Yup, solid hydrogen, i.e., hydrogen below its melting point of 14K. It looks something like this. Unfortunately, the WISE mission site is flaky for me right now, but there’s a bit more information there about how it works. It’s only 6K less than liquid hydrogen, but I guess they needed the extra oomph for the mission.
Whoa, I thought that insane pressure was necessary to solidify hydrogen. What’s this stuff’s material properties like? Density? Electrical conductivity? Does it have some properties that make it more desirable for this mission than the usual liquid helium?
The preprint on arXiv regarding the WISE design says that the cryostat is basically of the same design as the one on WIRE. The reason it was used on WIRE, best as I can tell, was because solid hydrogen is a more efficient cryogen than liquid helium, per unit volume, and has a higher heat content, per unit mass, so they could make the whole cryostat smaller, and fit it into the payload fairing of their launch vehicle. As the hydrogen slowly sublimes, it’s vented, maintaining the (very low) temperature. I don’t know all that much about it, but I think you’re thinking of the metallic phase, which lives at very high pressures, not the regular solid phase(s), which have been produced for over a century.
I’ve never worked with solid (low pressure) hydrogen, but if it’s like solid noble gases, it’s transparent and very much like clear Jello in appearance .
It actually might be bonded a little more tightly, since hydrogen can bond better than noble gases, which in their solid phase are held together only through van der Waals bonding. But I’ll bet it’s still pretty Jello-like.
I gather that the point of using hydrogen is that in sublimating it absorbs more heat than the evaporation of liquid helium. So it’s a case of being cold enough and a more durable heat sink than helium.