It was 1610 when Galileo Galilei observed four satellites orbiting Jupiter [1]. This radical discovery challenged the early consensus that all celestial objects orbit the earth and changed our understanding of our solar system forever [1]. Since then, many intriguing findings contributed to the motivation for the Galileo mission, including the massive size and energy demonstrated by Jupiter [1]. Its huge gravity significantly affects its satellites and it emits more thermal energy than it absorbs from the sun [1]. Jupiter’s magnetosphere is the largest in the solar system, extending further than the size of the sun and its corona [1]. Additionally, the predominantly helium and hydrogen composition of Jupiter is suspected to be largely unchanged since its formation [1]. Closer observation of Jupiter was therefore considered useful in elucidating planetary processes comparable to our early solar system and how planets evolve [1]. The Galileo mission was designed to send an atmospheric probe and orbiter spacecraft to Jupiter for a two-year observation of Jupiter, its satellites and its magnetosphere [7].
Mission Mechanics
The Galileo Spacecraft
The Galileo spacecraft contained an orbiter with ten scientific instruments and an atmospheric probe containing a radio and six scientific instruments for deployment into Jupiter’s atmosphere [2]. The two-tonne spacecraft and its Inertial Upper Stage rocket were launched in October 1989 by the Space Shuttle Atlantis [1].

Figure 1. Diagram of the Galileo spacecraft and its instruments [6]. Labels in blue and red indicate the locations of scientific instruments. Labels in green indicate the locations of hardware required for movement, protection, navigation and data transmission.
Journey to Jupiter

Figure 2. Diagram of the trajectory of Galileo’s multiple gravity assist and orbit around Jupiter [4]. Blue lines indicate the planetary orbits. The orange line indicates the path travelled by Galileo. Some of the major events of the journey are highlighted, including the two asteroid encounters and the Shoemaker-Levy 9 collision.
The Venus-Earth-Earth Gravity Assist (figure 2) provided the momentum required for the spacecraft to reach its destination [2]. Unfortunately, the high-gain antenna located between the two sun-shields (figure 1) failed to open correctly following the first Earth flyby in April 1991 [1]. This antenna was intended to transmit data at about 134,000 bits per second [3]. After several unsuccessful attempts to engage the essential umbrella-like instrument for use, a set of alternative solutions were formulated [3]. The smaller low-gain antenna on the spacecraft, capable of sending a mere ten bits per second, was deployed [3]. To compensate for the weak transmission and static interference, data was sent by the antenna at a minimal rate [3]. Additionally, a number of tracking antennas in Australia, USA and Spain were used simultaneously to form arrays that were able to assemble overlapping signals [1, 3]. The limited transfer capacity resulted in a significant decrease in the number of images that could be sent back to Earth [1]. Nonetheless, much of the scientific data was successfully collected with only a few images [1]. To further improve the signal strength, a new set of software was sent by radio in March 1996 to assist in compressing future data into lossless packet files before transmission [1]. These modifications eventually increased the maximum transmission rate to 160 bits per second [3].

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During its transit to Jupiter, Galileo made some interesting observations. Its flyby past Gaspra in 1991 was the closest of its kind [2]. Observations in 1993 of another asteroid named Ida also achieved a space exploration first, with the discovery of its moon-like object Dactyl [2]. Then in July 1994 Galileo had the only direct view of the Shoemaker-Levy 9 comet fragments collision into the surface of Jupiter [5]. One year later, Galileo initiated its primary missions and in December 1995 it received data from the atmospheric probe [1]. The probe travelled 150 kilometres through a hotspot, discovering a much drier atmosphere than expected and wind speeds of 720km/hour [2]. During its 58 minute descent, the recorded temperatures increased from -145°C to 153°C and the pressure rose to 22 bars, after which point the probe ceased transmission due to its destruction by the conditions [1]. Immediately after receiving data from the probe, Galileo’s trajectory was adjusted for its entry into Jupiter’s orbit [1].
Three Missions
During the primary mission, the Galileo spacecraft performed a series of eleven orbits around Jupiter with the aim of observing the giant Jovian planet and its four largest satellites – Io, Europa, Ganymede and Callisto [2]. The three sets of data collected from the orbits were of the atmosphere, magnetosphere and satellites [1]. The mission raised several new questions about Jupiter and its satellites [2].

Figure 3. Diagram of the Galileo spacecraft orbits during its primary mission [7]. Each orbit involved a close encounter period with one satellite and a cruise period of a few months [1]. Orbits are labelled with the first letter of the satellite Galileo encountered most closely [2].
Due to its maintained strength and plethora of backup instruments, a second two-year mission known as the Galileo Europa Mission was initiated in 1997 [2]. This mission performed fourteen extra orbits, focusing on the ice of Europa, the thunderstorms of Jupiter and the volcanoes of Io [2]. The flybys past Jupiter’s inmost satellite, Io were scheduled for the end of the second mission due to the assumption that the increased radiation exposure would damage the orbiter [2]. The success of this second mission and the survival of Galileo prompted a third mission [2].
The Galileo Millennium mission performed extra measurements on Europa and Io, analysed Ganymede’s magnetosphere and performed a joint observation of Jupiter with the Cassini spacecraft in 2000 [1, 2]. In 2003, the spacecraft had finally run low on propellant and many of its components had received detrimental amounts of radiation [8, 1]. The decision to crash it into Jupiter on its 35th orbit served to protect Europa from possible accidental contamination by the spacecraft [8]. Through Galileo’s exploration of the Jovian system, it was discovered that Europa may contain liquid water below its icy surface [9], and could therefore harbour extra-terrestrial life [8].
Major Findings
Europa
(Ocean)
Jupiter
(Storms and rings)
Io
(Volcanoes)
Ganymede
(Magnetic field)
(Ocean)
Callisto
(Ocean)
Beyond Galileo
Return to our understanding (conclusion?)

Include the simultaneous ground observations
Later modelling using Galileo data
Later missions with what we learnt from Galileo

References

Meltzer, M 2007, Mission to Jupiter: A History of the Galileo Project, NASA History Division, Washington DC.

NASA 2010, Solar System Exploration Galileo Legacy Site, accessed 16 May 2014, http://solarsystem.nasa.gov/galileo/index.cfm>

Sarkissian, J 1997, The Parkes Galileo Tracks, Parkes Radio Observatory, accessed 16 May 2014, https://www.cs.tcd.ie/Stephen.Farrell/ipn/background/five-antennae-for-galileo.html>

Harland, D 2013, Galileo (spacecraft), Encyclopaedia Britannica, accessed 16 May 2014,http://www.britannica.com/EBchecked/topic/224112/Galileo>

Hamilton, C 1996, Shoemaker-Levy 9/Jupiter Impact, View of the Solar System, accessed 16 May 2014, http://astro.if.ufrgs.br/solar/impact.htm>

NASA 2010, The Spacecraft, Solar System Exploration Galileo Legacy Site, accessed 16 May 2014, http://solarsystem.nasa.gov/galileo/images/mission-space-intro-enlarged.gif>

Brandt, N 2014, Astro 1: `Slides’ for Class 39 – The Jovian Planets and Pluto, Department of Astronomy & Astrophysics Pennsylvania State University, accessed 17 May 2014, http://www2.astro.psu.edu/users/niel/astro1/slideshows/class39/slides-39.html>

Mullen, L 2013, The End of Galileo, Astrobiology Magazine, accessed 17 May 2014, http://www.astrobio.net/news-exclusive/the-end-of-galileo/>

Wolf, P 2007, The Galileo Spacecraft, Laboratory for Atmospheric and Space Physics University of Colorado, accessed 17 May 2014, http://lasp.colorado.edu/education/outerplanets/missions_galileo.php#galileo>