Chapter +12: Galileo by Jove

Many years ago, at the beginning of the 17^th century, a 46-year old astronomer trained his newly improved telescope on the planet Jupiter. On January 7^th, 1610, he made note of three very faint stars in a curiously straight line either side of Jupiter. By January 13^th, he had identified a fourth faint star and had noted that the these were not part of the fixed background of stars but were generally carried along with Jupiter – though their exact distance from Jupiter varied. From these observations, he deduced that these were not stars but actually moons in orbit around Jupiter. He certainly understood the importance of this find. Undoubtedly he would have been honored and pleased to know that the four large moons would be eventually be referred to by his name. But he would have been even more surprised if he could have known that four-hundred years later those moons would be joined in their orbits by a man-made artificial moon – a spacecraft – also named after him. The four large Jovian^[1] moons are referred to as the Galilean moons and the spacecraft is called simply Galileo. This gentleman we are talking about was, of course, the famous Italian astronomer, Galileo Galilei^[2] (1564 – 1642) from Padua, Italy.

How far away were the moons that Galileo was observing? Galileo had no idea – a long way, no doubt – even a few million kilometers. The geometry of the Solar System was known very accurately but the scale was not. It was not until 60 years later in 1672 that a parallax measurement of Mars was made by Giovani Cassini (in Paris) and Jean Richer (in French Guiana) that the scale of the Solar System was established. Jupiter and the four moons that Galileo was observing lie at an average distance of nearly 800 million kilometers (500 million miles) from Earth.

It is worth re-emphasizing that while Jupiter is a long way away, Voyager-1, the focus of the previous chapter, is a really long way away. It is about 25 times further away than Jupiter. If we were to zoom in a factor of ten from Voyager, this would still only bring us to the outer reaches of the Solar System. If we want to identify the actual one billion kilometer mark (10⁺¹² meters), then that milestone lies between the gas giants Saturn and Jupiter. Saturn averages 1,430 million kilometers (890 million miles or 9.56 AU) from the Sun and Jupiter is about 779 million km (484 million miles or 5.21 AU) from the Sun. Both Voyager-1 and Voyager-2 visited both the Jovian system and the Saturnian system with Voyager-2 going on to complete the Grand Tour by visiting Uranus and Neptune. The Voyagers are perhaps the most famous and the most distant spacecraft, but there have been a number of other space-craft that have visited Jupiter and Saturn.

Pioneer 10, launched in 1972, was the first probe to visit Jupiter. Similarly, Pioneer 11, launched thirteen months later, visited Jupiter but also took advantage of a gravitational boost or sling-shot around the massive planet and went on to visit Saturn. Both spacecraft were spin-stabilized^[3] and the limited images that were taken were created with a 1-pixel (not 1 Megapixel) camera using the spacecraft spin and its relative motion to provide the image scanning mechanisms. The two Voyagers discussed in the previous chapter were launched in 1977, five years later than the Pioneers. In the 1990’s the Voyagers overtook the Pioneers to claim the title of humanity’s most distant spacecraft. Communications have failed on both Pioneers. The Pioneers and Voyagers were sent on a direct trajectory to Jupiter using a very powerful solid-fuel third-stage rocket. That was not the norm for later missions that generally used slower, but more energy efficient trajectories with gravitational (sling-shot) assists around Venus and Earth.

Since the time of the two Voyagers, there have been several more missions to the outer planets. The Galileo spacecraft was launched in 1989 to orbit Jupiter and the Cassini^[4] spacecraft was launched in 1997 to orbit Saturn. Both spacecraft carried successful ‘landing’ probes. The Galileo probe was actually an atmospheric probe descending deep into Jupiter’s atmosphere by heat-shield and parachute and the Cassini probe actually made a soft landing and transmitted images from Saturn’s largest moon Titan. Ulysses was launched in 1990 and used a Jupiter sling-shot to attain an orbit close to 90 degrees to the plane of the ecliptic (plane of Earth’s orbit) so that it could study the Sun’s polar regions. Finally, Juno is the most recent spacecraft to arrive at Jupiter (August 2016) having been launched five years earlier in August 2011. Juno is aimed at studying Jupiter itself rather than the moons. It is in a tight, highly-elliptical polar orbit and is heavily shielded to better survive the crippling radiation environment. Finally, the New Horizons spacecraft launched in 2006 was also in the news in 2016 having completed a ten-year trip (via Jupiter) to visit the dwarf-planet Pluto and its moon Charon.

This chapter focusses on Jupiter and the Galileo spacecraft for a several reasons. The Jovian system is at about the right distance for a chapter that targets a scale of one billion kilometers (10¹² meters). Galileo’s interesting ‘sling-shot’ route included a gravity-assist from Venus (once) and from Earth (twice). There was much controversy over the launch of the spacecraft because of the radioactive power source and risks inherent in the two subsequent near-Earth encounters. Galileo was the first spacecraft to orbit Jupiter and the first to provide detailed information on the Jovian environment and the many moons in the Jovian system. Jupiter presents an extremely harsh and challenging environment for a spacecraft because of the high radiation levels. Galileo suffered a major failure early in the mission but was still able to cover most of the objectives. It is regarded as one of the most successful missions ever. More than anything, the device that really saved the mission was the magnetic tape recorder.

Jupiter and the Jovian System

Jupiter is by far the largest planet in the solar system. It is more than twice as massive as all the other planets combined. In some ways it is more like a star than a planet. Its composition is similar to a star, mainly hydrogen and helium and it also radiates more energy than it receives. However Jupiter is not nearly massive enough to ignite nuclear fusion and qualify as a star. Its internal energy source is believed to be its continuing very slow gravitational contraction. Jupiter and Saturn are referred to as “gas giants”

in contrast to Neptune and Uranus which are “ice giants”. The distinction being that, although the ice giants have a thick atmosphere, they are predominantly solid ices and rocks with a well-defined surface. Jupiter and Saturn may have no solid surface at all. As one goes deeper into Jupiter’s atmosphere, the pressure rises and the hydrogen increases in density and temperature until it finally becomes a metallic fluid^[5]. This has never been seen in the laboratory, but there is a good theoretical basis for believing that, as the density increases and the atoms get closer together, the electron shells will start to overlap. At this point, the electrons will then be able to move freely from atom to atom throughout the volume. In essence, the hydrogen has become an electrically conducting fluid.

Jupiter has four large moons and myriad (~60) smaller moons^[6]. The four large moons are named Io, Europa, Ganymede, and Callisto in increasing distance from Jupiter. They are referred to as the Galilean moons and named for characters in Roman mythology. Both Io and Europa are comparable in size to our moon while Ganymede and Callisto are considerably larger. The four moons follow closely circular orbits, all very near Jupiter’s equatorial plane and the plane of the ecliptic^[7]. This is the reason the moons and Jupiter always appear almost exactly in a straight line viewed from Earth. All four moons are gravitationally locked (like our Moon), always presenting the same face towards Jupiter. The short orbital periods of 1.77, 3.55, 7.15 and 16.69 days make the moons’ motions easy to observe. Interestingly, the inner three moons are locked in a gravitational resonance with orbital periods in the exact ratio 1:2:4.

Jupiter has powerful radiation belts enveloping the inner moons and a very highly-extended magnetosphere. This magnetic field is generated by electrical currents in the core. The current scientific consensus is that perhaps over half of the bulk of Jupiter is in the form of liquid^[8] metal hydrogen. Earth also has a large portion of its mass in the form of liquid metal except that in Earth’s case it is iron-nickel not hydrogen. In both cases, the presence of convection currents in the conducting fluid in the rotating body leads to a self-generated self-sustaining magnetic field. However, Jupiter’s magnetic field at its surface is more than ten times stronger than the Earth’s. Given that Jupiter has about 1300 times the volume of Earth, this means that its equivalent internal magnet is some 18,000 times stronger than Earth’s. The Sun’s influence at this distance is also much weaker with the net result that Jupiter’s magnetosphere^[9] (region dominated by Jupiter’s field) is truly gigantic. If it were a structure visible to the human eye, it would appear surrounding Jupiter in our night sky as an object roughly five times the size of the full moon^[10].

Whether a magnetic field is described as small or large depends very much on the context. We can describe Jupiter’s magnetic field as large because it is much larger than the Earth’s field and very much larger than the interstellar fields that Voyager-1 is currently measuring. However these fields are puny compared with man-made fields even such as those that attract a refrigerator magnet to your refrigerator door. Fields used in magnetic recording are even more intense but highly localized on the scale of micro- or even nanometers. Some of the strongest large-volume fields commonly created on Earth are produced for medical applications. The huge fields used in Magnetic Resonance Imaging (MRI-scans) are created using large electrical currents in coils of superconducting wire. The superconducting magnets in the high-energy particle super-collider at CERN (famous for identifying the Higgs Boson, sometimes dubbed the “god-particle”) reach close to 9 Tesla^[11]. Some examples of magnetic field strength or flux density are shown in the table below^[12].

Jupiter’s huge magnetosphere is associated with intense and deadly radiation as electrons and ions get whipped into a frenzy circulating around Jupiter (see colored box at end of chapter on charged particles in a magnetic field). At the orbit of Io (the innermost of the original four moons that Galileo Galilei discovered in 1610), a few hours exposure to the radiation would mean certain death. The radiation is similarly damaging for spacecraft visiting the Jovian system^[13]. We will shortly be describing Galileo’s difficulties in detail

The Galileo Spacecraft

The Galileo spacecraft towered over 6 meters (20 feet) tall and weighed a massive 2.5 tonnes^[14] fully-fueled – more than double Voyager (0.8 tonnes) and dwarfing the earlier Pioneer probes (250 kg or 570 lbs). The spacecraft was nuclear powered like its predecessors and had a similar array of instruments though more advanced. The telescope/camera used a solid-state charge-coupled-device (CCD) sensor for the first time in space. The 800×800 pixel sensor was radiation-hardened with a 10 mm (0.4”) thick tantalum shield. For attitude control, Galileo took a hybrid ‘dual-spin’ approach. The larger higher-inertia portion of the spacecraft (including the high-gain antenna and the two long booms) was spin-stabilized (like Pioneer but unlike Voyager). The rotation was at a slow three rpm (once per 20 seconds) around the antenna axis. A smaller section of the spacecraft, including the scan-platform that mounted the cameras, was ‘de-spun’ so that it was in a stable fixed orientation and did not rotate.

Both sections required careful two-plane balancing (like a car tire) to avoid wobble. The two parts of the spacecraft were connected with high-vacuum ball-bearing rotating joints. Conventional slip-rings and brushes conveyed the electrical power. An array of axial rotary transformers^[15] was used to provide for the multiple signal paths.

The Galileo spacecraft was finally launched on October 18, 1989 from the cargo bay of the Space Shuttle Atlantis. This was several years later than originally planned and, also, was not on the direct trajectory that was originally planned. Much of the delay was associated with the Space Shuttle Challenger disaster that occurred in January 1986 and resulted in the tragic deaths of seven astronauts. There were no Space Shuttle flights for 32 months following the disaster and new safety regulations precluded the use of the powerful liquid-hydrogen upper stage that would have enabled a direct trajectory to Jupiter. Instead a much slower but lower-energy trajectory was taken that included a flyby of first Venus and then Earth and then Earth for a second time. The gravitational-assist from these three encounters caused the spacecraft to gain enough energy to reach Jupiter. This circuitous voyage would last six years.

Problems with Plutonium

Galileo’s launch and the double Earth flyby were not without controversy. Like the Voyagers, Galileo relied on a Radioisotope Thermoelectric Generator (RTG) containing 7.8 kilograms (17 pounds) of highly radioactive plutonium-238. Plutonium decays by emitting alpha particles (helium nuclei) and is dangerous only at short range. But there is a huge risk if particles are ingested or, especially, if inhaled into the lungs. Obviously, spaceflight is not without risk. Apparently as many as one in fourteen launches fail^[16] and there have been many instances of satellites or spacecraft meeting an untimely end by re-entering and burning up in the atmosphere. In fact there have been several well-documented incidents with both US and Russian RTGs that have led to the release of radioactive waste. However the Apollo 13 RTG provides an interesting counter-example. The RTG was carried on the lunar lander and was supposed to stay on the Moon to power continuing experiments. However, after the oxygen tank explosion, the lunar lander became the lifeboat for the return trip. Just prior to re-entry, the lunar lander was detached and sent on a trajectory into the South Pacific. The RTG survived re-entry intact and ended up at the bottom of the Tonga Trench in the South Pacific Ocean where it sits to this day – with no measurable release of radiation. The Apollo 13 re-entry was the fastest (i.e. worst-case) re-entry that has ever occurred and yet no radiation was released.

The earlier RTG designs were actually intended to burn up completely in the atmosphere during re-entry (dispersing the plutonium very widely but dilutely). But later designs including those for Galileo and Apollo 13 were intended to survive re-entry and contain the plutonium intact. In these designs, the plutonium is in the form of plutonium dioxide, a hard ceramic material with a very high melting point of 2,390 °C. The golf-ball sized pellets of plutonium dioxide are clad in iridium to block the alpha radiation. Iridium is extremely dense (23 g/cc) and very corrosion resistant with also a very high melting point of 2,446 °C. The iridium-clad pellets are finally encapsulated in a graphite heat-shield material such that they can withstand re-entry and possibly impact without the pellets disintegrating. Graphite or carbon has the highest melting point of any element at 3500 °C (at atmospheric pressure, it actually sublimes directly to a vapor).

The highest risk scenario considered for Galileo was not at the launch, where the plutonium containment system was deemed adequate, but in an accidental re-entry during, especially, the second very high speed, very close Earth flyby. The probability of an accidental re-entry and failure of containment was estimated at one in two million with a worst case outcome of a possible additional 1000 cancer deaths. As astronomer Carl Sagan said “there is nothing absurd about either side of this argument”^[17]. Lawsuits were filed by environmental groups including the German Green Party and protests were held outside the heavily-armed Florida launch site at Cape Canaveral.

In the end, of course, Space Shuttle Atlantis, STS-34, launched successfully on October 18^th 1989 and six hours later Galileo was floated safely out of the cargo bay. The Venus fly-by occurred four months later and then the first Earth fly-by 10 months after that. The second and most critical high-speed Earth fly-by occurred on December 8^th 1992 at a distance of only 300 km (200 miles or 2.5 % of Earth’s diameter). To everyone’s great relief, the fly-by was uneventful. The encounter added a final 3.7 km per second (8,300 miles/hour) to Galileo’s speed to boost it on its long journey (see colored box at end of chapter on Gravitational assist). Three years later to the day, on December 8, 1995, Galileo fired its main engine (400 Newton, 90 pounds-force) for 49 minutes and thus became the first artificial satellite of Jupiter.

Antenna Problems

On April 11, 1991, prior to the flyby of the asteroid Gaspra, the command was sent to deploy the large 4.8 meter (16 feet) high-gain antenna dish that would allow high-data-rate transmission of planetary images. The antenna had been previously kept in its stowed position to avoid damage from solar radiation and heat when closer to the sun. The antenna dish was supposed to unfurl like a large umbrella with 18 ribs. It quickly became evident that the drive-motor had stalled and the antenna deployment had failed^[18]. From subsequent analysis of the telemetry of the motor current, the spacecraft rotation rate and wobble, and from experiments on a sister set-up on the ground, it was concluded that three of the 18 ribs had not been released properly due to abnormally high friction and were still in their stowed position.

Multiple ingenious attempts were made to overcome the excessive friction and fix the problem. Unfortunately, the option of reversing the drive-motor and simply trying again was not available. That firmware device-driver code was one of the pieces that had been sacrificed in order to fit the code into the limited read-only memory. The various attempts to free the stuck ribs included heat-stressing by exposure alternately to sun or shade; pulsing the motors to try to excite a resonant mode; ramming one of the movable parts of the spacecraft against its hard stop to shock the entire spacecraft; and finally spinning the rotating part of the spacecraft extra fast (10.5 rpm). Ultimately after several frustrating months, it was concluded that the high-gain antenna was a lost cause.

Detailed analysis indicated that the molybdenum-disulfide dry-lubricant surface-coatings had probably been damaged during shipping across the country. The damage was likely aggravated by subsequent testing in air though this was not revealed by tests in air because a relatively low-friction titanium-dioxide surface-layer quickly forms in air. However, after prolonged exposure to vacuum and perhaps aggravated by a closer approach to the heat of the Sun than intended compared with the originally-planned direct trajectory, the damaged raw metal surfaces gave rise to the very high friction that prevented the ribs from releasing.

This failure was a very serious setback. The high gain antenna was critical to the mission of surveying the Jovian system. It was capable of transmitting information back to earth at data-rates up to 134,000 bits per second. This relatively high data-rate was essential for transmitting all the images and data gathered during the close encounters with the various Jovian moons.

The low gain antenna was still working fine. The converse of ‘low gain’ is that the antenna is not very directional. The low-gain antenna thus provides a robust means of receiving commands from Earth and for transmitting simple telemetry, even if the spacecraft orientation is uncertain. However, because it is ‘low-gain’, the data-rate through the antenna is a mere 40 bits per second – not nearly enough to transmit high-resolution images in real time. Some heroic efforts had to be made to salvage as much as possible of the mission. First, the repetitive transmission of engineering data and information from monitors that normally went through this low-gain channel was suspended. This information was only sent if a change was detected. Second, new compression software was uploaded to the spacecraft. This included both lossless data compression (as seen in today’s .zip files) and also lossy data compression (as in .jpg images) as well as techniques explicitly designed for planetary imaging where the black backdrop of empty space could be simply discarded. Third, by using the multiple dishes of NASA’s Deep Space Network to best advantage, peak data-rates of 160 bits/seconds were achieved under optimal conditions during the daily cycle. Finally, the tape recorder became the workhorse – capturing image data at full-rate then very slowly feeding it back to Earth over periods of months.

So, perhaps more than anything, it was the availability of the Galileo tape recorder that saved the mission. The trajectory of the spacecraft in orbit around Jupiter involved large highly-elliptical orbits with occasional brief encounters with the Jovian moons. Data could be recorded on tape at high-speed during these important events and then played back at the spacecraft’s leisure. Each brief encounter would last just a few hours while the interval between encounters would typically last two or more months. Even averaging only 100 bits per second, roughly 65 MegaBytes of information can be transmitted over a period of two months – enough for several high-resolution images from the fly-by of each moon plus much of the other scientific data. Had the tape recorder failed, only miniscule amounts of this data would ever have been recovered. Of course, the tape recorder did fail. In fact it failed twice. Remarkably, on both occasions it was possible to fix the problem remotely by sending commands from Earth. We will say more later about the tape recorder, its nerve-wracking ‘anomalies’ (failures), and the remarkable long-distance repairs!

Despite the crippled high-gain antenna and the hiccups with the tape-recorder, the mission proved to be one of the most successful ever and is estimated to have finally achieved 70% of its planned objectives.

The Mission

The mission’s contributions actually started well before the Galileo spacecraft’s arrival at Jupiter. During the first Earth flyby, Carl Sagan devised an experiment to see if Galileo could detect the presence of life on Earth and what would be its signature. The conclusions were ‘YES’ and the things to watch out for were 1) absorption of red light on the surface due to chlorophyll in plants, 2) absorption bands due to atmospheric oxygen (also due to plants), and 3) detection of modulated narrowband radio wave signals from no known natural source. Galileo’s trajectory also took it in the vicinity of two asteroids: 951-Gaspra in October 1991 after the first Earth flyby and 243-Ida in August 1993. Galileo’s images revealed both asteroids to be irregular cratered objects 15 and 30 km in average diameter, respectively.

One of the early and unexpected highlights arose due to the fortuitous timing and positioning of Galileo as it approached Jupiter in July 1994. The famous impact of comet Shoemaker-Levy 9 on Jupiter occurred on the night-side of Jupiter and was not visible from Earth. It was, however, visible to Galileo. The comet broke into several fragments due to tidal forces (high gravitational field gradient) as it approached Jupiter. The total energy released in the huge impact explosions was equivalent to about 200,000,000 Megatons of TNT (the Tsar Bomba^[19], the largest hydrogen-bomb ever exploded, yielded a mere 50 Megatons). The impacts were clearly visible to Galileo even from a distance of 239 million km (145 million miles). The dark plumes from impacts remained for many months and could be easily seen with Earthbound telescopes as the planet rotated.

Five months before arriving at Jupiter, Galileo released the atmospheric probe. The probe was directed to hit Jupiter’s atmosphere at the shallow angle of 8 degrees and then descend into the atmosphere while the Galileo spacecraft, passing about 214,000 km (133,000 miles) overhead would receive the probe’s telemetry and relay it to Earth. The heavily heat-shielded probe did operate as planned, surviving the fastest ever entry (47.8 kilometers or 29.7 miles per second) and deceleration (250g) into a planetary atmosphere before discarding the heat-shields and deploying its parachute. The probe transmitted data to Galileo for 57 minutes, descending 156 kilometers (97 miles) through Jupiter’s atmosphere before the transmitter failed due to the increasing temperature. (After descending several more hours, temperatures were certainly high enough to vaporize the entire probe, including the titanium casing.)

The Galileo spacecraft spent eight years exploring the Jovian system. The main objectives of the mission were to observe Jupiter and map out the structure of its magnetosphere and also to explore the four Galilean moons. The elongated elliptical orbit of the spacecraft was designed to allow exploration over a wide range of distances from Jupiter but without spending too much time in dangerous environment close to the planet. A wealth of scientific data was returned over the eight years. Amazing images were obtained of Jupiter and its massive weather systems and of the four very diverse Galilean moons. Each moon turned out to be totally different in appearance and structure from its neighbors. Io was found to be the most volcanically active body in the Solar System. Tantalizing clues suggested that huge underground oceans of liquid water exist on Europa and Ganymede and possibly Callisto.

As the years progressed, increasingly adventurous orbital trajectories were employed as more of the mission objectives were accomplished and as the spacecraft electronics sustained more and more damage from Jupiter’s radiation belts. In particular, both the star-tracker (used to determine the spacecraft’s orientation) and the CCD sensors in the cameras suffered badly with each pass through the radiation belts. Towards the end of the mission, the trajectories deliberately included encounters with the moons Io and Amalthea. Io is the innermost of the large Galilean moons and is 350,000 km from Jupiter’s cloud-tops. Amalthea is a tiny moon (~200 km or 125 miles diameter) orbiting only 110,000 km above Jupiter’s cloud-tops. (From Amalthea’s surface, Jupiter appears truly enormous, occupying over 45 degrees of the sky!). Allowing the spacecraft so close to Jupiter risked even more damage to the spacecraft’s electronics. The camera’s CCD sensor was already severely damaged, so that by Jan 2002, it was switched off. In November 2002, Galileo flew within 160 km (99 miles) of Amalthea. As well as additional information about Amalthea itself and better establishing its mass, the team was especially interested in the behavior of Jupiter’s inner magnetosphere. As long ago as 1955, Jupiter was recognized as a very strong source of high-frequency radio waves (3 to 30 MHz, like short-wave radio). The signals have a characteristic 10 hour periodicity because of the planet’s rotation and share some common features with objects known as pulsars which are believed to be rotating neutron stars. Scientists believed the signals originate in the inner magnetosphere where electrons are accelerated to very high energies. The data to help understand this was gathered successfully during the close pass of Amalthea and Jupiter. However, when commanded, the tape recorder failed to play back the data – necessitating one of the most remarkable long-distance repairs!

The Galileo Tape recorder

Designing a tape-recorder for spaceflight is no trivial task. High levels of acceleration and vibration occur during launch. The external environment is a harsh vacuum sustained for many years with ionizing radiation and with large swings in temperature. And, of course, in this application, long-term reliability is absolutely paramount.

The tape recorder used on the Galileo spacecraft was designed by Odetics^[20] (Anaheim, California) with the magnetic tape from Ampex^[21] (Redwood City, California). The tape recorder is shown in the picture below^[22]. The entire recorder operated in a hermetically sealed environment with an atmosphere of nitrogen and helium and with 30% to 40% relative humidity. In addition, the temperature of the recorder was actively maintained close to room temperature by electrical heaters powered by the Radioisotope Thermoelectric Generator (RTG).

The tape is 560 meters (1850 feet) long and ¼-inch (6 mm) wide. The tape substrate is made from a strong film of polyethylene terephthalate (Mylar). A thin layer of the magnetic recording medium (gamma-ferric-oxide particles in a polyester urethane binder) is coated onto one side of the tape substrate. A thin conducting back-coat is applied to the other side. The back-coat helps avoid build-up of static electricity and also provides good friction for driving the tape through the tape path. Control of humidity is important. Too little humidity and static electricity build-up can become an issue. Too much humidity and the binder material can hydrolyze and create sticky deposits known as ‘brown stain’ that can adhere to the read and write heads.

The magnetic material, gamma-ferric-oxide is the cubic crystalline phase of Fe₂0₃. It is readily prepared as a fine particulate suspension with a brown color (depending on particle size). The particles are chemically very stable at normal temperatures. The gamma phase is magnetic. The bulk remanence (diluted by the binder) is around 200 milliTesla (2000 Gauss). The particles are acicular (elongated) and well dispersed (not in contact with each other) which gives rise to them having a useful coercivity around 25,000 A/m (300 Oersted). A ‘useful’ coercivity means low enough that the medium can be written with fields produced by a recording (writing) head but high enough to retain the magnetization for long periods in the presence of other extraneous fields. A ‘useful’ remanence means the medium retains sufficient magnetization such that the flux and induced voltages in the playback head are enough to be easily read and detected. Due to the coating process and as the coating solvent dries and with additional help from an orienting magnetic field, the particles tend to align in the plane of the coating and along the tape direction. Better orientation means a ‘squarer’ hysteresis loop (higher remanence and coercivity) and a better quality recording.

The complexity of the tape path can be gauged from picture above of the Galileo tape recorder. There are two spools or reels coaxially mounted one above the other and arranged to rotate in opposite directions (this helps reduce angular momentum transfers to the rest of the spacecraft). Altogether on the tape-path there are 18 places where contact is made with the tape as it moves from one reel to the other. Two of these (closest to the front in the picture) are the tilted rollers which translate the tape from the plane of the upper reel to the plane of the tape deck and then down to the plane of the lower reel. The tape is driven by three identical capstans that contact the back of the tape with a 180-degree wrap. Plus there are eight passive rollers that position and guide the tape on its journey. Then there are two write heads (for redundancy) and two read heads (for redundancy). Finally there is one dummy head that replaces an erase-head that was used on some other configurations of the machine. We will hear more about that dummy head later!

In the Galileo tape recorder, data is recorded simultaneously on four tracks spaced across the width of the ¼-inch tape. Inductive heads are used for both writing and reading (magneto-resistive read heads are now used in modern data recorders). The heads are built from laminations of an AlFeSi alloy known colloquially as ‘alfesil’. This material is magnetically-soft but mechanically-hard and resists wear from the abrasive iron-oxide recording tape. There are several operating modes for the tape recorder offering a wide range of data rates from 7.68 kbit/s to 806.4 kbit/s. These are achieved largely through changes in tape running speed. The total available capacity of the tape was 900 MegaBytes.

The Tape Recorder ‘Anomalies’

In October, 1995, as it started approaching Jupiter, Galileo took three images of the giant planet and recorded them on the beginning of the tape. On October 11, the command was sent to rewind the tape ready for the very slow playback via the low-gain antenna. The rewind should have been completed in only a few seconds. However, the following day, 15 hours later, the recorder had not yet reached the end of tape and was apparently still rewinding. This is generally a very bad sign. Was the tape broken, or had it run off the end of the spool, or jumped off the tape guides, or somehow jammed? Any of these nightmare scenarios essentially doomed the mission – only a tiny amount of the scientific data and certainly none of those beautiful high-resolution images of Jupiter’s moons would ever make it back to Earth.

The spacecraft was immediately commanded to stop rewinding while the scientists and engineers worked frantically to find out what went wrong and see if it could be fixed remotely. The Jupiter arrival and all the data from the atmospheric probe was due on December 7^th, less than two months away.

From the spacecraft telemetry, the motor current indicated that the drive capstans had not stalled and had continued running. However, the beginning-of-tape sensor never encountered the transparent section at the start of the tape. After examining all the telemetry and also data from some earlier misbehavior, it seemed that the tape had become stuck somewhere between the drive-capstans and the beginning-of-tape sensor. The identical recorder available on the ground as a reference had never had any such ‘stiction’ problems. However, the problem was reproduced in a similar Odetics tape recorder from the Magellan mission to Venus.

The final conclusion was that the tape-usage prior to the final cleaning and sealing of the drive just before launch was the critical element. Newer tapes tended to generate debris until they had been run for some time. The debris adhered to the dummy head which was made of harder sapphire and did not wear away as much as the real heads made of alfesil. The debris included hydrolysis products from tape binder and was naturally sticky and would finally cause the tape to become stuck to the dummy head. Had an older ‘worn’ tape been used, the stiction problem would not have arisen.

For normal operation, it is important to maintain the correct tension on the tape between the two reels. There is a passive spring mechanism in the Galileo recorder that provides the tension. One side-effect from the particular design was that the reels always wanted to spool themselves towards the center of the length of tape. So when operating near the beginning of tape, this built-in bias reduced the force available in the rewind direction but increased the force in the forward direction. So it was suggested that it might still be possible to unstick the tape by trying to move it in the forward direction. The big question however, was how badly had the tape been damaged while it was stuck with the three capstans rotating and rubbing on the back of the tape for 15 hours. Would the tape break?

With some trepidation, on October 20, 1995, the command was sent to play the tape in the forward direction. After a momentary high pulse of motor current signaling an ‘unsticking’ event, the telemetry indicated the tape recorder to be operating normally. After a huge collective sigh of relief, an additional 25 turns were then wound onto the spool on the beginning-of-tape end to cover and isolate the potentially damaged tape section. That section of tape was never used again. The future protocol was to always move the tape slightly in the ‘downhill’ direction, towards the middle of the tape, before trying to move it ‘uphill’ against the bias.

The tape recorder then worked flawlessly in sending back all the invaluable information from the Jupiter atmospheric probe and then continued to operate well for the next eight years sending stored images and scientific data from each of the encounters with the Jovian moons. Then, almost at the end of the mission, came the daring flyby of the small inner moon Amalthea – deep in Jupiter’s radiation belt. During the flyby, the damage was such that spacecraft went into ‘safe’ mode. In this mode the spacecraft basically shuts everything down and focusses completely on recovering its orientation and re-establishing communication with Earth. Fortunately, the spacecraft was brought out of safe mode successfully, but, when the time came to retrieve the data gathered during the flyby, the recorder failed to respond.

This time the team pinpointed the problem as a failure of the light-emitting diode (LED) and photodetector that together sensed the capstan motor positioning (and thus tape position). Again some clever detective work led to a truly ingenious remote repair effort. One member of the team, Greg Levanas, speculated that high-energy proton radiation had knocked atoms out of their correct positions in the crystal lattice of the gallium-arsenide LED material causing the light output to drop well below normal. His proposed fix was to run a moderate current through the damaged diode for prolonged periods to heat-anneal the semiconductor material and gently jostle the atoms back into their correct positions. The appropriate commands were sent and after a cumulative 90 hours of treatment, the tape recorder was finally started to operate satisfactorily and the important information from the Amalthea flyby was all successfully recovered – much to the scientists delight.

The Amalthea flyby occurred on the 34^th orbit by Galileo. On the 35^th orbit the decision was made to send the crippled spacecraft directly into Jupiter’s atmosphere and to allow it to burn up. Part of the motivation was to avoid possible contamination of any of Jupiter’s moons with Earthly bacteria. So, on September 21, 2003, at 18:57 GMT on the completion of its 35^th orbit, Galileo sped into Jupiter’s atmosphere at 173,736 kilometers per hour (48 km/s or 107,955 mph) and met its fiery destiny after nearly eight years of faithful service to mankind.

Charged Particles in Magnetic Fields

A charged particle moving through a magnetic field experiences a force at right-angles to both the direction of motion and the direction of the field. This is very different to the behavior of objects in a gravitational field or of charged particles in an electric field. In both those cases, the force applies along the direction of the field and is independent of the direction of motion.

Since there is no force along the direction of motion, the particle’s speed is unchanged by the presence of a magnetic field. However, the particle will be continuously accelerated at right angles to its path and the field direction. As a result, the particle will take a helical spiral path along the direction of the field. In a uniform field, the particles will ‘drift’ along the field direction with a constant velocity equal to the parallel component of their initial velocity.

In a non-uniform field from a bar magnet or a giant planet where the field is weaker at the equator and stronger at the poles (where flux lines converge), the particles can become trapped and spiral endlessly, reflected back and forth between the north and south poles. It is this behavior that gives rise to the belts of dangerously concentrated radiation that surround Jupiter and the Earth. The belts around the Earth are called the Van Allen belts after their discoverer. The trapped particles come closest to the surface of the planet at the north and south poles and give rise to visible aurora in the winter polar regions on both Earth and Jupiter.

The international space-station (400 km altitude) operates safely below the altitude of the Van Allen belts while the geostationary broadcast satellites (35,786 km altitude) operate beyond the bulk of the radiation zone.

Mass spectrometers also rely on particle deflection (proportional to charge/mass) in electric and magnetic fields to distinguish elements and compounds.

Gravitational Assist

The concept of gravitational assist has been likened to the idea of throwing a tennis ball directly at the face of an oncoming train. From the perspective of the train, the ball arrives with velocity, v_ball + v_train, and bounces back with the same speed but opposite direction, -(v_ball + v_train). From the perspective of the stationary person, he or she threw the ball with velocity, v_ball, but finds that it bounces back with velocity -(v_ball + 2v_train). Depending on the speed of the train, this can mean a very large boost in energy. The source of the energy is of course the train which will slow down imperceptibly after the collision with the tennis ball.

Similar principles apply to the gravity boost with spacecraft. Typically everything is rotating in the same direction (prograde or counterclockwise viewed from the north pole) and the change in direction of the spacecraft may be much smaller than the 180 degrees seen in the tennis ball example. As with the train, at least in theory, the Earth was slowed down very slightly in reaction to the gravity-assist given to the Galileo spacecraft to boost it on its journey to Jupiter.

Gravity-assist obviously played a vital role in the once-in-a-lifetime Grand Tour of the outer planets that was performed by Voyager-2. It was also essential for the Venus-Earth-Earth sling-shots that propelled Galileo towards Jupiter in a very economical fashion. Finally, gravity-assist from Jupiter’s moons was integral in the design of the orbital trajectories around Jupiter. This was done to ensure maximum value from the close encounters with the Jovian moons, while consuming the minimum amount of precious fuel.

Footnotes:

The planet is named after Jupiter also known as Jove who was king of the gods in ancient Roman religion and mythology. “Jovian” is the corresponding adjective. ↑
Galileo was an Italian astronomer and scientist who later fell afoul of the Catholic church because of his belief in a heliocentric (sun-centered) solar system. This belief was bolstered by his observation that not everything revolved around the sun. ↑
An object spinning on an axis of minimum or maximum moment of inertia will passively recover its orientation in space when subject to small perturbations. The Pioneer spacecraft were spin-stabilized at 4.8 rpm around their maximum inertia axis which, by design, coincided with the axis of the dish antenna ↑
Giovanni Domenico Cassini, another Italian astronomer and scientist, discovered four new moons of Saturn and was the first to observe structure within Saturn’s rings. The Cassini division refers to a large gap between the inner and outer rings. ↑
https://en.wikipedia.org/wiki/Metallic_hydrogen https://commons.wikimedia.org/wiki/User:Kelvin13 ↑
https://solarsystem.nasa.gov/planets/jupiter/overview/ https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=3604 http://photojournal.jpl.nasa.gov/catalog/PIA01299 ↑
The ecliptic is the plane defined by Earth’s orbit. The Galilean moons orbit within about 1 degree of the ecliptic. and so always appear to be on a straight line viewed from Earth. ↑
Typically described as ‘liquid-metal” but at these extreme pressures and temperatures, hydrogen is well above its ‘critical point’ where liquid and gas become indistinguishable. ↑
Jupiter’s magnetosphere exists within the Sun’s magnetosphere which in turn exists within the interstellar medium. There is generally a sharp boundary between regions at which both the magnetic field and charged particles (plasma) change abruptly. ↑
C T Russell, “Planetary magnetospheres”, Reports on Progress in Physics, Vol. 56, No. 6, IOP Science, June 1993 ↑
The ‘Tesla’ (named after Nikola Tesla, the famous Serbian-American inventor) is the SI unit of magnetic flux-density. By definition, a flux-density changing at one Tesla per second will induce one volt in a perpendicular wire loop of one square meter area ↑
Data from http://www.coolmagnetman.com/magflux.htm ↑
As of 2016, the newer Juno spacecraft is explicitly exploring this environment. All of Juno’s critical electronic systems are radiation hardened and shielded inside a 400-lb. (180 kilograms) titanium vault with 0.5-inch-thick (1.75 centimeters) walls. ↑
1 metric tonne = 1000 kilograms = 0.984 (long) tons = 2204.6 lbs. ↑
In rotary transformers, the primary and secondary are free to rotate relative to each while closely maintaining a constant high level of magnetic coupling. Ferrite rotary transformers were a key feature of helical-scan rotating-head tape-recorders such as the VHS video cassette recorders (VCR) widely used for recording video and movies in the last quarter of the twentieth century. ↑
A. Gould, L. Allen, M. Orin, “Estimating Satellite Insurance Liabilities”, Casualty Actuarial Society (2000) ↑
Carl Sagan “To launch or not to launch”, Oct. 9^th, 1997 https://www.dartmouth.edu/~chance/course/Syllabi/97Dartmouth/day-6/sagan.html ↑
M. Johnson, “The Galileo High-Gain Antenna deployment Anomaly”, JPL, May 1, 1994 https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940028813.pdf ↑
https://en.wikipedia.org/wiki/Tsar_Bomba ↑
http://www.computerhistory.org/collections/catalog/102740348 ↑
https://en.wikipedia.org/wiki/Ampex ↑
Courtesy William Noack: https://www.flickr.com/photos/timeportal/646441693/ ↑