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THE INCREDIBLE MISSIONS OF In 1977, two Voyager spacecraft were launched from Earth: Voyager 2 on August 21 and Voyager 1 a few days later, on September 5. Their mission? To probe the gas giant planets (Jupiter, Saturn, Uranus and Neptune) and beyond. Amazingly, and beyond all expectations, their mission continues 41 years later (albeit with much of the on-board equipment shut down to conserve dwindling power). Voyager 2 is now humanity’s most distant object and travelling away from Earth at a speed of 62,000km/h (17km/second!). Radio signals to or from Voyager, at the speed of light, take 20 hours – one way! The “Grand Tour” by Dr David Maddison 12 Silicon Chip Australia’s electronics magazine This background image, the crescent view of Jupiter, was taken by NASA Voyager 1 on March 24, 1979 – almost four decades ago! Regrettably, there will be no more pictures from Voyager – to save power its cameras were turned off in February 1990 – already way past its planned life! siliconchip.com.au B oth Voyager spacecraft are still operational and sending back valuable data, using what would be regarded today as vintage electronics. Voyager 2 is also now humanity’s third most distant object, surpassed only by Pioneer 10, by a relatively small margin. But communications with Pioneer 10 were lost in January 2003. Voyager 1 is now in interstellar space, ie, mostly beyond the influence of the Sun, including both its solar wind and magnetic field. It is in the space between star systems and as of going to press, Voyager 2 is now thought to be entering interstellar space as well. The Voyager spacecraft were launched as a result of a once-in-a-lifetime opportunity. In 1964, Gary Flandro of the Jet Propulsion Laboratory (JPL) in California made the observation that a particular alignment of outer planets Jupiter, Saturn, Neptune and Uranus (the gas giants) would enable a single spacecraft to visit all of them on a single mission, using the gravitational slingshot effect to go from planet to planet without needing extra fuel. This trajectory became known as the “Grand Tour”. This special planetary alignment only occurs once every 175 years and was to occur in the later 1970s. The alternative was to send individual spacecraft to each of these four planets, at much greater expense. NASA decided to send two spacecraft on the Grand Tour, with some slight differences between the two trajectories (see Fig.1). This would significantly reduce the time taken to visit the planets of interest and also allow additional post-launch options, such as the possibility for Voyager 1 to visit Pluto instead of Saturn’s moon Titan. It also reduced the risk of a launch failure derailing the whole mission. Voyager 2 was launched on 20th August 1977, before Voyager 1, which was launched on 5th September 1977. This is because they were numbered based on their ex- Fig.1: trajectories of the Voyager spacecraft, showing their close encounters with the gas giants which gave opportunities for taking photos and scientific observations as well as using the gravitational slingshot effect to make their way to the outer planets and beyond the solar system. Voyager 1 visited Jupiter and Saturn and made a close flyby of Saturn’s moon Titan (considered more important than passing Pluto) while Voyager 2 visited Jupiter, Saturn, Uranus and Neptune. pected arrival at Jupiter. Even though Voyager 1 was launched 16 days after Voyager 2, due to different trajectories, Voyager 1 arrived at Jupiter four months before Voyager 2. The different trajectories provided the option for Voyager 2 to make close passes of Uranus and Neptune if desired, depending on scientific findings which were to be made along the way . Fig.2: the trajectory of Voyager 2 for its Jupiter encounter, showing the many navigational considerations that had to be taken into account to maximise the information to be obtained. siliconchip.com.au Australia’s electronics magazine December 2018  13 Fig.3: a depiction of Voyager showing some of the primary spacecraft systems and instruments. A much longer mission than intended The Voyager mission has been so successful that it has been extended a couple of times. The original primary mission of the Voyager program was to visit Jupiter and Saturn. Along the way, the probes made many important discoveries such as detecting volcanism on Jupiter’s moon Io and finding unexpected intricacies in Saturn’s rings. The mission was then extended to allow Voyager 2 to visit Uranus and Neptune, which it did in 1989. Uranus and Neptune had not been visited before or since. After that, a further mission extension was granted to both spacecraft; known as the Voyager Interstellar Mission (VIM), its purpose is to explore the outer limits of the Sun’s influence and further beyond. The VIM is planned to extend to 2020 and possibly longer, subject to the availability of electrical power on the probes. The journey to interstellar space The graphic opposite shows the location of the Voyager spacecraft relative to our solar system. The heliosphere is the ‘bubble’ surrounding the Sun, extending well past the orbit of Pluto. It has its origins in the solar wind, the stream of charged particles constantly emitted from the Sun. It is not a sphere; it is distorted into a teardrop shape due to the interaction of the heliosphere with the interstellar wind, the atomic particles moving past from interstellar space. Within the heliosphere, there is the termination shock, which is the sudden slowing of the solar wind from a speed of 300-700 kilometres per second to a much slower speed as it encounters the interstellar wind. The heliosheath is the outer layer of the heliosphere, where the solar wind slows further, becoming denser and hotter as it interacts and ‘piles up’ against the interstellar wind. The heliopause is the point at which the pressure of the solar and interstellar winds are in balance and the solar wind turns around and flows down the teardrop tail of the heliosphere. The bow shock is formed much like the bow wave of a boat, as the solar system moves through the atomic particles of the interstellar medium. Voyager 1 is heading above the plane of the planets while Voyager 2 is heading below the plane. Voyager 1 is in the interstellar medium and has been since August 2012. 14 Silicon Chip As of 5th October 2018, Voyager 2 is believed to be about to exit the heliopause due to an observed increase in cosmic ray activity. The exact time of transition cannot be predicted as the shape of the heliopause varies due to solar activity and its location with respect to the asymmetric heliosphere. Pluto has an average distance from the Sun of 39.5 astronomical units (AU), where 1AU is the average Earth-Sun distance. Voyager 1 is currently at a distance of 144AU from the Earth and Voyager 2, 119AU. For more details, see: https://bgr.com/2018/10/08/voyager-2heliopause-interstellar-space/ Also see: www.jpl.nasa.gov/news/news.php?feature=7252 Australia’s electronics magazine siliconchip.com.au Fig.4: the Multi-Hundred Watt Radioisotope Thermoelectric Generator (MHW-RTG) as used on both Voyager spacecraft. At the start of the mission each unit provided 157W of electrical power (2400W thermal) and each spacecraft had three generators providing 471W at launch, diminishing all the time due to radioactive decay. The objective of the VIM is to obtain useful information on interplanetary and interstellar fields, particles, and waves. Between 2020 and 2025, the probes’ remaining instruments will need to be shut down to preserve electrical power. After 2025 (some reports say 2030), the decay of the nuclear fuel onboard the spacecraft will reduce their power supplies to the point that neither will be able to function and they will finally “go dark”. Spacecraft design When they were designed in the early-to-mid 1970s, no spacecraft had yet been made to operate at such distances from the Earth. Both spacecraft are identical and after ejection of their propulsion module weighed 825kg, 117kg of which is the scientific instruments (see Fig.3). All spacecraft systems were designed with high reliability and redundancy in mind. The craft are stabilised on three axes to ensure the antennas remained pointed toward Earth; the Sun and Canopus are used as guide stars. Three separate onboard computer systems are used for different tasks, each having a backup system. Their magnetic tape data storage capacity is 536 megabits (a whopping 67 megabytes); enough to store 100 full resolution (800 x 800 pixel) 8-bit (256 grey scale) photos. For power, each spacecraft has three plutonium-based radioisotope thermoelectric generators which initially provided a continuous 470W of electric power, although the power output is continuously diminishing due to radioactive decay. A 3.66m high-gain antenna dominates the structure of siliconchip.com.au Fig.5: the plutonium fuel spheres within the MHWRTG assembly, along with layers of protection to avoid contamination in the event of a launch accident. each spacecraft and they also have a coaxial low-gain antenna for radio science observations. The bulk of the onboard electronics is contained within ten boxes which form a ten-sided structural “bus”. They also carry hydrazine fuel for 16 thrusters. Of the 16 thrusters, 4 are for trajectory correction and 12 are for attitude control. There are three pairs of primary attitude control thrusters and three more pairs of secondary thrusters for redundancy, giving a total of 12. All thrusters are the Aerojet model MR-103, which are still in production today. They deliver 0.89N or 0.09kgf (kilogram-force) of thrust. The attitude control thrusters on the Voyagers have been fired hundreds of thousands of times during the mission but typically only “puffs” are emitted for milliseconds at a time, to make the tiniest corrections. As a testament to the reliability of the thrusters, it was noticed in 2014 that the thrusters on Voyager 1 had been degrading in their performance and using more fuel than they should. It was decided to switch to the trajectory correction thrusters, which had not been turned on in 37 years (since the spacecraft’s encounter with Saturn) and they worked perfectly. This measure saved fuel, extending the mission life of the craft by 2-3 years. External to the bus are booms for the radioisotope generators, to keep their slight radiation as far from the sensitive instruments and spacecraft electronics as possible. There is also a scientific instrument boom, 2.3m long, containing most of the instruments (with a steerable platform at the end for the optical instruments) and a 13m long magnetometer boom. The instruments are mounted on a boom as they are Australia’s electronics magazine December 2018  15 Fig.7: the Flight Data System (FDS) computer used in the Voyager spacecraft. Fig.6: this is what a radio telescope image of the radio signal from Voyager 1 looks like. The Very Long Baseline Array (VLBA) was used to capture this image on February 21st, 2013. The elongated shape is a consequence of the antenna configuration. The width of the radio signal shown is 1 milliarcsecond, or at the distance of 18.5 billion kilometres when the image was produced, about 80km. radiation-sensitive and also sensitive to magnetic fields from the spacecraft. The nearest boom-mounted instrument to the generators is 4.8m away, with the spacecraft in between, and the closest platform-mounted instrument is 6.4m from the generators. The steerable platform on Voyager 2 once got stuck as it swung around Saturn but the problem was fixed by sending a sequence of commands to turn the platform one way and then the other multiple times, to free it. The thrusters are mounted on the outside of the bus, along with a combined planetary radio astronomy and plasma wave antenna system, comprising of two 10m-long elements mounted at right angles to each other. (Plasma is the fourth state of matter and is a gas in which atoms which have had some or all electrons stripped from them coexist with those electrons.) There are also two star trackers, a calibration instrument and a golden record containing sites and sounds of Earth and other information about the origin of the spacecraft, in Interesting Voyager Facts Five trillion bits of data have been jointly transmitted by both Voyager spacecraft. That’s enough data to fill seven thousand music CDs or over 4.5 terabytes. The power of the radio signal currently received from the Voyager spacecraft on Earth is between about 10-14W and 10-19W. A modern basic digital watch consumes about 10-6W (1 microwatt) so the signal power received is between 100 million times and 10 trillion times lower. Here are some informative documentaries about the Voyager probes on YouTube: https://youtu.be/xZIB8vauWSI https://youtu.be/seXbrauRTY4 16 Silicon Chip case an alien civilisation finds it (see opposite). The high gain antenna is coloured white but the rest of the spacecraft is black and blanketed for thermal control and micrometeorite protection, while some areas are coated in gold foil and according to one claim, some areas are even wrapped in domestic kitchen-grade aluminium foil. Appropriate operating temperatures for the electronics are maintained by a combination of electrical heaters, thermal blankets, radioisotope heaters (which generate about 1W of heat through radioactive decay) and thermostatically-controlled louvres in four of the ten electronics compartments. Power system Due to the extreme distances from the Sun and the long duration of the mission, currently expected to be 48 years total, there is no possibility of using solar panels or batteries for spacecraft power. The only viable power source is a type of nuclear reactor called a Radioisotope Thermoelectric Generator (RTG). At the start of the mission, the Voyager probes needed 400W of electrical power and the device to produce this is called the Multi-Hundred Watt RTG or MHW-RTG (see Fig.4). This power source has no moving parts and works by converting radioactive decay heat to electricity by many thermocouples arranged in thermopiles. Each thermocouple generates a small direct current from the temperature difference across the junction of two dissimilar metals. The heat comes from the radioactive decay of spheres containing plutonium-238 (Fig.5). When the outputs of these thermocouples are combined, a substantial amount of electrical power is produced. Would the Voyagers be much different if built today? If the Voyager spacecraft were built today, they would be similar in many respects; the basic layout, type of instruments, thermal control and power source would likely be very similar. But the computers would probably be very different, given the chips with much larger computing power and memory available today. The cameras would also be much more sensitive to light and have higher resolutions as they would use solid-state imaging sensors rather than tubes. Australia’s electronics magazine siliconchip.com.au Fig.8: the Voyager Digital Tape Recorder. It was designed with extreme longevity in mind. Safety was always a consideration, so to avoid the possibility of radioactive contamination in the event of a launch accident, the fuel is surrounded by many strong protective layers. Telemetry system Signals from Earth are sent on the S-band (2-4GHz) and signals are sent back to the Earth on the X-band (8-12GHz) at up to 21.3W. There is also a 28.3W S-band backup for the downlink. During the Jupiter encounter, data was sent back to Earth at 115,200bps and from Saturn at 44,800bps. The difference is due to the extra distance to Saturn as received power decreases due to the inverse square law, hence the Fig.9: the 11 science instruments (which include the radio antenna), a photo calibration target and the radioisotope thermoelectric generator, mounted far away from the scientific instruments to avoid interference. lower data rate. Today, data is received at just 160bps due to the extreme distance. Data is received by the NASA Deep Space Network (DSN) which comprises receivers in Goldstone, California; Madrid, Spain; and Canberra (see Fig.6). Voyagers’ Golden Record In case an alien civilisation ever encounters these spacecraft, there is a gold-plated copper record that contains 115 images (plus a calibration image) and a variety of sounds of Earth along with a plaque with instructions for playing the record and indicating the origin of the spacecraft. The record is also coated with ultra-pure uranium-238, which decays into other elements over time, enabling the age of the spacecraft to be determined. As a courtesy to aliens, a stylus is even supplied with the record! siliconchip.com.au The audio stored on the record is about 54 minutes long and the images have a resolution of 512 lines. A video showing the images (with the video author’s own soundtrack) can be seen at: https://youtu.be/50HN6HAmeis Parts of the audio track can be found on YouTube, but not a complete playlist. There is a video of the story of making the record at: https://youtu.be/Mx0eNqINNvw A copy of the record can be purchased from various sources including https://ozmarecords.com/ Australia’s electronics magazine December 2018  17 These receivers have occasionally been supplemented by others such as Parkes Radio Telescope, NSW and the Very Large Array, New Mexico. Also, the antennas of the DSN have been upgraded over time, plus new software has been sent to the Voyagers to implement some data compression. Onboard computer systems The Voyager computer systems are based partly on the computer system used on the Viking Orbiter spacecraft which went to Mars in 1976, a decision based on budgetary restrictions and a desire for standardisation. For Voyager, this computer was called the Computer Command System PGH-Rate [Ions (>70MeV/Nucleon) per second LA-1 Rate [Ions (>0.5MeV/Nucleon) per second Fig.10: the Voyager Cosmic Ray System. It consists of three different types of instruments: four low-energy telescopes (LETs), facing in a variety of directions; two double-ended high-energy telescopes (HETs) at far left and far right; and the electron telescope (TET), directly beneath LET A. Fig.11: data from 2012 showing Voyager 1 crossing through the heliosheath into the interstellar medium. Voyager 2 is seeing similar radiation patterns now as it enters the interstellar medium. You can see live updates for the radiation measurement instruments for both spacecraft at https://voyager.gsfc.nasa.gov/data.html Source: Wikipedia user Stauriko. (CCS) with additional computers added being the Flight Data System (FDS) and the Attitude Articulation Control System (AACS). None of the computers on Voyager use dedicated microprocessors; they are instead built from discrete logic ICs. The Voyager computers have a total of 69.656kB memory if both memory banks in each computer are counted. The CCS is the “master” computer and is responsible for memory management and commands sent to the FDS and the AACS. It uses almost identical hardware to the Viking computer but runs heavily revised software. Due to its capability of in-flight reprogramming, the code has been im- Preparing the spacecraft for the Voyager Interstellar Mission (VIM) Both spacecraft have exceeded their expected mission durations by a long margin. Many preparations have been made to upload new software and shut down various instruments and services to reduce the electrical load, to compensate for the diminishing power output of the nuclear power sources. Their power output is diminishing by about 4W/year. The most important mission requirement is to maintain each spacecraft’s High Gain Antenna pointed to Earth. This requires that the thrusters which make tiny changes to spacecraft attitude continue working. A second requirement is that software instructions must be sent to enable the spacecraft to continue to operate autonomously, with programmed sequences of events to perform and to return data, even if the spacecraft lose their ability to receive command signals from Earth. The table at right shows the electrical loads on Voyager 1 that have so far been turned off to save power since the VIM started. Further planned shutdowns include termination of Digital Tape Recorder operations (already shut down on Voyager 2) and shutdown of the gyros for normal operations, to be powered up only when needed. After 2020, the remaining operational instruments will be turned off permanently or periodically turned on and off to share the remaining electrical power. There is enough fuel for attitude control to last until 2025. Beyond 2025, there is just one remaining task for the Voyagers and 18 Silicon Chip that is to carry information to possible intelligent spacefaring alien species, who may find the spacecraft and discover that they are not alone. Voyager 1 Load Power Turned Saved Off IRIS Flash-off Heater 31.8W 1990 WA Camera 16.8W 1990 NA Camera 18W 1990 PPS Supplemental Heater 2.8W 1995 NA Optics Heater 2.6W 1995 IRIS Standby A 7.2W 1995 WA Vidicon Heater 5.5W 1998 NA Vidicon Heater 5.5W 1998 IRIS Science Instrument 6.6W 1998 WA Electronics Replacement Heater 10.5W 2002 Azimuth Actuator Supplemental Heater 3.5W 2003 Azimuth Coil Heater 4.4W 2003 Scan Platform Slewing Power 2.4W 2003 NA Electronics Replacement Heater 10.5W 2005 Pyro Instrumentation Power 2.4W 2007 PLS Science Instrument 4.2W 2007 IRIS Replacement Heater 7.8W 2011 Scan Platform Supplemental Heater 6.0W 2015 UVS Replacement Heater 2.4W 2015 UVS Science Instrument 2.4W 2016 Australia’s electronics magazine siliconchip.com.au Fig.12: LECP data for Voyager 1, showing an increase in galactic cosmic rays as the spacecraft enters interstellar space. The data points are obtained from many different angles by rotating the detector platform. Source: NASA/ JPL-Caltech/JHUAPL. proved continuously over time. The CCS can execute 25,000 instructions per second and has two independent memory banks of 4096 18-bit words of non-volatile plated-wire memory (a variation of core memory). As mentioned earlier, there is a duplicate of each computer system on each spacecraft, in case one fails. The CCS is also compartmentalised so that if one part of one CCS fails, it can use the good part in the other CCS. The duplicate CCS computers can operate in three modes: individual, where each CCS performs independent tasks; parallel, where each CCS works on a task together; or tandem, where the same task is performed by each CCS and the results are cross-verified. The latter was used during close encounters with the planets where an error could be disastrous. The FDS is the system which collects, formats and stores all engineering, scientific and telemetry data. If the amount of data collected exceeds the capacity to transmit it back to Earth, excess data is stored on magnetic tape until downlink capacity is available. The FDS contains two banks of 8192-word 16-bit CMOS RAM and can execute 80,000 instructions per second (see Fig.7). The FDS was the first spacecraft computer to use volatile CMOS RAM which requires constant power to maintain the memory. Even a momentary loss of power would mean a complete loss of memory. To ensure constant power to the FDS, each unit has a dedicated power line from the radioisotope generators. It was decided that no further redundancy was required because if power was lost from those for whatever reason, the mission had no hope to continue in any case. The reason for having separate CCS and FDS systems is the high data rate from sensors such as cameras. The CCS may have been overwhelmed by the amount of data but the FDS was explicitly designed to handle it. However, these were the last spacecraft where the two functions were handled by separate computers. Like the CCS, the FDS can be reprogrammed in flight. siliconchip.com.au Fig.13: the key elements of the Low Energy Charged Particle instrument. The AACS is a modified CCS and is used to control the scan platform stepper motors, thruster actuators, handle attitude control and implement thruster logic. It has a crucial task which is to keep the spacecraft antennas pointed toward Earth. The AACS has two banks of 4096, 18-bit words plated wire memory. All the software was originally written in Fortran 5. Later software was written in Fortran 77 and later again in C. One problem in later years of the mission was to find programmers who were familiar with these languages. For more information, see: http://forums.parallax.com/ discussion/132140/voyager-1-2 The data storage system For times when data was being acquired faster than it can be transmitted back to Earth, such as during planetary encounters where many photos were being taken, excess data was recorded on a digital tape recorder (DTR) – see Fig.8. In addition to image data, every week, each spacecraft records 48 seconds of high-rate plasma wave system (PWS) data at 115.2kbps. This data is recorded on the tape and transmitted back to Earth once every six months. The long delay between transmissions is due to competing resources in the NASA Deep Space Network (DSN) required to receive the data and the fact that the primary mission of the spacecraft has been completed. The operation of the DTR on Voyager 2 was ended in 2007 due to a failure of the PWS, which occurred in 2002. The operation of the DTR has either been terminated (or soon will be) on Voyager 1 this year due to the inability to receive its data at 1.4kbps, which is the minimum speed it can transmit on its telemetry channel. At a distance from Earth of 19 light hours, the maximum data rate which can be received is much lower than this. As mentioned above, the currently possible rate is around 160bps on the 34m radio telescopes within the DSN; it is somewhat higher on a 70m radio telescope. The tape recorders were designed to be extremely robust and reliable. The tape heads were designed to last for several thousand kilometres of tape travel. Australia’s electronics magazine December 2018  19 Fig.14 (left): the actual Low Energy Charged Particle instrument in Voyager. Fig.15 (above): a 70s-era photograph of the Fluxgate magnetometer system used in Voyager spacecraft. Scientific instruments The Voyager spacecraft have ten dedicated scientific instruments and also used the spacecraft’s communications system for certain investigations, for a total of eleven (see Fig.9). A description of each system follows. Four instruments are still operational on Voyager 1 and five on Voyager 2. 1. Cosmic Ray System (CRS) (operational)    The CRS is still running on both spacecraft and measures both cosmic rays and other energetic particles from outside the galaxy, the Sun and particles associated with the magnetospheres of planets. It has a wide range of energy resolutions and one of its functions is to study the solar wind. It comprises three different types of instrument, to measure different energy levels and also to determine the direction of the particles detected (Fig.10). All instruments in the CRS are based around solidstate detectors.    The CRS was instrumental in determining the location of the heliosphere’s termination shock, the heliosheath, the heliopause and Voyager’s entry into interstellar space (see Fig.11). 2. Low-energy charged-particle (LECP) experiment (operational)    This instrument is still running on both spacecraft. It detects sub-atomic and atomic particles such as electrons, protons and alpha particles along with elements around planets, in interplanetary space and now interstellar space. These particles may originate from the Sun, galactic cosmic rays or planets.    It consists of two subsystems, the Low Energy Magneto-spheric Particle Analyzer (LEMPA) and the Low Energy Particle Telescope (LEPT) – see Figs.12, 13 & 14.    This instrument helped establish the shape of the mag20 Silicon Chip netospheres of Saturn and Uranus and establish the point of transit of the spacecraft into interstellar space, along with the CRS. 3. Magnetometer (MAG) (operational)    The magnetometer instrument is still running on both spacecraft. Each spacecraft carries two low-field magnetometers that measure from 0.002nT to 50,000nT and two high-field magnetometers that measure from 12nT to 2,000,000nT (2000µT/2mT). By way of comparison, the Earth’s magnetic field is between 25,000nT (25µT) and 65,000nT (65µT) at the surface.    The magnetometers are located at various positions along a 13m-long boom to minimise interference from spacecraft electronics. The purpose of the magnetometers is to measure the magnetic field of the Sun, planets, moons and currently, interstellar space.    Among the many discoveries made by the MAG were the magnetic fields of Uranus and Neptune, which are not aligned with the planets’ rotational axes, and are of a similar strength to Earth’s. It has also detected strong magnetic fields outside the solar system. 4. Plasma Science (PLS) experiment (operational on Voyager 2 only)    This system is still running on Voyager 2 but has failed on Voyager 1. The purpose of this experiment is to determine how the solar wind varies with distance from the Sun, study the magnetospheres of the planets, study the moons of the planets and detect interstellar charged particles (see Figs.16 & 17). 5. Plasma Wave Subsystem (PWS) (operational)    The PWS uses two 10m-long dipole antennas mounted at right angles to detect the electric field from plasma near planets and the interplanetary and now interstellar medium, in the frequency range of 10Hz to 56kHz. The same antenna system is also used by the PLS. A recording of plasma waves as Voyager 2 encountered Neptune may be heard at https://youtu.be/dJ8Dz5ZmqGM 6. Imaging Science System (ISS) (switched off)   The Voyager spacecraft each have a wide-angle and narrow-angle video camera mounted on a moveable scan platform. Each camera is equipped with several different Australia’s electronics magazine siliconchip.com.au Voyager’s future encounters Fig.16: the Solar wind pressure on Voyager 2 throughout the mission, as measured by the PLS. Note the dramatic decrease in 2007. This happened after the spacecraft passed the termination shock and entered the heliosheath. It did this much earlier than Voyager 1 due to the asymmetric shape of the heliosphere, caused by the interstellar magnetic field. filters that can be selected as necessary, which are selective for specific wavelengths of light, including wavelengths associated with chemical elements and compounds. The wide-angle camera has filters on a colour wheel selective for Blue, Clear, Violet, Sodium (589nm), Green, Methane (541nm and 619nm) and Orange. The narrow-angle camera has filters for Clear, Violet, Blue, Orange, Green and Ultraviolet. The wide-angle camera has a 200mm focal length with a 60mm objective and aperture of f/4.17 while the narrow-angle camera has a 1500mm focal length with a 176mm objective and aperture of f/11.8. The cameras use a monochrome vidicon TV tube (model B41-003; see Fig.18) made by General Electro-dynamics Co, which is a storage tube that can store a high-resolution video image for 100 seconds. The image area in the tube is 11.14mm x 11.14mm, consisting of 800 lines with 800 pixels per line. After a picture is taken, 48 seconds is required to electronically read the image, after which the image is cleared by flooding the tube with light to prepare for the next picture. The greyscale images are sampled with eight bits per pixel, so they required 5,120,000 bits of storage space (640kB) on magnetic tape for transmission back to Earth. As mentioned earlier, images of Jupiter could be transmitted back to Earth at 115,200bps while images of Saturn were sent at 44,800bps, so each image of Jupiter took 44 seconds to transmit, and 114 seconds for Saturn. Colour images were generated by merging images taken with various filters on the colour wheel. Some of the many discoveries made with the ISS are the great turbulence in the Jovian atmosphere, the intricate patterns in Saturn’s rings, vulcanism on Jupiter’s moon Io and an indication of an ocean beneath the ice of Jupiter’s moon Europa.    The cameras on both spacecraft were turned off decades ago due to a lack of sufficient light for useful imaging, the lack of objects to image and to save power. Voyager 1 took its last photo (mosaic) in 1990, the famous “Solar System Family Portrait” while Voyager 2 took its last photos when it encountered Neptune in 1989. siliconchip.com.au In about 40,000 years, Voyager 1 will come within 1.7 light years of the star Gliese 445 and in 56,000 years it will pass through the Oort Cloud, a collection of icy objects and a possible source of solar system comets. This will be followed by close encounters with the stars GJ686 and GJ678 in 570,000 years. An interesting calculation concerning the encounter with Gliese 445 is shown at: http://mathscinotes.com/2013/06/ voyager-1-and-gliese-445/ Voyager 2’s next closest encounter, apart from interstellar dust and gas clouds will occur in about 40,000 years when it will come within 1.7 light years of the star Ross 248. At that time, Ross 248 will be the closest star to the Sun and just 3.02 light years from Earth. Then in 60,000 years, it will pass the Oort cloud. In about 296,000 years it will come within around 4 light years of the star Sirius. It is difficult to predict with certainty where either spacecraft will go next. Fig.17: the plasma detector, which comprises two Faraday Cups. 7. Infrared interferometer spectrometer and radiometer (IRIS) (non-operational)   Infrared light is outside the visible range, at the red end of the spectrum. It is absorbed by various molecules and the extent of absorption at various wavelengths can be used to determine their chemical composition. The IRIS has three functions. It can determine the presence of various compounds in planetary and moon atmospheres, determine the temperature of the various bodies and can measure the total amount of light reflected from the bodies. 8. Photopolarimeter Subsystem (PPS) (failed)    When non-polarised light from the Sun is reflected or refracted by various materials, such as ice crystals in a planet’s atmosphere, it acquires a polarisation. Polarising filters block light with certain types or orientations of polarisation, selectively allowing light with a specific polarisation through. Voyager’s PPS was designed to image planetary atmospheres, rings and their moons’ surfaces using a 150mm focal length telescope and various colour and polarising filters (a total of 40 combinations Why didn’t Voyager explore the Kuiper Belt? There are three mains reasons why the Voyager probes did not gather data on the Kuiper Belt, a region between about 30AU and 50AU from the Sun which contains many small bodies, remnants from the formation of the solar system. 1) The Kuiper belt was unknown when the spacecraft were launched; it wasn’t discovered until 1992, Voyager 1 had already passed it when it was discovered and Voyager 2 was well into it. 2) The Voyager imaging system would not have been sensitive enough to make out the small objects in the Kuiper Belt. 3) The only telescope that could have found objects for Voyager to investigate was not working correctly at the time (Hubble). NASA’s New Horizons mission is currently investigating these objects. Further details are at: https://blogs.nasa.gov/pluto/2018/02/28/the-pisperspective-why-didnt-voyager-explore-the-kuiper-belt/ Australia’s electronics magazine December 2018  21 Fig.18: a Vidicon tube, as used in the Voyager cameras, along with sample images. Courtesy www.digicamhistory.com were possible). It was used to distinguish between rock, dust, frost, ice and meteor material and obtain information about textures, compositions and distribution of particles such as in clouds and rings. Unfortunately, the instrument on Voyager 1 failed before the Jupiter encounter and none of the data was ever archived, so it was turned off.    The PPS on Voyager 2 also suffered multiple failures and was of limited use but it was used to watch stars dip behind the rings of Saturn, Uranus and Neptune, to examine their structure and behaviour. 9, Planetary Radio Astronomy (PRA) (non-operational) The PRA experiment is a radio receiver that covers two frequency bands, from 20.4kHz to 1345kHz and from 1.2MHz to 40.5MHz. It was designed to detect radio emissions from the planets, including those from lightning and plasma resonance. It uses and shares with the PWS the two 10m-long antennas mounted at right angles to each other, in a “V” shape. 10. Radio Science System (RSS) (non-operational) The RSS used the Voyager communications system to pass radio signals through planetary and moon atmospheres and ring systems, which were then picked up by receivers in the Deep Space Network to determine atmospheric and ring properties. This technique is generally known as radio occultation. The system can also be used to precisely determine the spacecraft trajectory so the shape, density and mass of nearby bodies could be determined. 11. Ultraviolet spectrometer (UVS) (non-operational) UV light is just outside the visible spectrum at the blue end and is responsible for causing sunburn. The UVS was used to measure the distribution of major constituents in the atmospheres of planets and moons, the absorption of UV light by bodies with atmosphere as the sun is occulted, the UV “airglow” emissions of various bodies and the distribution of hydrogen and SC helium in space. Mission status, data and communications activity You can view the real-time mission status of the Voyage probes at: https://voyager.jpl.nasa.gov/mission/status/ Data from all instruments are freely available on a variety of websites, so if you have a theory you want to test, you are welcome to do so. A good place to start is https://voyager.jpl.nasa.gov/ mission/science/data-access/ but be aware that many data links are outdated or not working. However, if you look hard enough, you will find current data. If you want to check if the Deep Space Network is transmitting or receiving data with Voyager, you can go to https://eyes.nasa. gov/dsn/dsn.html and look for codes VGR1 (Voyager 1) or VGR2 (Voyager 2). See recent image below. Fig.19: the Deep Space Network status on 8th October 2018, showing the Canberra DSN station receiving data from Voyager 2 at 8.44GHz with a power level of -108.42dBm (1.44 x 10-14W). The typical data rate is currently 160bps. Data is transmitted from Earth at around 19kW. On 9th October 2018, the Goldstone DSN station in California received data from Voyager 1 with an astonishingly low received power of -152.44dBm or 5.70 x 10-19W! 22 Silicon Chip Australia’s electronics magazine siliconchip.com.au