This cooling was done using helium II and mission designers had to overcome significant challenges such as how vent helium vapour when it is mixed in with blobs of liquid in a low-gravity environment.

IRAS was a watershed mission in astronomy because nobody had so extensively observed the universe in these infrared wavelengths before. Astronomers could peer through dust clouds and see objects that had been invisible to other telescopes.

IRAS observed the universe for 300 days before its superfluid ran out, but a decade later NASA was able to transfer liquid helium in space. How was that done?

Yes, that was a project called Superfluid Helium On-Orbit Transfer (SHOOT), which carried superfluid helium onboard a Space Shuttle. The demonstration involved transferring superfluid from a full dewar to an empty dewar in microgravity. This was done using a pump that made use of the “fountain effect” in helium II.

How does the fountain effect work?

The effect can be understood in terms of the two fluid model, which describes helium II as having a superfluid component and a normal fluid component. These aren’t real physical phases within helium II, but rather provide a convenient way of understanding many of its mechanical and thermal properties.

The effect occurs when two regions of helium II are separated by a porous plug with micron-sized channels. If the helium II in one region is heated and the other region is cold, the superfluid component will move through the porous media towards the heater. This is possible because the superfluid component has zero viscosity and can move without resistance through the tiny channels – something that the normal fluid component cannot do.

In the heated region, some of the superfluid component will become normal. However, the normal component is viscous and cannot exit the warm region via the porous plug, so pressure builds up. This pressure can be used to pump helium II without the need for mechanical components.

SHOOT was an important demonstration of how helium II could be used transferred in space. However, researchers realized that it is more cost efficient to launch experiments with larger dewars and lower heat loads, than to refill a dewar during a mission.

Helium II also has the ability to flow up the wall of a dewar, but despite its exotic properties a superfluid is relatively easy to handle in bulk. Why is that?

Research done in the 1970s and 80s showed that bulk helium II has essentially the same fluid mechanical properties as a conventional fluid – something that can also be explained by the two fluid model. When helium II flows, quantized vortices in the superfluid component interact with the viscosity of the normal fluid component. The result is that the bulk properties are the same as a conventional fluid.

This is tremendously helpful to engineers like me, I suppose we can be thankful that sometimes the universe is kind. The standard engineering rules that are used to design fluid-handling systems also apply to helium II – rules that help use chose components such as pipes, pumps and valves for a given system. The only instances when we need to consider the special properties of helium II are when we are transferring heat, using porous media or creating thin films of the superfluid.

There are several Nobel Prizes for Physics that were made possible by helium II cooling. Do you have a favorite?

For me it’s the 1996 prize, which went to David Lee, Douglas Osheroff and Robert Richardson for their discovery of superfluidity in helium-3. The superfluid that we have been talking about so far in this interview is helium-4, which is by far the most abundant isotope of the element. Helium-4 is a boson and bosonic atoms are able to condense into the lowest quantum energy state of the system, creating a superfluid.

Helium-3 atoms are not bosons, but are fermions. These atoms cannot undergo this Bose–Einstein condensation directly to create a superfluid. However, it the early 1970s Lee, Osheroff and Richardson showed that helium-3 can condense into a superfluid at the much lower temperature of 2.7 mK. The physical mechanism for this is similar to what occurs in superconductors, where at low temperatures, fermionic electrons pair up. These “Cooper pairs” are bosons, so they can condense to create a superconductor in which the electrons can flow without resistance.

Because of its magnetic properties, superfluid helium-3 is a much more complicated substance than superfluid helium-4. It has three different superfluid phases, rather than the one phase of helium-4.

What I like about this discovery is that the trio weren’t searching for superfluity in their experiment. Instead, they were studying the properties of solid helium-3 at very low temperatures and high pressure. I really like the fact that they were looking for one thing and found something entirely different. Often, the most exciting scientific discoveries are made this way.