SETI bioastro: FW: Today On Galileo - May 22, 2001

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From: Larry Klaes (
Date: Tue May 22 2001 - 10:44:15 PDT

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Sent: Tuesday, May 22, 2001 11:43 AM
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Subject: Today On Galileo - May 22, 2001

Today on Galileo
Tuesday, May 22, 2001

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.

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.

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:

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