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Heat Flow

NSSDCA ID: 1971-063C-06

Mission Name: Apollo 15 Lunar Module /ALSEP
Principal Investigator:Dr. Marcus G. Langseth


The Heat Flow Experiment (HFE), which was part of the Apollo Lunar Surface Experiment Package (ALSEP), was designed to determine the rate of heat loss from the lunar interior by temperature and thermal property measurements at the surface and in the subsurface. The experiment was carried on the Apollo 15, 16, and 17 missions and was essentially identical on all three missions. The experiment apparatus consisted of two probes connected by 8 meter long cables to an electronics box which was in turn connected by a flat ribbon cable to the ALSEP station. The astronauts would drill two holes with the Apollo lunar surface drill (ALSD). The ALSD was equipped with borestem caps and retainers, borestems, bore bits, a bore bit/drill adapter, a treadle, and a bore stem/core stem wrench. The borestem assemblies used in drilling consisted of lengths of hollow fiberglass tubes, 2.5 cm in diameter, which would be connected together as the drilling progressed, and remained in the holes to provide a casing to prevent collapse of the hole walls during insertion of the probes. Nominally the holes were to be drilled to a depth of 3 meters but in practice no holes reached this depth. The probe would be lowered down into the borestem until it came to rest on top of the drill bit at the bottom of the hole. The borestem tube would project out of the surface a distance dependent on the depth of the hole.

Heat Flow Probes

Each heat flow probe was constructed of two rigid cylinders connected by a flexible joint. Each cylinder was 50 cm in length and held four platinum resistance elements which were electrically connected in pairs to form two accurate (+/-0.001 K) differential thermometers. The first pair of elements were located with one element near the top and one near the bottom of the cylinder, 47 cm apart from each other, and were connected in a bridge circuit. These sensors were designated the gradient bridge (DTG). The other two sensors were located 9 cm below the top DTG sensor and 9 cm above the bottom DTG sensor, 29 cm apart from each other connected by a bridge circuit. This pair was designated the ring bridge (DTR).

A thermocouple (the probe thermocouple) was mounted near the top of the upper cylinder, colocated with the top gradient sensor. Attached to the top of the upper cylinder was a long cable which connected the probe to the electronics box. The cable contained 3 more thermocouples spaced 65 cm, 115 cm, and 165 cm from the probe thermocouple. The thermocouples were spaced so that at least some of them were outside the borehole on the lunar surface.

1000 ohm Karma-wire platinum resistor heaters surrounded each of the four gradient bridge sensor housings on each probe. These were used for conductivity experiments and could be energized at either 0.002 W (low conductivity mode) or 0.5 W (high conductivity mode). The heaters would be turned on for approximately 36 hours for the low conductivity experiments and 6 hours in high conductivity mode.

The probes return absolute temperature data, differential temperature data (across the bridges), low- and high-thermal conductivity data, and thermocouple temperature data. Specifically, the experiment measured lunar temperatures of the following types, with corresponding accuracies noted in parentheses -- gradient bridge high-sensitivity temperature difference measurements (0.001 K) low-sensitivity temperature difference measurements (0.01 K), and absolute temperature measurements (0.05 K) over the temperature range 190 - 270 K; ring bridge temperature difference measurements (0.002 K) and absolute temperatures (0.05 K) over the range 190 - 270 K; thermocouple temperatures (0.07 K) over 70 - 400 K; and the reference bridge temperature (0.01 K) over 23 - 363 K.

The electronics box contained two multiplexers and amplifiers, dc/dc converter, and an isothermal block which contained a reference thermocouple and reference bridge. The electronics box was nominally kept at 278 to 328 K using heating elements, power control thermostats, a layered aluminized mylar insulation bag. fiberglass case, radiator plate, and sunshield. The instrument was powered by 29 volt d.c. from the central station.

Sensor Naming Convention

The sensors on probe 1 were individually designated as follows; for the upper cylinder the second number is "1", the upper gradient bridge sensor was DTG11A and the lower sensor was DTG11B; the upper ring bridge sensor was DTR11A and the lower DTR11B. For the lower cylinder the second number was "2", the upper gradient bridge sensor was DTG12A, the lower DTG12B, and the upper ring bridge sensor was DTR12A, the lower DTR12B. The bridge sensor pairs were designated DTG11, DTR11, DTG12, and DTR12. The sensors on probe 2 had the same format except that the first number represented the probe number, the first "1" in each designation was replaced with a "2", so the upper cylinder, upper gradient bridge sensor was DTG21A. Two numbering conventions exist in the literature for the thermocouples. We are using the convention that TC14 designates the thermocouple at the top of probe 1, TC13 is the cable thermocouple closest to probe 1, followed by TC12 and TC11. Probe 2 would have TC24, etc. (Another convention, seen in the Preliminary Science Reports, has the probe thermocouple designated as TC11, followed successively in the cable by TC14, TC13, and TC12.)

Instrument Operation

In normal operating mode a 7.25 minute measurement sequence is used to collect the ambient high- and low-sensitivity differential data from the gradient sensors and the thermocouple outputs. The same measurement sequence would be used when the heaters were commanded on for the low conductivity (0.002 W) mode, with the heaters activated in turn for typically 36 hours. For the high-conductivity (0.5 W) sequence, the ring bridge sensors were used and were read every 54 seconds. This mode nominally would last 8 hours. This mode could also be done without the heaters on, with measurements simply being made by the ring bridge sensors. This mode, known as a ring bridge survey, would be used approximately every 6 hours at first and less frequently later in the experiment.

Apollo 15 Operational History

The Apollo 15 heat flow electronics box was set up 9 meters north of the central station, with the hole for probe 1 drilled 4 meters east of the box and the hole for probe 2 drilled 4.5 meters west of the box. Difficulty was encountered in drilling both holes due to the resistant nature of the subsurface regolith. The probe 1 borestem only penetrated to 162 cm and the probe 2 borestem to 160 cm. Additionally, an obstruction was encountered when emplacing probe 2, possibly a break in the borestem, at a depth of about 1 meter, leaving the top of the upper section of probe 2 above the surface and rendering the readings from this section unusable for heat flow studies. For probe 1, the sensors were at the following depths: DTG12B - 138 cm; DTR12B - 129 cm; DTR12A - 100 cm; DTG12A - 91 cm; DTG11B - 83 cm; DTR11B - 74 cm; DTR11A - 45 cm; DTG11A - 36 cm. All three cable thermocouples were above the ground, TC13 was in the borestem projecting above the surface. For probe 2, the sensors were at depths: DTG22B - 96 cm; DTR22B - 87 cm; DTR22A - 58 cm; DTG22A - 49 cm; DTG21B - 41 cm; DTR21B - 32 cm; DTR21A - 3 cm; DTG21A - approximately 6 cm above the surface. All three cable thermocouples were above ground and out of the borestem.

Probe 1 was placed in the hole on 31 July 1971 at approximately 18:47 UT. The instrument was turned on at 19:28 UT on 31 July and the first reading from probe 1 came at 19:47:59. (The turn on time is according to the operational history in the Apollo Scientific Experiments Data Handbook. The time is actually listed as "19.47 hours" which could be an error, possibly meaning 19:47). Because of the troubles with the obstruction in the probe 2 hole, probe 2 was not placed permanently in the hole until the end of the second EVA on 1 August at approximately 17:17 UT. Since the experiment was turned on the previous day readings from probe 2 were coming in before it was emplaced in the hole. In December 1975 the experiment began returning anomalous data and only operated intermittently starting on 28 April 1976. The experiment was commanded off from Earth on 15 January 1977.

Heater schedule

The heaters were turned on and off in the low power (0.002 W) mode in 1971 as follows, with the heater designation followed in parentheses by the depth of the heater, the date and time the heater was turned on, and the date and time the heater was turned off, in UT. For probe 1: H11 (36 cm, 30 Aug. 17:00 - 31 Aug. 17:00), H12 (83 cm, 4 Sep. 05:04 - 5 Sep. 16:55), H13 (91 cm, 26 Aug. 04:58 - 27 Aug. 16:57), H14 (138 cm, 2 Sep. 05:01 - 3 Sep. 16:55). For probe 2 only the bottom two heaters were used: H23 (49 cm, 24 Aug. 05:00 - 25 Aug. 17:01), H24 (96 cm, 7 Sep. 05:00 - 8 Sep. 17:00). In 1973 probe 1 heaters H11 and H12 were rerun in low power mode because they were strongly affected by diurnal variations. These times were chosen to minimize the effects of transient variations near the surface: H11 (36 cm, 13 Mar. 15:01 - 15 Mar. 15:00), H12 (83 cm, 14 Feb. 14:42 - 16 Feb. 02:26).

Alternate Names

  • Apollo15ALSEP/HeatFlow
  • Heat Flow Experiment
  • S037
  • urn:nasa:pds:context:instrument:hfe.a15a

Facts in Brief

Mass: 5.5 kg
Power (avg): 3.9 W
Bit rate (avg): 0.01656 kbps

Funding Agency

  • NASA-Office of Space Science (United States)


  • Planetary Science: Geology and Geophysics

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. Sydney P. Clark, Jr.Other InvestigatorYale University
Dr. Marcus G. LangsethPrincipal InvestigatorLamont-Doherty Geological Observatory

Selected References

  • Langseth, M. G., et al., Revised lunar heat-flow values, Proc. Lunar Sci. Conf. 7th, 1143-1171, 1976.
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