Why did the orbiting Salyut 7 get so cold when its electrical systems malfunctioned and its batteries drained empty?

A spacecraft's thermal control system is designed to handle normal heat loads, balancing heat generated by electrical systems inside (and humans in the case of crewed vehicles) with heat loss through the skin and heat gain from the sun. While I'm not familiar with the specifics of the Salyut 7 case you cited, if they lost electrical power then all of the heat generated by the onboard systems stopped while the heat loss continued. As the vehicle began to cool, electric heaters probably turned on to protect the propulsion lines from freezing. In short order, the batteries were drained (especially if the vehicle's attitude control system was also offline, meaning the solar arrays weren't correctly pointed towards the sun). This is very similar to the situation on Apollo 13. When they turned off so many electrical systems, the heat loss through the vehicle's skin was greater than the heat generated by body heat and heat gain from the sun. As a result, it got really cold inside.

I used to control DSCS-III military communications satellites. The thermal control subsystem consisted of passive and active devices to maintain the proper operating temperature. The passive methods included reflective mylar coatings, insulation, and optical solar reflectors (similar to mirrors) mounted on parts of the satellite. Being a communications satellite, there were a lot of heat generating components on the inside. There were also thermostatically controlled heaters on critical components like the propellant lines. If the satellite lost Earth-lock, it went into survival mode. This caused it to load shed as much as possible. The communications system, attitude control system*, and many other electrical loads were turned off. As a result, the heat produced inside the satellite was much less than normal and the internal temperatures would start dropping fast. As they did, the thermostats would turn on heaters increasing electrical consumption. If that happened, the batteries would be dead in about 6 hours and the satellite would be lost. Our quick recovery plan was to attempt to put the satellite into sun-hold mode (solar arrays pointed at the sun without regard to Earth pointing attitude). Once you had it in sun-hold mode, your batteries could recharge and you had much more time to regain control of the satellite. Whether you'd ever get all of the components to turn back on or not was a crap shoot. Some of the old tube-type travelling wave tube amplifiers (TWTAs) didn't take well to being turned off.

*If you lost Earth lock, it was assumed that the attitude control system was malfunctioning. Therefore, there was no need to keep it on wasting electricity.

You are of course right.

I was trying to turn the tables, making SLF:JAQ SFDJS think how does this Salyut 7 scenario fit the HB minset in which spacecraft under the Sun can only heat up and in which cooling down is apparently viewed impossible. This seemingly contradictory thermodynamic behavior between Skylab and Salyut 7 should introduce some interesting cognitive dissonance in the hoax believer minds.

Obviously, Salyut 7 must have been in a considerably lower orbit. Slylab was in the upper levels of the Thermosphere where the temperatures are around the 1000C and above range. Salyut 7 must have been lower orbit closer to the Mesosphere where the temperature is much colder -100C.

I doubt the ambient temperature had much effect since there's so little at such altitudes to make a difference. In any case, the altitudes overlapped: Salyut at 220-280 km, and Skylab at 270-275.

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Wikipedia Salyut 7 page
Perigee: 219 km
Apogee: 278 km
Wikipedia Skylab page
Perigee: 434 km
Apogee: 442 km

Also from wikipedia:
The mesosphere is located from about 50 km to 80-90 km altitude above Earth's surface.
The thermosphere, named from the Greek θερμός (thermos) for heat, begins about 80 km above the earth.

So, as I suspected, that has nothing to do with it.

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Slylab was in the upper levels of the Thermosphere where the temperatures are around the 1000C and above range. Salyut 7 must have been lower orbit...

No. Convective heat transfer involving the atmosphere at orbital altitudes is negligible due to the extremely low mass density of the gas. Radiative heat transfer dominates the thermal solution in orbit, and it is because of the difference in radiative properties between the spacecraft that one grew hot while the other grew cold. Skylab got hot because it accidentally absorbed an excess of solar energy directly to its inner vessel, not because excess heat was transferred to it from the surrounding gaseous ambient. Salyut's thermal design, optimized for heat rejection, was still intact.

Compare the published orbital altitudes of these spacecraft with the generally accepted values of the atmosphere ranges you mention.

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Obviously, Salyut 7 must have been in a considerably lower orbit. Slylab was in the upper levels of the Thermosphere where the temperatures are around the 1000C and above range. Salyut 7 must have been lower orbit closer to the Mesosphere where the temperature is much colder -100C.


That had nothing to do with it. Skylab lost solar shielding during liftoff, causing it to absorb more heat from the sun than intended. This caused it to get very hot inside. Salyut 7 lost electrical power causing the internal equipment to shut down. This upset the thermal equilibrium (heat loss became greater than internal heat production) and caused it to get very cold inside. This was similar to but more extreme than what happened with Apollo 13.

I'm looking for the smiley face.

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I don't think this is quite right, though it depends on exactly what you meant. The average temperature of the earth isn't constrained by its solar insolation to a value below freezing. Were its optical properties significantly different, it could be either hotter or colder.

In thermal equilibrium, every object radiates exactly as much heat power as it receives and it will seek whatever temperature causes this to happen. In a homogenous environment, this equilibrium temperature will be that of the environment regardless of the object's optical properties (assuming it is not possible to isolate it completely).

But the earth is not in a homogenous environment. It's surrounded mostly by dark sky at just a few kelvins, but a tiny fraction of the celestial sphere -- 7.6e-5 steradians -- is at 5800K. Because of this huge temperature disparity, the heat input to the earth is mainly in the visible and near IR while the heat the earth radiates to the rest of the universe is in the far IR, and the earth's optical properties at these two wavelengths can differ. (The sun and the rest of the universe are BOTH essential for evolution and life to be thermodynamically possible.)

Hoax believers get in trouble because they apply intuition from a lifetime on the earth to a very different place, space. And heat transfer in space is probably its single most counterintuitive property.

Thread necromancy alert -- almost 2 years.

Fred

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so? the contribution is good.

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It's still polite to mention it.

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John Glenn did. IIRC It cycled up and down from 90 to 109 degrees. Its cycle matched the duration of his orbit so it was probably due to solar heating albeit offset in time from its cycle of exposure as the heating took time to soak in to reach the thermometer in question. Additionaly one of the Apollo's lunar ascent modules had its cooling system turned off, after it was cast off subsequent to the mission, to see how long its electronics would survive. It overheated fairly quickly and shut down.

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IDW was banned several years ago. There's not much point in correcting him now.

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I don't think IDW cares, since he was auto-banned back in 2007.

Fred

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Will check the datum tag next time I post a reply ;-)

Ex

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