Galileo Engineering

Galileo is 20 years old!!
 
It's hard to believe but they started building the Galileo Orbiter that has been orbiting Jupiter for the past three years way back in 1977. And at that they were using spare and leftover parts from the earlier Voyager missions. The computer in Galileo is an absolute midget compared to what people have on their desktops these days - the microprocessors (there are two) are RCA Cosmac 1802, the first low-power CMOS processor chip, quite on a par with the old 8-bit 6502 that was being built into the Apple II desktop at that time.

The 2,223-kilogram (2-1/2-ton) Galileo orbiter spacecraft carries 10 scientific instruments; there are another six on the 339-kilogram (746-pound) probe. The spacecraft radio link to Earth and the probe-to-orbiter radio link have served additionally as instruments for scientific investigations. The 2,223-kilogram (2-1/2-ton) Galileo orbiter spacecraft carries 10 scientific instruments; there are another six on the
339-kilogram (746-pound) probe. The spacecraft radio link to Earth and the probe-to-orbiter radio link serve as instruments


Orbiter Fact Sheet
 
At launch, the orbiter weighed 2223 kilograms, including 118 kilograms of science instruments and 925 kilograms of usable rocket propellant. The overall length from the top of the low-gain antenna to the bottom of the probe measured 5.3 meters; the magnetometer boom extends 11 meters from the center of the spacecraft.


Why the digital camera "owes" Galileo
 
The CCD integrated circuit was developed especially for the Galileo mission.


Solid State Images
 
How big is an SSI image?

The maximum size of an image is roughly 5 megabits, or about 640 kilobytes (800x800pixels 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 mode, for example, which uses about 1/4th the space of a full size image, is used to return many images.


Overcoming the Loss of the High Gain Antenna
 
The High Gain antenna works by the same principle as the satellite dishes used nowadays to beam TV and other broadcast signals up to the communications satellites in Earth Geo-Synchronous orbits. For Galileo, it was folded up like an umbrella for launch and so that it would protected behind a round shield from hard solar radiation as the spacecraft spun in toward Venus for its first gravity assist flyby. After the second and final Earth flyby, mission control sent the signal to un-furl but three pins had gotten stuck and the antenna was jammed. After numerous unsucessful attempts to open the umbrella, attention was turned to the Low Gain antenna.

Transmitting scientific data over the low-gain antenna involved significantly lower data rates: from over 134,000 bits per second to 10! This 10bps rate was ultimately raised into the 200bps neighborhood by use of on-board data processing and data compression, and by implementing various enhancements to the communications link performance, including new encoding systems and advanced technology on the ground.

By going over to more powerful receiver equipment Earth-side (namely the larger dishes available to the Deep Space Monitoring Network), by optimizing data compression techniques in the on-board computers, and by using the Tape Recorder for backup, seventy percent of the scientific objectives have been fulfilled. The sheer quantity of e.g. photographic images has suffererd but not the quality of observations able to be performed.

Overcoming this problem surely stands out as the one of the gutsiest troubleshoots in the history of engineering. We can compare it to the kinds of we-can-do-it problem solving that characterized the Apollo 13 mission.

Galileo's orbital science results will be transmitted to Earth over the low-gain antenna at significantly lower data rates than originally planned, because of the in-flight failure of the high-gain antenna to deploy as commanded in April 1991. The Project team has developed means to transmit the key scientific data and to accomplish the Project's Jupiter science objectives, using on-board data processing and compression, and various enhancements to the communications link performance, including new encoding systems and advanced technology in ground equipment.


Data Compression: Zip 'em up

 
Why weren't the new data compression methods originally planned to be used with the High Gain Antenna to get even more data back?

Galileo's computers are not fast enough to compress data at the rate which was to be transmitted through the HGA! It is important to remember that at the time Galileo was being developed, spacecraft designers were using fairly state-of-the-art computers, but those computers are extremely slow by contemporary standards. With the advent of increasingly faster and more densely packed microprocessors, such computationally intensive processes are becoming practical for spacecraft to perform and are being incorporated into future spacecraft designs.


Computing in 8 Bits
 
How fast are Galileo's computers compared to a 80486 or other home computer? (9/4/96)

As you might expect, the 32-bit 486 processor is much more powerful than Galileo's 8-bit processors. A commercial equivalent to Galileo's processors would be the 6502 processors that were used in the Apple II computers in the 1970's. They are both 8-bit processors.

The 1802 processors run at a clock speed of ~1.6 mHz, whereas a 486 will run at up to 66 mHz (typically). This would indicate that the 486 is approximately 41 times faster. Other factors found in the 486's newer technology (see the technical details for more information) increase the 486's speed advantage over Galileo's 1802 processors to an estimated factor of roughly 200.

Galileo does have a much higher degree of redundancy (see the technical details for more information) than is found in a home computer; though your home machine may be faster, it probably crashes far more frequently.


Command and Data Subsystem
 
The Command and Data Subsystem (CDS) (really the "brain" of Galileo) has several functions. First, it must carry out instructions from the ground to operate the spacecraft and gather science data. Second, some portions of the CDS memory can serve as a storage place for science data. Third, the CDS must package the data for transmission to Earth. Finally, the CDS must be alert for and respond to any problem with any of the spacecraft subsystems.


The Attitude and Articulation Control Subsystem (AACS)

 
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.

Data Memory Subsystem,
aka Tape Recorder
  • ...
  
Data are either transmitted to Earth as they are gathered (called "real-time data"), or they are stored aboard for future playback. One place data can be stored is Galileo’s Data Memory Subsystem (DMS), a four-track tape recorder that holds 900 megabits of data.

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).

How do you power something 500,000,000 kilometers from the sun?
 
Solar panels won't do much this far from the sun and batteries will only get you so far. The Galileo orbiter carries two radioisotope thermoelectric generators (RTGs), which are used to generate electrical power on board the spacecraft. There are 7.8 kilograms (17.2 pounds) of Plutonium-238 in each RTG.

The radioactive decay of plutonium produces heat that is converted to electricity. The RTGs produced about 570 watts at launch. The power output decreases at the rate of 0.6 watts per month and was 493 watts when Galileo arrived at Jupiter.


Radiation Hardening
 
The radiation'll get ya.


Propulsion

 
The Propulsion Subsystem consists of the 400-newton main engine and twelve 10-newton thrusters together with propellant, storage and pressurizing tanks, and associated plumbing. The fuel for the system is monomethyl hydrazine, which is burned using nitrogen tetroxide. The Propulsion Subsystem was developed and built by Daimler Benz Aero space AG (DASA) (formerly Messerschmitt–Bolkow–Blohm) and provided by Germany, the major international partner in Project Galileo.

The newton (N) is a unit of force used to measure, among other things, the thrust level of rocket engines. A thrust of 10 N would support a mass of about 1 kilogram (or 2.2 pounds) at the Earth’s surface.

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The Probe

 
 

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The Scientific Instruments
 
There are 12 scientific experiments aboard the Galileo orbiter.

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