Galileo FAQ - Navigation
When spacecraft engineers refer to attitude, they're discussing how the spacecraft is oriented, or pointed, with respect to some unmoving reference, like the stars. This is one of the jobs of the Attitude and Articulation Control Subsystem, or AACS, which performs a number of functions for the Spacecraft, including:
If you're interested in more information, look at the article on attitude and articulation found in JPL's Basics of Space Flight Workbook. There is also an older Galileo Messenger article on the AACS.
Why does Galileo's speed change? Why doesn't it stay constant?
As of June 29, 1995, Galileo was travelling at roughly 25,900 kilometers per hour (16,000 miles per hour) relative to the Sun, but this speed isn't constant--the spacecraft is actually slowing down. Since the spacecraft is essentially in an elliptical orbit about the Sun, its speed naturally drops as it moves away from the Sun (if not for Jupiter's influence, it would speed up as it started moving back towards the Sun). You notice the same behavior when throwing a long, high fly ball--the ball slows down and "hangs" at the top of its path.
In addition, as Galileo draws very near to Jupiter, its speed is changed quite dramatically by the gravity of the giant planet. However, this effect won't be significant until a few days before arrival at Jupiter (on December 7, 1995, and subsequently as the spacecraft draws near each encounter). Since Jupiter lies "behind" Galileo prior to Galileo "meeting up" with the giant planet (if Galileo and Jupiter were on a racetrack, Galileo would have significant head start, though it's doomed to lose the race), Jupiter's gravitational influence will act to slow down the spacecraft relative to the Sun.
Galileo's sun-relative speed will continue dropping until the day that it arrives at Jupiter, at which point we will fire the main engine to move Galileo into orbit around Jupiter. At that point, the sun-relative speed will shoot way up.
Over the course of Galileo's 23-month orbital mission, the spacecraft speed will change with each satellite flyby (in fact, that's one of the reasons that we have these flybys).
What are the orbital elements for the spacecraft?
This contains more technical information than most people are interested in, so it's been placed in separate files. The May 28, 1998 addition is:
No, this is not the case for two reasons. First, several TCMs have been canceled: TCMs 3, 13, 18, and 21. Also, TCMs are not numbered consecutively - i.e., there was a TCM-4A and a TCM-4B, and a TCM-9A and a TCM- 9B, and there was also a TCM-22A.
The net effect of all this is that TCM-23 is the 22nd TCM since launch. Since TCM-24 was cancelled, TCM-25 (ODM) is either the 23rd TCM since launch, if one does not count the ODM wake-up burn as a TCM, or the 24th if one counts the wake-up burn.
Could the probe have be retargetted to the SL-9 impact points?
If the probe was aimed at--and entered--the SL-9 impact site, it might not have survived long enough to carry out its observations! When Mission Designers considered the probe's mission, they realized that the probe's speed relative to the atmosphere is minimized by having it enter near Jupiter's equator; entering at higher latitudes would raise this relative impact speed. A lower impact speed makes it easier to design the probe (rather like it being easier to design car bumpers that pass a bashing test of 3 miles per hour rather than 10 miles per hour). Thus, the probe was designed to allow it to enter within a few degrees of the equator, with an entry speed up to 47.8 km/sec (about 104,000 mph).
Regardless, it was impossible to retarget the probe at the time that this question was asked. The probe, having been released, was on a purely ballistic trajectory. Also, retargetting the probe would have required a significant time-of- arrival change (hours), which would mean that the orbiter could not fly by Io as planned. Therefore, we would have lost both the Io science and the required Io gravity-assist that reduced the size of the Jupiter Orbit Insertion maneuver by 175 m/s (which would cause a tremendous hit on the orbiter's propellent margin).
The speed that is posted daily is the speed of the orbiter with respect to the Sun. The Galileo orbiter is currently in an orbit around the Sun and is just past aphelion (the furthest point from the Sun) where the orbiter's speed is the slowest in its orbit. However, if we look at a Sun-centered coordinate system, the orbiter is actually ahead of Jupiter (i.e. Jupiter is coming up from behind the orbiter). As Jupiter "catches up" to the orbiter, its gravitational attraction is actually slowing the orbiter down with respect to the Sun. Between one and two days prior to arrival the orbiter's "forward" motion around the Sun will actually stop. Then, moving in the opposite direction (backwards), the orbiter speeds up again as it moves around the back side of Jupiter. While all of this is happening, the orbiter speed with respect to Jupiter is continuously increasing due to Jupiter's gravitational attraction. The JOI maneuver slows the orbiter down and allows it to be captured in the desired orbit.
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 was 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 was successfully performed on March 14 1996 and was 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.