The last 5 Apollo missions that landed on the moon (starting from Apollo-12) deployed Apollo Lunar Surface Experiments Package (ALSEP) which was powered by radioisotope thermoelectric generators (RTG)

RTG assembly (on the ground) and the fuel element part inside the fuel cask (which Alan Bean is trying to retrieve with a special tool, prior to inserting it into the assembly) are shown in the cropped AS12-46-6790 image below: enter image description here

Alan Bean (Apollo 12) claims he could feel the heat from the RTG through his gloves:

116:50:05 Bean: That cask's going (in?). Okay, Houston. The fuel element is in the RTG. I can feel it radiate heat already! (To Pete) Put your hand over here.


116:50:54 Bean: Hey, feel the heat off that machine. That's amazing.

116:50:59 Conrad: 1400 degrees (Fahrenheit or about 760 Celsius). (In a conspiratorial tone) Almost as hot as the Sun! (Chuckles)

Some more quotes and comments:

117:07:05 Bean: Use your tongs to hold this (probably ALSEP package 2 which is the base for the RTG) up a minute. It (the RTG)'s a little hot, and I don't want to touch it. Watch (out for) this.

[Bean - (From the 1969 Technical Debrief) "We had no trouble putting down the RTG. I did notice, however, that you could feel the heat radiating from the RTG. When I removed the bracket that carried the power cable that ran from the RTG to the Central Station, it felt warm to the touch. I didn't want to keep my fingers there too long, so I handled it with the ALSEP tool (UHT) as opposed to just my gloved hands, as I had been doing in practice. Apparently that bracket can get pretty hot, although we only had the element in it a short time."]

[Readers will note that anything that can be felt as 'warm' through the gloves, must be quite hot and a potential threat to the gloves. It has been about 17 minutes since they fueled the RTG.]

(Emphasis mine)

From the context of Conrad's phrase, it seems (to me, at least) he was either exaggerating the temperature figure in an attempt to joke, or quoted the temperature of plutonium sitting in the very inside of the fuel element. 760 degC seems a bit too hot for the actual radiator surface.

As the above crop from AS12-46-6790 photo shows, the front surface of the fuel element part (that is visible in front of the fuel cask) doesn't glow red, which it should have if it was above 525 °C, according to this source:

In practice, virtually all solid or liquid substances start to glow around 798 K (525 °C) (977 ˚F), with a mildly dull red color, whether or not a chemical reaction takes place that produces light as a result of an exothermic process. This limit is called the Draper point.

This source lists the figure for the RTG thermal power:

The energy source for each device was a rod of plutonium-238 providing a thermal power of approximately 1250 W.

Question: How hot would surface of the RTG radiator (deployed by Apollo-12 crew) have gotten (I assume the radiator surface as Bean did not touch the actual power element with bare gloves), so that it was possible to feel it through thermally insulated gloves? Was Conrad's estimate a joke, or was it close to reality?

Bonus question: what temperature Apollo gloves could withstand before they get damaged?

  • 1
    $\begingroup$ 760 °C may be the temperature of the fuel elements. There's some temperature data here, but not specifically for the ALSEP RTG. $\endgroup$
    – Hobbes
    Dec 11, 2019 at 9:50
  • 1
    $\begingroup$ This is a really interesting question! The efficiency of TE conversion improves with temperature difference, so the cooler it is relative to the radioisotope thermal source, the better. However the total power radiated is almost the total heat output of the source, and the ability of the radiators to radiate depends on $\sigma \epsilon T^4$ times the area, so every 10% reduction in radiator temperature (absolute) means a 50% increase in radiator effective area. $\endgroup$
    – uhoh
    Dec 11, 2019 at 10:43
  • $\begingroup$ @Hobbes Maybe. In that case it must be temperature of plutonium material itself, hidden in the very inside of the element, as the surface of the element on the photo, evidently, doesn't glow red. I've updated question with some information about 'glowing thershold' temperature, the Draper point. $\endgroup$ Dec 11, 2019 at 11:50

2 Answers 2


I've found a couple of test reports on other RTGs designed for space. First, from A report on RTGs for Pluto and other missions post 2006:

The RTGs (Qualification Unit and flight units) were assembled in the Inert Atmosphere Assembly Chamber (IAAC) at Mound (except for F-8 which was assembled at INL). Mound and INL performed initial RTG functional performance measurements before the end domes were attached and before the RTGs were removed from the IAAC. These performance measurements included power output, load voltage, open circuit voltage, current, internal resistance, isolation (“insulation”) resistance, average outer case temperature (requirement #533 K) as measured by resistance temperature devices (RTDs) and bell jar temperature.

[emphasis mine]

Next, from 1980,

This report, submitted by the General Electric Company, describes work accomplished during the reporting period on the DOE Silicon Germanium RTG Program, Contract DE-ACOl79ET-32043. This program consists of the following three tasks: Task 1 - Multi-Hundred Watt RTG for the Galileo Probe Mission Task 2 - Reestablishment of Silicon Germanium Unicouple Capability Task 3 - General Purpose Heat Source RTG for the International Solar Polar and Galileo Orbiter Missions.

Average qual module shell temperatures in vacuum ranged from 113°C (6 Ib/min at 21°C) to 122°C (4 Ib/min at 30°C).

Another source related to Pluto missions says

Sensors attached to the outside of the RTG case before launch pegged the case temperature at about 245 C


An ALSEP document quoted in this answer https://space.stackexchange.com/a/39611/6944 gives the fuel capsule temperature as 1200 deg F. Doesn't give the radiator fin temp though.


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