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Jupiter - 10 Years Since Voyager 2 :
The Galileo Space Probe
By Greg Bryant

 Published in the September 1989 issue of Universe
 



Prelude

This article marks the beginning of a series of monthly articles that I will be writing for the Journal on subjects relating to the Solar System - the planets and their satellites in particular.  In coming months, this regular article will discuss such topics as Voyager 2's encounter with Neptune, what Pioneer and Voyager discovered at Satum, the discoveries from Uranus and growing support for a return to Mercury.  This month, we look at Jupiter and the Galileo mission.

Introduction

It has been over 10 years since Voyager 2 encountered the planet Jupiter (July 9, 1979).  Now the revolutionary $900 million Galileo Space Probe is ready to fly into orbit on board Shuttle Atlantis on October 12 of this year.  From orbit, Galileo will be launched on a 6 year journey to Jupiter.  This journey is longer than what is normally required because of safety constraints with regard to the propulsion system boosting Galileo from Earth orbit.  Before discussing the Galileo mission in further detail, it would be appropriate to look at what is already known about Jupiter, especially looking at the results of the Pioneer and Voyager probes.

General Characteristics

Jupiter is by far the largest planet of the Solar System, with an equatorial diameter of 142,800 km, and is fifth in order from the Sun at a mean distance of 778,360,000 km.  At this distance, Jupiter takes about 11.86 years to orbit the Sun.

Like all the other giant gaseous planets, Jupiter spins very quickly.  This causes an equatorial bulge, and hence Jupiter's polar diameter is only 134,200 km compared to the equatorial diameter of 142,800 km mentioned above.  In looking at Jupiter's rotation, the movement of the visible surface is so complex that the planet has been divided into two main regions for the sake of convenience.  System I, which covers the equatorial region from 9 degrees North to 9 degrees South, has a rotational period of 9 hr 50 min 30.003 sec, this value of course being an average of a number of belt values.  System II, which applies to the rest of Jupiter, has a rotational period of 9 hr 55 min 40.632 sec.  There is also a third system, System III, which refers to the source of certain radio emissions from the planet, and this has a rotational period of 9 hr 55 min 29.7 sec.

The first evidence that Jupiter has a magnetic field came in the mid 1950's through the detection of radio emissions from the planet by radio telescopes.  It is clear that the origin of the Jovian magnetic field is closely related to the structure of the planet's interior.  Indeed, Jupiter's interior is now believed to have the following structure, working from the surface inwards: Between the surface and a depth of about 1,000 km, there is a transitional zone in which hydrogen's state changes from gaseous to liquid. At this level, the temperature is about 2000 K, and the atmospheric pressure is 5,600 times Earth's sea-level atmospheric pressure! Dropping down further to a depth of 3,000 km, the temperature is 5,500 K and the pressure 90,000 Earth atmospheres. At a depth of 25,000 km the temperature reaches 11,000 K, with a pressure of 3 million Earth atmospheres. Here, hydrogen is known as liquid metallic hydrogen. From this depth towards the centre, the temperature and pressure rise until they reach 30,000 K and 100 million atmospheres respectively at the centre.  It is speculated that Jupiter has a small rocky core of about 10 or 20 Earth masses, composed mainly of iron and silicate materials.  However, the existence of this core has only been deduced from what is known about Jupiter's interior.

A vast region of space surrounding Jupiter is dominated by the Jovian magnetic field.  Indeed, this magnetic field, the strongest in the Solar System, is more than ten times the strength of Earth's magnetic field, and if this field was visible with the naked eye, its apparent size from Earth would be equivalent to that of a full moon!  This region, the magnetosphere, has quite a complex structure.  The bow shock is where the solar wind collides with Jupiter's magnetic field.  Within this region is an area of great turbulence called the magnetopause.  The Voyager space probes found effective temperatures of 300 to 400 million K, the highest found in the Solar System.  It is important to realise what is really meant by this.  The theory is that heat energy can be interpreted as the kinetic energy of particles due to their random movement.  However, in the highly rarefied magnetopause, the individual particles are so far away from each other that the collective kinetic energy of a given region is quite minimal, and hence spacecraft can pass quite safely through these areas.

The basic structure of Jupiter's atmosphere does resemble that of Earth, at least at tropospheric levels.  Knowledge of the higher levels (stratosphere, mesosphere, and thermosphere) is quite incomplete.  In looking at Jupiter's atmosphere in comparison to Earth's, there are a number of fundamental differences.  Firstly, Jupiter has no solid surface equivalent of Earth which can affect atmospheric motion.  Secondly, the temperature at Jupiter's poles is virtually the same as at the equator.  Thirdly, Earth has solar radiation as the primary source of energy for the weather system, while Jupiter has an internal source of heat energy.  Also, Jupiter's rapid rotation has an important role to play.  Below is given a table of the main constituents of Jupiter's atmosphere above the cloud tops.
 

Constituent Approximate % Volume
Hydrogen 90
HD 1.8 * 10^-3
Helium 4.5
Methane 7 * 10^-2
Deuterated methane 3 * 10^-5
Ammonia 2 * 10^-2
Ethane 10^-2
Acetylene 10^-2
Water vapour 10^-4
Phosphine 10^-6
Carbon monoxide 10^-7
German tetrahydride 10^-7

The appearance of Jupiter's southern hemisphere is dominated by the incredible Great Red Spot, and the three smaller white ovals.  Infrared observations of the Great Red Spot have shown that the Great Red Spot is colder than its surroundings, suggesting that it is an elevated high-pressure region, consistent with its anticyclonic rotation. Perhaps the best clue to understand the nature and origin of the Great Red Spot is by looking at the white ovals, which were observed at their formation in 1939. At this time, dark features were seen in the South Temperate Zone, and these dark features gradually evolved into 3 large white ovals, each nearly 100,000 km long. Since then, over the past 50 years, the white ovals have shrunk to a size of 11,000 km long. It would seem that these white ovals may disappear in the near future. The Great Red Spot may have formed in the same way, although its known existence since 1664 seems to rule out the simple idea of a mere transitory feature.  However, it is interesting to note that the Great Red Spot is shrinking: it is half the size of what it was a century ago.

The Jovian system of satellites numbers 16, and extends all the way out to a distance of 23,700,000 km (Sinope). The Voyager probes found 3 satellites, Metis, Adrastea and Thebe, all of which orbit within Io's orbit. Pioneer 10 and 11 obtained images of the Jovian system in December 1973 and 1974 respectively, but it was left to the Voyager missions for a more comprehensive survey. Probably one of the most startling discoveries of the entire mission (some even say the entire planetary exploration space program!) was the observation of volcanic eruptions on Io, the innermost of the 4 Galilean satellites. Surprisingly enough, this discovery had been suggested before the encounter, but it wasn't until after the encounter that the discovery was made. On March 9, 1979, analysis of an image taken by the Voyager 1 probe showed an umbrella-shaped plume on the edge of the satellite. Io is the most volcanically active body known in the Solar System, and the sulphurous volcanoes are most likely -roduced by the tidal heating caused by Jupiter and Europa's gravitational attraction. The material ejected was found to be ionised and forming a plasma ring about Jupiter. It was also discovered that Io is connected to Jupiter through a "flux tube" which carries as much electrical power as Earth's generating capacity.  Europa was observed to be covered with a surface of ice at least 100 km thick. Tidal heating may maintain a liquid ocean of water beneath the icy crust. Ganymede has two types of crusts - old cratered terrain, and what appears to be grooved terrain that has modified the old cratered areas like terrestrial plate tectonics. Finally, Callisto is the most heavily cratered of the Galilean satellites, and indeed its apparently unmodified icy surface shows the planetary bombardment of 3.5 billion years ago. Next year, an indepth article will appear on the Jovian satellites.

The discovery of rings around Jupiter was an unexpected discovery from the Voyager missions (now, it is known that all the gaseous real planets, even Neptune, have at least one complete ring around them). Jupiter's rings were first detected by Voyager 1 17 hours before closest approach to Jupiter, and Voyager 2 was programmed to take better pictures. The ring particles seem to extend nearly all the way down to Jupiter itself. Maybe the ring particles are a source of oxygen that is related to the carbon monoxide that was unexpectedly found in the Jovian atmosphere. Jupiter's ring is orange in colour, and the ring particles have a radius of about 4 micrometers, as opposed to the Satumian ring particles of several centimetres in radius.  The newly discovered satellites, Metis and Thebe, seem to play an important role in defining the ring limits.

The Galileo Mission

As mentioned earlier, the Galileo space probe will be launched on October 12, 1989, aboard the Shuttle Atlantis.  Upon reaching orbit, Galileo and its attached inertial upper stage booster will be ejected from the payload, and an hour later, the booster will ignite to launch Galileo on its trajectory, which includes three planetary flybys before arriving at Jupiter.

The history of Galileo is quite incredible.  Plans for the Galileo mission were first considered even before the Voyager launches in 1977.  Although Galileo was designated a top priority planetary mission, the mission has been plagued with a number of mishaps.  At one stage, the mission was actually cancelled, but was subsequently reinstated.  The Galileo mission was originally planned for launch in January, 1982.  However, budget cuts saw the launch delayed till May 21, 1986 aboard Shuttle Atlantis.  Using a modified Centaur upper stage booster, the plan was for Galileo to pass within 20,000 km of the 200 km wide asteroid, Amphitrite (number 29), on December 7, 1986, thus becoming the first spacecraft to encounter an asteroid.  The Galileo spaceprobe would have reached Jupiter in December, 1988.  Needless to say, the tragic Challenger disaster of January, 1986 saw the launch delayed until now.  Also, following an indepth review of all aspects of the Shuttle program (the space transportation system: STS), it was decided that the Centaur booster was too dangerous to be placed in the Orbiter's payload, and hence the Centaur program was cancelled in June, 1986.  As such, the new Galileo mission has a weaker booster in the form of the IUS.  However, Galileo will use the gravity of Venus and Earth to give the velocity necessary to get to Jupiter.  As explained below, the flybys give Galileo a change in velocity that is 6 times greater than the 1.6 km per second velocity change which could be achieved using all of the probe's 932 kg of propellant. This complicated trajectory is known as VEEGA : Venus-Earth-Earth Gravity Assist.

In February, 1990, the Galileo probe will glide by Venus at a distance of 19,000 km.  This flyby will provide the first bonus of the new mission, in that Galileo will utilise many of its instruments in studying Venus and the surrounding environment. Indeed, it will be a preview to the arrival of Magellan in August of 1990. During this time, one of the things that Galileo will look for is lightning in the Venusian atmosphere.  However, because of Galileo's proximity to the Sun, the main antenna, 4.8 metres wide, will remain folded like an umbrella for protection.  In addition, to make the best use of the solar protection, Galileo's orientation will be such as to be pointed away from Earth.  As such, communication will be with a small omnidirectional antenna on one of Galileo's attached booms, and hence the data return will be limited.

A week after this encounter with Venus, Galileo will approach within 106 million km of the Sun. Travelling within the inner Solar System has posed problems for Galileo, as the probe was originally designed to operate out in the coldness of the outer Solar System.  At its closest approach to the Sun, Galileo will experience temperatures twice as strong as that found in Earth orbit. As such, some of Galileo's hardware has had to be revised. Once black, the probe now glistens with golden foil sun shields, which cover the scientific instruments, the booms for the small nuclear generators and parts of the main body.  Also, a grey collar protects the delicate gold mesh of the main antenna until it unfurls in the coldness of the outer Solar System.

After Galileo's close encounter with the Sun, the probe will make its second planetary flyby: Earth in December 1900.  Galileo will fly by at a distance of 3,600 km, and this will be used to increase the probe's speed by 5 km per second. However, rather than then heading out towards Jupiter, the probe will move inside Earth's orbit at 35 km per second in a very elliptical orbit before heading out beyond Mars.

Out in space beyond Mars, Galileo will be travelling within the realm of the asteroids.  An opportunity does exist for Galileo to encounter an asteroid, and indeed, on October 29, 1991, the probe will pass asteroid 951 Gaspra (15 km wide) at a distance of 1,000 km. Studying the asteroids is an exciting element of the mission.  Indeed, one scenario that has been suggested is that Mankind may explore the asteroids before Mars.

In December, 1992, Galileo will return to Earth, cruising past at an incredible distance of only 300 km!  One wonders as to whether we will be able to see the flyby in the night sky. I imagine that we will be able to.  However, I have yet to see the Space Shuttle fly overhead, so maybe I need some m-ore facts.  This flyby marks the end of Galileo's travels through the inner Solar System, and the velocity change will send Galileo out towards Jupiter.  On the way to Jupiter, the probe will be able to encounter asteroid 243 Ida (30 km wide) on August 21, 1993.  Both Gaspra and Ida are considered S-type objects.  These are a class of bodies common to the inner asteroid belt whose surfaces are somewhat reddish and moderately reflective.

Before describing what will happen upon arrival at Jupiter, now is an appropriate point to look at what the Galileo probe actually consists of. The numerous delays in the program have given designers ample time to improve many aspects of the probe.  Galileo, described by many as the most complex spaceprobe since the Viking missions, consists of two main elements : a planetary orbiter and an atmospheric probe.  The Galileo probe marks a landmark in spacecraft technology : it is the first dual-spin planetary spacecraft.  The "spun" section will rotate at 3 revolutions per minute, and will provide the scanning motion necessary for the five particle and field sensors to make measurements in all directions, like the Pioneer spacecraft.  The "despun" section, on the other hand, will provide the inertially stable platform for the camera and remote sensing instruments, like the Mariner, Viking and Voyager probes.

Communication between Galileo and Earth will be through a 4.8 metre wide antenna made of gold plated mesh.  Seeming as gold has a low melting point, the antenna will be protected until the probe is at least beyond Earth orbit.  Galileo is capable of transmitting data at a rate of 134,000 bits per second on the X-band frequency (8,415 MHz).  Transmission of data can also be done on the S-band (2,295 MHz).

Galileo's various electronic components are hardened against the strong Jovian radiation fields.  High energy particles can penetrate and change the contents of a memory cell, thus leading to errors and loss of data.  To get an understanding of the radiation levels involved, a chest X-ray is 0.045 rads (a rad is a measure of the amount of energy absorbed), and a lethal dose for humans is about 600 rads.  During the Pioneer 10 mission, the probe was subjected to 56,000 rads from protons, and 200,000 rads from electrons!  The hardened components have been protected to a level of 150,000 rads.  One is unfortunately led to believe that '2001" and "2010" were truly science fiction - Io is a very radioactive environment.

19 microprocessors aboard the probe control the various systems and scientific instruments.  All of Galileo's programs accept commands from Earth, and most are completely reprogrammable.  Needless to say, NASA procedures will ensure that Galileo will not meet the fate that the Phobos craft did.

Galileo's inventory of scientific instruments is quite impressive.  Included is an 800-line by 800-element CCD, which is over 100 times as sensitive as the TV tubes being used by Voyager 1 and 2. Indeed, resolution in the order of metres can be expected from some images.  Other instruments include :
 

In addition to this, radio equipment will make measurements of Jupiter's atrnosphere, as well as Jovian and solar gravitational fields, and extrasolar gravitational radiation!

Power for the Galileo orbiter comes from two radioisotope thermoelectric generators (RTG's), each mounted on separate 5 m long booms.  The RTG's produce 570 watts of total power at the start of the mission, but this will decrease throughout the mission.  Indeed, by the time that Galileo arrives at Jupiter in 1995, power levels will be down to 500 watts.

The atmospheric probe, which is located in the despun section, consists of two elements : a deceleration module and a descent module.  The deceleration module includes the heat shields that have to protect the scientific instruments during entry into the Jovian atmosphere, which will be the fastest atmospheric entry by any human craft ever!  During atmospheric entry, about 60% of the shield material will burn away.

The descent module, on the other hand, is the part that is supposed to survive entry.  It includes n atmospheric instrument to study the temperature, density, and pressure within the atmosphere; a neutral mass spectrometer for measuring the composition of the various gases; a helium-abundance interferometer to determine the ratio of hydrogen to helium; and various other instruments to study cloud particles, atmospheric lightning and energetic particles.

In June of 1995, the orbiter will line up the atmospheric probe and release it at Jupiter on a trajectory designed to intercept the Jovian atmosphere at an angle of between 7 and 10 degrees.  As is the case on Earth, a shallower entry path would cause the probe to bounce off the atmosphere (like a stone skipping over water), and a steeper angle would destroy the probe very quickly.  3 days after releasing the probe, the orbiter will fire its main engine to change trajectory so as to be positioned over the probe as it enters the Jovian atmosphere.

In November 1995, the Galileo spaceprobe, now in two parts, will finally reach Jupiter.  The orbiter will fly past Io at a distance of 1,000 km, nearly 20 times closer than the Voyager probes.  This will probably be Galleo's only encounter with Io, in that the radiation levels at this distance from Jupiter are very high, but it is necessary to slow the orbiter down into an orbit around Jupiter.  4 hours after this flyby, and 6 hours after being "woken" by a timer, the probe will hit the atmosphere.

The probe will enter the Jovin atmosphere at an altitude of 450km above the cloud deck, and just north of the equator, in order to avoid ring particles orbiting Jupiter in its equatorial plane.  Its entry speed will be a staggering 48 km per second (172,800 km/hr!).  Although the probe will enter the atmosphere in the same direction as the planet is rotating, heating will still be quite considerable: 8,300 K.  This will last for about twenty seconds.  About two minutes later, the descent module will descend through the atmosphere using the brakes of a Dacron parachute.

As the probe descends, the above orbiter will receive transmissions from the probe and directly relay them to Earth.  About an hour after entry, the probe will be below the cloud deck, and this combined with rising pressures and temperatures will lead to considerable losses in data.  The orbiter, however, is programmed to "listen" for another 15 minutes for any further information before continuing the rest of the mission.

After the probe mission, the orbiter will fire its engines to move into the first of 11 orbits around Jupiter.  The mentioned Io flyby will send the orbiter into a 230 day long elliptical orbit.  The next flyby will be the first with Ganymede, and this will reduce the orbital duration to 90 days.  With this flyby, perijove will be moved out beyond the danger of Jupiter's radiation.  After this, the next two definite flybys are Ganymede again, to remove any orbital inclination, and Callisto.  Over the next 22 months, the orbiter will carry out its primary orbital mission, making close flybys with Europa, Ganymede and Callisto.  Besides obtaining incredible informaion from these distances, the close encounters will enable the orbiter to change its orbit from orbit to orbit.  The actual orbital trajectories have yet to be determined : they will only be determined once Galileo leaves Earth orbit.  although only one flyby is scheduled for Io, the original flyby way back upon arrival, the orbiter may make additional flybys towards the end of the 22 months if the craft is in good occasion.

As mentioned above, the orbiter is scheduled to spend 22 months in orbit around Jupiter : November 1995 to September 1997.  It is important to realise that this mission is not just a follow-up to the Pioneer and Voyager missions.  They were just flybys - this is a true planetary mission to determine the nature of the jovian system.  The Galileo mission will truly provide a quantum leap in our knowledge and understanding of Jupiter, and indirectly, our Solar System.  indeed, the Galileo mission is destined to revitalise NASA's Solar System Exploration Program.  Already, a mission is being planned for Saturn that involves a probe entering Titan's atmosphere, and hopefully similar missions like this will be scheduled for Uranus and neptune in the decades to come.  Galileo will be the first mission in history to send a probe into the atmosphere of a planet of the outer Solar System, and it will also be the first mission to place a spacecraft in orbit around a planet of the outer Solar System.  One can only say that Galileo is truly a fitting memorial to Galileo Galilei, who documented the first observations of Jupiter's moons way back on January 7, 1610.

References

Astronomy : March 1986, January 1987, March 1988, January 1989
The Atlas Of The Solar System
National Geographic : January 1980
Sky and Telescope ; April 1987, February 1988
Universe : December 1987