The Opik-Oort Cloud, which I’ll dub the OOC (oo-cee), begins from the far reaches of the Scattered Disk and extends out to almost a light-year or so. The effective temperature of any object in the OOC is going to be low. But what interesting marker-posts are there on the way to the CMB (Cosmic Microwave Background at 2.735 K – the very coldest anything can permanently get in this Universe)?
Hydrogen, in its two stable isotopes of protium and deuterium, first liquefies, then freezes from 38.35 K (@16.65 bar) downwards. The two are separate in their marker points by a few degrees and a few bar, which makes for ease of separation when they’re cold enough.
Protium, Hydrogen deuteride & Deuterium Marker Points
Marker Point |
Protium |
Hydrogen deuteride |
Deuterium |
Critical Point |
32.97 K @ 13.15 bar |
… K @ … bar |
38.35 K @ 16.65 bar |
Melting Point |
14.01 K @ 1.01325 bar |
… K @ … bar |
18.55 K @ 1.01325 bar |
Boiling Point |
20.28 K @ 1.01325 bar |
22.13 K @ 1.01325 bar |
23.67 K @ 1.01325 bar |
Triple Point |
13.8033 K @ 0.07042 bar |
16.60 K @ 0.1237 bar |
18.73 K @ 0.1715 bar |
Protium is plain hydrogen (H2), hydrogen deuteride (HD) is a protium atom plus a deuterium, since hydrogen is found as a diatomic molecule almost always. And deuterium (D2) is the handy fusion fuel isotope.
Deuterium is the Sun’s main power source. In the Sun plain old ionised protium (i.e. protons) is first fused into deuterium in a weak-force interaction, which spits out a neutrino as one of the protons swallows an electron and becomes a neutron. This is a pretty low energy and low probability reaction. The Sun’s core has a low energy-making rate per unit mass, but there’s a LOT of it, so the output is huge.
Then the deuterium has two options – bang into a proton and make Helium-3 (3He) or bang into another deuterium and make helium-3 or tritium. That reaction, D + D, and the similar 3He + 3He reaction, make most of the energy of the Sun, and happen very quickly. In very low mass stars the core temperature is too low for quick 3He reactions, so it builds up until the core heats up a bit more over trillions of years.
Back to the OOC… according to some cosmogonic modelling, there may be several Mars to Earth mass planets out there, thrown outwards by the migration of Neptune and Uranus to their current orbits. Such planets would have retained their original hydrogen/helium atmospheres, and if they’ve sufficiently chilled to ambient, then lakes and seas of different hydrogen isotopes could well exist on them. Imagine the boon that such easy access to liquid deuterium or its ice would be. However one romantic vision – the Enzmann Starship – can’t be obtained. To recap, Robert Enzmann imagined huge starships using D-D fusion with the deuterium stored as a great big frozen ball and the starship attached. Problem is that frozen hydrogen isn’t hard. Instead it’s a soft solid, perhaps more like soft butter than the hard frozen water we typically call ‘ice’. No starship could push against the stuff and not sink into it…