3 ms·
The limiting factor there is radiative temperature x radiative area. For something the size of a small star, sure, you can pick up 300K. For something the size
by glimcat 13y ago
The limiting factor there is radiative temperature x radiative area. For something the size of a small star, sure, you can pick up 300K.
For something the size of a plausible ship, the surface area is astronomically smaller, while the temperature of most of its surface would likely be kept at or lower than the range which is compatible with macro-scale organisms and biochemical processes. A ship would also be trying to manage its heat, probably using an insulating exterior.
Meanwhile it would be pointing its thruster - the only part meaningfully above 300K - away from you for at least half the trip if directly oncoming, with angular efficiency coefficients if heading somewhere else. Recall that the ship itself is insulated & temperature-regulated, and therefore shields the emissions of the thruster. The thruster would also likely be focused as much as possible, so we could expect thermal radiation at steeper angles to drop off sharply.
Even something like a nuclear pulse rocket would be hard to spot until it was within a very small neighborhood, and then it would be up to whether you're actually observing that part of the sky during the time when it might be visible.
Ships are very hard to detect until they're almost on top of you.
- Pinckney 13y agoAny engine worth using will produce large amounts of waste heat, which the ship will need to reject somehow. The most plausible option seems like large radiators; efficiency is as the fourth power of temperature, so it is very desirable to run radiators as hot as possible. 600-2000K are fairly plausible values. Thermal insulation isn't necessary or helpful, except as it separates crew spaces from hot spaces and gives the radiators time to reject heat. Also, the exhaust plume will be very hot and very bright. Even if the ship doesn't radiate, the exhaust will
- deleted 13y ago[deleted]
- dalke 13y ago> In order for a radiator to do its job, it has to be immersed in some sort of working medium ... You can sort of make a "radiative transfer" radiator, but they don't work quite the same. You should re-read Pinckney's essay on the assumption that it specifically concerns the latter type of radiator. Consider that "efficiency is as the fourth power of temperature" is a reference to the Stefan–Boltzmann law for black-body radiative heat transfer. Consider too that the linked-to essay, in the part about radiative cooling and with worked-out details about Stefan-Boltzmann, refers to a working temperature of 2200 K, where the T-to-the-fourth really kicks in.