December 4th, 1995. Copyright, 1995, Graphic News. All rights reserved

GALILEO: DESTINATION JUPITER
By Nicholas Booth, Science Editor

LONDON, December 4th, Graphic News Ð Engineers at NASAÕs Jet Propulsion Laboratory (JPL) in Pasadena, California are calling it ÔThe Longest DayÕ. For this Thursday, December 7th, they will command their Galileo spacecraft to perform a series of delicate manoeuvres so it will become the first man-made object to enter orbit around Jupiter. And because there are so many commands which have to be executed in the space of a few hours, it promises to be the most anxious time in a distinctly ill-starred project which began twenty years ago.
     Galileo suffered repeated delays and was finally launched in 1989 since when it has taken a long, involved journey to reach the largest planet in our Solar System. En route its main radio dish jammed before it was fully opened and thereby reduced the amount of data which can be transmitted to Earth. Once in orbit around Jupiter, Galileo will return only 4,000 images over the next two years as opposed to the 50,000 originally intended. Engineers at JPL have worked around this problem by making greater use of the spacecraftÕs onboard tape recorder. Instead of transmitting data directly back to Earth, it will first be recorded and then played back over a period of hours. It is akin to speaking slower to enunciate more clearly. 
     In October, the tape recorder jammed unexpectedly, but JPL engineers were able to unjam it and now believe that they have solved the problem. They will, however, ensure that the reel-to-reel device is not overtaxed, particularly on Thursday when much has to be done in the space of a few hours. Galileo will enter orbit around Jupiter by making close passes of two of its largest moons, Europa and Io, whose gravity will catapult the spacecraft into orbit without need of a large rocket motor. The craft will also pass through the planetÕs dangerous radiation belts and then have to quickly re-orientate itself to record data from a separate atmospheric probe which was released from the main spacecraft in July. 
     The timing of their separate arrivals is crucial. The probe will enter the dense atmosphere of Jupiter on Thursday afternoon, and in just two minutes, decelerate from  30.5 miles per second (49 kilometres per second) to 0.3 miles per second (0.4 kilometres per second). It will then return 75 minutes worth of data about the jovian atmosphere before it is crushed by the immense pressure. Because it has only limited battery power, the probe has not been in communication with the main craft since release. Mission officials at JPL will command the main spacecraft to look towards the region of Jupiter where the probe is scheduled to enter and lock onto the signal. 
     The probe data will be recorded and then played back to Earth over a period of months to give us our first ever detailed information on the mysterious atmosphere of Jupiter. 1,300 Earths could be fitted into the volume the planet occupies and its surface is characterised by swirling cloud patterns whose chemical composition is not well understood at present. 

Source: Galileo Project Office, Jet Propulsion Laboratory.