Today on Galileo
Friday, May 25, 2001
Galileo's Mission at Jupiter - Day 4 of the Callisto 30 Encounter:
The Meat and Potatoes
Today's activities include the main focus of attention for this orbit, the
close flyby of Jupiter's moon Callisto. This occurs at 5:14 a.m. PDT [See
Note 1] at a distance of only 123 kilometers (76 miles) above the icy
surface. At that time, Galileo is flying at a speed of 9.7 kilometers per
second (6.0 miles per second or 21,700 miles per hour) relative to that
satellite. Coincidentally, this flyby occurs on the fortieth anniversary of
President John F. Kennedy's famous speech to the U.S. Congress, on May 25,
1961, in which he committed the nation to a manned moon landing by the end
of the decade. At that time, the U.S. had only launched one spacecraft that
had successfully escaped Earth orbit and fulfilled its mission -- the
Pioneer 4 Lunar flyby in March of 1959. The U.S. and NASA were still over a
year away from launching our first successful planetary mission, the
Mariner 2 flyby of Venus. See how far we've come! In contrast, the Galileo
spacecraft had five successful planetary encounters (one of Venus, two of
Earth, and two of asteroids) even before reaching its primary goal of Jupiter!
But now the spacecraft is approaching Callisto's night side. This makes it
difficult to observe Callisto, because to do so would also mean looking
very near the Sun, which is hazardous to our remote sensing instruments. So
we take this opportunity to continue our observations of Jupiter itself.
The Near Infrared Mapping Spectrometer (NIMS) performs the last two of its
three limb-to-limb, pole-to-pole global maps of the disk of Jupiter,
looking for compositional variation in the atmosphere of the planet.
The science instruments then shift their attention to Callisto. Shortly
after 3 a.m. PDT [See Note 1], Radio Science configures the spacecraft for
an occultation. On Tuesday night, the body that came between Earth and
Galileo was Jupiter. This time, Callisto itself blocks our communications
with Galileo for about an hour. The radio signal to Earth is changed to a
pure tone, with no telemetry modulation, and the science team will be
looking for the effects on the radio signal of the extremely tenuous
charged-particle atmosphere of the satellite. They will be measuring the
vertical distribution of free electrons above the surface. While no
telemetry is being sent to the ground, engineering and science measurements
will be stored both in on-board computer memory, and on the tape recorder.
In particular, the Fields and Particles instruments [Energetic Particle
Detector (EPD), Heavy Ion Counter (HIC), Magnetometer (MAG), Plasma
instrument (PLS), and Plasma Wave Subsystem (PWS)] will be recording
continuous high-rate, high-time-resolution data for about an hour centered
on the closest approach. This recording will assist with studies of the
Callisto's induced magnetic field and its interaction with the Jovian
magnetosphere. The measurements from the various instruments will
contribute to an understanding of particle pickup processes near Callisto,
and thermal and non-thermal plasma interactions in the region.
About 45 minutes before closest approach, the Photopolarimeter Radiometer
instrument (PPR) performs a brief calibration of its signal before
embarking on a high-resolution scan from east to west across the night side
of Callisto. This observation begins 14 minutes before closest approach and
its purpose is to see how quickly different types of terrain cool off at night.
A scant three minutes before closest approach, the spacecraft once again
appears from behind the satellite as seen from Earth. This will be the
first signal we will have heard in an hour, but it will only be the pure
tone of the Radio Science occultation experiment as it probes the tenuous
atmosphere from the ground up. The first telemetry is still an hour away!
Just at closest approach, the Solid State Imaging camera (SSI) snaps the
highest resolution near-terminator images ever taken of any icy satellite!
From this vantage point, we may be able to see some boulders on the
surface, and will be able to see small impact craters as well. Travelling
at over 21,000 miles per hour at the time, the pictures would definitely be
smeared by the motion while the shutter is open. However, the spacecraft
employs a technique of moving the camera in the opposite direction of the
spacecraft motion to try and keep the desired surface features steady.
Shortly after this, SSI takes the first of two sets of pictures of a domed
crater. Eighteen minutes later, another set is taken from a different
angle. This will allow scientists to reconstruct a stereo view of the
surface, and to accurately measure the heights of the features they see.
NIMS next undertakes a high-resolution observation of a multi-ringed impact
structure called Asgard, to study the composition of the surface materials.
This is followed by a similar observation of a region near the crater Bran.
SSI also views Bran crater with moderate resolution, to provide context for
closer images taken during the 20th flyby in the Galileo series.
Context observations are important because they allow for more precise
location of high resolution images and also reveal the kinds of terrain and
the geologic features (such as large impact craters) that may have
influenced the area seen at high resolution. SSI takes several
opportunities as the spacecraft recedes from Callisto to look back and
collect such context images.
SSI also takes pictures of an area called the Valhalla Antipode. Valhalla
is a gigantic, continent-sized impact structure on Callisto, and the
antipode is that area of the satellite which is exactly on the opposite
side of the body. It is thought that the tremendous seismic energy
generated by the impact would be focused on the point on the surface
diametrically opposed, and may disrupt the surface in unusual ways. These
images will determine if there is any 'weird terrain' caused by this
seismic disruption at this location.
PPR performs two scans from east to west across the satellite to measure
temperatures and determine how the surface materials warm up during the
course of a Callisto day (one Callisto day lasts for over 16 Earth days).
The final science observation of the day, at 8:30 a.m. PDT, is a PPR scan
from pole to pole, to determine the temperatures at the poles. These areas
are likely to be the coldest places on Callisto, and may therefore be a
place where exotic volatile materials might collect, similar to the way ice
may collect at the poles of Earth's Moon and Mercury.
Though this may seem like at least a full day's effort, the entire block of
Callisto observations described here take place over a scant four hours --
five and a half hours, if you include Radio Science!
On the engineering side of the house, this evening sees a standard test of
the spacecraft gyroscopes, in preparation for an Orbit Trim Maneuver which
will be executed next Wednesday.
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. All times quoted above are in Earth