Galileo FAQ - General Questions
A few of the inventions we now enjoy were originally developed for Galileo. Charge-coupled devices like those in Galileo's television systems are used in some of our home video cameras, yielding sharper images than ever conceived of in the days before the project began. In addition, radiation-resistant components developed for Galileo are now used in research, businesses, and military applications where radiation environment is a concern. Another advance, integrated circuits resistant to cosmic rays, has helped to handle disturbances to computer memory that are caused by high-energy particles; these disturbances plague extremely high-speed computers on Earth and all spacecraft.
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 electronic 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. 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.
What is the cost of the Galileo mission?
The total cost of the Galileo mission, from the start of planning in 1977 through the end of mission in December 1997 is $1.354 billion. This value does not include launch costs, Deep Space Network tracking costs, and foreign contributions. (The latter is estimated at about $110 million, a very substantial international contribution indeed!) The total breaks down into $892 million in development costs (through about 30 days after launch, and $462 million in operating costs. Galileo has cost each citizen of the US only 27 cents a year during its 20 year life.
Not all, but several are. Mars Pathfinder, Mars Global Surveyor, and Pluto Express have all expressed interest in the telemetry subsystem being developed for Galileo. The ultracone is usable by any spacecraft which transmits in S- band. It is also fair to say that Galileo's emphasis on data compression and encoding has spurred new research and development which is leading to the incorporation of sophisticated data compression schemes on projects under development.
Do scientists known much about Jupiter or is there still a great deal of mystery? If so, what?
There are many mysteries about the Jupiter system. Among them are:
What is unique about the Galileo mission?
The Galileo mission makes major advances over previous missions:
How will learning about Jupiter help scientists understand Earth? The Solar System?
The Jovian system contains clues to much of the early history of the solar system. Advances in such fields as meteorology, geochemistry, geology and geophysics, atmospheric sciences and space physics are expected. These are all disciplines which benefit all the earth sciences and understanding both our own planet and our place in the universe.
What are the most fascinating aspects of Jupiter compared to other planets?
Previous missions (Pioneer and Voyager) provided brief reconnaissance - Galileo is the first thorough study of system.
What can be learned by orbiting the gaseous planet that could not be gained by the earlier flybys?
Gravity is what holds Jupiter -- and all the other planets -- together. Most people don't worry about the Earth falling apart, because we have a solid surface under our feet, but the Earth also contains a fair amount of gas as well --the atmosphere --which isn't floating away into space (and a good thing, or we wouldn't have any air to breathe). Seeing that gravity binds the atmosphere to Earth makes it easier to understand that gravity can also hold together gas giants like Jupiter and Saturn.
The easy answer to that is: the Galileo Orbiter transmits either at 2295.0 or at 2296.5 megahertz (MHz). The spacecraft transmission is 24 hours per day, and the frequency at any particular time depends on how we are using the downlink. The lower frequency comes from an on-board source called the ultrastable oscillator and gives the best telemetry. The higher frequency originates on the uplink from the tracking station to the spacecraft and is turned around (transponded) by the spacecraft. This resulting "two way" frequency gives us the best tracking data for calculating the spacecraft's velocity and position as a function of time.
The real answer to your question is: you probably won't be able to track Galileo with your radio telescope unless you have a really big one, such as at Arecibo (Puerto Rico) or Parkes (Australia). That's because the Galileo high gain antenna failed to open properly in 1991. The downlink has always been transmitted on a low gain antenna. The actual gain of this antenna is about 7 decibels above isotropic (+7 dBi) when it is oriented within 10 degrees of earth which is the case for most of the rest of the mission. A gain of 7 dBi is only about two or three times as good as the average "rabbit ears" antenna on top of the TV. The low gain antenna on the spacecraft means the downlink has to be planned to take advantage of the most sensitive antennas and receivers of NASA's Deep Space Network.
You may have also heard that the Galileo transmitter power is about the same as a refrigerator light bulb. Well, almost: measured at the low gain antenna, the actual power is about 15 watts. Starting with these numbers (15 watts transmitter and +7 dBi antenna gain), a communication engineer can figure out that the arrival day power picked up by the largest of NASA's deep space stations was -197.5 decibels relative to one watt (-197.5 dBW). We work with decibels in communications work to avoid having to multiply and divide very large and very small numbers. That -197.5 dBW received power requires a 70- meter diameter ground antenna. To detect the science and engineering telemetry data on the downlink requires a sensitive receiver. The pre- amplifier at the 70-meter stations is a cooled maser that has a sensitivity (system noise temperature) of about 15 kelvins.
So, unless your radio telescope has a gain and sensitive comparable to these numbers, you won't be able to pick up Galileo's downlink. It happens that the spacecraft-to-earth communications distance was the largest of the mission on arrival date. But even if you wait another six months, when Jupiter and the earth are much closer together and on the same side of the Sun, the communications distance is still 66% (-3.6 dB) of what it was on arrival day. We will take advantage of the distance decrease to transmit the science and engineering data at a higher rate.
Everything would have been different if the Orbiter's high gain antenna were functional. We would have an X-band downlink (8415 MHz) as well as the S-band (2295 MHz). The spacecraft antenna gain would be +50 dBi for the X-band or + 38 dBi for the S-band, compared with +7 dBi for the low gain antenna. With each decibel representing a 26% increase in capability, the difference between 50 dBi and 7 dBi is a factor of 20,000. That's why we struggled with receiving an 8 bps downlink on arrival day as compared with the original plan for a maximum 134,400 bits per second X-band downlink rate. If we had the high antenna, we would have transmitted (relayed) the Probe data back to the earth at 28,800 bits per second on the 2295 MHz downlink at the same time it was received on board the Orbiter instead of storing it and reading it out later at 8 bits per second as we in fact did.
If you *should* happen to have the Arecibo or Parkes antenna handy, then there would be several other FAQs: What's the real downlink frequency from Galileo, taking into account the doppler frequency shift caused by the motion of the spacecraft relative to the tracking station? And, what's the schedule for transmitting the 2295.0 MHz and the 2296.4 MHz downlinks? And what's the telemetry modulation scheme on the downlink, going from carrier to data symbols to data bits to telemetry words to data frames? But since there aren't too many people who have access to radio telescopes of this caliber, these aren't frequently asked questions.
The Venus and Earth flybys speeded Galileo up, but the Io encounter slowed it down. How do the two types of encounters with the planetary bodies differ to speed up or slow down the spacecraft?
Take a look at the section on gravity assists in the Basics of Space Flight Home Page.
There are two possibilities for what will happen to Galileo after the two years of our primary mission are over in December of 1997. The first is that we will go into an "extended" mission and will continue observing Jupiter's atmosphere, magnetosphere, and moons. For this to happen, the spacecraft must remain healthy with all critical parts working, we must have enough propellant left to control our trajectory and keep the communications antenna pointed at Earth, and the project must be funded.
If this is not possible, the spacecraft will be turned off and left in orbit around Jupiter.
In the past, many missions have been continued after their primary missions have ended (e.g. the Magellan mission to Venus, which performed some exciting science well after its primary mission was over. Maybe some excellent scientific ideas can be planned for Galileo at the end of 1998!
Eventually, Galileo will run out of propellant, or the electronics on board the spacecraft will stop functioning because of damage from the high radiation around Jupiter, or NASA will choose to stop funding the mission. But Galileo will continue to orbit Jupiter for a very long time after the mission ends. Eventually, sometime between several hundred to a thousand years in the future (these things are very hard to predict), Galileo will suffer one of the following fates:
Although Galileo can send some data directly on to Earth immediately after it is collected (i.e. in "real time"), most of the data taken during a satellite encounter will be recorded on Galileo's tape recorder, and then played back in the weeks following the encounter period. As you might therefore expect, the tape recorder's capacity limits the amount of data that can be collected, though time may limit the amount of data that gets returned to Earth.
Since the tape recorder only has one tape, the same tape gets used for each encounter. Therefore, the data from one encounter will be overwritten by the data from the next encounter, much like someone reusing the same videotape to record a favorite television show each week will record over the previous week's show. If the "cruise" period between encounters is long enough, or if the spacecraft can transmit its data to Earth at a high enough rate, then more of the data on the tape recorder will be played back to Earth. However, if the cruise period is short, or the data rate is low, then less of the data will be returned. We never actually return all of the data that is recorded on the tape recorder in any orbit. Most of it gets processed in some way on board Galileo so that we don't waste our resources on the bits of data that we know in advance do not contain useful information. If we returned a whole tape load of data without any on-board processing, it would actually take several orbits worth of time to play it back! Typically, the spacecraft compresses or edits the contents of the tape recorder to about 1/8 of its original volume for each prior to playing back the data. Galileo flight team members carefully identify the most important observations to ensure that the most valuable data gets back to us.
I hear the term "Real Time" used a great deal. What does it refer to? (9/4/96)
"Real Time" is a fuzzy term we use for looking at things "as they happen." Of course, we need to wait for the signal to reach us here on Earth, so it takes one light-time before we see "real time" events.
There is also a significant lag in the data return (from minutes to as much as 10 hours or more) as a result of buffering processes on the spacecraft and on the ground.
The reason that we refer to "real time" is because Galileo's scientists and engineers do work in both real time and non-real time. For example, when we do a big engineering event like Jupiter Orbit Insertion, engineers perform both real time analysis (sitting in the control room and watching the events unfold, making sure that as the spacecraft does each programmed command, the right thing happens and nothing has gone wrong) and non-real time analysis (looking over the data afterwards in more detail). A good analogy would be taking an exit poll on Election Day, compared with analyzing the election results over the following weeks.
Similarly, scientists may receive their data in "real time," or they may receive it days or weeks after it was initially recorded. Images, which may take up a megabyte of memory, cannot be sent in real time, since Galileo's downlink rate (the rate at which it sends data to Earth) is relatively low. Fields and Particles information, on the other hand, does not use as much memory, and can often be sent in real time as a near-continuous stream.