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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 which were then known.
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 (December 2010) is
the farthest human-made object, at a distance from the sun of about 18 billion
kilometers (or 118 AU). In March 2009 Voyager 2 was about 13.3 billion
kilometers (or 88.1 AU) from the sun. This means that they are still only 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 16 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 (1st May, 2009)
Voyager 2 has followed Voyager 1 into
the heliosheath, where the solar wind runs up against the thin gas between the
stars. However, Voyager 2 took a different path, entering this region on
30th August, 2007. Because Voyager 2 crossed the heliosheath
boundary, called the solar wind termination shock, about 10 billion miles away
from Voyager 1 and almost a billion miles closer to the sun, it confirmed that
our solar system is " squashed" or " dented" - that the
bubble carved into interstellar space by the solar wind is not perfectly
round. Where Voyager 2 made its crossing, the bubble is pushed in closer
to the sun by the local interstellar magnetic field. Voyager
1 may have also had only a single shock crossing and it happened during a data
gap, but Voyager 2 had at least five shock crossings over a couple of days (the
shock " sloshes" back and forth like surf on a beach, allowing
multiple crossings) and three of them are clearly in the data. In a normal
shock wave, fast-moving material slows down and forms a denser, hotter region as
it encounters an obstacle. However, Voyager 2 found a much lower
temperature beyond the shock than was predicted, probably indicating that the
energy is being transferred to cosmic ray particles which were accelerated to
high speeds at the shock. Scientists also found that the
wind speed beyond the shock boundary was much less than expected, and at times
it appeared to be flowing back inward toward the sun. They 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.
LATEST NEWS (14th December, 2010)
The
Voyager
1 spacecraft has now reached a distant point at the edge of our solar system
where there is no outward motion of solar wind. Now
heading for interstellar space some 17.4 billion kilometers (10.8 billion miles)
from the sun, Voyager 1 has crossed into an area where the velocity of the hot
ionized gas, or plasma, emanating directly outward from the sun has slowed to
zero. Scientists suspect the solar
wind has been turned sideways by the pressure from the interstellar wind in the
region between stars. The event is
a major milestone in Voyager 1's passage through the heliosheath, the turbulent
outer shell of the sun's sphere of influence, and the spacecraft's upcoming
departure from our solar system. Our
sun gives off a stream of charged particles that form a bubble known as the
heliosphere around our solar system. The solar wind travels at supersonic speed
until it crosses a shockwave called the termination shock. At
this point, the solar wind dramatically slows down and heats up in the
heliosheath. Launched on Sept. 5,
1977, Voyager 1 crossed the termination shock in December 2004 into the
heliosheath. Scientists have used data from Voyager 1's Low-Energy Charged
Particle Instrument to deduce the solar wind's velocity. When
the speed of the charged particles hitting the outward face of Voyager 1 matched
the spacecraft's speed, researchers knew that the net outward speed of the solar
wind was zero. This occurred in
June, when Voyager 1 was about 17 billion kilometers (10.6 billion miles) from
the sun. Because the velocities can
fluctuate, scientists watched four more monthly readings before they were
convinced the solar wind's outward speed actually had slowed to zero. Analysis
of the data shows the velocity of the solar wind has steadily slowed at a rate
of about 20 kilometers per second each year (45,000 mph each year) since August
2007, when the solar wind was speeding outward at about 60 kilometers per second
(130,000 mph). The outward speed has remained at zero since June. Scientists
believe Voyager 1 has not yet crossed the heliosheath into interstellar space. Crossing
into interstellar space would mean a sudden drop in the density of hot particles
and an increase in the density of cold particles. Scientists
are putting the data into their models of the heliosphere's structure and should
be able to better estimate when Voyager 1 will reach interstellar space. Researchers
currently estimate Voyager 1 will cross that frontier in about four years.
Voyager 2, launched on Aug. 20, 1977 has reached a position 14.2 billion
kilometers (8.8 billion miles) from the sun. Both
spacecraft have been traveling along different trajectories and at different
speeds. Voyager 1 is traveling
faster, at a speed of about 17 kilometers per second (38,000 mph), compared to
Voyager 2's velocity of 15 kilometers per second (35,000 mph). In
the next few years, scientists expect Voyager 2 to encounter the same kind of
phenomenon as Voyager 1.
LATEST NEWS (6th December, 2011)
Voyager 1 has now entered a new ‘stagnation region’ in the outermost layer
of the bubble surrounding our solar system, and between the sun and interstellar
space. Data obtained over the last year (2011) reveal that in this new region
the “wind” of charged particles streaming out from the sun has calmed and
the solar system's magnetic field has piled up. Yet although the
spacecraft is about 11 billion miles (18 billion kilometers) from the sun, it is
not yet in true interstellar space. In the latest data, the direction of the
magnetic field lines has not changed, indicating Voyager is still within the
heliosphere, the bubble of charged particles the sun blows around itself. The
data do not reveal exactly when Voyager 1 will make it past the edge of the
solar atmosphere into interstellar space, but suggest it will be in a few months
to a few years.
The latest findings, come from Voyager's Low Energy Charged Particle instrument,
Cosmic Ray Subsystem and Magnetometer. Scientists previously reported the
outward speed of the solar wind had diminished to zero in April 2010, marking
the start of the new region.
Mission
managers rolled the spacecraft several times this spring and summer to help
scientists discern whether the solar wind was blowing strongly in another
direction. It was not. Voyager 1 is plying the celestial seas in a
region similar to Earth's doldrums, where there is very little wind.
During this past year, Voyager's magnetometer also detected a doubling in the
intensity of the magnetic field in the stagnation region, showing that inward
pressure from interstellar space is compacting it. Voyager has also been
measuring energetic particles that originate from inside and outside our solar
system. Until mid-2010, the intensity of particles originating from inside our
solar system had been holding steady. But during the past year, the intensity of
these energetic particles has been declining, as though they are leaking out
into interstellar space. The particles are now half as abundant as they were
during the previous five years. At the same time, a 100-fold increase has
been detected in the intensity of high-energy electrons from elsewhere in the
galaxy diffusing into our solar system from outside, which is another indication
of the approaching boundary. Scientists have been using the flow of
energetic charged particles at Voyager 1 as a kind of wind sock to estimate
the solar wind velocity. They have found that the wind speeds are low in this
region and gust erratically - for the first time, the wind even blows back at
us. Scientists had suggested previously that there might be a
stagnation layer, but now we have proof.
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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
Cassini first went to Venus and also flew past the
Earth quite a while after it was launched. It's even went to
Jupiter! 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 and 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 has being analyzed. Full details can be found on the various
web sites for NASA and JPL, but in total 21,987 images were 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 reached Saturn.
What Happens on 1st July, 2004?
Since that time Cassini entered Saturn orbit as
planned, separated the Huygens probe as planned on 6th November, 2004, and the
probe descended onto the surface of Titan. The data returned are still
being analysed but our knowledge of Titan is now hugely wider than it was before
this mission.
Cassini meanwhile has been
orbiting Saturn ever since, and at the time of writing 1st
May, 2009, it is still sending back high
quality images of Saturn's amazing ring system and satellites (moons). You
can see a selection of them at this link
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