This Week on Galileo
September 13-19, 1999
Encounter activities begin on Monday, September 13, at 1:00 pm PDT [see Note 1], and span the next three days, ending Thursday, September 16, with the close flyby of Callisto, at 10:26 am PDT. During this time, the spacecraft is approximately 628 million kilometers (390 million miles) from Earth and it takes radio signals just under 35 minutes to travel between the spacecraft and Earth.
The spacecraft has distant flybys of Ganymede, Europa, Jupiter, and Io on Tuesday, September 14, before its close flyby of Callisto later in the week. At 09:49 am PDT on Tuesday, the spacecraft passes within 583,000 kilometers (363,000 miles) of Ganymede's surface. At 11:25 am PDT, Galileo flies past Europa's surface at a distance of 1.1 million kilometers (0.7 million miles). At 12:57 pm PDT, the spacecraft flies past Jupiter at a distance of 6.5 Jupiter Radii (465,000 kilometers, 289,000 miles) from the planet's center. Finally, at 2:48 pm PDT, Galileo flies past Io's surface at a distance of 439,000 kilometers (273,000 miles).
Two days later, Galileo flies past Callisto at 10:26 am PDT at a distance of 1057 kilometers (657 miles) from the moon's surface. The spacecraft's flight path past Callisto is such that Galileo passes behind the moon as seen from the Sun and Earth. As Galileo passes behind the moon, its radio signal will be weakened and refracted by Callisto's tenuous atmosphere. These changes will be measured by the Radio Science team back here on Earth, which will allow scientists to study the structure of Callisto's atmosphere.
With the start of the encounter comes the resumption of the magnetospheric survey performed by the Fields and Particles Instruments. The Fields and Particles Instruments are comprised of the Dust Detector, Energetic Particles Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave Instrument. During the survey, the instruments take measurements of plasma, dust, and electric and magnetic fields. These measurements have been repeated from encounter to encounter and will allow scientists to study the long term variations within the inner portions of Jupiter's magnetosphere. This particular leg of the survey will last for the next two weeks.
All of the encounter's recorded science observing is performed by the Fields and Particles instruments. The recordings are part of a series performed during each encounter of the Perijove Reduction Campaign (Galileo's previous three encounters, and this encounter). During these encounters, the spacecraft's Perijove distance, or closest distance to Jupiter for a given orbit, has been reduced from orbit to orbit. These recordings will provide high resolution data on the inner magnetosphere and Io torus environment, contributing toward studies of the dynamic processes that occur in these regions. The Io torus is a region of intense plasma and radiation activity, in which there are strong magnetic and electric fields. Constantly replenished by the volcanic activity on Io, it is a vital part of the Jovian magnetosphere.
In related observations, the Plasma Wave instrument performs an observation dedicated to the detection of Chorus emissions. The Chorus signal is seen in the electromagnetic fields measured by the Plasma Wave instrument when plasmas are being accelerated due to a particularly efficient type of wave-particle interaction. By detecting and analyzing Chorus emissions, scientists hope to understand a significant process by which energy is transferred from Jupiter's magnetic field into plasmas within the Io torus and from there into outer portions of the magnetosphere.
Processing and transmission to Earth of these recordings, which are stored on the spacecraft's onboard tape recorder, is initiated on Wednesday, September 15, at 9:30 am PDT. Later that day, the Utraviolet Spectrometer performs an observation of auroral activity on Jupiter. Unlike the Fields and Particles data, it is not recorded on the spacecraft's onboard tape recorder, but rather it is returned to Earth in near real time. The Extreme Ultraviolet spectrometer joins the Ultraviolet Spectrometer toward the end of the day, and they perform a joint observation of the Io torus. Again, the observation is made and returned to Earth in real time. Playback of the Fields and Particles observations is interrupted to perform these spectrometer observations. Once resumed, playback continues for the remainder of the week.
Note 1. All times listed correspond to the Pacific Time zone (currently daylight time) and spacecraft event time. Radio signals indicating that an event has occurred on the spacecraft reach the Earth 33 to 50 minutes later, depending on the time of year. Currently, Pacific Daylight Time (PDT) is 7 hours behind Greenwich Meridian Time (GMT), and it takes radio signals 35 minutes to travel between the spacecraft and Earth.
For more information on the Galileo spacecraft and its mission to Jupiter, please visit the Galileo home page.
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