today i thought we'd talk about how you
actually steer and point
a 12-ton spacecraft that's up in orbit
around the earth such as such as the
hubble space telescope look at that our
pride and joy so where is that
i mean obviously that's on your desk
where's the real one
no one's up there orbiting around the
earth it was designed so that it could
be in an orbit that would be accessible
by the space shuttle so that if and when
things went wrong we could send
astronauts up there to fix it and thank
goodness that we did because there have
been several issues with hubble in its
wonderful long lifetime that these
astronauts
risking life and limb to help us out
have managed to solve which has made it
such an incredibly long lasting and
successful mission
mike massimino's reflection
in the
aft shroud of the hubble space telescope
as he prepares to open the doors
protective doors over the fixed head
star trackers and the rate sensor units
well i want to talk about the physics
behind how you actually get a telescope
like this to point in the direction that
you want it to point and keep it there
and it's actually surprisingly simple
physics and it's all down to the
conservation of angular momentum
now you might see in in some science
fiction shows like battlestar galactica
or something like that uh that they
maneuver spacecraft with thrusters and
you've got these jets and propellants
and newton's law says that if you squirt
a jet of propellant in one direction you
move in the opposite direction but for
many reasons that's not ideal for a
telescope like hubble first of all
you don't want to be spreading
some sort of material like propellant in
the atmosphere around your telescope it
will cause a fog it'll get condensed on
your instruments and fundamentally it's
a finite
resource and so eventually you'll run
out the pointing system behind hubble
consists of several elements
first to
help tell how it's pointing we use
gyroscopes and you might be familiar
with gyroscopes like this this was a
christmas present in our house this year
and the thing about a gyroscope is that
once you get it spinning
it stays pointing in the same direction
and that's because of the confirmation
of angular momentum
so once you get the gyroscope spinning
it doesn't like to move in a different
direction so i can lift this coffee cup
up and i can tilt it but you'll notice
that the axis
of the spin keeps pointing the same
direction
and so the gyroscopes on the hubble
space telescope work in the sense that
they keep pointing in one direction and
if the telescope starts to move
the sensors in the telescope will feel a
force you feel the gyroscope pushing
back resisting that motion
and that's a diagnostic that tells the
sys the spacecraft system
that it's pointing in it's trying to
it's trying to move or point in a
different direction
so the normal complement of gyroscopes
on the hubble space telescope is six and
that gives you extra redundancy because
normally you need three to be operating
one pointing in each perpendicular
direction to tell you which way you're
going but the gyroscopes are not the
only pointing mechanism that the whole
space telescope has
it actually has these four
black bars that you see here on the
outside
and those are called magnetic torque
bars
and they're kind of neat because they
use the fact that the hubble space
telescope is operating in earth orbit
which means it can feel the magnetic
field of the earth and so there are
electromagnets that can be switched on
and off and actually provide a torque
perpendicular to the magnetic field of
the earth and help move the telescope in
that way the magnetic torque bars
link up to the final piece of the
steering puzzle
because we've talked about how you know
where you're pointing and how you know
if you start moving in orientation
but how do you actually make the
telescope spin on its axis and for that
there are four flywheels they're called
reaction wheels that can be spun up or
spun down by an electric motor and so
again it's all just simple conservation
of angular momentum you cause the
reaction wheel to spin up in one
direction the rest of the telescope has
to move in the other direction to
compensate everything i've talked about
with hubble
really relied on intervention by people
to keep it going to replace the gyros
and the flywheels as well the reaction
wheels were replaced two of them failed
over 18 years and were replaced as well
all of these things have moving parts
right
and so all of those things are subject
to failure
when you think of other telescopes that
are sent up now
they may not be so accessible they might
be going to a lagrange point or being in
an earth trailing orbit so for example
the james webb telescope exactly it's
going to be much further away and
completely outside any hope of
intervention in the future so instead of
these very vulnerable gyros that are you
know subject to working parts and wires
that are corroding and things like that
james webb actually uses a different
kind of gyroscope which has no moving
parts at all and it's called a
hemispherical resonator gyroscope and
essentially it's not much different
than a wine glass so if you take a wine
glass or anything else that sort of
resonates and you get it going
if you put it on a turntable
you'll notice something this is this is
exactly what g.h brian discovered back
in 1890
that if you think about what's happening
with the
vibrations in this wine glass if i
strike it if you could look at it in
very slow motion you'd see it deforming
this way and then deforming that way it
would be squashing in and out in two
different directions if you put that on
a turntable and you spun it around
you would expect to hear a beat a sort
of a wah wah wah sound as you turned it
around as each one of those nodal points
came into view
what he found was that as you did that
you didn't hear the four
beats per revolution that you would
expect you only heard about 2.4
which means the vibrating pattern
is trailing the rotation of the physical
object what that means is that you have
just like with the gyroscope which
exerted a force when you tried to move
it you now
by
noting the vibrations of this
hemispherical object
you can again get a sense of whether it
is moving or not
and so on james webb
they essentially have
a little quartz hemisphere these are
these are very very finely machined and
incredibly sensitive instruments and
it's vibrating at a resonant frequency
and then just little electrical sensors
will will be able to sense whether or
not it's moving and the wonderful thing
about this is it works better in a
vacuum right and so it'll actually work
better in space than it does on the
ground and it has no moving parts and
again you know it's it's much less
likely to fail than the old mechanical
spinning gyroscope so fingers crossed
that when james webb goes up uh it will
carry the full complement of these
gyroscopes um and that they'll work for
the lifetime of the mission
until it reaches the very top at which
point it will stop
and the thing is like a really good putt
yeah like a really really good put yeah
um that just sort of stops right on the
edge and then falls in in that time
the antenna will fold out so it can talk
to earth and then the whole thing moves
upwards