A rebreather is a type of breathing apparatus that allows the user to reuse some or all of the exhaled breath by removing carbon dioxide (CO₂) and replenishing oxygen (O₂). This makes it different from traditional open-circuit breathing systems, where the exhaled air is expelled into the environment. Here's how it works step by step:

1. Inhalation: The user breathes in from the breathing loop, which contains a mix of gases, usually oxygen or an oxygen-nitrogen mix.

2. Exhalation: The exhaled air contains unused oxygen, nitrogen, and carbon dioxide. Instead of being expelled, it is directed into the rebreather’s circuit.

3. CO₂ Scrubbing: The exhaled gas passes through a chemical scrubber, usually containing a material like soda lime. The scrubber absorbs the carbon dioxide, preventing it from building up to toxic levels in the breathing loop.

4. Oxygen Replenishment: As the user consumes oxygen during respiration, the oxygen levels in the breathing loop drop. The rebreather has a mechanism (manual or automatic) to add oxygen to maintain the proper levels. Some advanced rebreathers can add oxygen based on the depth of the dive to keep the oxygen partial pressure safe.

5. Recycled Air: After scrubbing the CO₂ and replenishing the oxygen, the cleaned and oxygen-enriched gas is fed back into the breathing loop, ready for the user to inhale again.

Advantages of Rebreathers:

• Efficiency: They allow longer dive times since the oxygen is not wasted with each breath.
• Stealth: They are quieter and produce fewer bubbles than open-circuit systems, making them useful for military, technical, or wildlife diving.
• Compactness: Because gas is reused, the cylinders carried can be smaller, making the setup more lightweight.

Types of Rebreathers:

• Oxygen rebreathers: Used in shallow diving, they add pure oxygen.
• Semi-closed circuit rebreathers (SCR): These add a gas mix based on the user's consumption but vent some of the gas to maintain the balance.
• Closed-circuit rebreathers (CCR): These completely recycle exhaled gases and manage oxygen levels precisely, making them suitable for deep and technical diving.

Safety is crucial, as improper oxygen or CO₂ management can be dangerous in rebreathers.

Do astronauts use a CCR?

Yes, astronauts use a form of closed-circuit rebreather (CCR) technology in their life support systems, although these systems are more advanced and specifically designed for space environments. The Extravehicular Mobility Unit (EMU), which is the spacesuit worn by astronauts during spacewalks (extravehicular activities or EVAs), uses a closed-loop life support system to maintain breathable air. Here's how it relates to a CCR and the specifics for astronauts:

How the Life Support System Works in Space:

1. Closed-Circuit Rebreather Functionality: Similar to a CCR, the spacesuit’s life support system recycles exhaled air. The astronaut breathes oxygen from the suit's tanks, and when exhaling, the system collects the exhaled gas, removes carbon dioxide (CO₂), and replenishes oxygen as needed.

2. CO₂ Scrubbing: Like in diving rebreathers, CO₂ must be removed from the breathing loop. In spacesuits, this is done using lithium hydroxide (LiOH) canisters, which absorb CO₂. The scrubbed air, now free of CO₂, is recirculated back into the breathing loop.

3. Oxygen Replenishment: The suit continuously monitors oxygen levels. As the astronaut consumes oxygen during respiration, the system releases more from onboard tanks to keep oxygen levels optimal for breathing.

4. Cooling and Humidity Control: The EMU also has to manage other environmental factors that aren’t an issue underwater. The suit’s life support system removes excess moisture (humidity) from the exhaled breath and body perspiration. It also has a cooling system, usually liquid-cooled garments, to prevent overheating in the vacuum of space.

5. Pressure Control: Unlike underwater CCRs, which balance gas pressures to avoid issues related to depth, the spacesuit maintains a constant internal pressure, since astronauts operate in a near-vacuum environment where maintaining suit integrity is critical to prevent decompression and protect the astronaut’s body.

Why a CCR-Type System is Used in Space:

• Oxygen Conservation: In space, resources are limited. A closed-circuit system minimizes the need for large oxygen reserves by recycling and reusing the air the astronaut breathes.
• CO₂ Removal: Just like in diving, the accumulation of CO₂ in a closed environment can quickly become toxic. Effective scrubbing is essential for astronaut safety.
• Suit Mobility: Space suits need to be compact and efficient to allow for maximum mobility during spacewalks. A closed-loop system means less oxygen needs to be stored, reducing the suit's bulk and weight.

While the basic principles of gas recycling and CO₂ scrubbing are shared between a CCR used by divers and an astronaut’s life support system, the engineering of space suits is far more complex, addressing the unique challenges of operating in a zero-gravity vacuum.

What were the specifics of the system used by the Apollo crews?

The Apollo astronauts used a sophisticated life support system both in their spacecraft and within their spacesuits during extravehicular activities (EVAs). Here's a breakdown of the specifics of the system used by the Apollo crews, especially in relation to the Portable Life Support System (PLSS) used during lunar surface operations:

Portable Life Support System (PLSS) for EVAs

The Apollo astronauts used a closed-loop system similar to modern closed-circuit rebreathers (CCR), but adapted for the unique challenges of space exploration. Here's a detailed look at how it worked:

1. Oxygen Supply:

• Primary and Backup Tanks: The PLSS carried two oxygen tanks—a primary tank that provided a continuous flow of oxygen to the astronaut's suit and a secondary tank as a backup in case of emergencies.
• Constant Flow: The system delivered a constant flow of oxygen, which the astronaut breathed in. Oxygen was provided at a rate of about 0.84 to 1.2 kilograms per hour.

2. CO₂ Scrubbing:

• Lithium Hydroxide (LiOH) Canisters: The exhaled air, rich in carbon dioxide (CO₂), was passed through filters containing lithium hydroxide (LiOH) to chemically absorb the CO₂. This ensured that dangerous levels of CO₂ did not build up inside the suit.
• Scrubbing Efficiency: The LiOH canisters were highly efficient, capable of scrubbing CO₂ for up to 7 hours during a lunar EVA, with spare canisters available for replacement.

3. Cooling System:

• Liquid-Cooled Garment (LCG): To prevent overheating, astronauts wore a liquid-cooled garment under their spacesuits. The LCG was a network of tubes that circulated water cooled by a sublimator.
• Sublimator Cooling System: The sublimator used the vacuum of space to boil away water into vapor (sublimation), removing excess heat from the circulating water. This heat exchange helped maintain a stable body temperature during physically demanding EVAs on the lunar surface.

4. Humidity and Moisture Control:

• The life support system also removed moisture from the exhaled air. Humidity was a concern in the closed environment of the spacesuit, as too much moisture could lead to fogging of the helmet visor and affect the astronaut’s comfort. Excess moisture was condensed out of the air and managed by the suit’s system.

5. Suit Pressure and Structural Integrity:

• The spacesuit maintained a pressure of around 3.7 psi (25.5 kPa), which was a partial pressure of oxygen, providing enough to breathe without risking decompression sickness. The suit had to maintain this pressure while allowing enough flexibility for the astronaut to move on the Moon's surface.

6. Waste Management:

• The Apollo astronauts had to deal with waste products while wearing the suit for long periods. Urine was collected in a special device called the Urine Collection Transfer Assembly (UCTA), which channeled liquid waste away from the body into a storage unit in the suit.

7. Communication and Power:

• The PLSS also housed the communication equipment, with the system relaying voice communications back to the Lunar Module (LM) and the mission control on Earth. It was powered by batteries that provided enough energy for the life support and communication systems during lunar walks.

Environmental Control System in the Command Module (CM):

When inside the Command Module (CM), the life support system was designed to handle the unique conditions of space, maintaining a controlled cabin environment.

1. Oxygen Supply:

• The Command Module had oxygen tanks that supplied the astronauts with breathable air. It operated on a closed-loop system where oxygen levels were maintained by onboard controls.

2. CO₂ Scrubbing:

• Lithium Hydroxide (LiOH) Canisters: As in the PLSS, the Command Module used LiOH canisters to scrub CO₂ from the air. After the Apollo 13 incident, where an oxygen tank explosion compromised the Command Module, the crew had to adapt the LiOH canisters from the Lunar Module to scrub CO₂ from the air while they awaited rescue.

3. Temperature and Humidity Control:

• The CM had an environmental control system that maintained comfortable temperature and humidity levels. Excess heat generated inside the spacecraft by both the astronauts and onboard equipment was managed by a combination of radiators and heat exchangers.

Lunar Module (LM) Life Support:

The Lunar Module (LM), which took astronauts to the Moon's surface, also had its own life support system.

• Oxygen Supply and CO₂ Scrubbing: The LM had its own oxygen supply and CO₂ scrubbing system similar to the Command Module, relying on LiOH canisters to remove CO₂.
• Water and Power: The LM carried water for drinking and batteries to power its systems, but was designed for short-duration missions, meaning life support systems only needed to last for a few days.

Emergency Systems:

The Apollo spacesuits and modules had built-in redundancy. In case of failure, astronauts had emergency oxygen supplies and backup CO₂ scrubbers to ensure survival.

Key Takeaways:

• Efficient CO₂ Removal: Lithium hydroxide canisters were the primary method for CO₂ scrubbing both in the PLSS during EVAs and in the spacecraft.
• Closed-Loop Oxygen System: The astronauts used a highly efficient closed-loop system that recycled air, replenished oxygen, and removed CO₂.
• Cooling System in Vacuum: The PLSS used a sublimator system to remove excess heat, leveraging the vacuum of space to dissipate heat through water sublimation.

This system allowed Apollo astronauts to survive in the vacuum of space and the extreme conditions on the lunar surface for extended periods during their moonwalks.

How did sublimation work through the outer spacesuit from the internal tubing?

The sublimation cooling system in the Apollo astronauts’ Portable Life Support System (PLSS) was a crucial mechanism for controlling body temperature during extravehicular activities (EVAs) on the Moon. The system relied on the principle of sublimation, which occurs when a substance transitions directly from a solid to a gas, bypassing the liquid phase. Here's a breakdown of how this process worked through the layers of the spacesuit:

Sublimator in the Cooling System

1. Liquid-Cooled Garment (LCG):

   • The astronaut wore a Liquid-Cooled Garment (LCG) under their spacesuit, which consisted of a network of narrow plastic tubes woven into a fabric that circulated water over the astronaut's skin. This garment absorbed excess body heat during physical activity or exposure to the Sun on the lunar surface.
   • The water was cooled via a sublimation process, which was carried out in the PLSS.

2. Heat Transfer to Water:

   • The heated water, having absorbed heat from the astronaut’s body, was pumped through the tubes and into a heat exchanger in the PLSS.

3. Sublimator Unit:

   • The heart of the cooling system was the sublimator, a device located within the PLSS. This sublimator was filled with a porous metal plate or porous material, which held ice formed from water introduced into the system.
   • The ice was kept in a special compartment within the sublimator where it could be exposed to the vacuum of space.

4. Sublimation Process:

   • Once in the vacuum of space, the water ice in the sublimator was exposed to an environment with extremely low pressure. In this vacuum, ice does not melt into liquid water; instead, it sublimates directly into water vapor.
   • This phase change from solid ice to water vapor required heat energy. The heat was drawn from the circulating water that had absorbed the astronaut's body heat. As the ice sublimated, the heat was removed from the water, cooling it down.

5. Venting Water Vapor:

   • The resulting water vapor, formed from sublimation, was vented directly into space. The continuous sublimation process provided a steady removal of heat as long as ice and water were supplied to the system.

6. Cooling the Circulating Water:

   • As the water vapor was vented out, the remaining water in the heat exchanger was cooled, and this cooler water was then circulated back through the tubes in the LCG. The astronaut's body heat was once again absorbed by the circulating water, and the cycle repeated.

Key Points of the Sublimation Process:

• Ice as a Heat Sink: The sublimator acted as a heat sink by using the astronaut's body heat to transform solid ice into vapor. The cooling effect was direct, and no mechanical pumps or refrigeration units were necessary to expel heat, making it efficient in the vacuum of space.

• Porous Plate Design: The porous structure in the sublimator ensured that the water formed ice evenly across the plate's surface, maximizing the exposure of ice to the vacuum and making the sublimation process highly efficient.

• Vacuum of Space: The lack of atmospheric pressure in space enabled sublimation, as water ice directly transitions to vapor in such conditions, which wouldn't be possible in Earth’s atmosphere.

• Continuous Operation: As long as the PLSS had a supply of water for the sublimator and the environment remained a vacuum, the system could continue cooling effectively. The system worked by maintaining a balance between body heat removal and the sublimation of ice.

Spacesuit Layer Interaction:

While the sublimator was located in the PLSS backpack and not directly integrated into the outer layers of the spacesuit, the cooling was transferred to the astronaut through the water circulating within the tubes of the Liquid-Cooled Garment (LCG). The cooling water was isolated from the external environment by the multiple layers of the spacesuit, which provided thermal insulation and protection. These outer layers did not interfere with the sublimation process happening in the PLSS backpack because the vented water vapor exited through designated ports in the PLSS.

In summary, the sublimation cooling system removed heat by using the vacuum of space to sublimate ice, cooling water that was then circulated through the astronaut’s Liquid-Cooled Garment. The process was efficient and vital for keeping astronauts comfortable during EVAs in the harsh lunar environment.

Ice Sublimated from the Backpack/PLSS?

Yes, that's correct! The ice that sublimated during the cooling process was located inside the Portable Life Support System (PLSS), which was the backpack worn by the Apollo astronauts during extravehicular activities (EVAs) on the Moon. Here's a clearer explanation of how the sublimation process worked within the PLSS:

Sublimation Process in the PLSS:

1. Ice Formation in the PLSS Sublimator:

   • The PLSS had a dedicated sublimator unit, which was essentially a heat exchanger that used ice to cool the astronaut’s body via a Liquid-Cooled Garment (LCG).
   • Water from the astronaut’s suit cooling system was fed into the sublimator, where a portion of it was frozen into ice on a porous plate.

2. Exposing Ice to the Vacuum:

   • This ice, located within the PLSS sublimator, was then exposed to the vacuum of space via a venting mechanism. In the vacuum, the ice could not melt into water due to the extremely low pressure. Instead, it sublimated directly into water vapor.
   • Sublimation is the phase transition where a solid (in this case, ice) changes directly into a gas (water vapor) without passing through the liquid phase. This phase change absorbed heat from the system.

3. Cooling the Circulating Water:

   • The sublimation of ice absorbed the heat from the water circulating in the system. This cooled the water, which was then pumped back through the tubes of the Liquid-Cooled Garment worn by the astronaut to regulate body temperature.

4. Venting Water Vapor into Space:

   • The water vapor generated by the sublimation process was vented out directly into space through a small exhaust port in the PLSS. This allowed the system to continually remove heat from the astronaut’s body as long as ice was available.

Why Ice Was in the PLSS:

The reason the ice was located in the PLSS (and not directly in the suit) is that the PLSS housed all of the astronaut’s life support equipment, including the oxygen supply, CO₂ scrubbing system, communication systems, and cooling systems. The sublimator was an efficient way to remove excess heat in the vacuum of space by harnessing the properties of sublimation.

Since the Moon has no atmosphere, the vacuum environment around the astronaut allowed the ice to sublimate directly, creating an effective method of cooling without requiring complex mechanical refrigeration systems. The heat from the astronaut’s body was carried by the water in the LCG to the sublimator, where it was released as water vapor into space, thus cooling the astronaut.

In summary:

• The sublimator unit in the astronaut’s PLSS froze water into ice, which was exposed to the vacuum of space.
• The ice in the PLSS sublimated (turned directly into vapor) when exposed to the vacuum, absorbing heat from the circulating water.
• The water vapor was vented out into space, while the cooled water was recirculated through the astronaut’s cooling garment to regulate body temperature.

This process allowed astronauts to stay cool while performing physically demanding tasks on the Moon, where extreme temperature fluctuations occur.

How Could They Still Have Ice After Four Days in Direct Solar Radiation?

Great question! It seems surprising that the Apollo astronauts could still have ice in the Portable Life Support System (PLSS) after days in the intense solar radiation on the Moon. However, several factors allowed the PLSS sublimator to maintain ice and perform effective cooling, despite these extreme conditions. Here's how it worked:

1. Water, Not Pre-Frozen Ice:

• The astronauts did not start with ice in the PLSS; instead, they carried liquid water in a tank inside the PLSS.
• The sublimator created ice on-demand by freezing a small portion of this water in a porous plate inside the sublimator, which was constantly exposed to the vacuum of space.
• Since the water was not exposed directly to the Sun and was stored in an insulated tank, it remained in liquid form. Ice was formed only when needed during the cooling process.

2. Vacuum of Space and Sublimator Design:

• The vacuum of space around the astronauts played a crucial role. In the vacuum, water can sublimate (turn directly from ice into vapor) without first melting. The sublimator harnessed this principle.
• The sublimator had a porous metal plate where water from the tank was slowly fed. As the water entered the sublimator, the extreme cold of space caused the water to freeze immediately on the plate.
• This ice was then exposed to the vacuum of space, causing it to sublimate (turn directly into water vapor). This phase change absorbed heat from the water circulating in the astronaut’s Liquid-Cooled Garment (LCG), cooling it.

3. Thermal Control of the PLSS:

• The PLSS and the spacesuit had multiple layers of thermal insulation that protected the astronaut and the equipment from extreme temperatures on the lunar surface.
• The outer layer of the spacesuit was designed to reflect solar radiation using a highly reflective surface made from aluminized Mylar and other insulating materials. This helped prevent excessive heating of both the astronaut and the equipment, including the water reservoir inside the PLSS.

4. The Moon’s Day-Night Cycle:

• Although the Moon experiences extreme temperature changes (ranging from about +127°C (260°F) in direct sunlight to -173°C (-280°F) in shadow), each Apollo mission was carefully timed to take place during the lunar day, when the Sun was lower in the sky.
• The astronauts were exposed to less intense solar radiation because they were conducting operations during the early lunar morning or late afternoon, avoiding the hottest part of the day.

5. Efficiency of the Sublimation Process:

• The sublimator process was extremely efficient in cooling the circulating water by sublimating small amounts of ice, only when heat needed to be removed.
• The water supply in the PLSS was adequate for the duration of the lunar EVAs (which lasted about 7 hours per EVA). The cooling system was designed to maintain enough ice through sublimation to keep the astronaut cool without the need for large amounts of pre-stored ice.

6. Duration of the Moonwalks:

• The Apollo astronauts were not continuously exposed to the Sun for four straight days while on the lunar surface. Each lunar EVA (Extravehicular Activity) lasted around 4 to 7 hours, after which they would return to the Lunar Module (LM), which had its own life support and thermal management systems.
• Between EVAs, the astronauts were inside the Lunar Module, where they had access to a separate thermal control system and could replenish their resources.

Conclusion:

• The astronauts carried liquid water in their PLSS, which was insulated and protected from direct solar radiation by the suit’s reflective and insulating layers.
• The sublimator inside the PLSS created ice on-demand from this water and sublimated it in the vacuum of space, cooling the astronauts through the heat exchange process.
• The thermal control measures of the PLSS and careful timing of the missions ensured that ice could be continuously generated, even after days of exposure to sunlight.

In essence, the design of the PLSS and the sublimator allowed for the creation and sublimation of ice only when needed, rather than relying on pre-stored ice, making the cooling system highly efficient and well-suited for the extreme environment of the lunar surface.

I Meant Four Days in Translunar Space

Ah, I see! You're asking how the Apollo astronauts managed to have liquid water for the sublimator system after spending several days in translunar space, where they were exposed to direct solar radiation without the Moon's shadow. This is a great point, and the answer lies in the thermal management systems of both the Command Module (CM) and the Portable Life Support System (PLSS). Let's break down how they dealt with the extreme temperatures in deep space:

1. Thermal Management in Translunar Space

In space, especially during the journey from Earth to the Moon (translunar space), there is no atmosphere to regulate temperature. The spacecraft and life support systems were exposed to extreme conditions—solar radiation on one side and the cold of space on the other. To maintain a suitable environment for both the astronauts and their equipment, including the water for the PLSS, the Apollo spacecraft used several strategies:

Active Thermal Control in the Command Module (CM):

• During the multi-day journey to the Moon, the astronauts were inside the Command Module (CM), where the water for the PLSS was stored.
• The Command Module had a heat exchanger system that regulated the temperature of the spacecraft's interior and prevented any single component from overheating or freezing.
• The CM used a combination of radiators and a liquid cooling system to dump excess heat from the spacecraft into space. This system kept the spacecraft’s interior within a comfortable temperature range (around 18–24°C or 65–75°F), ensuring that the water in the PLSS remained in liquid form during transit.

The "Barbecue Roll" Maneuver:

• One of the most well-known techniques used during the Apollo missions was the Passive Thermal Control (PTC) maneuver, commonly referred to as the "barbecue roll."
• This maneuver involved rotating the entire spacecraft along its long axis slowly and continuously (about 1 revolution per hour). This slow roll ensured that the spacecraft's surface was evenly heated by the Sun, rather than one side baking in the sunlight and the other side freezing in deep space.
• By rotating the spacecraft, the Apollo engineers prevented excessive heating or cooling on any part of the spacecraft, maintaining stable internal temperatures for the astronauts, the equipment, and the life support systems.

2. Water Storage and Insulation in the PLSS

• The water tank for the PLSS was stored in the Command Module during the journey to the Moon and was transferred to the PLSS only when the astronauts prepared for their extravehicular activities (EVAs) on the lunar surface.
• During transit, the water tanks were insulated to protect the contents from external temperature fluctuations. Even though space has extreme temperatures, the insulation and thermal control measures in the spacecraft prevented the water from freezing or boiling.

3. Solar Radiation Protection:

• The Command Module’s outer shell was equipped with thermal blankets and insulation layers that reflected solar radiation and minimized heat absorption from the Sun.
• These materials protected the spacecraft from excessive heating due to direct sunlight, allowing the internal systems (including the water supply) to remain within safe temperature ranges.

4. Water Cooling in the PLSS After Lunar Orbit Insertion:

• Once the astronauts were in lunar orbit and preparing for their lunar EVA, the water for the PLSS sublimator was transferred to the PLSS backpack.
• By that point, they were closer to the Moon, which presented different thermal conditions compared to the translunar phase. However, the design of the PLSS ensured that once the water was inside the system, it remained stable, only sublimating small amounts of ice as needed during the EVA.

Key Points:

• Thermal control systems inside the Command Module, such as heat exchangers, radiators, and the rotating "barbecue roll" maneuver, prevented extreme temperatures from affecting the astronauts and their equipment during the trip to the Moon.
• Water storage tanks inside the Command Module were well insulated and maintained at safe temperatures for the duration of the mission.
• Upon reaching the Moon, the water was transferred to the PLSS backpack, where it was used to generate ice and cool the astronauts via the sublimation process during EVAs.

Conclusion:

The thermal management systems aboard the Apollo spacecraft were specifically designed to handle the extreme environment of space. By carefully regulating the spacecraft's internal temperature through a combination of insulation, active cooling, and passive maneuvers like the "barbecue roll," the Apollo missions ensured that essential resources, such as the liquid water needed for the PLSS sublimator, remained in a usable state throughout the multi-day journey to the Moon.

Space isn't Cold, It's Just a Vacuum

You're absolutely right! Space itself isn’t "cold" in the way we typically think of it; it’s a vacuum, which means it doesn't conduct heat because there are no molecules to transfer thermal energy. Instead, the way objects in space gain or lose heat is primarily through radiation, not through conduction or convection as on Earth.

Here’s a breakdown of what this means and how it affects temperature management in space, especially during Apollo missions:

1. How Heat Works in a Vacuum:

• In a vacuum like space, objects can only exchange heat through radiation, meaning they emit infrared energy to lose heat or absorb it if exposed to sunlight.
• Radiative heat transfer is the only way for an object to cool down or heat up. If an object is in the shade, it cools down by radiating its heat into space. If it's in direct sunlight, it absorbs the Sun's energy and heats up.

2. Solar Radiation in Space:

• Without an atmosphere to filter or block sunlight, solar radiation is extremely intense. Objects in direct sunlight can get very hot, while objects in the shade or facing away from the Sun can cool to very low temperatures because they radiate heat away into the vacuum of space.
• For instance, objects in direct sunlight in space can reach temperatures over 120°C (248°F), while objects in shadow can plummet to temperatures as low as -150°C (-238°F) or even lower, depending on their environment and distance from a heat source.

3. Thermal Regulation in Spacecraft:

• To maintain livable temperatures in spacecraft like the Apollo Command Module (CM) or to keep equipment (like the PLSS water supply) stable, thermal regulation systems are critical.
• These systems manage both the intense solar heating and the cooling effects of radiating heat into space. They use techniques like:
  • Insulation: Thermal blankets and multi-layer insulation (MLI) reflect solar radiation and prevent excessive heat from being absorbed.
  • Radiators: Heat from inside the spacecraft is actively pumped to radiators, which emit it as infrared radiation into space.
  • Thermal control coatings: Special coatings on the spacecraft’s surface reflect solar radiation or emit heat efficiently to maintain a stable temperature.

4. Thermal Control for Astronauts and Equipment:

• In translunar space, as you mentioned, the Apollo astronauts were exposed to direct solar radiation for days. However, the Command Module’s heat shields and insulation layers protected the internal environment, keeping it within a comfortable temperature range. This was essential for the astronauts and the water in the PLSS.
• The "barbecue roll" maneuver helped spread the heat evenly around the spacecraft, preventing one side from overheating and the other from getting too cold due to excessive radiative cooling.

5. Why Water Didn't Freeze or Boil:

• Water in the PLSS or the Command Module didn’t freeze or boil in space because of the combination of thermal insulation and the spacecraft’s internal temperature control systems. Although space is a vacuum and water would behave unpredictably if exposed directly, the Apollo spacecraft maintained a controlled environment to prevent this from happening.
• Thermal management systems ensured that the interior of the spacecraft (including the water tanks) stayed in a temperature range that allowed water to remain in its liquid state, despite the intense solar radiation outside.

In Summary:

• Space isn’t "cold" in the traditional sense; it’s a vacuum, meaning there’s no matter to conduct heat.
• In space, temperature is managed by radiation, with objects heating up in sunlight and cooling down when radiating heat into the vacuum.
• The Apollo missions used thermal insulation, active heat management systems, and maneuvers like the "barbecue roll" to protect both the astronauts and the equipment from extreme temperature variations.
• The water in the PLSS remained in liquid form because it was stored in a carefully regulated environment, protected from both excessive heating and cooling.

So, while space itself doesn't "freeze" objects, managing heat in space is all about balancing radiation absorption (like from the Sun) and heat emission (through radiators or insulation).

But it's a Closed System Though, Radiation Out=Radiation In

You're touching on a key principle of thermal equilibrium, but the situation in space isn't quite as simple as radiation out always equaling radiation in, especially in the context of spacecraft and life support systems like those used on the Apollo missions. Let’s dive deeper into the nuances of thermal management in a closed system in space:

1. Closed System vs. Open System:

• Closed system: In the context of the Apollo spacecraft, the Command Module (CM) or Portable Life Support System (PLSS) was indeed a closed system in terms of mass (i.e., no matter or gases entering or leaving). However, energy transfer still occurred between the spacecraft or suit and the external environment, primarily through radiation.
• Radiation as the Only Transfer Mechanism: In space, since there's no air or matter for conduction or convection, the spacecraft can only gain or lose heat by radiating energy to or absorbing energy from the environment (like solar radiation).

2. Radiation In ≠ Radiation Out (Active Management):

• Solar Radiation Input: The spacecraft or astronaut's suit would naturally absorb solar radiation if exposed to direct sunlight. This absorption increases the internal energy (temperature) of the system.

• Radiation Output: The spacecraft or astronaut’s suit also radiates infrared energy out into space to balance the energy input. The problem is that without active thermal control, the amount of radiation out may not match the amount of radiation in.

  • If radiation in > radiation out, the system heats up.
  • If radiation out > radiation in, the system cools down.

3. Thermal Management in Space:

• In space, achieving thermal equilibrium (where radiation out = radiation in) is not automatic. Without intervention, one side of the spacecraft could become extremely hot from sunlight exposure while the other side, in shadow, could become extremely cold from radiating heat away into space.

• This is where active and passive thermal control systems come into play:

  • Reflective Insulation: Materials like multi-layer insulation (MLI) on spacecraft reflect much of the incoming solar radiation, reducing the amount of heat that is absorbed.
  • Radiators: Active radiators emit excess heat from the interior of the spacecraft into space. This helps prevent the spacecraft from overheating by increasing radiation out.
  • Barbecue Roll: As mentioned earlier, the slow roll of the Apollo spacecraft ensured that all sides were evenly heated and cooled, avoiding localized hot or cold spots. This maneuver distributed the solar input and radiative losses evenly, helping to balance radiation out and in.

4. Apollo Thermal Management:

• Inside the Command Module (CM) or Lunar Module (LM) during the journey to the Moon, thermal control systems maintained a stable internal environment. The crew compartment was kept at a comfortable temperature by regulating heat.
• This was not simply a matter of letting radiation out equal radiation in. The spacecraft actively controlled how much heat was retained or radiated using heat exchangers, thermal blankets, and radiators.

5. Why Apollo Wasn't a Perfectly Closed Radiative System:

• Controlled Heat Rejection: While the Apollo spacecraft and suits were "closed" in terms of keeping in air and other life support components, they were not a perfectly closed radiative system because heat rejection was carefully controlled. The goal was not to let natural radiative equilibrium occur passively, but to manage the balance between heat gain and heat loss.
• The sublimator in the PLSS used sublimation (converting ice into water vapor) as an additional method to remove heat, providing active cooling rather than relying solely on radiative cooling.
• Excess heat from equipment and the astronauts' bodies was transferred via cooling systems (water or glycol loops) to the radiators or sublimators, which then released that heat into space.

6. Radiation in Translunar Space:

• During the translunar phase, the spacecraft received heat primarily from the Sun. Without the Earth or Moon's atmosphere to filter out solar radiation, the spacecraft was exposed to significant energy.
• The Apollo Command Module's thermal systems had to reject this heat to maintain a livable environment. This involved reflective surfaces (to minimize heat absorption) and radiative panels (to increase heat loss) to ensure that radiation out was balanced with the incoming energy.
• If the spacecraft was left in one position, facing the Sun for too long, one side could overheat. The "barbecue roll" distributed this exposure more evenly, preventing localized temperature extremes.

7. Astronauts' PLSS (Portable Life Support System) during Lunar EVA:

• The PLSS wasn't just relying on radiative cooling to maintain temperature equilibrium. The system had an active cooling mechanism via the sublimator to ensure the astronauts didn’t overheat during EVAs.
• The sublimator vented water vapor into space, actively cooling the water that circulated through the Liquid-Cooled Garment (LCG) worn by the astronauts. This allowed the astronauts to control their temperature in a very dynamic environment, where both solar radiation and body heat could cause rapid overheating if not managed.

Conclusion:

While the Apollo spacecraft and suits were closed systems in terms of mass, they were not simple "radiative equilibrium" systems. They used active thermal control to balance heat gain and loss, with reflective surfaces, radiators, and even maneuvers like the "barbecue roll" to avoid overheating or excessive cooling.

In short, radiation out ≠ radiation in naturally, but the Apollo systems were carefully designed to manage this balance and maintain stable, safe temperatures for both astronauts and equipment.

In a Vacuum you can Perfectly Reflect Radiation Without Gaining Energy?

In a vacuum, perfect reflection of radiation without gaining energy into a system is not possible in practical terms. While highly reflective materials can significantly reduce the absorption of radiation, there is no material that can reflect 100% of all incoming radiation across all wavelengths. Let’s explore this idea in more detail:

1. Reflection vs. Absorption in a Vacuum:

• Reflection in the vacuum of space works similarly to reflection in any other environment. If an object is covered in a highly reflective material (like the thermal blankets used on spacecraft), it can reflect a large portion of the incoming radiation, particularly in the visible and infrared spectra.
• However, even highly reflective materials still absorb some fraction of the radiation. The reflected radiation helps reduce the net energy gain, but it is not a perfect shield. Some amount of energy is always absorbed, and this is where thermal management systems become important.

2. Perfect Reflectors Do Not Exist:

• Perfect reflection of radiation, meaning 100% reflection without any absorption, is not achievable with current materials. Even highly reflective materials, such as aluminized Mylar used in spacecraft, are not perfect reflectors.
• These materials can reflect most of the incident radiation (up to 95-99% depending on the wavelength and material), but they still absorb a small portion of it. This absorbed energy causes the object to heat up over time unless there’s a mechanism to remove the excess heat.

3. Energy Balance in Space:

• In space, the balance between the energy coming into a system (through solar radiation) and the energy leaving the system (through thermal radiation emitted by the object) determines the temperature of the object.
• Even with reflective surfaces, some amount of radiation will be absorbed, and that energy will cause the system to gain heat unless it is actively managed.
• The concept of thermal equilibrium comes into play: if an object absorbs more energy than it radiates, it heats up; if it radiates more energy than it absorbs, it cools down. Reflective materials help shift this balance by reducing the absorption of solar energy, but they don’t eliminate it.

4. Albedo and Emissivity:

• Reflective materials work by having a high albedo, meaning they reflect a large fraction of the incoming radiation. However, they still have some degree of emissivity, which measures how well an object emits thermal radiation.
• Even objects with a low emissivity (like polished metals) radiate energy, which means they can lose heat to space. In the vacuum of space, this radiation is the only mechanism through which an object can cool down.
• The albedo-emissivity ratio plays a significant role in determining how a material behaves thermally in space. The goal in spacecraft design is to balance these factors so that the system doesn't gain or lose too much heat.

5. Thermal Management on Spacecraft:

• Spacecraft use highly reflective surfaces to minimize the amount of solar radiation absorbed. Materials like gold, silver, or aluminized Mylar are commonly used because they reflect much of the Sun’s energy, particularly in the visible and infrared wavelengths.
• However, spacecraft also need to radiate heat away, which is why they have radiators that emit infrared radiation to prevent overheating.
• The balance between reflecting incoming solar energy and radiating away excess heat is essential to maintain a stable temperature inside the spacecraft.

6. Practical Example – Apollo Spacecraft:

• The Apollo Command Module and the Portable Life Support System (PLSS) used reflective materials to reduce solar heating. However, they were not immune to heat gain. That’s why the Apollo spacecraft employed active thermal control systems, such as radiators and heat exchangers, to maintain a balanced temperature.
• The barbecue roll maneuver was also used to expose the spacecraft to solar radiation evenly, ensuring that no side absorbed too much energy or radiated too much heat.

7. Active Heat Management is Necessary:

• Since no material is a perfect reflector, and some energy is always absorbed, active thermal management systems are critical in space.
• These systems ensure that any absorbed heat is removed, either by radiating it into space (through radiators) or, in the case of spacesuits, by sublimating water (in the PLSS) to carry heat away.

Conclusion:

In space, while highly reflective surfaces can significantly reduce the amount of solar radiation absorbed by a spacecraft or spacesuit, they cannot completely eliminate heat gain. Even in a vacuum, some radiation is absorbed, and this will lead to a net energy gain in the system unless active cooling methods (such as radiators or sublimators) are used.

In short, perfect reflection without energy absorption isn’t achievable in practice. Therefore, spacecraft rely on a combination of reflective materials and active thermal control systems to manage heat in the vacuum of space.

Why Were they So Cold in Apollo 13?

The extreme cold experienced by the crew of Apollo 13 was primarily due to the loss of electrical power, which crippled the spacecraft's ability to actively manage its thermal environment. Here’s a detailed breakdown of why the Apollo 13 astronauts faced such cold temperatures:

1. Loss of Power and Life Support:

• The Apollo 13 accident occurred when an oxygen tank exploded in the Service Module (SM), which contained the primary power and life support systems for the spacecraft. This explosion disabled the fuel cells that generated electricity by combining hydrogen and oxygen.
• With the fuel cells offline, the Command Module (CM) lost most of its power. The heaters and active thermal management systems were powered by these fuel cells, and without electricity, the systems couldn't function properly.

2. No Active Thermal Control:

• Normally, the Environmental Control System (ECS) in the Command Module and the Lunar Module (LM) would regulate the internal temperature of the spacecraft using radiators and active heat exchangers. These systems helped balance heat gain and heat loss, maintaining a livable temperature for the crew.
• After the explosion, these systems could not operate at full capacity due to limited power, so the spacecraft was unable to reject excess heat effectively or maintain an internal temperature.

3. Shutdown of Command Module:

• To conserve the limited battery power remaining in the Command Module, the crew was forced to shut down all non-essential systems, including those responsible for maintaining temperature.
• The astronauts moved into the Lunar Module (LM), which was designed to be a temporary habitat for use on the Moon, not for prolonged travel in space. The LM was not equipped to provide long-term thermal control for the extended period they had to remain in it.

4. The Moon’s Shadow and Cold of Space:

• The spacecraft had to navigate back to Earth after the accident, passing through the cold vacuum of space. The Lunar Module’s Passive Thermal Control (PTC) system, which relied on radiation to manage heat, was not sufficient to keep the crew warm under these conditions.
• Space is a vacuum, and while it doesn't have a temperature in the traditional sense, objects in space radiate their heat away into the vacuum. Without sufficient active heating, the Lunar Module and the Command Module became cold as they radiated more heat than they absorbed.

5. Lack of Solar Heating:

• Although the spacecraft was exposed to some sunlight, they were no longer using the barbecue roll maneuver, which was designed to distribute solar radiation evenly across the surface of the spacecraft. This led to uneven heating and allowed the spacecraft to lose more heat in shadowed areas.
• The astronauts didn’t receive enough solar radiation to offset the heat they were losing through radiation into space, especially after power was lost and active heating was no longer functioning.

6. Cold from Radiative Cooling:

• Without power to run the heaters or maintain environmental controls, the temperature inside the spacecraft dropped significantly. The spacecraft was designed to lose heat through radiative cooling into the vacuum of space, and without the ability to add heat back in, it continued cooling down.
• Temperatures inside the Lunar Module dropped to near-freezing levels, creating an extremely uncomfortable and challenging environment for the crew.

7. Water and Humidity Issues:

• As the temperature dropped, condensation began to form on the interior walls of the spacecraft. This was because the colder temperatures caused the water vapor in the air to condense, leading to high humidity levels.
• The cold, combined with the condensation, posed risks of electrical short circuits if power systems were reactivated. It also contributed to the discomfort of the crew, who were cold, wet, and trying to conserve energy.

8. Lack of Waste Heat:

• Normally, the systems onboard the spacecraft (especially the electrical systems) would generate waste heat that helped maintain a comfortable internal temperature. With most systems shut down, this source of heat was no longer available.
• The astronauts themselves were generating some body heat, but it was insufficient to counteract the heat loss from the spacecraft radiating heat into space.

Summary of Why They Were So Cold:

• The explosion of the oxygen tank caused a loss of electrical power, which disabled the heaters and thermal control systems that were necessary to maintain a livable temperature in the spacecraft.
• The crew was forced to shut down non-essential systems to conserve power, which meant the active thermal control system was also shut down.
• Without these systems, the spacecraft radiated heat into space and was unable to replace the lost heat, leading to extreme cold.
• The Lunar Module was not designed for long-term occupancy or thermal management in deep space, contributing to the cold environment.

In conclusion, the extreme cold during Apollo 13 was caused by the loss of power and the resulting inability to actively manage the spacecraft’s internal temperature. This left the astronauts at the mercy of radiative heat loss into the vacuum of space.

That Doesn't Really Seem to Fit with Our Discussion Up Until Now

You're right—it seems like the previous answer didn’t fully align with the concepts we were discussing regarding thermal regulation and radiation in space. Let's try to reconcile the details of the Apollo 13 situation with the broader principles of thermal management in space.

Revisiting Key Concepts:

In our earlier discussion, we talked about how radiation is the primary mechanism for heat transfer in space, where there’s no air for conduction or convection. Objects in space gain or lose heat primarily through the absorption and emission of radiation. A spacecraft, even in the vacuum of space, can manage its internal temperature using reflective materials, radiators, and active control systems to ensure that the heat balance is maintained. When those systems fail or are disrupted, temperature regulation becomes a serious challenge.

Why Apollo 13 Got So Cold:

Given that, let's take a closer look at Apollo 13 and why it became so cold, while keeping in mind the principles we’ve discussed.

1. Loss of Power → Loss of Active Thermal Control:

   • Active thermal control is crucial in space because you have to manage heat flows carefully. For Apollo 13, once the oxygen tank exploded, the crew had to shut down the Command Module (CM) to conserve battery power for re-entry. The active thermal systems, which normally would balance the heat generated internally and manage heat loss to space, were no longer operational.
   • Without these systems, the spacecraft was unable to control its internal temperature. It couldn’t regulate heat loss, so it began to radiate more heat into space than it could retain, causing the internal temperature to drop.

2. Heat Loss by Radiation:

   • Even though space itself is a vacuum and doesn’t "conduct" cold, objects in space naturally radiate heat away into the vacuum. Normally, the Apollo spacecraft would radiate excess heat through radiators, which were designed to manage this balance.
   • With the spacecraft in a low-power state and without active thermal regulation, it radiated heat out into space faster than it could replenish heat. Passive radiation in space can cause rapid cooling, especially when systems that generate heat (like electronics) are turned off or operating at low power.

3. The Role of the "Barbecue Roll":

   • The "barbecue roll" maneuver (slowly rotating the spacecraft) helped maintain thermal balance by evenly distributing the heat from the Sun around the spacecraft. During normal operations, this roll helped prevent one side from becoming too hot and the other from becoming too cold.
   • After the explosion, the maneuver was likely disrupted or abandoned, meaning that parts of the spacecraft were exposed to the coldness of space (radiative heat loss) for prolonged periods. This uneven exposure could lead to a faster rate of cooling in certain areas, compounding the overall drop in temperature.

4. No Heat Generation From Equipment:

   • Under normal conditions, spacecraft equipment, like computers and other electronics, generate heat as they operate. This heat contributes to maintaining the spacecraft’s internal temperature. However, once the Apollo 13 crew shut down non-essential systems to conserve power, there was very little waste heat being generated.
   • This lack of internal heat generation meant that the spacecraft had less heat to "lose" through radiation, which accelerated the cooling process.

5. Conservation Mode in the Lunar Module:

   • The astronauts moved into the Lunar Module (LM), which wasn’t designed for prolonged occupancy in deep space. The LM had a much smaller life support system, and it wasn’t equipped to handle the long-term thermal regulation needed for this emergency situation.
   • The Lunar Module’s systems were designed to work during lunar surface operations, where different thermal challenges exist (such as exposure to sunlight on the Moon or lunar surface cooling). In translunar space, with low power and no active thermal control, the LM wasn’t able to provide sufficient heat.

6. Limited Solar Heating:

   • Although the Sun was still present during parts of Apollo 13's journey back to Earth, the crew was not able to manage their orientation as carefully as they would under normal circumstances. They couldn’t optimize their exposure to the Sun for passive heating.
   • Without the barbecue roll or careful orientation, parts of the spacecraft were not receiving as much sunlight, meaning the crew couldn’t take advantage of solar heating.

So Why Were They So Cold? (In Light of Our Discussion)

In essence, Apollo 13 became so cold not because space itself is "cold," but because the systems designed to regulate heat were offline. Here’s why this matters in relation to radiation:

• Radiative Heat Loss: In space, objects radiate heat into the vacuum. Without active control, the spacecraft kept losing heat through radiation, but there was no significant source of heat generation or retention because most systems were powered down.
• Lack of Active Thermal Management: The shutdown of the spacecraft’s active thermal control systems (like the Command Module's radiators and heaters) meant that the spacecraft could no longer regulate how much heat it was losing or gaining.
• No Significant Heat Source: The lack of operational equipment meant the crew didn’t have any internal systems producing waste heat. Normally, this waste heat helps maintain the internal temperature.
• No Use of Solar Energy: The failure of the barbecue roll and the inability to manage solar radiation exposure meant the crew couldn’t utilize the Sun’s heat to offset the cooling.

Conclusion:

Apollo 13 became extremely cold because the crew had to shut down most of the spacecraft’s systems to conserve power. This resulted in uncontrolled radiative cooling, where the spacecraft radiated heat into space without generating or retaining enough heat to maintain internal temperatures. This aligns with our previous discussion of heat management in space: radiation is the primary way heat is transferred, and without active thermal control, radiative cooling can lead to significant drops in temperature, as happened to Apollo 13.

Was the LEM in the Shadow of the CM?

No, during the Apollo 13 mission, the Lunar Module (LM) (or LEM as it’s sometimes called) was not consistently in the shadow of the Command Module (CM). The spacecraft did not stay in a fixed position relative to each other, and the overall configuration during the mission was designed so that both the Command Module and the Lunar Module would be exposed to space and sunlight as needed.

Key Points on the LM and CM During Apollo 13:

1. Docked Configuration:

   • After the explosion in the Service Module, the Lunar Module (LM) became the "lifeboat" for the crew. The LM remained docked to the Command Module (CM) throughout the return journey to Earth, but the LM wasn’t tucked in the shadow of the CM consistently.
   • The LM and CM were docked nose to nose, so both modules were exposed to space. Depending on their orientation, parts of both spacecraft were exposed to sunlight, while other parts were radiating heat into the cold vacuum of space.

2. No Consistent Shadowing:

   • Due to the docking configuration, the LM was not constantly shielded by the CM. The two modules didn’t stay in a fixed orientation relative to the Sun. Instead, the spacecraft slowly rotated to ensure that heat was distributed more evenly—a maneuver known as Passive Thermal Control (PTC) or the "barbecue roll."
   • The "barbecue roll" maneuver ensured that both the LM and CM were alternately exposed to the Sun and shadow, preventing one side from overheating or overcooling. However, after the explosion and the loss of power, maintaining precise thermal control became difficult.

3. Post-Explosion Effects:

   • After the oxygen tank explosion, power and life support systems were compromised. This disrupted the spacecraft's ability to maintain a controlled thermal environment. While some parts of the spacecraft would have been exposed to sunlight, the overall thermal management system wasn’t functioning properly to handle heat retention or rejection.
   • With limited power available, the thermal systems in both the LM and CM couldn’t operate effectively to manage the heat balance, leading to the crew experiencing the cold, regardless of whether they were in sunlight or shadow at any given moment.

Why It Matters:

• In the vacuum of space, radiative heat loss occurs even in shadowed regions, meaning that without power to manage heating and cooling, both the LM and CM would lose heat over time.
• While the LM may have been in sunlight during parts of the journey, the lack of proper thermal regulation (due to power loss) meant that the spacecraft couldn’t retain enough heat to keep the interior warm.
• The crew could not rely on passive solar heating because the systems that would normally balance heat gain and heat loss were offline or limited.

Conclusion:

The LM was not consistently in the shadow of the CM during Apollo 13. Instead, both modules were intermittently exposed to sunlight and the vacuum of space, especially given the spacecraft’s rotation and the "barbecue roll" maneuver. However, the extreme cold the crew experienced was a result of the loss of power and the failure of thermal control systems, not the spacecraft's position relative to the Sun or the CM's shadow.

Didn't the LM Have Windows? Surely the Heat Would've Kept them Warm from the Sun

Yes, the Lunar Module (LM) did have windows, but the exposure to sunlight through the windows wasn’t enough to keep the crew warm during the Apollo 13 mission. Here’s why the windows didn’t significantly help with heat, and why the crew still experienced extreme cold despite being exposed to sunlight:

1. Limited Solar Heating Through the Windows:

• The LM had two small triangular windows designed primarily for visibility during lunar landing operations. These windows were not large enough to let in a substantial amount of sunlight to provide significant heating for the cabin.
• The surface area of the windows was relatively small compared to the rest of the LM, so even if sunlight entered the windows, it wouldn’t have been enough to compensate for the overall heat loss occurring in the spacecraft.

2. Thermal Insulation and Reflection:

• The windows and walls of the LM were designed with insulation and reflective coatings to prevent excessive heat absorption. This was crucial during normal operations on the lunar surface, where the spacecraft would be exposed to the Sun's intense heat.
• The windows and walls reflected much of the incoming sunlight to avoid overheating during lunar landings, where temperature management was vital. In the cold vacuum of space, the LM’s design prioritized insulation and radiative heat rejection to maintain balance rather than allowing heat to accumulate.

3. Heat Loss Through Radiation:

• In the vacuum of space, heat is lost primarily through radiation, and the LM’s systems were designed to radiate heat away to avoid overheating in the harsh lunar environment. During the Apollo 13 emergency, the active thermal management systems were offline, meaning the spacecraft couldn’t retain heat effectively.
• Even though sunlight was entering the windows, it wasn’t enough to offset the continuous heat loss that the LM was experiencing through radiation into the vacuum of space.

4. Internal Heat Loss and Power Shutdown:

• The main reason the crew felt so cold was that the LM was designed for short-term use, not prolonged space travel. With most systems shut down to conserve power, there was very little waste heat from electronics or life support systems to help maintain the cabin temperature.
• The heat generated by equipment and life support systems (which typically contributes to keeping the cabin warm) was severely reduced because many systems were turned off.

5. Sunlight Didn’t Penetrate Effectively:

• The Apollo 13 crew needed to carefully manage their exposure to the Sun. During parts of their journey, they were in orientations where the windows wouldn’t receive direct sunlight due to the spacecraft’s attitude and slow rotation (when they could still execute the "barbecue roll" maneuver). This limited how much solar heating the windows could provide.
• The windows weren’t intended to act as a source of heat but rather were functional for navigation and landing operations. Their contribution to the overall cabin temperature was minimal.

6. Overall Loss of Thermal Control:

• The core issue wasn’t sunlight exposure but the loss of active thermal management. The LM, and especially the Command Module (CM), was designed to manage temperatures through a combination of heaters, radiators, and insulation. These systems were either damaged or turned off during the power-saving efforts after the oxygen tank explosion.
• Even with some sunlight coming through the windows, the spacecraft as a whole continued to radiate more heat into space than it absorbed, leading to a continuous drop in temperature.

Conclusion:

The windows in the Lunar Module did allow some sunlight into the cabin, but this wasn’t nearly enough to keep the crew warm. The small size of the windows, combined with the loss of active thermal management systems, meant that the LM continued to lose heat faster than it could gain from sunlight. The insulated walls and reflective coatings of the spacecraft also minimized heat absorption, prioritizing thermal balance over passive solar heating. As a result, the crew endured extreme cold during the Apollo 13 mission.

As Temperature Dropped, So Would Heat Loss. If They Were Insulated, Their Suits and Body Heat Didn't Keep Them Warm in Constant Solar Radiation?

You're correct that as temperature drops, the rate of heat loss would slow down due to the decreasing difference between the internal temperature of the spacecraft and the environment. Additionally, the insulation of the Lunar Module (LM) and the astronauts' spacesuits should, in theory, help retain some heat. However, there are several factors at play during Apollo 13 that made it difficult for the astronauts to stay warm, despite insulation, body heat, and periodic exposure to solar radiation. Let’s break it down:

1. Insulation and Radiative Heat Loss:

• The LM and spacesuits were designed with multiple layers of insulation, primarily to protect astronauts from extreme heat and cold in space and on the Moon. These materials (like aluminized Mylar) worked by reflecting radiant energy and minimizing heat conduction.
• In a vacuum, radiation is the only mechanism for heat transfer. Even with insulation, objects will radiate heat into space as long as they are warmer than their surroundings. The Lunar Module and the astronauts were constantly radiating heat into space, and without an active heating system to replenish that lost heat, they continued to cool down.

2. Spacesuit Body Heat Management:

• The astronauts’ spacesuits were designed for use on the lunar surface, where they would be exposed to direct sunlight and would need to reject excess body heat. The suits had built-in systems, like the Liquid-Cooled Garment (LCG), to manage body heat by circulating water and removing excess heat through sublimation.
• During Apollo 13, however, the astronauts weren’t using the full cooling features of the suits since they weren’t on the lunar surface, and the LM was not actively heating. While the suits offered insulation, they were not designed to retain significant amounts of heat over long periods because they were optimized for removing heat, not conserving it.

3. Solar Radiation and Limited Heating:

• Solar radiation was present, but it wasn’t consistently heating the crew or the spacecraft. After the explosion, the Apollo 13 crew had to manually control the spacecraft’s attitude, and they weren’t able to maintain the barbecue roll maneuver as they would in normal circumstances. This resulted in parts of the LM and Command Module being unevenly exposed to sunlight.
• Moreover, solar radiation is effective in heating objects that absorb it directly, but the astronauts inside the LM weren’t in direct sunlight for most of the time. They were shielded by the LM’s insulated walls and windows, which were designed to reflect and minimize the absorption of solar heat to prevent overheating on the lunar surface. As a result, the insulation that protected them from overheating during lunar operations also limited the amount of warmth they could gain from sunlight.

4. Body Heat Not Enough to Counter Heat Loss:

• Body heat does provide warmth, but in the context of the Lunar Module, it was insufficient to maintain a comfortable temperature. The LM was designed to rely on active thermal systems to maintain cabin temperature, and when those systems were no longer available due to the power loss, body heat alone could not compensate for the continuous heat loss through radiation.
• Even though the astronauts were insulated in their suits, the overall environment of the LM was still losing heat to space, and the LM wasn’t designed to retain enough of the astronauts’ body heat to significantly warm the cabin. The astronauts themselves would have been losing heat to the surrounding environment faster than they could generate it.

5. Limited Heat Sources and Power Constraints:

• Typically, the Command Module (CM) and the Lunar Module (LM) would have internal heat sources, such as the electronics and life support systems, which generate waste heat that helps keep the cabin warm. However, after the explosion, most of these systems were either damaged or shut down to conserve power, so there was very little waste heat being generated.
• With the loss of these systems, the spacecraft was unable to actively generate or retain enough heat to maintain a stable temperature. The cold was exacerbated by the inability to reheat the cabin using onboard systems.

6. Thermal Inertia and Delayed Cooling:

• Initially, the insulation in the LM and the astronauts’ suits would have helped slow down the rate of heat loss. However, as the temperature inside the spacecraft gradually dropped over time, the rate of radiative cooling also slowed down (because the temperature difference between the spacecraft and space decreased).
• Despite this, the thermal inertia of the LM and the lack of any significant internal or external heat sources meant that the cabin continued to cool down until it reached a very uncomfortable temperature.

Why Body Heat and Solar Radiation Weren’t Enough:

• Insulation alone (whether from the LM or spacesuits) was not sufficient to maintain warmth because the LM was still losing heat through radiative cooling into space.
• Body heat from the astronauts wasn’t enough to meaningfully warm the large volume of the LM, and their suits, while insulated, were designed to regulate body temperature rather than trap significant amounts of heat.
• Solar radiation wasn’t effectively heating the LM because the windows were small, and the spacecraft’s reflective insulation was designed to prevent heat gain from the Sun, which was helpful on the Moon but counterproductive in this situation.
• With the loss of active heating systems and the reduced internal heat generation from powered-down electronics, the LM essentially became a cold environment with limited ways to retain or generate heat.

Conclusion:

Despite the insulation in the LM and their spacesuits, the astronauts on Apollo 13 experienced extreme cold because of the continuous radiative heat loss into space and the lack of active heating systems. Their body heat and the small amount of solar radiation entering through the windows were insufficient to maintain a comfortable temperature, especially over an extended period. The insulation prevented rapid heat loss but couldn’t stop the steady cooling that occurred without a source of heat to counterbalance the radiative cooling.

How Hot Does a Geostationary Satellite Get in Orbit?

A geostationary satellite in orbit can experience significant temperature variations due to its exposure to the Sun and the vacuum of space. The temperatures depend on whether the satellite is in direct sunlight or in the Earth's shadow. Here’s a breakdown of the temperatures that a geostationary satellite typically encounters:

1. Direct Sunlight:

• When a geostationary satellite is exposed to direct sunlight, it absorbs solar radiation. The solar constant at geostationary orbit (around 35,786 km or 22,236 miles above Earth) is about 1,360 watts per square meter.
• The surfaces of the satellite that are in direct sunlight can reach temperatures of about +120°C (248°F) or even higher, depending on the material and surface coatings (which can influence how much heat is absorbed or reflected).

2. In the Earth's Shadow (Eclipse):

• When the satellite passes into the Earth’s shadow, it can cool down significantly because it is no longer receiving direct solar radiation. In the vacuum of space, heat is lost only through radiation, so the satellite can cool to very low temperatures, especially on surfaces that aren’t radiating much energy.
• During periods in the Earth’s shadow (such as during an eclipse), temperatures on the satellite can drop to around -180°C (-292°F) or lower, depending on the duration of the eclipse and the satellite's thermal design.

3. Thermal Management:

• Geostationary satellites are equipped with thermal control systems to manage these extreme temperature variations. These systems include:
  • Thermal blankets and insulating materials (like multi-layer insulation or MLI) to help manage heat absorption and prevent heat loss.
  • Radiators that allow excess heat to be radiated away from critical components.
  • Heat pipes that help distribute heat from hot areas (in sunlight) to cooler areas (in shadow).
  • Surface coatings that either reflect sunlight or help radiate excess heat.

4. Thermal Design and Average Temperatures:

• The average temperature of a geostationary satellite typically remains within a manageable range due to its thermal control systems. For most spacecraft, critical components are kept within operational temperature ranges, generally between -40°C and +60°C (-40°F to 140°F).
• While certain external surfaces may experience extreme temperatures (up to +120°C in sunlight and -180°C in shadow), the internal components are usually kept within much narrower limits thanks to active and passive thermal control systems.

Factors Affecting Satellite Temperature:

• Albedo (Reflectivity): How reflective the satellite's surfaces are can significantly affect how much solar radiation is absorbed. Surfaces with high albedo reflect more sunlight and absorb less heat.
• Thermal Emissivity: This measures how effectively a material radiates heat. Surfaces with high emissivity can radiate more heat away, helping the satellite cool more effectively.
• Orientation: The orientation of the satellite relative to the Sun and Earth can also influence its thermal balance. Some surfaces may receive more direct sunlight while others remain in shadow.

Summary:

• In direct sunlight, a geostationary satellite can experience temperatures as high as +120°C (248°F) or more.
• In the Earth’s shadow, temperatures can drop to around -180°C (-292°F) or lower.
• The satellite’s thermal control systems help maintain a stable internal temperature, usually keeping critical components within a much narrower range of -40°C to +60°C (-40°F to 140°F).

This wide range of temperatures is one of the biggest challenges in satellite design, requiring sophisticated thermal management to ensure that the satellite’s components function correctly over long periods in orbit.

And Apolllo 13 Was Without Shade for 9 Days

Yes, Apollo 13 spent much of its journey back to Earth in direct sunlight for extended periods, but there are important factors that explain why the crew still experienced extreme cold despite being exposed to solar radiation during much of the 9-day mission. Let's break down the key points:

1. Apollo 13 Was Not Constantly in Direct Sunlight:

• While the spacecraft was not in the Earth's shadow for most of the mission, it was also not continuously exposed to direct sunlight. After the explosion, they lost the ability to perform the normal "barbecue roll" (Passive Thermal Control maneuver), which is essential for evenly distributing solar radiation around the spacecraft.
• Without this slow roll, different parts of the spacecraft would have experienced alternating periods of sunlight and shadow, leading to uneven heating and cooling. Some surfaces may have been exposed to solar radiation, but others were radiating heat away into space.

2. The Spacecraft’s Design Was Meant to Prevent Overheating:

• The Command Module (CM) and Lunar Module (LM) were designed to operate in both sunlight and the vacuum of space, where radiative cooling is the only way to lose heat. The spacecraft’s exterior was highly reflective, using materials like aluminized Mylar and thermal coatings that were intended to minimize heat absorption from the Sun. These materials prevented the spacecraft from overheating during normal operations, but in the context of Apollo 13, they also limited how much heat the spacecraft could absorb when they needed warmth.
• The LM and CM had thermal control systems that could regulate temperatures in space and on the Moon, balancing heat absorption and rejection. With those systems partially offline, the spacecraft could not regulate heat as intended.

3. Radiative Heat Loss in a Vacuum:

• In space, the only way to lose heat is through radiation. After the oxygen tank explosion, Apollo 13 lost power to many systems, including much of the thermal control systems. Without those systems, the spacecraft was still radiating heat away into space faster than it could be replenished.
• Even in direct sunlight, the spacecraft continued to lose heat from its dark surfaces and other components exposed to space. In the vacuum, radiative heat loss becomes a major factor, and without an active heating system, the interior of the spacecraft continued to cool down over time.

4. Solar Radiation Alone Was Not Enough:

• Solar radiation in space is indeed intense (about 1,360 watts per square meter at Earth’s distance from the Sun), but that doesn’t necessarily translate into warming the spacecraft’s interior. The reflective outer layers of the spacecraft minimized how much solar energy was absorbed.
• Furthermore, the astronauts were inside the LM and CM, which were insulated against both heat gain and heat loss. The insulation, while crucial for preventing overheating, also reduced how much of the Sun’s heat penetrated the spacecraft to warm the interior.

5. Internal Heat Sources Were Shut Down:

• During normal operations, much of the heat inside the spacecraft came from waste heat generated by onboard systems, such as electronics, life support, and propulsion systems. After the explosion, many of these systems were shut down to conserve power, leading to a significant loss of internal heat generation.
• The Lunar Module was not designed to operate in deep space for extended periods, so its thermal systems weren’t robust enough to handle the longer journey. Without enough active systems to produce waste heat, the spacecraft continued to cool.

6. Body Heat Was Insufficient:

• Although the astronauts were generating some body heat, it was not nearly enough to warm the spacecraft's interior. The insulation in their spacesuits and the spacecraft was designed to keep the astronauts safe from extreme temperatures, but without active heat sources, their body heat alone couldn’t offset the overall heat loss to space.
• The size of the spacecraft and the heat loss from radiative cooling overwhelmed the small amount of heat produced by the astronauts themselves.

7. Water Condensation and Humidity:

• As the temperature inside the spacecraft dropped, condensation formed on the walls. This indicated that the interior was cold enough to cause moisture in the air to condense, further evidence that the spacecraft was losing heat faster than it was gaining it.
• The high humidity also contributed to the crew’s discomfort, making them feel colder in an already cold environment.

Conclusion:

Even though Apollo 13 spent much of its journey in direct sunlight, several factors led to the spacecraft becoming cold:

• The reflective exterior of the spacecraft was designed to prevent overheating, minimizing solar heat absorption.
• The loss of active thermal control systems meant the spacecraft could not balance heat gain and heat loss, leading to continuous radiative cooling.
• The shutdown of internal systems resulted in a lack of waste heat, which would normally help keep the spacecraft warm.
• Body heat and limited solar heating were insufficient to offset the heat being lost to space.

In the end, the combination of radiative heat loss, lack of internal heat generation, and limited solar absorption caused the Apollo 13 crew to experience extreme cold despite being exposed to solar radiation for much of the mission.

Parts of Apollo 13 Must Have Exceeded 248°F For The Whole Journey

Parts of Apollo 13's spacecraft, particularly those surfaces exposed to direct sunlight, would have experienced extremely high temperatures—possibly exceeding 248°F (120°C)—during the journey. This is consistent with how spacecraft in space, without atmospheric interference, can absorb intense solar radiation.

So, why didn’t this heat keep the crew warm, even though parts of the spacecraft were exposed to such high temperatures?

Let’s break this down by considering the thermal design of the spacecraft and the specific situation with Apollo 13.

1. Thermal Design of the Spacecraft:

• Reflective Surfaces: The Command Module (CM) and Lunar Module (LM) were covered in highly reflective materials, such as aluminized Mylar, to reflect most of the solar radiation. This was critical to prevent overheating of the spacecraft during normal operations, particularly on the lunar surface. These surfaces were designed to reflect solar radiation rather than absorb it.
• Insulation: Both the LM and CM had multi-layered insulation designed to protect internal components from extreme temperatures, both hot and cold. This insulation helped ensure that the heat absorbed by the spacecraft's external surfaces did not transfer efficiently to the inside. It prevented the high temperatures on the spacecraft’s outer surface from affecting the internal cabin environment significantly.

2. Uneven Heat Distribution:

• Even though parts of the spacecraft were in direct sunlight, the lack of the "barbecue roll" maneuver after the explosion meant that other parts of the spacecraft were in constant shadow for extended periods. This uneven exposure led to some areas heating up significantly, while others radiated heat into space and cooled down.
• Surfaces facing away from the Sun would have cooled to very low temperatures, potentially as low as -250°F (-157°C) in shadow, as they radiated heat away without any incoming solar radiation to offset the heat loss. This stark contrast between hot and cold surfaces would make it difficult to achieve an even temperature throughout the spacecraft.

3. Radiative Cooling of the Interior:

• The interior of the spacecraft was still radiating heat away into space, even though the outside was exposed to the Sun. Without active thermal control systems to manage heat distribution, the spacecraft continued to lose heat from the inside.
• The vacuum of space allows for continuous radiative cooling, and without sufficient power to run the heaters, the LM and CM couldn't retain heat effectively. This explains why, despite external surfaces being hot, the internal temperature kept dropping.

4. Limited Heat Transfer to the Inside:

• Although the external parts of the spacecraft facing the Sun were absorbing heat, the heat transfer from these surfaces to the internal cabin was very limited due to the insulating materials and reflective coatings designed to minimize heat conduction.
• The spacecraft was built to isolate its internal environment from the extreme external temperatures—both hot and cold—so even though some parts of Apollo 13's exterior could have reached extreme temperatures in sunlight, this heat did not penetrate into the cabin where the crew was situated.

5. Loss of Internal Heat Sources:

• Normally, heat generated by the electrical systems, life support, and equipment inside the spacecraft would help maintain a comfortable temperature for the crew. After the oxygen tank explosion, these systems were either damaged or shut down to conserve power, leading to the loss of internal heat generation.
• With minimal internal heat sources and limited heat transfer from the external surfaces, the interior of the spacecraft became cold, even though some external parts were very hot.

6. Solar Radiation Alone Was Insufficient:

• While the Sun is a powerful heat source, the spacecraft’s design ensured that most of the solar radiation was reflected rather than absorbed. The interior insulation and reflective layers acted as barriers, limiting how much solar heat could reach the crew.

7. Thermal Inertia and Gradual Cooling:

• The internal components of the spacecraft had thermal inertia, meaning they held onto heat for a while but gradually cooled down as radiative heat loss continued.
• Without active systems to replenish the heat lost, the internal temperature continued to drop despite exposure to the Sun. The crew’s body heat and the limited waste heat from equipment could not compensate for the radiative losses.

Conclusion:

Yes, parts of Apollo 13—especially the exterior surfaces in direct sunlight—would have reached very high temperatures, possibly exceeding 248°F (120°C), for much of the journey. However, the crew was still cold because:

• The spacecraft’s insulation and reflective materials were designed to keep the interior isolated from extreme external temperatures.
• The thermal control systems were offline, so heat couldn't be actively managed or redistributed.
• Heat transfer from the hot external surfaces to the interior was minimal due to the spacecraft’s thermal design.
• Radiative cooling continued to lower the internal temperature, and the lack of internal heat generation compounded the problem.

As a result, despite parts of the spacecraft being extremely hot on the outside, the crew inside experienced intense cold.

Without Roll Capability, Radiation Exposure Should Have Ruptured the Hull

You raise an important point about the role of thermal management and exposure to extreme heat in space. However, in the case of Apollo 13, while the spacecraft did indeed lose the ability to perform the "barbecue roll" (also known as Passive Thermal Control (PTC)), the spacecraft was designed to endure significant thermal stress and prevent catastrophic damage, even without the roll. Let's explore why thermal radiation exposure didn’t rupture the hull, despite the loss of the roll maneuver:

1. Thermal Design and Materials of the Apollo Spacecraft:

• The Command Module (CM) and Lunar Module (LM) were constructed with highly reflective surfaces, such as aluminized Mylar and Kapton, which are designed to reflect the vast majority of solar radiation.
• These materials were chosen to withstand high levels of thermal radiation from the Sun, especially during long-duration missions. The spacecraft were built to endure prolonged exposure to direct sunlight and space environments without suffering significant thermal degradation or overheating.
• Heat-resistant materials in the outer hull and protective thermal coatings ensured that the spacecraft could handle both extreme heat (from solar radiation) and cold (from shadow or deep space exposure). Even without active thermal control, these surfaces could reflect and radiate heat efficiently to prevent damage to the structural components.

2. Thermal Limits and Design Safety Margins:

• The Apollo spacecraft were designed with significant thermal safety margins to handle the extreme temperatures of space. Even with direct exposure to solar radiation, the outer surfaces of the spacecraft could reach temperatures of +248°F (120°C) or higher, but the hull materials were capable of withstanding much higher temperatures than this.
• The structural materials (such as titanium, aluminum alloys, and stainless steel used in the hull) were engineered to tolerate these temperatures without risk of rupture or failure. These materials do not melt or deform at the temperatures reached by the spacecraft’s exterior in sunlight.

3. Heat Distribution and Surface Exposure:

• Although Apollo 13 lost its ability to perform the barbecue roll, it didn’t result in a single side of the spacecraft being continuously exposed to direct sunlight for the entire journey. The spacecraft likely still rotated slowly or adjusted its orientation from time to time as part of maneuvers, leading to partial exposure to the Sun and shadow at different points.
• Even without continuous roll, the spacecraft would have experienced varying levels of exposure, allowing for some passive heat dissipation on the shaded side through radiative cooling. The hull’s thermal inertia (its ability to absorb and release heat over time) also helped spread the heat around the spacecraft, avoiding catastrophic overheating on one side.

4. Thermal Expansion Without Structural Damage:

• The materials used in the Apollo Command Module and Lunar Module were designed to handle thermal expansion and contraction without suffering structural failure. Thermal expansion occurs when materials expand as they heat up and contract when they cool down.
• The engineers accounted for this effect in the design, ensuring that the spacecraft could tolerate the thermal cycling (alternating between hot and cold) experienced during spaceflight. This includes exposure to sunlight and the cold vacuum of space without rupturing the hull.
• Even though parts of the spacecraft may have heated up significantly due to direct exposure, the hull could expand slightly without any structural damage, thanks to the selection of heat-tolerant materials and the overall design.

5. Internal Insulation Prevented Heat Transfer:

• While the exterior of the spacecraft could reach high temperatures, the multi-layer insulation (MLI) and reflective coatings were specifically designed to prevent heat from transferring into the interior of the spacecraft.
• The insulation helped ensure that the interior of the Command Module and Lunar Module remained relatively stable in temperature, even if the outer hull was exposed to intense sunlight. The insulation prevented the buildup of heat inside the spacecraft that could damage sensitive electronics or the structural integrity of the module.

6. No Power for Internal Systems to Overheat:

• Another key factor is that many of the internal systems in Apollo 13 were powered down to conserve battery power after the explosion. This reduced the amount of internal heat generation from electrical systems, life support, and other equipment.
• While the barbecue roll was essential for distributing heat during normal operations, the fact that many of the internal heat-generating systems were off also helped prevent overheating from occurring. Essentially, there was less heat to manage overall, which made the spacecraft less prone to thermal damage from internal heat buildup.

7. Prolonged Exposure to the Sun:

• Although direct sunlight can cause significant heating, the spacecraft’s thermal design allowed it to reflect and radiate heat, preventing the hull from overheating to a dangerous level. While parts of the spacecraft were indeed exposed to the Sun for long periods, this exposure did not reach the critical point where the hull would rupture because of the materials used and the passive heat dissipation.

• Heat rejection through radiation was still happening on parts of the spacecraft that were in shadow, and while Apollo 13's loss of the barbecue roll increased the thermal challenges, the spacecraft’s design prevented catastrophic thermal damage.

Conclusion:

Despite the loss of the barbecue roll maneuver and the prolonged exposure to solar radiation, Apollo 13 did not experience a hull rupture because:

1. The spacecraft was designed with highly reflective and heat-resistant materials to handle prolonged exposure to solar radiation without overheating.
2. The thermal insulation and design safety margins allowed the spacecraft to manage heat without transferring too much of it into the interior or damaging structural components.
3. Thermal expansion was accounted for in the design, so the hull materials could tolerate heat without rupturing.
4. The spacecraft was not in continuous, unchanging exposure to direct sunlight. Even without active thermal control, there were varying levels of exposure and passive radiative cooling on the shaded side.
5. With many of the internal systems powered down, there was less internal heat generation, reducing the risk of overheating from within.

The spacecraft's thermal design and materials were robust enough to handle the thermal stresses encountered during the mission, preventing catastrophic failure, even in the challenging circumstances of Apollo 13.