The Voyagers

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.  Voyager 2 is currently about 14.7 billion kilometers 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 17 Km per second since 1977 can still only be 16.7 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 August, 2017)

The spacecraft achieve 40 years of operation and exploration in August and September 2017.  They continue to communicate with NASA daily and have set numerous records.  In 2012, Voyager 1, which launched on Sept. 5, 1977, became the only spacecraft to have entered interstellar space.  Voyager 2, launched on Aug. 20, 1977, is the only spacecraft to have flown by all four outer planets - Jupiter, Saturn, Uranus and Neptune.  Their numerous planetary encounters include discovering the first active volcanoes beyond Earth, on Jupiter's moon Io; first hints of a subsurface ocean on Jupiter's moon Europa; the most Earth-like atmosphere in the solar system, on Saturn's moon Titan; the jumbled-up, icy moon Miranda at Uranus; and icy-cold geysers on Neptune's moon Triton.  Though the spacecraft have left the planets far behind - and neither will come remotely close to another star for 40,000 years - they still send back observations about conditions where our Sun's influence diminishes and interstellar space begins.

Voyager 1, now almost 13 billion miles from Earth, travels through interstellar space northward out of the plane of the planets. The probe has shown that cosmic rays are as much as four times more abundant in interstellar space than in the vicinity of Earth.  This means the heliosphere effectively acts as a radiation shield for the planets. Voyager 1 also hinted that the magnetic field of the local interstellar medium is wrapped around the heliosphere.

Voyager 2, now almost 11 billion miles from Earth, travels south and is expected to enter interstellar space in the next few years. The different locations of the two Voyagers allow scientists to compare right now two regions of space where the heliosphere interacts with the surrounding interstellar medium using instruments which measure charged particles, magnetic fields, low-frequency radio waves and solar wind plasma.  Once Voyager 2 crosses into the interstellar medium they will also be able to sample the medium from two different locations simultaneously.

The twin Voyagers were well equipped for their journeys. Both carry redundant systems which allow the spacecraft to switch to backup systems autonomously when necessary, as well as long-lasting power supplies.  Each Voyager has three radioisotope thermoelectric generators using the decay of plutonium-238 and only half of it will be gone after 88 years.  Space is almost empty, so the Voyagers are not at a significant level of risk of bombardment by large objects. However, Voyager 1's environment is not a complete void. It's filled with clouds of dilute material remaining from stars which exploded as supernovae millions of years ago. This material doesn't pose a danger to the spacecraft, but is a key part of the environment the Voyager mission is helping scientists study and characterize.  Because the power decreases by four watts per year, engineers are learning how to operate the spacecraft under ever-tighter power constraints.  And to maximize lifespan they also have to consult documents written decades earlier, describing commands and software, in addition to the expertise of former Voyager engineers.

Team members estimate they will have to turn off the last science instrument by 2030.  However, even after the spacecraft go silent, they'll continue on their trajectories at their present speed of more than 30,000 mph (48,280 kilometers per hour), completing an orbit within the Milky Way every 225 million years.

LATEST NEWS (16th December, 2014)
The Voyager 1 spacecraft has experienced three shock waves.   The most recent shock wave, first observed in February 2014, still appears to be going on.  The "tsunami wave" that the spacecraft began experiencing earlier this year is still propagating outward, according to new results. It is the longest-lasting shock wave that researchers have seen in interstellar space. Most people thought the interstellar medium would be smooth and quiet, but these shock waves seem to be more common than was thought.  A "tsunami wave" occurs when the sun emits a coronal mass ejection, throwing out a magnetic cloud of plasma from its surface. This generates a wave of pressure. When the wave runs into the interstellar plasma -- the charged particles found in the space between the stars -- a shock wave results that perturbs the plasma.

This is the third shock wave that Voyager 1 has experienced. The first event was in October to November of 2012, and the second wave in April to May of 2013 revealed an even higher plasma density. Voyager 1 detected the most recent event in February, and it is still going on as of November data. The spacecraft has moved outward 250 million miles (400 million kilometers) during the third event.  It is unclear to researchers what the unusual longevity of this particular wave may mean. They are also uncertain as to how fast the wave is moving or how broad a region it covers. The second tsunami wave helped researchers determine in 2013 that Voyager 1 had left the heliosphere, the bubble created by the solar wind encompassing the sun and the planets in our solar system. Denser plasma "rings" at a higher frequency, and the medium that Voyager flew through, was 40 times denser than what had been previously measured. This was key to the conclusion that Voyager had entered a frontier where no spacecraft had gone before: interstellar space.

It seems that density of the plasma is higher the farther Voyager goes, but it is not know if that is because the interstellar medium is denser as Voyager moves away from the heliosphere, or if it is from the shock wave itself.

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.

LATEST NEWS (15th June, 2012)
Latest Data from Voyager 1 indicate that the spacecraft has encountered a region in space where the intensity of charged particles from beyond our solar system has markedly increased. Voyager scientists looking at this rapid rise draw closer to an inevitable but historic conclusion – that humanity's first emissary to interstellar space is on the edge of our solar system. The laws of physics say that someday Voyager will become the first human-made object to enter interstellar space, but we still do not know exactly when that someday will be. This latest data indicate that the spacecraft is clearly in a new region where things are changing more quickly. We are approaching the solar system's frontier. The data making the 16-hour, 38 minute, 11.1-billion-mile (17.8-billion-kilometer) journey from Voyager 1 to antennas of NASA's Deep Space Network on Earth detail the number of charged particles measured by the two High Energy telescopes aboard the 34-year-old spacecraft. These energetic particles were generated when stars in our cosmic neighborhood went supernova. From January 2009 to January 2012, there had been a gradual increase of about 25 percent in the amount of galactic cosmic rays Voyager was encountering. More recently, there has been a very rapid escalation in that part of the energy spectrum. Beginning on May 7, the cosmic ray hits have increased five percent in a week and nine percent in a month. This marked increase is one of a triad of data sets which need to make significant swings of the needle to indicate a new era in space exploration. The second important measure from the spacecraft's two telescopes is the intensity of energetic particles generated inside the heliosphere, the bubble of charged particles the sun blows around itself. While there has been a slow decline in the measurements of these energetic particles, they have not dropped off precipitously, which could be expected when Voyager breaks through the solar boundary. The final data set that Voyager scientists believe will reveal a major change is the measurement in the direction of the magnetic field lines surrounding the spacecraft. While Voyager is still within the heliosphere, these field lines run east-west. When it passes into interstellar space, the team expects Voyager will find that the magnetic field lines orient in a more north-south direction. Such analysis will take weeks, and the Voyager team is currently crunching the numbers of its latest data set.

Launched in 1977, Voyager 1 and 2 are in good health. Voyager 2 is more than 9.1 billion miles (14.7 billion kilometers) away from the sun. Both are operating as part of the Voyager Interstellar Mission, an extended mission to explore the solar system outside the neighborhood of the outer planets and beyond. NASA's Voyagers are the two most distant active representatives of humanity and its desire to explore.

LATEST NEWS (5th December, 2012)
Voyager 1 has now entered a new region at the far reaches of our solar system that scientists feel is the final area the spacecraft has to cross before reaching interstellar space.  They refer to this new region as a magnetic highway for charged particles because our sun's magnetic field lines are connected to interstellar magnetic field lines.  This connection allows lower-energy charged particles that originate from inside our heliosphere -- or the bubble of charged particles the sun blows around itself -- to zoom out and allows higher-energy particles from outside to stream in. Before entering this region, the charged particles bounced around in all directions, as if trapped on local roads inside the heliosphere.  The Voyager team infers this region is still inside our solar bubble because the direction of the magnetic field lines has not changed. The direction of these magnetic field lines is predicted to change when Voyager breaks through to interstellar space. and scientists believe this is the last leg of the journey to interstellar space - possibly only a few months to a couple of years away.  The new region isn't what they had expected, but they've come to expect the unexpected from Voyager.

Since December 2004, when Voyager 1 crossed a point in space called the termination shock, the spacecraft has been exploring the heliosphere's outer layer, called the heliosheath.  In this region the solar wind abruptly slowed down from supersonic speeds and became turbulent.  Voyager 1's environment was consistent for about five and a half years but the spacecraft then detected that the outward speed of the solar wind had slowed to zero.  The intensity of the magnetic field also began to increase at that time.  Voyager data from two onboard instruments that measure charged particles showed the spacecraft first entered this magnetic highway region on July 28, 2012.  The region ebbed away and flowed toward Voyager 1 several times, the spacecraft entering the region again on 25th August, and the environment has been stable since.

LATEST NEWS (13th September, 2013)
PASADENA, Calif. -- NASA's Voyager 1 spacecraft officially is the first human-made object to venture into interstellar space. The 36-year-old probe is about 12 billion miles (19 billion kilometers) from our sun. New and unexpected data indicate Voyager 1 has been traveling for about one year through plasma, or ionized gas, present in the space between stars. Voyager is in a transitional region immediately outside the solar bubble, where some effects from our sun are still evident. A report on the analysis of this new data, an effort led by Don Gurnett and the plasma wave science team at the University of Iowa, Iowa City, is published in Thursday's edition of the journal Science. "Now that we have new, key data, we believe this is mankind's historic leap into interstellar space," said Ed Stone, Voyager project scientist based at the California Institute of Technology, Pasadena. "The Voyager team needed time to analyze those observations and make sense of them. But we can now answer the question we've all been asking -- 'Are we there yet?' Yes, we are." Voyager 1 first detected the increased pressure of interstellar space on the heliosphere, the bubble of charged particles surrounding the sun that reaches far beyond the outer planets, in 2004. Scientists then ramped up their search for evidence of the spacecraft's interstellar arrival, knowing the data analysis and interpretation could take months or years. Voyager 1 does not have a working plasma sensor, so scientists needed a different way to measure the spacecraft's plasma environment to make a definitive determination of its location. A coronal mass ejection, or a massive burst of solar wind and magnetic fields, that erupted from the sun in March 2012 provided scientists the data they needed. When this unexpected gift from the sun eventually arrived at Voyager 1's location 13 months later, in April 2013, the plasma around the spacecraft began to vibrate like a violin string. On April 9, Voyager 1's plasma wave instrument detected the movement. The pitch of the oscillations helped scientists determine the density of the plasma. The particular oscillations meant the spacecraft was bathed in plasma more than 40 times denser than what they had encountered in the outer layer of the heliosphere. Density of this sort is to be expected in interstellar space. The plasma wave science team reviewed its data and found an earlier, fainter set of oscillations in October and November 2012. Through extrapolation of measured plasma densities from both events, the team determined Voyager 1 first entered interstellar space in August 2012.

The new plasma data suggested a timeframe consistent with abrupt, durable changes in the density of energetic particles that were first detected on Aug. 25, 2012. The Voyager team generally accepts this date as the date of interstellar arrival. The charged particle and plasma changes were what would have been expected during a crossing of the heliopause.

Voyager mission controllers still talk to or receive data from Voyager 1 and Voyager 2 every day, though the emitted signals are currently very dim, at about 23 watts -- the power of a refrigerator light bulb. By the time the signals get to Earth, they are a fraction of a billion-billionth of a watt. Data from Voyager 1's instruments are transmitted to Earth typically at 160 bits per second, and captured by 34- and 70-meter NASA Deep Space Network stations. Traveling at the speed of light, a signal from Voyager 1 takes about 17 hours to travel to Earth. After the data are transmitted to JPL and processed by the science teams, Voyager data are made publicly available. Scientists do not know when Voyager 1 will reach the undisturbed part of interstellar space where there is no influence from our sun. They also are not certain when Voyager 2 is expected to cross into interstellar space, but they believe it is not very far behind.

The cost of the Voyager 1 and Voyager 2 missions -- including launch, mission operations and the spacecraft's nuclear batteries, which were provided by the Department of Energy -- is about $988 million through September.

LATEST NEWS (7th July, 2014)
Voyager 1 has experienced a new "tsunami wave" from the sun as it sails through interstellar space. Such waves are what led scientists to the conclusion, in the fall of 2013, that Voyager had indeed left our sun's bubble, entering a new frontier. "Normally, interstellar space is like a quiet lake," said Ed Stone of the California Institute of Technology in Pasadena, California, the mission's project scientist since 1972. "But when our sun has a burst, it sends a shock wave outward that reaches Voyager about a year later. The wave causes the plasma surrounding the spacecraft to sing." Data from this newest tsunami wave generated by our sun confirm that Voyager is in interstellar space -- a region between the stars filled with a thin soup of charged particles, also known as plasma. The mission has not left the solar system -- it has yet to reach a final halo of comets surrounding our sun -- but it broke through the wind-blown bubble, or heliosphere, encasing our sun. 

Our sun goes through periods of increased activity, where it explosively ejects material from its surface, flinging it outward. These events, called coronal mass ejections, generate shock, or pressure, waves. Three such waves have reached Voyager 1 since it entered interstellar space in 2012. The first was too small to be noticed when it occurred and was only discovered later, but the second was clearly registered by the spacecraft's cosmic ray instrument in March of 2013. Cosmic rays are energetic charged particles that come from nearby stars in the Milky Way galaxy. The sun's shock waves push these particles around and data from the cosmic ray instrument tell researchers that a shock wave from the sun has hit.

Meanwhile, another instrument on Voyager registers the shock waves, too. The plasma wave instrument can detect oscillations of the plasma electrons. "The tsunami wave rings the plasma like a bell," said Stone. "While the plasma wave instrument lets us measure the frequency of this ringing, the cosmic ray instrument reveals what struck the bell -- the shock wave from the sun." This ringing of the plasma bell is what led to the key evidence showing Voyager had entered interstellar space. Because denser plasma oscillates faster, the team was able to figure out the density of the plasma. In 2013, thanks to the second tsunami wave, the team acquired evidence that Voyager had been flying for more than a year through plasma that was 40 times denser than measured before -- a telltale indicator of interstellar space. Why is it denser out there? The sun's winds blow a bubble around it, pushing out against denser matter from other stars.

Now, the team has new readings from a third wave from the sun, first registered in March of this year. These data show that the density of the plasma is similar to what was measured previously, confirming the spacecraft is in interstellar space. Thanks to our sun's rumblings, Voyager has the opportunity to listen to the singing of interstellar space -- an otherwise silent place.

Voyager 1 and its twin, Voyager 2, were launched 16 days apart in 1977. Both spacecraft flew by Jupiter and Saturn. Voyager 2 also flew by Uranus and Neptune. Voyager 2, launched before Voyager 1, is the longest continuously operated spacecraft and is expected to enter interstellar space in a few years.

LATEST NEWS (30th October, 2015)
Observations from the Voyager 1 after it left the heliosphere were puzzling with regard to the magnetic field around it, as they differed from what scientists derived from observations by other spacecraft.
  A new study offers fresh insights into this mystery. Writing in the Astrophysical Journal Letters, Nathan Schwadron of the University of New Hampshire, Durham, and colleagues reanalyzed magnetic field data from Voyager 1 and found that the direction of the magnetic field has been slowly turning ever since the spacecraft crossed into interstellar space. They believe this is an effect of the nearby boundary of the solar wind. "This study provides very strong evidence that Voyager 1 is in a region where the magnetic field is being deflected by the solar wind," said Schwadron, lead author of the study.

Researchers predict that in 10 years Voyager 1 will reach a more "pristine" region of the interstellar medium where the solar wind does not significantly influence the magnetic field.  Observations from Voyager's instruments have found that the particle density is 40 times greater outside this boundary than inside, confirming that it had indeed left the heliosphere. But so far, Voyager 1's observation of the direction of the local interstellar magnetic field is more than 40 degrees off from what other spacecraft have determined. The new study suggests this discrepancy exists because Voyager 1 is in a more distorted magnetic field just outside the heliopause, which is the boundary between the solar wind and the interstellar medium.

"If you think of the magnetic field as a rubber band stretched around a beach ball, that band is being deflected around the heliopause," Schwadron said.

In 2009, NASA's Interstellar Boundary Explorer (IBEX) discovered a "ribbon" of energetic neutral atoms that is thought to hold clues to the direction of the pristine interstellar magnetic field. The so-called "IBEX ribbon," which forms a circular arc in the sky, remains mysterious, but scientists believe it is produced by a flow of neutral hydrogen atoms from the solar wind that were re-ionized in nearby interstellar space and then picked up electrons to become neutral again. The new study uses multiple data sets to confirm that the magnetic field direction at the center of the IBEX ribbon is the same direction as the magnetic field in the pristine interstellar medium. Observations from the NASA/ESA Ulysses and SOHO spacecraft also support the new findings.

Over time, the study suggests, at increasing distances from the heliosphere, the magnetic field will be oriented more and more toward "true north," as defined by the IBEX ribbon. By 2025, if the field around Voyager 1 continues to steadily turn, Voyager 1 will observe the same magnetic field direction as IBEX. That would signal Voyager 1's arrival in a less distorted region of the interstellar medium. "It's an interesting way to look at the data. It gives a prediction of how long we'll have to go before Voyager 1 is in the medium that's no longer strongly perturbed," said Ed Stone, Voyager project scientist, based at the California Institute of Technology in Pasadena, who was not involved in this study.

While Voyager 1 will continue delivering insights about interstellar space, its twin probe Voyager 2 is also expected to cross into the interstellar medium within the next few years. Voyager 2 will make additional observations of the magnetic field in interstellar space and help scientists refine their estimates.


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:-

  • Imaging Cameras for taking pictures in visible, near infra-red and near ultra-violet light

  • Radar to map the surface of Titan through the clouds and measure height of surface features

  • Radio Science package to search for gravitational waves, studying the atmosphere, rings and gravity fields of Saturn and its moons

  • Ion and Neutral Mass Spectrometer to examine neutral and charged particles near Titan, Saturn and other satellites

  • Visible and IR Spectrometer to measure chemical composition of moon and planet surfaces and atmospheres

  • Composite IR Spectrometer to measure IR radiation from the surface of moon and planet

  • Cosmic Dust Analyzer to study ice and dust grains in and near the Saturn system

  • Radio and Plasma Wave Spectrometer to analyze plasma waves generated by ionized gases flowing from the Sun

  • Plasma Spectrometer to explore highly ionized gas within Saturn's magnetic field

  • UV Imaging Spectrograph to measure UV energy from atmosphere and rings, to study their structure, chemistry and composition

  • Magnetospheric Imaging Instrument to image and measure interactions between Saturn's magnetosphere and the solar wind

  • Dual Technique Magnetometer to study Saturn's magnetic field and its interactions with the solar wind and moons

Carried on the Huygens probe are:-

  • Doppler Wind Experiment to study Titan's winds and their effect on the probe during its descent

  • A Surface Science Package to investigate the physical properties of Titan's surface

  • A Descent Imager and Spectral Radiometer to photograph the atmosphere and measure particle temperatures

  • An Atmospheric Structure Instrument to explore the structure and physical properties of the atmosphere on Titan

  • A Gas Chromatograph and Mass Spectrometer to measure the atmospheric composition

  • 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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