Frequently Asked Questions

Q1:	Is it true that this is the first time in the history of space exploration that two robotic spacecraft will actively observe the same planet at the same time from such close range? What about the Viking Orbiters?
A1:	In fact, Earth has been studied with multiple spacecraft since the start of the Space Age, and Mars was studied by the Viking Orbiters at the same time. It is also true that HST, Galileo, and Voyager all viewed the Shoemaker-Levy 9 impact of Jupiter at the same time, albeit not from close range. Both Galileo and Cassini also arranged to have simultaneous observations made with Earth-orbital spacecraft during their respective flybys of Earth. It is important to note the cooperative efforts that have been undertaken to coordinate the observations of the two independent missions (Galileo and Cassini) at Jupiter. The Viking observations were planned as if they were elements of a single mission, which indeed they were. Not so for Galileo and Cassini. A more elaborate and complete way of stating the situation would have been, "This is the first time in the history of deep space exploration that spacecraft from two independent robotic missions will actively and cooperatively observe the same gas giant planet at the same time from such close range." The "deep space" eliminates the Earth-orbital missions, the "spacecraft from two independent robotic missions" eliminates the Viking orbiters, "gas giant" specifies the four large planets beyond the asteroid belt, and the "same time from such close range" eliminates the Galileo and Voyager observations of Shoemaker-Levy-9's Jupiter impact. Also, the "actively and cooperatively" might take care of some more recent (or future) Mars Missions that might just happen to be collecting Mars data simultaneously, albeit not cooperatively.
Q2:	Has Cassini contacted Galileo at this time?
A2:	The science teams have been collaborating for at least a year on what activities are going to be done at Jupiter. The spacecraft have not talked with each other and there is no way for them to do so.
Q3:	Is there going to be any overlap between Galileo and Cassini science data?
A3:	Cassini is expected to remain outside Jupiter's magnetosphere throughout the period Oct. 2000 - Mar. 2001 whereas Galileo will transition between Jupiter's magnetosphere and the solar wind, depending on where it is in its orbit. This provides scientists with the opportunity to study the solar wind and its effects from both within and without the magnetosphere. The fields and particles instruments on both spacecraft will therefore be doing joint studies.
Q4:	Was the encounter happening at solar maximum planned or is the timing coincidental?
A4:	It's purely coincidental due to the launch date changes for the two spacecraft.
Q5:	Will there be any photography of Jupiter?
A5:	The Hubble Space Telescope will be making observations and ground-based observations in both visual and infrared wavelengths are planned. Radio measurements will be made from the ground and from Cassini. And, of course, Cassini's cameras will be quite busy throughout the flyby period. Galileo will be collecting images of Jupiter's cloud tops and the satellites. The key difference is that Cassini can image the whole planet (requires 3x3 images at close approach) and put the postage stamp pictures of Jupiter by Galileo into context. On the other hand, Galileo will get much better resolution than Cassini of the satellites.
Q6:	What about auroral images?
A6:	Both Jupiter's and Ganymede's auroras will be imaged with the Galileo Solid State Imager (SSI). Cassini will be taking images throughout the 5-day and other templates developed for the flyby, except during downlink windows when data will be transmitted back to Earth.
Q7:	Is Jupiter somewhat like a slow rotating pulsar because of the physical similarity to Jupiter's magnetosphere?
A7:	Because of the rotation speed of Jupiter's magnetic field, the particles are moving at near relativistic speeds. This is linked to Jupiter's synchrotron radiation. But Jupiter is in no way similar to a pulsar or neutron star. Both pulsars and Jupiter have tilted dipole magnetic fields. But that's where the similarity stops. Pulsars have enormously stronger magnetic fields (literally millions of times stronger) and rotate at a much faster rate, taking less than 10 seconds for one rotation of the 10 mile (16 km) neutron star, while Jupiter is hundreds of times less massive and rotates in just less than 10 hours.
Q8:	Will we be getting any realtime data from Cassini?
A8:	Cassini has no scan platform. So, unless some very fortuitous geometry presents itself, the remote sensing pallet with the cameras and spectrometers can't be pointed at Jupiter while the antenna is pointed at Earth. Therefore we cannot usually send and collect remote sensing data simultaneously. Instead, the spacecraft will be collecting data for about 4.5 days and then spending about 0.5 day downlinking them. During the downlink period, Cassini's fields & particles instruments will be acquiring data that will be downlinked near the end of the downlink period, in near-real time.
Q9:	What are you measuring to determine the strength of the magnetosphere?
A9:	The magnetosphere is somewhat bullet-shaped. The solar wind blows the magnetic field lines from Jupiter back, away from the Sun so you get this very elongated shape. As Galileo goes from the sun side to the night side, the s/c will see dramatic changes in the field strength and particle density. The Magnetometer, Energetic Particle Detector, and Heavy Ion Counter are all used on Galileo to study Jupiter's magnetic field. Cassini is expected to remain outside Jupiter's magnetosphere, so it will be monitoring the solar wind. Those results can be combined with Galileo's to learn about the influence of the solar wind on Jupiter's magnetosphere.
Q10:	What is the wattage strength of the Cassini Transmitter?
A10:	The Cassini transmitter (called a travelling wave tube amplifier, TWT) is 20 watts.
Q11:	What groundbased radio observations are planned?
A11:	Along with Cassini and Galileo, Goldstone Apply Valley Radio Telescope (GAVRT) students will be measuring changes at Jupiter by looking at the changes coming from Jupiter's radiation belts. We will be learning about the short-term changes in Jupiter's radio emission and how those changes (if any) correlate with Galileo and Cassini data.
Q12:	Can Cassini and Galileo see each other at closest approach?
A12:	No, they aren't close enough. At closest approach the two spacecraft are about 4.5 million miles apart.
Q13:	What will be the distance of Cassini from the top of Jupiter's atmosphere at closest approach?
A13:	At closest approach, Cassini will be 9,721,846 kilometers from Jupiter's cloud tops.
Q14:	How much acceleration will Cassini get from the Jupiter flyby?
A14:	Cassini's speed, relative to the Sun, will increase by 2.2 km/s. The actual acceleration is a small number, since the change occurs over several weeks' time.
Q15:	How much of a threat, if any, does Jupiter's intense magnetosphere pose for instrumentation aboard Cassini?
A15:	Cassini is not expected to enter Jupiter's magnetosphere at all. Even if it were to, through some extreme effect of the solar wind, it is staying so far from Jupiter that it is would not be exposed to the intense radiation belts found close to the planet.
Q16:	Will Cassini pass close enough to any of the large Jovian moons to collect data from them?
A16:	Cassini is not passing close to any of the Galilean satellites, but it will be collecting data on them, either by itself or jointly with the Galileo spacecraft. The moons will be tiny as seen by Cassini but there are still good, and even unique, data to be collected, even from a large distance.
Q17:	What do scientists hope to learn from this unique encounter?
A17:	Data collected during this encounter will be used to better understand Jupiter's weather, its magnetosphere, its satellites, its rings, and its dust streams. There will be an increase in our knowledge of this planet and its system in some aspects of virtually all of its components.
Q18:	Can the two spacecraft be tracked by the same Earth antenna at the same time?
A18:	Theoretically, two spacecraft within the field of view of an antenna, and transmitting at different radio frequencies, could be tracked simultaneously. In reality, Cassini and Galileo are never close enough together in the sky for this to happen.
Q19:	Will there be any navigation or other radio science measurements performed during this unique opportunity?
A19:	Navigation tracking for both Cassini and Galileo will be performed throughout the flyby period, during planned navigation tracks and during data downlink periods. Cassini's Radio Science Team has no experiments planned because there are no real opportunities for measurements (they get their chance later this year and on other occasions during cruise when they will be searching for gravitational waves). Cassini's RADAR Science Team will make observations of Jupiter to both study its radiation belts and calibrate the receiver (in conjunction with groundbased observations including those made by Goldstone Apple Valley Radio Telescope (GAVRT) classrooms). Cassini's Radio and Plasma Wave Science instrument team will be making observations of the planet's emissions and calibrating their instrument too.

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