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>> The Hubble Space Telescope
has had five servicing missions
Since its launch in 1990,

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>> from Servicing Mission
1 in 1993 to Servicing Mission 4
in 2009.

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>> Wait, what? Five servicing
missions but the last one is
called Servicing Mission 4?

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>> While it might seem strange
at first, there’s a reason for
that,

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>> and that’s because Servicing
Mission 3 has a very interesting
history.

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>> Originally scheduled for
launch in mid-2000, Hubble’s
third servicing mission was

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>> going to upgrade and
refurbish the telescope

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>> just as the first
two servicing missions had done
before. 

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>> But in quick succession,
Hubble’s all-important
gyroscopes began to fail.

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>> So, why are Hubble’s
gyroscopes so important? 

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>> Hubble deputy project manager
Jim Jeletic can explain.

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>> So, while we're turning
Hubble we need to know exactly

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>> which direction we're turning
it. So we use a sensor known as
a gyroscope,

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>> or a gyro. They use the
conservation of angular momentum
to tell us

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>> if Hubble is turning in which
specific direction, 

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>> and how fast it's turning in
that direction.

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>> So essentially without the
gyroscopes, we have no way to
know where Hubble is pointing. 

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>> Which, for a telescope, is
kind of important.

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>> In 1999, Hubble needed at
least three of its six
gyroscopes

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>> to be running in order to do
science, 

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>> and with only three gyros
still functioning, a Hubble
emergency was 

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>> declared and Servicing
Mission 3 was quickly split into
two separate launches.

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>> SM3A’s main priority would be
switching out all the gyroscopes

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>> with new and improved
versions, while SM3B

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>> was scheduled for a few years
later and would work on the
updates

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>> that Hubble still needed, but
weren’t as urgent.

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>> Due to this timing change,
the astronauts for SM3A had less
time

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>> to train for their important
mission, but they went into
overdrive

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>> and were ready in time for
their launch date of Oct. 14,
1999.

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>> However, delays caused by
final inspections and wiring
repairs on the shuttle

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>> continued to push the launch
date deeper into the end of the
year. 

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>> And then, as if things
couldn’t get any worse, in
mid-November,

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>> a fourth gyroscope on Hubble
failed. Hubble was put into safe
mode,

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>> and all science came to a
screeching halt.

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>> Thankfully, the Hubble team
had decided to split SM3 into
two missions 

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>> months earlier, otherwise
Hubble would have been totally
silent until the

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>> originally planned mid-2000
mission. 

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>> So after a few more delays
due to more inspections,
rewiring,

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>> engine replacements, and bad
weather, the crew of SM3A,

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>> sitting in the cockpit of the
Space Shuttle Discovery, was
ready to launch. 

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>> The 8-day mission had 3
spacewalks planned, the first
was

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>> the all-important gyro
switch-out. There were a few
minor problems

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>> that came up during the
8-hour, 15-minute-long
spacewalk, 

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>> but by the end of the day
Hubble’s gyro troubles were
over.

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>> The next two days had their
own spacewalks, updating Hubble
with

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>> a more advanced computer,
adding a new fine guidance
sensor, 

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>> installing a new solid-state
digital recorder, and replacing 

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>> Hubble's old outer insulation
with new panels in order to
protect 

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>> Hubble’s scientific equipment
from getting too hot or too
cold.

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>> When all the work was
finished, Hubble was released
back into orbit

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>> on Christmas day, and as the
crew of Discovery landed at
Kennedy Space Center,

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>> it became clear that this
mission was a resounding
success.

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>> Servicing Mission 3B would
also be a success later in March
of 2002, 

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>> followed by the fifth and
final flight, Servicing Mission
4 in May of 2009.

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>> Thanks to everyone involved
with servicing mission 3A, 

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>> Hubble was able to quickly
get back on schedule with its
ground-breaking science,

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>> and continue forward with its
mission of uncovering the
mysteries of the universe.

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>> So I'm sure you know who I
am! You know today is the
busiest day of 

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>> the year for me, I'm
delivering presents to good
little girls and boys

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>> I heard the Discovery was up
in space working over the
holidays

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>> so I decided to stop by and
visit my astronaut friends

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>> and deliver presents, because
they've been good girls and
boys!

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>> It's a very busy time of the
year, so I must be on my way! 

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>> But I would like to wish all
the boys and girls around the
world

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>> a Merry Christmas!
Especially those ones down in
Houston

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>> in Mission Control at the
Kennedy Space Center who have
done so much

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>> work during the holidays to
make this mission such a great
success!

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>> It's a busy, busy year so
I've gotta go! Merry Christmas
boys and girls!

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>> Merry Christmas!