close by one of the four Galilean satellites either on the inward or outward legs. The orbital design also minimized the time that Galileo spent close to Jupiter, and thus within its highly energetic and damaging radiation belts. The orbital design, however, had a further, unforeseen, and wholly serendipitous advantage. The main communications high-gain antenna (HGA) of Galileo had a novel, lightweight, deployable design, rather like an umbrella. Unfortunately, when this was commanded to open soon after launch, it became stuck and could not subsequently be moved. This meant that Galileo had to communicate with Earth via its much smaller low-gain antenna, which at Jupiter's distance from Earth limited the communication speed to initially only 10 bits/s! It was feared that this would greatly reduce the amount of data that could be returned and thus the scientific value of the mission. However, since each orbit lasted approximately two months, and encounters with Jupiter and the satellites took only a few days, the spacecraft was comparatively idle for most of the time. Hence, data were recorded onto an onboard tape recorder during each encounter and then subsequently "trickled" back to Earth in the relatively inactive parts of the orbit at the low data rate. This solution, together with the use of sophisticated data compression techniques and improvements in the Deep Space Network receiving stations meant that the Galileo mission achieved scientifically almost everything it set out to achieve.
The Galileo orbiter operated for many years longer than originally planned. However, the spacecraft and its instruments slowly became more and more damaged by particle impacts from passing though Jupiter's radiation belts and the spacecraft slowly ran out of attitude control fuel. Hence, it was decided to terminate the mission by crashing the spacecraft into Jupiter's atmosphere where it burned up and vaporized in September 2003. The advantage of destroying Galileo in this way rather than leaving it in orbit about Jupiter was to avoid collision with the Galilean satellites and thus contamination of those worlds with any terrestrial organic matter that might have been on the spacecraft. This is of particular importance with respect to Europa, which is believed to have a substantial salty ocean beneath its icy crust and could thus, as some researchers suggest, have possibly evolved its own forms of life!
The Galileo orbiter had four remote-sensing instruments suitable for atmospheric study placed on its remote-sensing platform and a further instrument mounted on the spinning section, which will now be reviewed, together with the probe mission.
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