cj#335> Science can still be a thrill


Richard Moore

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by Jeff Foust
American Reporter Space Editor
Cambridge, Mass.

                                 by Jeff Foust
                         American Reporter Space Editor

        PASADENA -- After a six-year, 2.3-billion-mile journey, the
Galileo spacecraft successfully arrived at the planet Jupiter Thursday, as
the main spacecraft entered orbit and the probe entered Jupiter's
        "Is this a great day or what?" NASA administrator Dan Goldin said
Thursday night at the Jet Propulsion Laboratory (JPL), mission control for
Galileo.  Goldin spoke at a press conference after Thursday's events.
        Two critical events took place late Thursday: the entry of the
Galileo Probe into Jupiter's atmosphere, and a thruster firing by the main
Galileo spacecraft to put the vehicle into orbit around the planet.
        The failure of either event would have been a serious blow to the
mission.  Had the probe not correctly entered Jupiter's atmosphere and
established contact with the orbiter, no atmospheric data would have been
returned.  If the thrusters had not fired, the spacecraft would have kept
going away from Jupiter, never to return.
        Scientists and engineers at JPL waited anxiously as the probe
approached Jupiter at 170,000 kmph (106,000 mph).  After the probe entered
Jupiter's atmosphere and slowed down to 400 kmph (250 mph), the probe
would establish contact with the main spacecraft, which was about to enter
orbit around Jupiter.
        The orbiter would record all the data sent by the probe.  However,
due to the low rate of the orbiter's antenna, it would only transit two
"bursts" of information to Earth to let controllers know that the probe
had established contact with the orbiter.
        The first burst of data was due shortly after 6pm EST.  At 6:10pm,
JPL reported that it had received the burst from the orbiter, reporting
that it was receiving data from the probe.
        The full amount of data collected will not be know for some time.
This Sunday Galileo will begin to playback some of the data received by
the probe.  Scientists hope to get up to the first 40 minutes of data by
        After Wednesday the orbiter will lose contact with the Earth as
Jupiter passed behind the Sun as seen from Earth.  Contact will be
reestablished in January when Jupiter reappears.  The rest of the data
will be downloaded by February.
        After the orbiter finished collecting data from the probe, it
prepared itself for a thruster firing called "Jupiter Orbital Insertion
(JOI)" that would place the spacecraft into orbit around the planet.
        The main thruster on Galileo had to fire at least 46 minutes to
place the spacecraft into the proper orbit.  If the thrusters fired less
than that time, the spacecraft would end up in the wrong orbit, or would
escape Jupiter's gravity altogether.
        JOI began at about 8:20 p.m. EST.  Controllers monitored the firing
by watching the Doppler shift of the spacecraft's transmissions and
watched the spacecraft's trajectory change.
        The thrusters burned for almost 49 minutes, three minutes longer
than needed and within 22 seconds of the optimal time.  Another, smaller
thruster firing is scheduled for Sunday to make any corrections to the
spacecraft's orbit.
        After Galileo finishes returning the probe data, controllers will
upload new programs that will give the spacecraft improved data
compression capabilities.  The compression software, coupled with
improvements in the ground stations, will allow Galileo to transmit data
at a faster rate, improving on the poor rate of the low-gain antenna.
        As Galileo approached Jupiter, it passed less than 900 km (540 mi)
of Io, one on Jupiter's four largest moons.  Mission controllers had
originally planned to take high-resolution images of Io during this time,
but due to problems with the spacecraft's tape recorder, a higher priority
was placed on recording the probe data.
        There will be no close approaches to Io for the rest of the
mission, but planners have suggested sending Galileo back past Io after
the main mission is completed.
        The success of the probe and the orbit insertion was cause for
celebration at JPL.  It was a great accomplishment for many of the people
involved with the mission, who had been a part of the Galileo project
since its inception 20 years ago.
        The success also eased the nerves of many workers.  At Thursday
night's press conference, one high-ranking official remarked, "We were
never worried."  His comment was met with roars of laughter.


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 Posted by Richard K. Moore (•••@••.•••) Wexford, Ireland
 Cyber Rights co-leader | Cyberlib=http://www.internet-eireann.ie/cyberlib