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This Week on Galileo
Today on Galileo
Monday and Tuesday, November 4-5, 2002
DOY 2002/308-309

Encounter with Amalthea

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.

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