Today on Galileo
Wednesday, May 23, 2001
Galileo's Mission at Jupiter - Day 2 of the Callisto 30 Encounter
Today begins with an ending. The Radio Science Jupiter-Earth Occultation
experiment, which began yesterday at 6:20 p.m. PDT, concludes at 12:50 a.m.
PDT (yawn). [See Note 1] At this time the spacecraft resumes sending
telemetry bits. Although Galileo itself is about 10.5 Jupiter radii from
the planet (750,000 kilometers or 466,000 miles), the radio signal sent
from the craft clears the top of the visible atmosphere by only 25,000
kilometers (15,500 miles) on its way to Earth, and the last of the effects
that the atmosphere has on the signal should have dissipated.
Just before 6 a.m. PDT, the Fields and Particles instruments [the Energetic
Particle Detector (EPD), Magnetometer (MAG), Dust Detector (DDS), Heavy Ion
Counter (HIC), Plasma Wave instrument (PWS), and Plasma instrument (PLS)]
complete their configuration for the upcoming close pass by Jupiter. Now
these instruments begin an 18-hour period of continuous data collection
surrounding perijove, the closest point to Jupiter. These data will give
information about the current state and variability of the Io Torus. The
ionized gases which make up the Torus originate with the volcanic activity
on Io, and form a doughnut-shaped ring that encircles Jupiter and extends
from Io's orbit outward.
At 6 a.m. PDT the Photopolarimeter Radiometer (PPR) instrument is turned on
to be ready to observe the planet and its satellites. The two other optical
instruments, the Solid State Imaging camera (SSI) and the Near Infrared
Mapping Spectrometer (NIMS) are already on and ready. Their first
opportunity is at 7 a.m. PDT, when Io passes into Jupiter's shadow. All
three instruments observe the satellite as it passes into darkness,
studying how the surface cools, and looking for hot spots.
Three and a half hours later, after Io has left the shadow, PPR, NIMS and
SSI take another look. PPR checks to see how the surface is warming up in
the sunlight, NIMS does a global observation to monitor volcanic activity,
and SSI takes some color pictures of the leading hemisphere of the
satellite to help in the planning of observations in upcoming orbits when
Io is the prime target of interest. All four of the large Galilean
satellites (Io, Europa, Ganymede, and Callisto) are tidally locked to
Jupiter, as the Earth's Moon is to Earth, and always show the same face to
their parent planet. This means that one side of the globe is always in the
lead as the body orbits, hence the name 'leading hemisphere'.
Closest approach to Io comes at 11:08 a.m. PDT, at a distance of 341,771
kilometers (212,412 miles), a far cry from the 198 kilometer (123 mile)
distance of the 27th orbit in February 2000. Shortly afterwards, at 11:24
a.m. PDT, Galileo reaches perijove, the closest point to Jupiter, at a
distance of 6.3 Jupiter radii (450,400 kilometers, or 279,865 miles) above
the cloud tops. Here the spacecraft is also relatively close to the small
inner satellite Amalthea, and we take this opportunity to take several
pictures of this body. These pictures will help the Navigation Team refine
our knowledge of the orbit of the satellite. The long-term plan for the
Galileo spacecraft includes a much closer flyby of that tiny body in
In the afternoon, a lighter schedule of activities includes PPR looking at
a white oval storm in the atmosphere of Jupiter. This white oval is what
remains from the merger of three smaller ovals during the past two years.
This measurement will provide temperatures of the feature at unprecedented
spatial resolution, and will help define models for convection and energy
transfer in the atmosphere.
Closest approach to the icy satellite Europa is at 5:41 p.m. PDT, at a
distance of 781,513 kilometers (485,707 miles). NIMS takes this opportunity
to perform an observation of the body to complete the global mapping which
began in Galileo's Prime Mission.
The day's activities wrap up in the evening with more NIMS observations of
the Jupiter atmosphere. At 7:00 p.m. PDT, NIMS looks at cloud structures
called "brown barges" in the northern hemisphere of the planet. These
measurements provide information about the composition of the clouds, and
how they change over time. At 8:00 p.m. and 9:00 p.m. the instrument also
looks at a region of hot spots just north of the equator, again studying
the composition and dynamics of the clouds.
The day concludes at 10 p.m. PDT with SSI once again taking color pictures
of Io to look for changes in the surface of the satellite. Io is the most
volcanically active body in the solar system, and significant changes in
its colorful surface have been noticed even in the geological blink of an
eye represented by Galileo's Tour of the Jovian system.
A busy day, with much more yet to come!
Note 1. Pacific Daylight Time (PDT) is 7 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 50 minutes to travel
between the spacecraft and Earth.