Imaging data will make up less than 25% of all of the "bits" of scientific data returned ("downlinked") to Earth during the orbital tour. The remainder of the downlink budget will be used to return data from other scientific instrumentation, such as infrared, ultraviolet, and fields and particles observations.
It can be argued that more has been learned about the solar system from images than from any other type of instrumentation. Most astronomical discoveries through the ages have been made through either visual observations or using cameras attached to powerful telescopes. The same is true for solar system exploration using robotic spacecraft. That is why virtually every planetary exploration mission has included a camera in its payload. The value of images for public information is a wonderful bonus, but if it were not for the scientific utility of these cameras, few would be flown.
A simple review of some of the major discoveries and scientific studies that have been made possible through spacecraft images illustrates the valuable contributions pictures have made to our understanding of the solar system.
So why aren't we sending back even more pictures, since they add so much to scientist's understanding of the solar system? As the question noted, it's expensive to send back pictures, compared to fields and particles data. However, it is the mix of both types of data that will yield the most complete and scientifically interesting picture of the jovian system.
Consider some of the goals of the imaging team: long-term studies of Io's active volcanoes and specific features in Jupiter's atmosphere, for starters. Both of these phenomena show changes from day to day, but the quick Voyager flybys didn't allow scientists to see how these changes evolved. We need pictures in order to, for example, map out the surface of the Galilean satellites (Jupiter's four largest moons), or to detect ring particles.
Pictures are also used to support other types of scientific observations. For example, the atmospheric Probe will send back data on the temperature, pressure, and composition of the atmosphere, but it will be images from the Orbiter that will help scientists to put the Probe observations into context. In another example, the fields and particles instruments on board the orbiter will map and characterize the distribution of magnetic fields, plasma, and particles, but pictures of Jupiter's auroral phenomena, which are intimitely connected with fields and particles, will add additional insight into the magnetic field's interaction with Jupiter's atmosphere.
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.
The maximum size of an image is roughly 5 megabits, or about 640 kilobytes (800x800 pixels x 8 bits/pixel); you could fit one or two full-size images on a floppy disk. Not all images are returned at this full size; a 2x2-pixel summation mode, for example, which uses about 1/4th the space of a full size image, is used to return many images.
Without the HGA, how long does it take to return a single image?
At maximum possible downlink rate, with no compression or editing being used, it will take just under 9 hours to return a single full-size image. All images will be compressed or edited by at least a factor of 2 and will be returned in far less time (typically 1 to 2 hours).
An average of 2-3 images per day will be returned starting in late June of 1996.
How much data is really lost when "lossy" compression is used?
Lossy compression is to be used on imaging and plasma wave data. Let's focus on imaging data for now. Each picture is made up of an array of up to 800x800 picture elements, or pixels. Each pixel has 256 possible values, depending on the brightness of that part of the image. The lossy compression takes a group of these pixels and approximates their brightness values with a formula. On the ground this formula is reconverted into a set of data which is supposed to be close to the original data set. The fidelity of the reconstructed image depends on the selected compression factor, which will range from 3:1 to 80:1. Tests of the compression routine on Galileo and Voyager images demonstrated that pixel values were correctly reconstructed to within about one percent, which is considered excellent for most investigations.
When will Galileo surpass Hubble's imaging capabilities?
The Hubble Wide Field/Planetary Camera has two imaging modes: either Wide Field or Planetary. The imaging systems on board Galileo have a resolution capability twenty times less than the Hubble Wide Field setting, and 45 times less than the Planetary.
On average, Jupiter is located 5 astronomical units (one astronomical unit = the average distance from the Sun to the Earth) away from the Earth. At that distance, Galileo passed up Hubble's Wide Field resolution of Jupiter on October 6, 1995, and passed up the Planetary on November 13, 1995. If you want to consider the best possible resolution that Hubble is capable of (namely, when Jupiter is closest to Earth--a mere four astronomical units away), Galileo passed the Wide Field performance on October 21, 1995, and the Planetary on November 18, 1995.
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