Encounter with Amalthea: Today on Galileo Monday and Tuesday, November 4-5, 2002
Early Monday morning begins our sprint into the inner reaches of the
Jupiter system to snatch the scientific secrets of that environment out
from under the nose of the gas giant, and to skirt by the tiny inner
satellite Amalthea. The science instruments that will focus on the inner
magnetosphere are the Dust Detector (DDS), the Energetic Particle Detector
(EPD), the Heavy Ion Counter (HIC), the Magnetometer (MAG), the Plasma
Subsystem (PLS), and the Plasma Wave Subsystem (PWS) instruments. The
Galileo spacecraft, however, may be unique among NASA’s planetary probes in
being the only mission to add a science instrument to its payload after launch!
The Attitude Control Star Scanner, an engineering system normally used to
provide information about the orientation of the spacecraft by sensing the
locations of stars, can double as a radiation sensor. Several years ago,
engineers noticed that the pesky radiation-induced noise that interferes
with the normal star sensing of the instrument could be used to provide a
measure of the intensity of that radiation. The sensor mechanism is most
sensitive to high-energy electrons. Though the instrument was never
designed or calibrated to provide an absolute physical measure of the
quantity of such electrons, when combined with the measurements taken by
the other science instruments, the relative noise level seen by the Star
Scanner can provide additional insight into the continuum of particles and
other radiation in the environment sensed by Galileo.
At midnight, the spacecraft is 20 Jupiter radii from the center of the
giant planet (1.43 million kilometers or 888,000 miles) and the science
instruments are studying the magnetospheric plasma sheet, which
periodically waves past Galileo as the planet rotates.
By 6:30 a.m., PST, the radiation from Jupiter is becoming strong enough to
cause a noticeable effect in the Star Scanner. At this point, the Attitude
Control system is told to rely only on a single bright star for knowledge
of the orientation of the spacecraft. The static in the sensor caused by
the radiation is enough to mask the signals from fainter stars. The single
bright star we are using for this encounter is Rigel Kentaurus, more
popularly known as Alpha Centauri, the nearest bright star to the Sun.
At 9:45 a.m., the EPD instrument turns its power off and on again, and
reloads its memory. During a small number of previous encounters, this
instrument has suffered upsets which can only be cleared by this technique.
Three times during this flyby the instrument is reset in this fashion, so
that if an upset occurs, the instrument will be able to continue to collect
science data without waiting for commands from Earth to correct the problem.
At 1:02 p.m., the Radio Science team begins an experiment to measure the
gravity field of the small satellite Amalthea. Though we are still 10 hours
away from the closest approach, the team uses this distant measurement of
the radio signal to establish a baseline against which they can compare the
changes seen as Amalthea’s gravity tugs on Galileo during the later flyby.
By measuring the extent and nature of this tug, the mass of Amalthea can be
determined. In addition, the flyby’s proximity will also yield knowledge of
whether or not Amalthea has a dense central region or core. This
information will give additional clues as to the composition of Amalthea
and may also help us to understand its origin.
At 2:55 p.m., the spacecraft is again expected to pass through Jupiter’s
plasma sheet, and detailed Fields and Particles measurements are written to
the tape recorder. The recorder is used to collect data faster than the
spacecraft can transmit in real time. At this time the spacecraft is only
10 Jupiter radii from the planet (715,000 kilometers or 444,000 miles).
After 45 minutes, the instruments revert to collecting data for real-time
transmission to Earth.
At 5:49 p.m., the Fields and Particles instruments switch from transmitting
all of their data in real-time to begin recording the data for later
playback. This allows the instruments to collect more data at a higher time
resolution than would be possible in real time. This recording continues
for the next 10.5 hours, through the closest approach to Amalthea and Jupiter.
At 6:07 p.m., the spacecraft changes its telemetry system to put more power
into the fundamental carrier frequency that is transmitted. This allows the
70-meter-diameter (230 foot) communications antenna located near Madrid,
Spain, to better track the Galileo signal during the upcoming close flyby
of Amalthea. It is the change in frequency (Doppler shift) of this
transmitted signal that provides the Radio Science and Navigation teams the
information about Amalthea’s gravity field.
At 7:18 p.m., the Near Infrared Mapping Spectrometer begins a 5-minute
period of real-time collection of engineering data. This peek into the
signals generated by the instrument as the radiation level rises will help
researchers understand detector behavior seen during observations taken on
previous orbits. This information can be used to help engineers design
instruments that will operate in similar radiation environments for future
missions.
At 7:41 p.m., Galileo reaches the closest point to the volcanic satellite
Io. At 45,250 kilometers (28,100 miles), this pass is over twice the
distance that Voyager 1 flew by in 1979, and is a distant cousin to the
101-kilometer (63-mile) altitude at the previous encounter in January of
this year. No observations of Io are planned during this passage. The
spacecraft is passing Io’s orbit at about 6 Jupiter radii (429,000
kilometers or 267,000 miles) from the planet on its way in to the inner system.
The radiation at this point in the orbit is becoming fierce enough that
even Alpha Centauri may no longer be seen by the Star Scanner, and the
attitude control software would not be able to determine the orientation of
the spacecraft. At 8:12 p.m., the software is told to enter hibernation. In
this state it will ignore the signals from the Star Scanner and remember
its last calculated orientation and spin rate, relying on the fact that we
don’t plan to change it. This configuration will last for the next nine
hours, while Galileo is within the distance of Io’s orbit.
Then, at 11:02:28 p.m., Galileo reaches its closest point to Amalthea. This
irregularly-shaped moon measures approximately 270 kilometers (168 miles)
across its longest dimension. Galileo will fly by with its closest distance
to the surface of the body of 160 kilometers (99 miles). The speed of the
spacecraft relative to Amalthea is 18.4 kilometers per second (41,160 miles
per hour) so it will take less than 15 seconds to pass by! At this speed,
Galileo could circle the Earth (at sea level) in 36 minutes, not counting
stops for the speeding tickets.
Ten minutes later, at 11:14 p.m., Galileo enters the shadow cast by Jupiter
from the Sun, and eleven minutes after that, at 11:25 p.m., the spacecraft
passes behind Jupiter as seen from Earth. The spacecraft will remain out of
view of ground controllers for about an hour, reappearing 23 minutes after
midnight on Tuesday morning, having cleared Jupiter’s shadow 10 minutes
earlier.
While the spacecraft is hidden from Earth, at eight minutes after midnight,
it will reach this orbit’s closest point to Jupiter. This is also the
closest Galileo has ever come to the planet. Galileo will pass 71,500
kilometers (44,500 miles) above the visible cloud tops. This is three times
closer than the previous Galileo record in 1995, which was set as we first
entered Jupiter orbit. Pioneer 11 still holds the ultimate record, however,
speeding by in 1973 only 43,000 kilometers (26,725 miles) above the clouds.
For a period of about two hours, starting about the time Galileo passes
Amalthea, the spacecraft will be passing through a region occupied by what
is known as the Amalthea Gossamer Ring. This very tenuous band of dusty
material circles Jupiter between Amalthea’s orbit and the start of the more
prominent main ring first noticed by the Voyager spacecraft in 1979. This
offers a unique opportunity to study a planetary ring system from the
inside! The Dust Detector instrument will be the primary student, but the
plasma environment is also likely to hold some interesting surprises.
On the outbound stretch of the Jupiter-Earth occultation, the Radio Science
team will use the radio transmission from Galileo to probe the layers of
the Jupiter atmosphere, studying how the signal changes as it passes
through increasingly thinner gases as the spacecraft recedes from its
closest point.
At 12:20 a.m., the EPD instrument reloads its memory again, as protection
against a possible upset in the high radiation environment. During this
single flyby the spacecraft may be subjected to up to 100 times the
radiation dose that would be lethal to a human being. It has already
received more than 4 times its planned spacecraft-lifetime dosage, and is
still ticking away.
At 12:37 a.m., the Radio Science occultation experiment is over, and
science telemetry is restored into the radio signal. For the past few
hours, the Fields and Particles science data have been stored on both the
tape recorder and in a computer memory buffer while the spacecraft has been
out of sight. Now the buffered data can be sent to Earth. The continuous
recording period ends at 4:04 a.m. Recorded data from the encounter will be
played back starting Thursday evening.
At 4:15 a.m., Galileo again crosses Io’s orbit, this time outward bound,
and the radiation levels have dropped to the point that the Star Scanner
should again be able to recognize Alpha Centauri. At this time the attitude
control software is told to come out of hibernation and re-establish its
lock on that single bright star. By 6:30 p.m., the radiation has dropped to
the level that will allow fainter stars to be seen, and the software is
told to look for the normal contingent of three stars.
Finally, (has this really only been two days?) the tape recorder is slewed
to a new position and a new series of plasma sheet observation recordings
is begun at 11:07 p.m. Tuesday night. The high-intensity pace of the
encounter has slowed to a more bearable crawl, the spacecraft has receded
again to 20 Jupiter radii from the planet, and the final flyby of the
mission is behind us.
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 44 minutes to travel
between the spacecraft and Earth. All times quoted above are in Earth
Received Time at JPL in Pasadena.
For more information on the Galileo spacecraft and its mission to Jupiter,
please visit the Galileo home page at one of the following URL’s:
