Up to Date
The following activity occurred over the last five months, April 7 through August 31,
1995. See related stories on Probe Release (see story, From
the Project Manager) and ODM (see story, "The
Morning After" Press Conference). The Orbiter is currently operating normally,
spinning at about 3 rpm and is transmitting telemetry at 10 b/s. The Probe, healthy at the
time of release, will not be heard from again until arrival at Jupiter.
Navigation
The planned TCM-24 for June was not needed because the Navigation Team found that
TCM-23 had already achieved the required probe aim point. TCM-26 (ODM was TCM-25)
fine-tuned the Orbiter's trajectory to the Io-Jupiter encounter aim point by removing the
small trajectory errors of ODM (which had a very slight 1.2 percent underburn).
Dust Streams Continue
Additional memory readouts were performed during August to collect data on the greatest
Jovian dust storm activity ever observed (it began July 28). The dust detector subsystem
(DDS) instrument that collects data for the size and speed of these tiny particles has had
impact rates ranging from a few hundred to almost 20,000 impacts per day. Compare this
to the typical impact rate in space--one particle per 3-day period. The 3-week storm in
April 1995 (reported in the May issue of The Galileo Messenger) collected a total of
2900 particles. Scientists look forward to learning the source of this tremendous dust
stream activity after Galileo enters orbit around Jupiter.
Plasma Science Checkout
The plasma science instrument (PLS) has been checked out after an August 7 turn-on. The
PLS is now configured for its measurements of the composition and the velocity
distribution of plasma in the Io torus.
Relay Radio Antenna
On August 23, the relay radio antenna (RRA) was moved to the initial "elevation" pointing
direction for probe relay on December 7 (the "azimuth" or "roll" direction will be
established at about 4 hours before relay starts).
Gravity Wave Experiment
Data-taking for the second and final, long round-trip-light-time Gravity Wave Experiment
(GWE) began May 20 and ended June 28. The experiment searched for low-frequency
gravitational radiation-ripples in the curvature of space-time that are predicted by Einstein's
general relativity. The GWE is motivated both as a test of physical law (gravity waves are
a prediction of general relativity, but have not yet been directly detected) and as a potential
new window for observational astronomy. The GWE used the Earth and Galileo as test
masses, with their relative velocity continuously measured by Deep Space Network
Doppler tracking. The observations required that the spacecraft be near opposition, the
Earth-Galileo distance be large, and Galileo be in interplanetary space. These conditions
were required to minimize the detrimental effects of the interplanetary plasma on the radio
signal. A gravitational wave incident on the Earth-spacecraft system will cause three small
perturbations in the Doppler frequency. While it is thought that gravitational waves will be
rare at the sensitivity level set by the Galileo radio system, analysis of the GWE data is
underway, and even a negative result will help to constrain models of gravity wave
phenomena.
Key Uplink Planning Activities
The Project approved the final updates to the sequences that cover spacecraft activities from
October 9, 1995 to January 3, 1996, the so-called Jupiter Approach and the Jupiter
Encounter time periods. Special testing of the critical engineering sequence, called Relay/
JOI will be performed on the spacecraft testbed in the period from September 6 through
October 11, 1995. This sequence is designed to ensure that, even in the face of faults,
Galileo will acquire the probe data and get into orbit around Jupiter.
Telecommunications/Ground System
Several tests of the Block V receiver (BVR) were successfully performed over DSS 14 and
DSS 43 in June in preparation for their first operational support of Galileo at probe release.
Telemetry rates of 8, 10, and 16 b/s were received and processed, as was two-way Doppler
data for navigation.
Anomaly Status
Analysis of data received since the July 27 ODM shows that one of the two checkvalves in
the helium pressurization system apparently remains open. The purpose of these valves
(one on the fuel side and one on the oxidizer side of the propellant system) is to limit the
amount of propellant vapor that can flow upstream, potentially condense, subsequently
react, and in a possible, but very unlikely scenario, damage the propellant feed system.
Project engineers plan to manage the propellant temperatures and pressures to minimize the
possibility of reaction. Even without this additional propellant temperature and pressure
management, the propulsion system design makes the potential for any harmful interaction
of the propellants very low. No impact is expected on planned maneuvers or on the
mission as a whole.
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