QUESTION: What is the most technologically advanced capability of the Galileo spacecraft? ANSWER from Dr. Richard B. Pomphrey writes on December 22, 1995: Thank you for your interest and your question. You ask a very important and yet very difficult question to answer!! There are many technologically advanced capabilities of the Galileo Spacecraft, and thus there are many opinions and many views as to "what ... is the most technologically advanced capability of the Galileo Spacecraft?". This is due to the fact that the spacecraft is the most sophisticated and complex system that JPL has ever flown. It involves many technological advances over our previous experience, and therefore is very technologically advanced for deep space exploration. However, please keep in mind that the Galileo spacecraft was designed and built in the late 1970's. Technology advances so rapidly that the much of the hardware technology used on the Galileo spacecraft is old relative to applications here on earth. At the same time, much of the software used to compress and code the image data is state of the art today. For all these reasons, it is difficult to identify any one piece of spacecraft hardware or any one spacecraft capability as the most technologically advanced. There are many opinions, and I will simply pass on to you some of those I think most important. Thanks again for your interest. Please feel free to ask questions about what we're sending you, or any other questions which are of interest to you. 1. AACS Software: The Attitude and Articulation Control Subsystem (AACS) is responsible for setting and maintaining the attitude (or orientation) of the spacecraft and for pointing the imaging instruments. Since the computers on Galileo were designed and built in the late 70s, the computers and computer software is not very big or powerful by 90's standards. As an example, the main computer of the Attitude and Articulation Control Subsystem's (AACS) has only 32 kilobytes (thousand bytes) of random access memory (RAM) on each of two redundant memories, while an average home computer today has 4 to 8 megabytes (MILLION bytes) of memory. However, software on board the Galileo spacecraft is very advanced, very compact, and highly efficient. Thus the AACS computer but is able to do all the complicated math to point the cameras accurately to about .057 degree! (When you look at the full moon at night, it covers approximately 0.5 degree in the sky. Thus the AACS can point accurately to 1/10 of that area). In addition, using very sophisticated AACS software, the Galileo spacecraft is the first deep-space craft which can accurately and autonomously determine its absolute attitude from any orientation. This means that no matter how the Galileo spacecraft is oriented in space, the spacecraft can very accurately figure out how it is oriented (or where it is pointing) relative to the stars, and take action to change where it is pointing. It can do all of this with NO help from the earth, and at ANY orientation of the spacecraft. In the past, spacecraft could only do this for a very limited part of the sky, and had to have ground data processing. 2. Date Dependent (Image) Compression Software: The Galileo spacecraft was designed to transmit data back to earth through its High Gain Antenna (HGA) at up to 134,000 bits per second. However, the HGA failed to open, and has forced the mission to operate through the Low Gain Antenna (LGA) at a small fraction of its intended data rate. Without enhancements, the Low Gain Antenna transmits data at 8 to 16 bits per second. Software techniques on the spacecraft and innovative hardware and software adaptations at earth-based receiving stations will increase the data rate from Jupiter by as much as 10 times, to 160 bits per second. One of the very advanced on-board computer techniques will automatically compress an image to a small fraction of its raw size, while still retaining the scientific content required by the Imaging Science Team. Moreover, how much the image is compressed is a function of the image. That is, the less contrast there is in an image, the more the image can be compressed, and vice-versa. For instance, in this way, the dark background of space can be eliminated from the data without losing scientific data. Further information on subject 2: Advanced Technology on Galileo One of the most advanced technologies on the Galileo Spacecraft is the image and data compression software algorithms (or formulas) which are used to compress the data from the spacecraft. The failure of the Galileo high gain antenna means that the number of bits of data that can be transmitted from the spacecraft to the Earth is limited. So, to get the most information from the limited number of bits, data compression is used. Two types of data compression are employed on Galileo, lossless compression and lossy compression. With lossless compression, no real information is lost; whereas with lossy compression, some information is lost. Lossless data compression is the same as the data compression that is done on a computer's hard disk. In this type of compression, the data is put through an algorithm and it removes redundancy in the data. To recover the original data, the compressed data is run through a decompression algorithm and the original data is reconstructed, exactly like it was in the beginning. Data compression lowers the number of bits that need to be transmitted. Compression ratios are approximately 2 to 1, so we have to transmit only 1/2 of the number of bits. On Galileo, we use a lossless compression algorithm invented by a scientist named Robert Rice, and it is known as the Rice compression algorithm. For transmitting pictures, a different type of compression is used. With pictures, losing a little bit of the data is okay, because the image doesn't get fuzzy until you lose a good portion of the information. Pictures from the Galileo camera are made up of individual dots, known as pixels. The Galileo camera takes pictures that are 800 pixels high and 800 pixels wide. That means that every picture from the camera contains 640,000 pixels. Since each pixel is represented by 8 bits, to transmit one picture from the spacecraft to the ground we would have to transmit 640,000 bytes of data, which would take almost 8 days for each picture! To cut down on the number of bits in each picture, we use a process called lossy compression. For example, if every other pixel in a picture is thrown away it would result in a lossy compression of 2 to 1. We will still see the image almost the same as before, only it will be slightly fuzzier. You can do this over and over again, keeping only every third pixel, or every fourth pixel, and as you reduce the number of pixels, the picture gets slightly more fuzzy each time. This is a very simple lossy compression algorithm. We use a more sophisticated algorithm known as Integer Cosine Transform (ICT) compression. With ICT compression, we can compress the number of bits that need to be transmitted to the ground by up to 80 to 1. Of course, a picture compressed that much would be pretty fuzzy, but would be useful for scientific investigation. We will pick the most appropriate compression ratio for each picture before we transmit it from the spacecraft to the ground. This way we get the best pictures for the least amount of bits transmitted from the spacecraft to the ground. In summary, one of the most advanced technologies on the Galileo spacecraft is the data compression algorithms in the spacecraft software. While the hardware is exciting, the spacecraft was built over ten years ago. The software however, has the latest advances in data compression, some of which were only invented a couple of years ago! 3. Radiation Hardening JPL is the main organization (in the world!) which designs and operates vehicles for deep space exploration. Our deep space hardware is the most advanced and usually only hardware which is designed to be operated remotely in the harsh environment of deep space. For instance, the entire Galileo spacecraft is "radiation hard", which means that it is built especially to withstand the harsh radiation environment near Jupiter. This means that all of Galileo's components (especially electronics) are specially designed and built to withstand high levels of radiation. This is very sophisticated technology and unique in the spacecraft field. 4. Spin-Bearing Assembly Galileo is a spin-stabilized spacecraft. As it spins about its axis of rotation, the spacecraft maintains a stable orientation in space, and is prevented from wobbling or tumbling (which a non-stabilized object in space will begin to do, due to gravitational effects on the body). However while the spacecraft spins, it must also point the cameras and remote sensing instruments. This is accomplished by NOT spinning the part of the spacecraft that holds the cameras. This is made possible by a Spin-Bearing Assembly (SBA). The SBA is one of the most complicated pieces of hardware on the spacecraft. It connects the top 90% of Galileo which always spins, with the bottom 10% which can be "de-spun" to point the cameras mounted to the bottom part of Galileo Orbiter spacecraft. The SBA is actually three separate devices that are assembled into a long cylinder the size of a couple of coffee cans placed end over end. It is 1. a complicated, very precise, redundant electric motor with optical encoders (sophisticated devices used to define the relative positions of the spun to the de-spun part of the spacecraft). 2 a bus or pathway through which both electrical power and computer information can be transferred between the spinning and the de-spun parts of the spacecraft. 3. fuel and oxidizer lines which carry fuel and oxidizer from tanks on the spinning side of the spacecraft to the main engine which is located on the despun portion of the spacecraft. 5. Heavy Ion Counter The HIC (Heavy Ion Counter) is a science instrument which is mounted on the Galileo Orbiter. It became a part of the spacecraft AFTER the initial launch window for Galileo (April, 1986) was missed due to the Challenger accident. Thus this instrument has much more recent hardware technology than other parts of the spacecraft. One of the hazards of deep space turns out to be the presence of fast moving charged particles (cosmic rays). Most of these are protons, which are very light. But some are nuclei of atoms which are composed of many protons. Thus these nuclei are much heavier, and they can damage on-board computers. As an example, just one of these particles can cause a command stored in a computer to change to some other command. This could be fatal to a spacecraft. The scientists supporting Galileo wanted to know just how many of these dangerous particles there were in the vicinity of Jupiter. But no such experiment was originally included on the craft. Some of us suggested putting an extra board of computer chips on the craft, and then measuring how many of the bits on these chips were changed by the heavy particles. That would have been useful, but it would not tell us the energy of the impacting particles which is a critical piece of scientific information. JPL's Director, Dr. Edward Stone, had been on the Voyager Science team that investigated these cosmic rays using a very sophisticated device. He encouraged NASA to install a similar, but more sophisticated experiment on Galileo. The new experiment is able to detect cosmic rays that are more than ten times as energetic as those measurable by even the Voyager experiment. This is done by using a clever new version of Voyager's device. The device is like a telescope. It lets in cosmic rays much as a telescope lets in light. The particles deposit their energy in a series of plates at the bottom of the scope. The technology advances consisted of modifying the geometry of the "telescope" and of using improved (and thicker) plates.