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This Week on Galileo?
Today on Galileo
Tuesday, May 22, 2001
DOY 2001/142

Galileo's Mission at Jupiter - Day 1 of the Callisto 30 encounter

Today the final commands in the cruise portion of the orbit are completed. The routine maintenance of the on-board tape recorder is finished, and the spacecraft is turned approximately 5 degrees so that the Star Scanner, which helps determine which direction the spacecraft is pointed, can view the star Achernar (Alpha Eridani - the brightest star in the constellation of Eridanus, the River). This is the sixth brightest star in the star catalog that we maintain for the spacecraft. Such a bright star is used when the spacecraft is within about 15 Jupiter radii (1 million kilometers or 700,000 miles) of the giant planet because the intense radiation near Jupiter creates noise in the Star Scanner, and a strong star signal is needed in order to be seen through this noise.

At 10:50 a.m. PDT [see Note 1] the sequence of commands that directs the spacecraft's activities for this encounter begins to execute.

First up, at 6:20 p.m. PDT Galileo turns off telemetry to Earth in preparation for a Jupiter occultation, which occurs when the spacecraft passes behind Jupiter as seen from Earth. The Radio Science Team uses these opportunities to study the structure of the atmosphere of the planet by measuring changes in the radio signal as it passes lower and lower through the increasingly dense layers of the atmosphere. Telemetry is turned off to provide the maximum amount of power to the pure tone of the transmitted signal carrier frequency. The telemetry data collected by the spacecraft sensors during this time are stored in a buffer area of computer memory for later retrieval. At 8:20 p.m. PDT even the carrier tone is expected to be lost and the spacecraft will be completely hidden behind Jupiter's vast bulk. This continues until 10:48 p.m. PDT when the spacecraft will reappear on the other side of the planet.

At approximately the same time, another type of occultation occurs: a solar occultation. This is when the spacecraft passes into the shadow cast by Jupiter. No specific observations are planned to mark this event, but the craft will be out of the warming rays of the Sun between 9:21 p.m. and 11:46 p.m. PDT. The on-board software that checks for health and safety of various spacecraft systems has been warned of the occultation and will not trigger any of its programmed responses during Galileo's passage through Jupiter's shadow.

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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.

 
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