Date: Fri, 29 Dec 2000 08:37:14 -0800 (PST)
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Subject: Today On Galileo - December 29, 2000
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Today on Galileo
December 29, 2000
The spacecraft makes its closest approach to Europa today, the third day of
the 29th encounter of Galileo's mission at Jupiter. Closest approach occurs
at 1:37 a.m. PST [see Note 1] at a distance of 794,000 kilometers (493,000
miles). Throughout the day, Galileo's instruments make observations of
Europa, Io, and Jupiter. Galileo also takes a break from its science
endeavors to perform two engineering activities. In the early morning, the
spacecraft executes the first part of a standard gyroscope test. In the
early evening, the spacecraft performs part two of the test, which involves
moving the spacecraft's scan platform to examine how the gyroscopes react to
the movement. The scan platform houses all of Galileo's remote sensing
instruments and is used to point those instruments at targets of interest
(like Jupiter and its moons).
The Photopolarimeter Radiometer (PPR) and Near-Infrared Mapping Spectrometer
(NIMS) are again major players in today's observation suite, performing 27
of the day's 29 observations. The Solid-State Imaging camera (SSI) performs
the remaining two observations. The first SSI observation is a global color
mapping observation of Io at low phase angle. SSI's second observation is
also global in nature, and is timed so as to capture the Prometheus volcanic
plume in profile on the edge of Io's disk. The shape and structure of the
plume will be compared with that seen in previous Voyager and Galileo
images.
PPR's six observations consist of four observations of Europa and two of Io.
All of these observations measure the polarization of light reflected off of
the surface of these Jovian moons. Like the polarimetery observations made
yesterday, these data will allow scientists to learn about the surface
texture and small-scale surface properties. NIMS performs the remaining 21
observations scheduled for today. One is of Europa, while the other 20 are
dedicated to studying Jupiter's atmosphere.
NIMS' observations of Jupiter's atmosphere cover the North Temperate Zone
(1), the South Tropical Zone (3), Jupiter's aurora (10), Jupiter's limb (5),
and a white oval (1). North Temperate and South Tropical Zone observations
will provide scientists with information on cloud activity, and thermal and
compositional characteristics. When compared to similar measurements from
previous orbits, they can give insight into the change in these
characteristics over long time scales. The auroral observations are designed
to study the auroral emissions at Jupiter's south polar region. Auroral
emissions are much like Earth's northern lights. NIMS' limb observations are
designed to look right at the edge of Jupiter's disk and will measure how
thermal emissions vary with altitude in the upper cloud levels in Jupiter's
atmosphere. Finally, the white oval observation is designed to look at the
last white oval. The three long-lived white oval features have merged over
the course of the last two years into a single storm. White ovals are storms
that occur between two adjacent zonal jet streams, and have lasted for
decades. The data obtained in this observation will provide scientists with
more information on the composition and cloud dynamics of the region.
Galileo still has a few days of exciting observations ahead of it. Come back
tomorrow and learn what is in store for the weekend!
Note 1. Pacific Standard Time (PST) is 8 hours behind Greenwich Mean 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 occurred is known as Earth Received Time (ERT).
Currently, it takes Galileo's 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 at one of the following URLs:
http://galileo.jpl.nasa.gov
http://www.jpl.nasa.gov/galileo
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