the outer solar system yet to be discovered.
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On 15th October, 1997 the Cassini spacecraft was
launched from Cape Canaveral, Florida using a Titan IVB/Centaur launch
vehicle. It was a perfect launch and since that time the spacecraft has
been waltzing around the solar system, taking lots of photographs, making
scientific measurements, and gaining speed. The final destination of this
mission is the planet Saturn, which will finally be reached on 1st July, 2004
and in addition to orbiting Saturn itself, there are plans to launch a
scientific probe to the large Saturnian moon, Titan. Titan was targeted
for this mission because it is one of the largest moons in the solar system, and
it has a thick atmosphere which is suspected to be similar to the primeval
atmosphere which is thought once to have existed on Earth.
About the Spacecraft
The combined Cassini Saturn orbiter and Huygens probe (the one which will
descend to the surface of Titan) form one of the largest, heaviest and most
complex interplanetary spacecraft ever built. The orbiter weighs 2,150
kilos and the probe 350 kilos. At launch they carried 3,132 kilos of
propellant, and only the two "Phobos" spacecraft sent to Mars by the
Soviet Union weighed more. The overall spacecraft stands 6.8 meters tall
and is more than 4 meters wide, it has 1,630 interconnect circuits, 22,000 wire
connections and more than 14 kilometers of cabling.
Instruments carried on the Cassini orbiter are:-
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Imaging Cameras for taking pictures in
visible, near infra-red and near ultra-violet light
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Radar to map the surface of Titan through the
clouds and measure height of surface features
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Radio Science package to search for
gravitational waves, studying the atmosphere, rings and gravity fields of
Saturn and its moons
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Ion and Neutral Mass Spectrometer to examine
neutral and charged particles near Titan, Saturn and other satellites
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Visible and IR Spectrometer to measure
chemical composition of moon and planet surfaces and atmospheres
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Composite IR Spectrometer to measure IR
radiation from the surface of moon and planet
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Cosmic Dust Analyzer to study ice and dust
grains in and near the Saturn system
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Radio and Plasma Wave Spectrometer to analyze
plasma waves generated by ionized gases flowing from the Sun
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Plasma Spectrometer to explore highly ionized
gas within Saturn's magnetic field
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UV Imaging Spectrograph to measure UV energy
from atmosphere and rings, to study their structure, chemistry and
composition
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Magnetospheric Imaging Instrument to image
and measure interactions between Saturn's magnetosphere and the solar wind
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Dual Technique Magnetometer to study Saturn's
magnetic field and its interactions with the solar wind and moons
Carried on the Huygens probe are:-
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Doppler Wind Experiment to study Titan's
winds and their effect on the probe during its descent
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A Surface Science Package to investigate the
physical properties of Titan's surface
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A Descent Imager and Spectral Radiometer to
photograph the atmosphere and measure particle temperatures
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An Atmospheric Structure Instrument to
explore the structure and physical properties of the atmosphere on Titan
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A Gas Chromatograph and Mass Spectrometer to
measure the atmospheric composition
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An Aerosol Collector Pyrolyzer to examine
clouds and suspended atmospheric particles
The Flight Path
You might have heard that Cassini has been to Venus and also flew past the
Earth quite a while after it was launched. It's even been to
Jupiter! Why is this? Wasn't it supposed to go to Saturn? Well
yes it was, but to have launched the spacecraft on a direct trajectory to Saturn
would have required a significant amount of fuel, so to make the whole business
more fuel efficient, NASA used a procedure which has been successfully used
since the early days of the space program - the slingshot. This is a
procedure where the spacecraft is flown close by a planet, but not so close as
to cause it to risk crashing into the planet itself. As the craft skims
the atmosphere it follows an arc caused by the gravitational attraction of the
planet, and this gives it added speed. When the craft reaches the other
side of the planet it zooms off in a completely new and different direction,
with the additional velocity which the slingshot maneuver has given to it.
In this way, with careful planning and calculation, it is possible to use
encounters with several planets to continually add more and more velocity, which
is why after launch Cassini headed first to Venus. You might remember all
the fuss which was created when Earth itself was used for one of the Cassini
slingshot maneuvers. Because the spacecraft is powered by nuclear fuel,
there was concern in some areas that a miscalculation could cause the spacecraft
to burn up in the atmosphere, and the radioactive fuel to spill, causing
significant atmospheric contamination. Luckily this did not happen, and
the encounter with Earth passed without incident, but there was always a risk of
something going wrong.
The next major milestone on its voyage was a slingshot maneuver
around Jupiter, but this was no chance meeting ands swift "kiss and
goodbye". Cassini carried out a significant amount of scientific work
as it swung past the giant planet on 30th December 2000, and all of this
information is being analyzed. Full details can be found on the various
web sites for NASA and JPL, but in total 21,987 images have been received from
Jupiter using the Imaging Science Package and 4,689 from the Visual and
Infra-Red Mapping Spectrometer. This proved to be an excellent opportunity
for the scientists to test the equipment and to make some adjustments and
modifications to their plans for when Cassini finally reaches Saturn.
What Happens on 1st July, 2004?
Well, the Cassini spacecraft itself will go into a permanent orbit around
Saturn, carrying out experiments using the onboard instruments. That in
itself is interesting enough, but life for the Huygens probe is going to get
quite exciting. On 6th November, 2004, 24 days before its encounter with
Titan, the probe will separate from the main spacecraft and will then
"coast" towards the moon with no systems active except for a timer
which will "wake it up" 15 minutes before contact with the
atmosphere. The main mission phase will be a descent by parachute through
Titan's atmosphere. The batteries and all other resources are sized for a
Huygens mission duration of 153 minutes, corresponding to a maximum descent time
of 2.5 hours plus at least 3 additional minutes (and possibly a half hour or
more) on Titan's surface. The probe's radio link will be activated early
in the descent phase, and the orbiter will "listen" to the probe for
the next 3 hours, which includes the descent plus 30 minutes after impact.
Not long after the end of this three-hour communication window, Cassini's
high-gain antenna (HGA) will be turned away from Titan and toward Earth, its
work with the Huygens probe complete.
So, late 2004 looks like being a very exciting time, and I for
one will be glued to my TV or any other device of the time which can keep me
updated on what the probe and spacecraft are seeing. Looking at Saturn
through our telescope is exciting enough, but I suspect that the events of late
2004 will be reminiscent of those in 1969, when as a teenager I first heard
those magical words, "Tranquility Base Here - The Eagle Has Landed."
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