Starting in May of 1996, the spacecraft's top data rate will be 160 bits per second. Even at that rate, it's not possible for the spacecraft to send pictures back in "real time" (i.e., no live imaging coverage, although some Fields and Particles data will be returned to Earth as soon as they are acquired). Instead, Galileo will return pictures on a "tape delay" system. During encounter periods (that is, during Jupiter and satellite flybys, and for several days before and after), and at other selected times during "cruise" (the periods between encounters), data will be collected from the instruments, and stored on the spacecraft's tape recorder for later processing and downlink to Earth.
What is the capacity of the tape recorder on Galileo?
There is a single tape recorder on board the spacecraft; it is a four-track digital model manufactured by Odetics Corporation that can store up to 914,489,344 bits of data (that's about 109 Megabytes, or about 300,000 pages of text; roughly as much storage as on the hard drive of the average new home computer).
Following the first in-flight system software update, Galileo's top data rate rose to 16 bits per second. This will be boosted following the final update to the system software in May of 1996 to 160 bits per second. This isn't to say that Galileo will always be communicating at such a high bit rate: the actual achieveable data rate depends on a variety of factors, including the Earth-spacecraft distance, how close Galileo and the Sun appear to be in the sky when seen from Earth, and the geometry between the spacecraft and the Deep Space Network antennas that are tracking Galileo. Most of Galileo's data will be sent to Earth using a downlink rate of 80 bits per second (the average value, over the entire two-year orbital mission, is closer to 50 bits per second, but this includes periods when the Sun is directly between Earth and Jupiter, and when the data rate consequently plummets).
Keep in mind that a great deal of the science data will be compressed before being sent down to Earth, boosting the effective data rate significantly.
Why does Galileo's speed change? Why doesn't it stay constant?
As of June 29, 1995, Galileo is travelling at roughly 25,900 kilometers per hour (16,000 miles per hour) relative to the Sun, but this speed isn't constant--the spacecraft is actually slowing down. Since the spacecraft is essentially in an elliptical orbit about the Sun, its speed naturally drops as it moves away from the Sun (if not for Jupiter's influence, it would speed up as it started moving back towards the Sun). You notice the same behavior when throwing a long, high fly ball--the ball slows down and "hangs" at the top of its path.
In addition, as Galileo draws very near to Jupiter, its speed is changed quite dramatically by the gravity of the giant planet. However, this effect won't be significant until a few days before arrival at Jupiter (on December 7, 1995). Since Jupiter lies "behind" Galileo prior to Galileo "meeting up" with the giant planet (if Galileo and Jupiter were on a racetrack, Galileo would have significant head start, though it's doomed to lose the race), Jupiter's gravitational influence will act to slow down the spacecraft relative to the Sun.
Galileo's sun-relative speed will continue dropping until the day that it arrives at Jupiter, at which point we will fire the main engine to move Galileo into orbit around Jupiter. At that point, the sun-relative speed will shoot way up.
Over the course of Galileo's 23-month orbital mission, the spacecraft speed will change with each satellite flyby (in fact, that's one of the reasons that we have these flybys).
Propellant, power (provided by the Radioisotope Thermal Generators), dollars and Jupiter's radiation environment are the most significant factors to define the mission's lifetime.
Energetic electrons and ions (both protons and heavy ions) trapped in Jupiter's radiation belts can cause interference and damage in elctronic parts in the Galileo Orbiter. In fact, if the spacecraft kept repeating its initial 200-day orbit, the spacecraft would "die" after several passes through the system from radiation effects (or might actually drop at its low point into the atmosphere due to gravitational perturbations of the Sun). The spacecraft contains enough radiation shielding to keep any part from failing during the baseline 2 year mission mission. After that, the risk of spacecraft components failing increases (somewhat like an auto manufacturer's warranty).
Galileo's budget covers two years of orbital operations. Without the support of the flight team--almost three hundred individuals that initiate Galileo's every action, continually monitor the spacecraft's health, and plan for the spacecraft's short and long-range future--the spacecraft's mission ends.
Galileo has only a limited amount of propellant available for spacecraft trajectory and attitude control during the satellite tour; without propellant, mission controllers can't send the spacecraft towards another satellite flyby, or keep Galileo's antenna pointing towards Earth. Effective communication from the Orbiter will then eventually (i.e. months later) be lost.
Galileo's navigation team currently estimates that there will be 20 kilograms of propellant left at the end of the "nominal" (i.e. baseline) mission, so propellant isn't currently the big worry; power is a little more worrisome. Total power output is directly related to the amount of heat generated by the plutonium in the RTGs. This in turn is a function of how old it is. The older the plutonium gets the less heat it puts out, so the less power we have.
At launch in October, 1989 the Galileo RTGs were producing about 570 watts of usable electric power. Right now (June, 1995) they're putting out 498 watts and, at the end of the mission in December 1997, we expect about 480 watts. If you think about it, we run our entire spacecraft on less than half the power of an average hair dryer!
There should be enough power to keep operating the spacecraft for years after the mission is officially over. But, as the available power declines there won't be enough power to run all of our scientific instruments. We would then have to make decisions on which ones to turn off. Ultimately, a time would come (probably within 10-15 years) when there wasn't enough power to keep the vital functions warm and the computers and transmitter running.