Heat Flow Experiment

Lore & Background

The experiment was developed after a small committee decided to focus on heat flow from the Moon's interior, ruling out a blanket technique due to complexity. The chosen method used a cylindrical heater paired with a temperature sensor, requiring a drill to place probes below the surface to avoid large temperature fluctuations. Bendix Corporation was the principal contractor, Arthur D. Little the sub-contractor, and Gulton Industries Inc. developed the electronic circuitry. The drill was led by Martin Marietta.

On Apollo 15, commander David Scott drilled to 170 cm after difficulties with compacted regolith, inserting one probe; the second hole reached only about 100 cm due to a broken drill bit. Apollo 16's experiment was never deployed; the astronauts could not get the drill to penetrate the regolith to the required depth, and the unit was returned to Earth unused.

Reader's Guide

The Heat Flow Experiment provided the first direct measurements of heat escaping from the Moon's interior, revealing a thermal gradient of 1.5–2.0 K/m and heat flow of about 17 mW/m2. These findings, consistent with seismic and magnetic data, suggested that temperatures at depths around 300 km are relatively close to melting, offering critical insights into the Moon's internal structure and thermal evolution. The experiment also demonstrated the challenges of lunar drilling, as compacted regolith on Apollo 15 required significant force, and the Apollo 16 failure highlighted the vulnerability of surface cabling. Despite these setbacks, the successful Apollo 17 deployment provided years of data, establishing a baseline for understanding lunar heat loss and informing future planetary heat flow studies.

Did You Know?

• The Apollo 16 instrument was rendered inoperable when John Young tripped over the cable, which was not designed to resist tearing motions.
• The Apollo 13 heat flow experiment burned up in Earth's atmosphere while still on board the Lunar Module.
• On Apollo 15, the drill became ineffective after 100 cm due to extreme compaction of regolith not disturbed for at least half a billion years.
• The experiment found that during lunar noon, 70% of all heat transfer in the near surface was radiative.

Design & Engineering

The HFE did not spring from a single inventor's vision. NASA solicited proposals from academia, industry, and its own science groups, and several independent teams suggested measuring lunar heat flow. A small committee was then tasked with determining how such a measurement could actually be executed. They settled on the goal of quantifying heat flowing upward from the Moon's interior rather than merely characterizing surface properties. Several approaches were weighed, including multi-probe arrays and a so-called blanket method, but the blanket idea was dropped because matching its thermal albedo to the lunar surface proved too complex. The winning design paired a cylindrical heater with a temperature sensor at a fixed distance. Crucially, the committee recognized that surface-level readings would be swamped by the Moon's day-night temperature swings, so the probe had to be buried below the regolith. That requirement dragged in an entirely new engineering problem: building a drill capable of penetrating lunar soil. Martin Marietta, which had prior NASA tool experience, led that effort. Bendix Corporation became principal contractor, with Arthur D. Little as sub-contractor and Gulton Industries handling the electronic circuitry.

Instrument Architecture

The HFE package was built around two probes, each made of two 50-centimeter sections. At each section's end sat a gradient thermometer capable of reading temperature at two points—28 and 47 centimeters from the end—yielding four measurement points per section. The connecting cables themselves carried four thermocouples positioned at 0, 65, 115, and 165 centimeters from the topmost gradient sensor. Each section end also housed a heater with two power settings: a low 0.002 watts and a high 0.5 watts, allowing the team to probe a wide range of possible material conductivities. Data acquisition ran on two cadences depending on heater mode—every 54 seconds in one mode and every 7.1 minutes in the other. The probes were meant to be driven to a depth of three meters below the lunar surface using the Lunar Surface drill, a depth chosen to escape the thermal noise of the day-night cycle. The entire assembly was designed to be deployed by astronauts during a surface EVA and then left to collect data autonomously, transmitting readings back through the ALSEP central station.

Mission Deployments

Of the four HFE instruments built, only two ever returned usable data. The Apollo 13 unit was lost when the mission was aborted in flight; the instrument burned up during reentry while still aboard the Lunar Module, and there was no time to transfer it to Apollo 14. On Apollo 15, commander David Scott personally drilled the boreholes. After 100 centimeters the drill lost effectiveness, and Scott had to press his full body weight into the handle to reach 170 centimeters before the first probe was inserted. The second hole stalled at 100 centimeters and was finished on the next EVA, with the probe never fully buried. The culprit was regolith undisturbed for at least half a billion years, making it extraordinarily compact. Apollo 16 saw Charles Duke drill to the full three meters using a modified drill, but the experiment died before it began: John Young tripped over the cable linking the probe to the ALSEP station, tearing it. The cable could resist tugging but not the lateral tearing motion, and repair was rejected as too time-consuming. Apollo 17 was the clean run—both holes drilled without issue, both probes installed, and data streamed for years.

Scientific Context & Findings

Before the Apollo era, scientists could already glean some lunar thermal properties from Earth-based observations. Infrared emissions measured through telescopes and microwave emission spectra had revealed surface temperature, thermal conductivity, and heat capacity. But these remote techniques were limited by low infrared signal levels, long wavelengths that capped data resolution, and an inability to probe how thermal properties changed with depth. The HFE committee therefore focused on what remote sensing could not deliver: a direct measurement of heat flowing upward from the Moon's interior. The experiment's results showed that in the top few centimeters of regolith, heat transfer was dominated by radiation rather than conduction, a consequence of the material's structure at that shallow depth. The loss of the Apollo 13 deployment was felt as a genuine scientific setback; the planned landing site contained a substantial presence of long-lived radioisotopes, and the principal investigator believed that an attempted deployment there would have forced earlier mitigations for the drill and compact-regolith problems that later plagued Apollo 15.

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