To add to @Cosmacelf's answer, I'll add a point of reference: the James Web Space Telescope is sitting at a Lagrange point with a sunshield, and the mirrors remain at 55K. Oxygen stays liquid between 54K and 90K. Methane is 91K to 112K. So if you can isolate from energy coming from Sun, Earth and Moon, you're in pretty good shape.
Right, which is why Blue Origin’s hydrogen propellant makes even less sense for deep space missions (Meaning anything to the moon or beyond).
JWST is not really a good baseline as it has a far more benign and directionally constant thermal loading together with lots of deep space to radiate to. Low earth orbit provides much larger thermal loads coming from a large arc which changes throughout the orbit.
Indefinitely, if you can deflect enough incoming radiation (from sun and Earth; the Moon's contribution is measurable but probably insignificant). A reflective Mylar sunshade or equivalent would do the trick, although it might also severely irritate astronomers. A potential approach is to keep the nose of the depot pointed directly at the sun, with a reflective sunshade keeping the entire ship in solar shadow. Then shade one side of the depot as well, facing the shaded side towards Earth at all times, except perhaps for a few minutes each orbit in twilight when it would be problematic for astronomy. Gyroscopic flywheels should be capable of performing the rotation; it shouldn't take reaction mass. JWST has shown that the sunshade approach can be extremely effective, although as noted, the dynamic lighting (Earth + sun) in LEO is much trickier than at L2.
Hey, astronomers are people too.
I was curious enough to do some math.
spectrum.ieee.org
Yeah, but you can dump a lot of that heat with radiators. It's not going to be nearly as bad as on Earth.
At first, yes. But if Elon's dreams come true of sending 1000 Starships to Mars every couple years, it will be far more than that. Of course, by that time we'll be able to place enormous telescopes in orbit equally well, so maybe the astronomers will be ok with it after all.
If you think reflective Starships might upset astronomers, just wait til you add several acres of solar panels to each one! Nuclear may be better suited as a power source for that reason, though that may make it a greater environmental hazard than just launching LOX+LH2 in the first place. And with H20 you have freezing to worry about as well as boiling; the net energy flux needs to be close to zero in either case to keep the fuel tank contents at a constant temperature. Once we're on the Moon or Mars, where we can obtain raw materials, then ISPP (in-situ propellant production) is essential. But probably not on-orbit, I think.
You seem to envisage a sunshade which is mounted away from the ship, as is the case with JWST and a number of other telescopes. My understanding is that the reason for having a shade distanced from the surface of the telescope is to reduce distortion of the telescope hence improving accuracy/performance. In the case of Starship that would not be an issue and surface mounting the insulation would be far preferable. This is already done with the thermal blankets and tiles which cover the reentry surfaces (and which do not appear to frost when fuel and LOX are loaded). I was surprised when the ships which were thought to be for testing ship to ship prop loading did not have the thermal blankets/tiles - fully tiling those parts of the orbital tanker ship which cover the fuel/LOX tanks would seem a simple way to use existing proven technology to reduce the heat absorbed by the ship.
A different way to think about it: at thermal equilibrium, the "average temperature" as seen in all directions from the Depot's point of view must be the same temperature as the fuel. (About -180 °C.) Ironically, the boiling point of LOX (-183 °C) is slightly lower than the freezing point of CH4 (-182 °C), so the whole ship can never exactly reach thermal equilibrium. "Deep space" is much colder than that; the Earth is much warmer, and the sun is obviously extremely hot. The equilibrium blackbody temperature at 1AU from the sun ("average temperature in all directions") is around -18 °C. To keep a spacecraft below this, radiation from the sun must be diverted, not just insulated; that's why the sun shield should ideally be separate from the spacecraft, and reflective, not just insulating. (Or it could be attached at a point, like an umbrella.) Gold foil or aluminized film (e.g. Kapton) is also very effective at reflecting infrared wavelengths, which could be helpful on the Earth-facing side, and possibly enough if it were on the nose, with the nose kept facing the sun. Stainless steel is not terrible (about 90% reflective), but Kapton would be an improvement, if it could be deployed post-launch. I doubt it could survive Max-Q if applied in advance, but maybe?
I doubt Starship is going to be anywhere close to 90% reflective in practice.
That is why I specifically referred to the existing thermal blanket and tiles used on Starship - they are designed to withstand launch.
I did not claim that it reduced the total exposure (because as you say, that would be completely incorrect). What it does is it reduces the heat transferred to the propellant and hence boil off (because it reduces localized high surface temperatures - point (a) in your list. Higher surface temperatures mean higher thermal gradients to drive the heat transfer to the propellant).
Sorry, this average temperature for thermal equilibrium is not a good way to think about it as it does not represent what Starship will experience. Firstly it is ignoring any radiative inputs from the Earth and secondly it assumes heat absorbed on the hot side of the craft is somehow transported to the cold side of the craft for radiation to deep space.
Earth is almost net zero in terms of its total thermal effect relative to it not being there. Its radiation warms up the spacecraft of course, but that's balanced by the spacecraft being in Earth's shadow half the time, which blocks the much hotter sun. It does change the geometric distribution of incident energy of course, and a sunshade floating independently from Starship blocking the entire side of the ship would probably be infeasible, so the attached thermal blanket is necessary there. But for the nose shade (if the nose is always pointed towards the sun), my idea was that thermal energy that does make it through the sunshade will re-radiate isotropically (and thus mostly to deep space) if the shade is placed at some distance, whereas if it's right on the ship's skin, all the energy that makes it through the MLI will heat the spacecraft. Perhaps that extra optimization is not necessary, but that's what I was thinking when I suggested it.
