Starting out from a low Earth orbit, a spacecraft needs to increase its speed by 9 kilometers per second (19,440 mph) in order to reach Jupiter. Navigators refer to a needed speed change as "delta V," where "delta" indicates "change" and "V" stands for velocity.
Keep in mind, though, that Jupiter's orbit about the Sun doesn't lie in the same plane as the Earth's, so a spacecraft going to Jupiter would have to move out of the plane of the ecliptic. This is known as a "broken-plane" maneuver. Couldn't the spacecraft go "directly" to Jupiter without having to make the broken-plane maneuver? Yes, but that usually means that the spacecraft needs to be going even faster to begin with -- around 11 km/sec.
By comparison, Galileo's Venus-Earth-Earth Gravity Assist (VEEGA) trajectory required that the spacecraft provide a delta-V of only 4.094 km/s to reach Jupiter. Of this total, 4 km/s was provided by the IUS booster; the other .094 km/s of delta-V came from Galileo's thrusters (the spacecraft also produced an additional 100 meters/sec of delta-V that was used to for science purposes on the way to Jupiter, e.g. for asteroid flybys). The additional delta-V needed to get to Jupiter was provided by the planetary flybys (2.0 km/sec (4,320 mph) from Venus, 5.2 km/sec (11,600 mph) from the first Earth flyby, 3.7 km/ sec (7,992 mph) from the second Earth flyby). Note that this doesn't add up to 9 km/sec total delta-V; that's because we're actually giving changes in velocity (which involves direction), not just speed, and velocity changes add as vectors.
As a bonus, Galileo didn't have to perform a broken-plane maneuver -- that was thrown in "for free" by the flybys.
What is the Perijove Raise Maneuver?
The Jupiter Orbit Insertion (JOI) maneuver placed Galileo into orbit around Jupiter. Much like shaping a ball of putty, however, more than a single 'action' or 'effort' is required to get the shape we want. The Perijove Raise (PJR) maneuver is scheduled for March 14 1996 and is designed to change the shape of the spacecraft's orbit around Jupiter.
Another term for 'closest approach to Jupiter' is "perijove" (from the Latin). Hence, a perijove raise maneuver is a maneuver designed to increase the closest approach distance to Jupiter, or in other words, "raise" perijove. This action will prolong the life of the spacecraft by moving it out of the high radiation belts surrounding Jupiter.
Galileo's arrival at Jupiter was planned very precisely. The trajectory was designed so that the spacecraft's closest approach to Jupiter occurred very close to Io and the Io torus. We wanted to examine Io close-up and also to pass through a special ring of charged particles (e.g. electrons and various ions) in orbit around Jupiter called the Io torus. This ring of charged particles, and the severe radiation environment close to Jupiter, while fascinating, are also dangerous to the health of the spacecraft. Therefore we do not want the spacecraft to pass through this region more than once. Yet since Galileo passes close to Jupiter on its "zeroth" orbit, it is fated to return to that region over and over again if the orbit remains unchanged.
More details on Galileo's Perijove Raise Maneuver.
What qualifications would I need to work on the navigation and flight dynamics team?
For our Navigation Team, we look for people with advanced degrees (masters or doctorate) in Aerospace Engineering or Applied Mathematics (or related fields), and that have experience in one or more of the following: orbital mechanics, maneuver analysis, trajectory optimization, estimation theory, numerical analysis, and computer programming. Certain colleges that have specific programs in spacecraft navigation are preferred.
Galileo puts out about 20 watts of power, slightly less than the power of a refrigerator lightbulb. By the time it reaches the DSN antennas on Earth, a 70 Meter antenna is able to scoop up only about one part in 10 to the 20th watt, in other words .00000000000000000001 watt. But it's enough to do great science.
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:
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