the Hubble Space Telescope has had some
difficulty with its gyros lately which
are essential to helping it maintain a
stable pointing but how does Hubble
actually Point welcome back to launchpad
I'm Christian Reddy your friendly
neighborhood astronomer the Hubble Space
Telescope has been in orbit since 1990
and for 28 years it's performed
incredibly well Hubble operates in
low-earth orbit and it's for this reason
that the space shuttles were able to
access it on five different occasions
over the last 28 years
and these servicing missions have been
critical for Hubble's longevity but how
does a 12-ton telescope actually point
itself and maintain that pointing well
at first you might think let's just go
ahead and use thrusters after all if you
fire a jet in one direction the force
exerted by the jet on the spacecraft in
return will point the telescope in the
opposite direction that's Newton's third
law but there are however two problems
with thrusters the first is the obvious
one thrusters require fuel and fuel runs
out over time but a second problem with
thrusters are that they would create a
cloud of gas around the telescope and
that gas could then contaminate the
sensitive instruments inside and render
the telescope useless so Hubble needs to
make use of a different but very simple
method of pointing it relies on two
fundamental concepts in physics the
conservation of angular momentum which
is to say the conservation of the
momentum of things that rotate and
Newton's third law for every force
there's an equal and opposite Li
directed reaction force so Hubble
actually points itself using two systems
the first are called reaction wheel
assembly's reaction wheels make use of
these two laws by spinning up they
rotate at some speed and there are four
reaction wheels on board by spinning up
any combination of any three of these at
different speeds and different rates the
telescope then rotates in the opposite
direction so for example if a wheel is
rotating clockwise the telescope will
then counter rotate counterclockwise and
this is done for all three of Hubble's
axis of roll pitch and yaw a telescope
is subject to external torques
for example the telescope may be trying
to rotate in this direction but pressure
from the sun's radiation pushing against
the solar panels may be counteracting
the angular momentum of the reaction
wheel a second problem is that Hubble
can drag against Earth's atmosphere now
that may seem strange because Hubble is
after all in space but remember Hubble
is in low-earth orbit and that means
that it can still encounter fine
particles of Earth's atmosphere over
time that drag builds up that means that
the reaction wheels even when they're
spinning at their maximum rpm will not
be able to overcome these two forces so
Hubble employs another trick to help out
the reaction wheels it makes use of the
fact that it's actually embedded inside
Earth's magnetic field and it has these
black bars here these magnetic torque
rods or magnet workers these are
electromagnets that can be energized off
and on as needed to push against or draw
the telescope toward Earth's magnetic
field and that gives Hubble a little bit
of extra torque to overcome these two
pressure and drag forces so that's how
Hubble points itself but how does Hubble
know where it's pointing well the first
priority of any spacecraft is safety and
in Hubble's case that means keeping it
away from the sunlight
that's why Hubble carries solar sensors
onboard which keep an eye out for the
Sun and warned when the telescope is
within 50 degrees of the Sun now we
don't want sunlight coming right down
the barrel of the telescope and ruining
the sensitive instruments inside but we
also don't want sunlight to even warm up
the interior of Hubble's telescope tube
because that would cause the interior to
expand
and that would just ruin the alignment
of the mirrors and render the telescope
completely useless so the Sun censors
act as a lookout to ensure that Hubble
is always no closer than 50 degrees to
the Sun but Hubble also makes use of
magnetometers
which are mounted around the top of the
telescope
these magnetometers are sensitive to
Earth's magnetic field and act as a kind
of magnetic compass that lets the
computer inside to know how Hubble is
oriented with respect to the magnetic
field
now as Hubble is rotating it needs to
know how fast it's moving and in what
direction and that's where these gyros
come in that we've been talking about
these operate on basic gyroscopic
principles and as the gyro is spinning
its becomes very sensitive and very
resistant to the motion of the telescope
around it essentially the telescope
pushes on the gyro and the gyro pushes
back as it's spinning and the amount of
the pushback of the spinning gyro tells
the computer how fast the telescope is
moving now Hubble also employs a set of
fixed head star trackers it's these
three items right here on the aft shroud
of the telescope the star trackers
measure the position and brightness of
stars and feed that information to
Hubble's computer which then matches the
patterns of those stars against a star
map to help it know where the telescope
is pointing and the final component of
Hubble's pointing control system are the
three fine guidance sensors the fine
guidance sensors are mounted inside the
telescope alongside Hubble's main camera
they look along the periphery of
Hubble's field of view to lock on to
guide stars these are pre-selected stars
whose positions and space are very well
known and are bright enough to easily be
tracked
they in turn tell the computer if the
guide stars are drifting and the
computer in turn spins up and spins down
the reaction wheels as needed up to 30
times a second to correct for Hubble's
position these fine guidance sensors are
so precise that they allow Hubble to
maintain a pointing accuracy that is the
equivalent of holding a
laser-beam steady on the head of a dime
in Washington DC while standing in New
York City and holding that laser stable
for 24 hours
and Hubble's pointing control system is
redundant throughout the entire system
it needs three reaction wheels to point
so it has four on board it needs three
magnetic torque ER bars to ensure
orientation it has four on board it
needs two fixed head star trackers so it
carries three it preferably needs three
gyroscopes to sense its direction and
pointing so it carries six it needs two
fine guidance sensors to lock on to
guide stars so it carries three this
built-in redundancy is another reason
why the Hubble Space Telescope has
lasted as long as it has and best of all
engineers have worked out some tricks to
make use of some of these subsystems
such as the extra fine guidance sensor
to make up for one of the missing gyros
so Hubble still has a lot of life left
in it and if you'd like to learn more
about the problem with Hubble's gyros
I've made some videos which I'll link to
here and in the description below and if
you have any questions about how the
Hubble Space Telescope works or how
other space telescopes like Chandra and
James Webb or please let me know in the
comments below and I'll be sure to check
them out and as always if you'd like to
join me on this journey through this
amazing universe of ours well please
make sure to subscribe and ring that
notification Bell so that you don't miss
out on any new videos until next time
i'm christian ready and keep watching
the skies