Mission Galileo

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GALILEO MISSION TO JUPITER
INTRODUCTION
The original target launch date for the Galileo mission was the spring of 1986. Due to the events of January 1986 and the Shuttle Challenger disaster, the Galileo mission did not begin until October 18, 1989. The launch trajectory was modified to use three planetary flybys in order to reduce fuel requirements in favor of payload: one past Venus and two past Earth, to gain enough energy for the trip to Jupiter, since the original date was scrapped. Along the way, Galileo provided some close-up photographs of asteroids (see Figure 1 below). On October 29, 1991, Galileo made the first fly-by of an asteroid, coming within 1,600 kilometers (1,000 miles) of Gaspra, first named by Grigoriy N. Neujamin in 1916. It is classified as an S-type asteroid, mostly likely made up of metal-rich silicates and perhaps even bits of pure metal. Its irregular shape suggests it is of recent origin, breaking off from a collision with a larger body (Hamilton, 2009).
Upon reaching Jupiter, it released the atmospheric probe, the main thrust of the mission. The probe lasted long enough to provide a great amount of data. For one, the atmospheric sample found the main constituents of hydrogen and helium to be in similar proportion as found in samples taken from the Sun. This data provides important clues as to the origin and makeup of other planetary bodies in our Solar System. An interesting observation was the scarcity of oxygen in the samples taken, indicating that Jupiter is drier than once postulated (Young, 1998).
The Orbiter remained around Jupiter more than fourteen years in contrast to its planned stay of two years, providing great amounts of data yet being researched. The mission far surpassed its desired goals, and provided information useful to future planetary missions.
EQUIPMENT DESCRIPTION
The Galileo orbiter was a first of its kind design utilizing a “dual-spin” technology: two separate satellite types combined into one unit. Previous missions, such as Pioneer, used a simple rotation of the entire craft to maintain balance and direction, and allow more data retrieval of particular fields of data. This was not useful for the remote sensing capabilities required by some of the Galileo mission parameters, as obtained using three-axis satellites like the Mariners and Vikings. The combined dual-spin design of the Galileo spacecraft provided both abilities, which were useful for the measurements taken during flybys and other times when focusing on particular elements (W.J. O'Neil, 1983).
Providing both types of data gathering in one satellite produced a challenging engineering design problem: how to transfer data and power between the two units. This problem was resolved by using two types of connecting rings, a slip ring for transfer of power between the two sections, and rotary transformers for the signal (data) transfer (W.J. O'Neil, p. 3).
The spacecraft consisted of a spun section and a de-spun section. The spinning section provided regular sampling of fields and particles; the de-spun section provided inertial stability and allowed a platform for remote sensing. Communications were originally planned using a furlable, high-gain antenna (HGA) for data transfer rates of 134 kb/s (kilobytes per second), similar to antennae used with tracking/data relay satellites—a proven system (W.J. O'Neil, pp. 2-3). However, this ‘proven’ system hit a snag upon launch. It was determined that stresses during the launch damaged one of the holding clips used to keep the antenna in an un-furled position while stowed, then upon release from the launch vehicle, allowed the antenna to open. The clip failed and left the antenna in its stowed position. It left engineers with only one alternative: utilizing the low gain antenna as their main communication link between mission control and Galileo (see Mission Overview below).
The Probe consisted of two units, a deceleration module that would position the second portion of the mission element of the Probe, the descent module. The descent module contained instrumentation to measure atmospheric pressures and density, helium abundance, and a net flux radiometer (to measure how deep solar energy and planetary emissions penetrate the Jovian atmosphere), among other instruments (W.J. O'Neil, p. 4).
MISSION OVERVIEW
The reason for an exploration of Jupiter draws from our understanding of the origin of the solar system (in 1975 from the National Academy of Sciences recommendations of in-depth studies of the universe) and that Jupiter, being the gaseous planet it is, could be representative of a primordial solar system. Since it retains what is considered a primordial composition, it is considered a better “cosmological ‘laboratory’” than the other planets (W.J. O'Neil, p. 1).
As the orbiter remained in a prolonged voyage around Jupiter proper, mission controllers used each Jovian satellite encounter to grab a gravity assist to provide it the energy to make the next Jovian satellite encounter. This process provided repeated visits to each of the four major moons of Callisto, Europa, Ganymede and Io but also provided the desired changes in the periods and orientation of orbits to allow the next satellite flyby. “This satellite-gravity-assist-tour is the foundation of the Orbiter mission design.” (W.J. O'Neil, p. 1)
IMPLICATIONS FOR FUTURE SPACE EXPLORATIONS
Information gathered from this most important mission to Jupiter continues to provide insight for missions to come. One important consideration for future missions was drawn from the demise of the Galileo orbiter. Unlike the Probe, whose planned mission was a plunge into the atmosphere whereupon it would disintegrate due to the intense pressure of the Jovian atmosphere, it was agreed to end Galileo in a similar fashion for another reason: evidence of water. Evidence of water was gleaned on one of the Jovian satellite flybys of the moon Europa. This was given as the reason to end Galileo’s mission with a purposeful, though un-planned, entry of the orbiter into Jupiter’s atmosphere in order to keep Europa from being contaminated, and maintain its purity for future exploration (E.E. Theilig, 2003).
As we delve deeper and deeper into our Solar System and into deep space, consideration for the well-being of other planets must remain a priority. It is similar to how we approach the Galapagos Islands here on earth, islands that are left undefiled and un-developed in order to preserve that ecosystem for future non-destructive exploration, and to leave it for future generations’ enjoyment.
Another valuable lesson taken from this historic mission is that of longevity of a spacecraft’s lifespan. The Galileo orbiter was designed for a much shorter mission than the fourteen years it remained productive, and even longer, if not for the consideration given Europa. Without purposefully designing for a fourteen year mission, the quality that was put into this particular craft allowed it to survive much longer than planned. This needs to remain a goal for all future spacecraft.
Another discovery by Galileo that provides important navigation information for future missions is the star Delta Velorum is actually a variable star, not a single entity. Delta Velorum is a grouping of five stars, of which one pair is binary. Deep space satellites identify navigable stars from their apparent brightness at any given time. Knowing if a certain star is variable (and changes in apparent brightness) is information that is highly useful for future missions, since changes in the brightness of a particular star could cause incorrect interpretation of navigable data for a satellite or spacecraft (E.E. Theilig, pp. 338-339).
Studies of Io’s volcanic activity indicated it was produced by conditions that are more similar to an early Earth. Original theories of the origin of Io’s volcanic activity from data gathered by the earlier Voyager mission were that it was produced by melted sulfur. Distant observations showed temperatures too high for this theory. The close observations of Galileo proved that Io’s core was silicate based, and heated by the gravitational tides of Jupiter (E.E. Theilig, p. 40). Observations like this could provide for go, no-go decisions as to which planet to approach for exploration or which one to avoid.
In all, the amount and type of data gathered by Galileo in its long and illustrious travels will continue to provide guidance for future exploratory missions, and the program was a major building block of our understanding of the Solar System and of space. Each deep-space mission builds on the next in a block by block fashion, giving a leg up on the next successive mission as we reach out beyond our own planet.
OVERCOMING PROBLEMS
One of the procedures used to overcome the effects of intransigent equipment and the harshness of the space environment was using the low gain antenna (LGA) for data transmission when the main antenna failed to perform to its design. It was eventually determined that either one or both of the rib restraint pins that kept the antenna folded during launch remained in their receptacles, preventing the antenna from being unfurled (Slatman, 1994). Although it was the designed method to be used for the mission, reverting to and then using the LGA required other adjustments as well: on board buffering of data prior to transmission and extensive processing, which in itself stood to overload onboard equipment. This required having an Earth-bound replica of the orbiter to perfect the processes used to successfully retrieve data from Galileo (Bruce A. McLaughlin, 1994).
“To allow extended high rate data acquisition without overflowing the multi-use buffer the Buffer Dump to Tape function has been implemented.” (Bruce A. McLaughlin, p. 1042) This was a method devised whereby the onboard Data Memory System (DMS - a type of tape recorder) temporarily stored the data that overflowed the buffer, since the speed of the downlink was sorely restricted using the LGA. It required buffer management by predicting rates of data acquisition and scheduling the buffer to dump to the DMS so that any overflow could be prevented, which could freeze the system and prevent downloading the data to mission control.
This type of foresight and engineering manipulation is something to be emulated on future missions when unforeseen events occur. This type of scenario is highly likely as space remains quite unpredictable, and is precisely why lessons learned from the Galileo mission are so valuable as we continue explorations of the Solar System.
Exposure to accumulated radiation in the further reaches of the Solar System at rates four times anticipated, far above design standards, degraded the performance of some of the subsystems. The Attitude and Articulation Control System’s (AACS) gyros, used to control the movements of the satellite and its orbital changes, suffered from this radiation, reaching degradation of nearly 100% after one particular pass by the moon Io (E.E. Theilig, 2003).
Levels of radiation beyond what we encounter close to Earth are a major concern for spacecraft, and methods of dealing with, and protecting from these levels must continue to be researched and incorporated into other missions. This will be especially true for spacecraft control systems that are quite delicate here on Earth, where our atmosphere provides a protective cocoon, and therefore must be shielded with anticipation of a very hostile environment that is completely inhospitable.
Command and Data and Data Memory Subsystems also suffered from the exposure to high levels of radiation. “Transient bus resets, presumably induced by radiation exposure…plagued the spacecraft [starting] in 1991.” (E.E. Theilig, p. 331) Cosmic particles can pass through a memory bus, impacting the electronic switches, causing them to either stop working or produce erroneous data (a “1” for a “0”, and vice versa). Software patches were written and uploaded many times to the orbiter to bypass these problems as they arose. A bus error occurs in a computer (or computer system) when its processor attempts to access memory and the location it is trying to access doesn’t exist (the system doesn’t recognize the memory location, a software error), or it doesn’t respond as designed (a hardware problem) (Unkown, 2010).
The NASA team alleviated most occurrences of bus resets by up-linking software patches that set specific memory bits to recognize certain errors as being spurious (E.E. Theilig, p. 331). Although not 100% successful, this bit of software ‘sleight-of-hand’ was able to resolve several instances where, had it not been for the patch, raw data would have been lost and commands to the spacecraft would not have been recognized at the correct times.
A glitch with Galileo that may have faded into history had digital recording media been available at that time, was the recording mechanism used to gather data and replay it at a rate to match the reduced downlink data transfer speed; essentially a magnetic tape recorder, albeit advanced enough to withstand the rigors of space. Due to radiation exposure and outliving its intended lifespan, the tape mechanism would get stuck from time to time, more so later in its lifespan. It was determined the tape was sticking at a tape guide, so a software patch was uploaded that anticipated this sticking, and allowed the moment of sticking to be bypassed. All digital recording devices used today have replaced this older technology.
Better protection from cosmic rays and higher levels of radiation will be required for future missions to shield spacecraft subsystems and allow them to fulfill their missions. A solution the Galileo team utilized to provide more protection was to place certain systems into hibernation when approaching a known high-radiation sector of space (E.E. Theilig, p. 337). Of course, this area was known since a previous pass through it had exposed the orbiter to elevated levels of radiation, providing evidence to ground controllers of its existence. In deeper space probes, this solution may not be viable if higher-radiation level spots are unknown, but it is well to keep it as an available option. It may be that deep-space probes will need planned periods of hibernation in order to bypass those known high-radiation areas, or simply to pass through portions of space prior to reaching a mission objective and to increase their longevity until the objective has been reached.
CONCLUSION
The Galileo mission was a resounding success since its original mission span was far exceeded, a testament to well-planned design and mission management. Lessons learned from overcoming the obstacles Galileo faced can be applied to tomorrow’s explorations, a building block upon which future blocks can be placed and draw direction from. As we send more and more unmanned spacecraft to explore our vast universe, lessons carried over from one mission to the next will improve success rates and mission achievements for years to come, lessons that will also be applied to manned explorations.


 
REFERENCES
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W.J. O'Neil, R. M. (1983). Galileo Mission Overview. Reno NV: AIAA.
Young, R. E. (1998). Galileo Mission (Jupiter). Access Science.