October 9, 1999
Galileo Returns to Io
Galileo returns to Io for the first time since December 1995. This is the thirteenth encounter
of the Galileo Europa Mission, a two-year extension of Galileo's two-year primary mission. The
close flyby of Io is the first of two scheduled in the next month and a half. These flybys of
the innermost Galilean moon were made possible by the four preceding flybys of Callisto, the
outermost Galilean moon, which were used to lower the orbit of Galileo towards Jupiter.
Encounter activities begin tonight, and span the next four days, ending Wednesday, October 13.
The bulk of the action, however, is set to occur on Sunday and Monday, with the peak surrounding t
he close flyby of Io on Sunday night at 9:33 pm PDT at the spacecraft's location. However, the radio
signal indicating that the flyby has occured won't be received on Earth until 33 minutes later,
or 10:06 pm PDT. The time difference is due to the fact that the spacecraft is approximately 598
million kilometers (372 million miles) from Earth and it will take radio signals just over 33
minutes to travel between the spacecraft and Earth [see Note 1].
The spacecraft has distant flybys of Jupiter and the other Galilean moons during this encounter,
all occuring Sunday, the same day as the Io flyby. A couple of observations are directed toward
Jupiter during the encounter, but by far, most observations are focused on Io, its interior, its
volcanic surface, and its electromagnetic environment.
From the data acquired during this encounter, scientists hope to learn more about how Io's
surface and atmosphere have been formed and shaped over the moon's 3.5 billion year history,
more about Io's interior structure, its gravity field, and its magnetic environment, and more
about the characteristics of the Io torus and the interactions between Io and the Jovian
magnetosphere. The magnetosphere is the region of space in which Jupiter's magnetic field is
stronger than the magnetic field due to the solar wind. The Io torus is a doughnut-shaped
region with its inner edge bounded by Io's orbit. It is a region of particularly intense plasma
flow and radiation activity inside the Jovian magnetosphere. The Io torus is constantly
replenished by volcanic activity on Io.
The images taken at Io by the Solid-State Imaging camera will be the highest resolution ever
collected for many features and regions on Io. Several images will provide image resolutions of
7 meters (23 feet) per picture element. Over 100 images of 10 targets will provide resolutions
of better than 500 meters (0.3 miles) per picture element, which was the best resolution achieved
during the Voyager spacecraft flybys of Io in the late 1970s. The Near-Infrared Mapping
Spectrometer will make observations with resolutions of as good as several hundred meters,
while the Photopolarimeter Radiometer's best resolution will be 13 to 56 kilometers.
Early today, one final activity is performed in preparation for the start of the encounter.
That activity is standard maintenance of the spacecraft's onboard tape recorder. The recorder
is used to store most of the data collected by Galileo's instruments for subsequent processing
and return to Earth.
Only one observation is initiated today. As with previous encounters, the start of this
encounter marks the resumption of the magnetospheric survey performed by Galileo's suite of
particles and fields 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 are not recorded on board, but rather are
processed and returned to Earth in near real time. This survey has been repeated from encounter
to encounter allowing scientists to study the long term variations within the inner portions of
Jupiter's magnetosphere. The measurements will also provide a broader context for higher
resolution measurements made by the Fields and Particles instruments later in the encounter period.
Come back tomorrow for the next edition of Today on Galileo, and learn what is in store for
Galileo as it zooms past Io and its volcanoes!
Note 1. Pacific Daylight Time (PDT) is 7 hours behind Greenwich Meridian Time (GMT). The time
when an event occurs at the spacecraft is known as Spacecraft Event Time (SCET). The time at
which radio signals reach Earth indicating that an event has occured is known as Earth Received
Time (ERT). Currently, it takes Galileo's radio signals 33 minutes to travel between the spacecraft