I am off to go find the coldest thing that I can get my hands on.
And I cannot think of a more fitting
place than this.
Speaking of cold things, we all know that canned air
can get things cold. But why? This can has
pressurized liquid difluoroethane.
Now difluoroethane has a really low boiling point. That means, when I
push the nozzle, it escapes really quickly because it is boiling away.
Now we use something else with a really low boiling point. In fact, it is
the lowest of all the boiling points, to cool things
on the James Webb Space Telescope. And that element is helium.
How does helium make stuff cold?
Helium's low boiling point has come
in handy in a number of ways on this mission. The Webb Telescope traveled
from Goddard Space Flight Center to John Space Center to be tested
in Chamber A. now this is the only thermal vacuum chamber
chamber large enough to fit Webb. Chamber A even got an upgrade
so it can use helium to get even colder.
We also use helium in a special refrigerator called a cryocooler on the telescope itself.
Now one of the reasons that Webb has a cryocooler
is because it has to operate at very cold temperatures. This is because it detects
in the infrared spectrum of light. Infrared is something we experience
as heat. And so the telescope needs to be as cold to avoid any sources of heat
that do not come from the cosmos.
Webb in fact is so sensitive to any infrared light, that the science instruments
need to be cooled to avoid detecting heat it creates by just operating!
Three of the instruments are cooled passively by sitting
on the cool side of the sunshield. The last instrument, MIRI
needs to be cooled more than the other instruments
to 7 degrees Kelvin using the cryocooler.
Now 7 degrees is unimaginably cold.
I have here the coldest thing that I could get my hands on
music
It is dry ice.
And this is still 194
degrees Kelvin. Much warmer than Webb will be at its
coldest point. Alright, we know that Webb has to
get cool using the cryocooler, but how does helium make it cool?
It's because the atoms are really small and can move really fast
Helium only has 2 protons and 2 electrons.
That means it is super light and it means the gasses move really quickly.
That’s why it tends to float away. And because
the gases expand so quickly, it also means that it cools
things really quickly. So when things go from a pressurized liquid
liquid to a gas they tend to cool. You see that same effect if you use a
pressurized pot. When its liquid and boiling it is very
very hot. The steam escaping it actually cools really rapidly.
If you were to put your hand above, right above where that steam is coming out
its gonna be hot. But even just one foot
above that, and that air is touchable.
This is called the Joule-Thompson effect. If you blow on your hand
with an open mouth, the air is going to be hot, but if you purse your lips,
the air has to expand more rapidly and it has a cooling effect.
The cryocooler’s liquid helium expands
and contracts and expands again in a chamber similar to this
And because it needs to get even colder, it has
has two chambers. And this is going to allow us to see
the developing Universe!