Apollo Lunar Surface Experiments Package

• 1. Introduction

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  The Apollo Lunar Surface Experiments Package, known as ALSEP, was a constellation of scientific instruments left behind on the Moon by five Apollo crews. While the astronauts themselves returned to Earth, the machines they planted kept working, transmitting data across space for years. The stations ran continuously until the 30th of September 1977, when mission support was officially terminated. And yet even after the plug was pulled, the signals did not stop. What exactly was ALSEP? How did it get to the Moon? And what did it find out about a world that no human has visited since?

• 2. Deciding the Science

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  In February 1966, scientists and engineers gathered to determine what questions a set of Moon-based instruments should answer. The decisions made that month shaped every Apollo landing from Apollo 12 onward. Seven major experiments were chosen, and the institutions behind them read like a roll call of American research. Frank Press at the Massachusetts Institute of Technology took responsibility for detecting moonquakes through the Passive Lunar Seismic Experiment. C. P. Sonett at Ames Research Center was assigned the task of measuring the Moon's magnetic field. Rice University had two separate experiments: J. W. Freeman Jr. led the Suprathermal Ion Detection work, while B. J. O'Brien took charge of studying low-energy solar wind particles. The Heat Flow Experiment fell to M. Langseth at Columbia University, working alongside S. Clark at Yale. Stanford University's R. L. Kovach led the Active Seismic work, which aimed to probe the Moon's interior by setting off small explosions. Once the experiments were assigned, the job of building and testing the hardware went to Bendix Aerospace in Ann Arbor, Michigan.

• 3. Power and Command

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  A radioisotope thermoelectric generator, or RTG, sat at the heart of every ALSEP station. It converted the heat from the radioactive decay of plutonium-238 into approximately 70 watts of electrical power, enough to keep the instruments running and to push data across the vacuum to Earth. The RTG's fuel element was stored in a protective cask during the journey from Earth, a cask engineered to survive a launch explosion or an accidental re-entry into Earth's atmosphere. That protection was tested in practice on Apollo 13, when the mission was aborted and the hardware re-entered without deploying. The Central Station, a 25 kilogram box with a stowed volume of 34,800 cubic centimeters, was the hub that distributed power and received commands relayed from mission control. A modified axial-helical antenna, 58 centimeters long and 3.8 centimeters in diameter, was mounted on top of the Central Station and aimed at Earth by the astronauts before they left. Thermal control relied on passive elements like insulation and reflectors, supplemented by heaters and power dissipation resistors. On Apollos 12 through 15, a Dust Detector mounted on the Central Station quietly tracked how lunar dust accumulated over time.

• 4. Getting It Out of the Lander

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  ALSEP traveled to the Moon packed into two separate subpackages stored in the Lunar Module's Scientific Equipment bay, which crews called the SEQ bay. The base of the first subpackage became the Central Station once deployed. The base of the second subpackage was the RTG itself. Pete Conrad opened the SEQ bay doors on Apollo 12 by pulling a system of lanyards and pulleys. Alan Bean then used a boom and pulley assembly to lower the second subpackage to the ground. By Apollo 17, crews found that the boom and pulley system created more trouble than it was worth, and the system was removed entirely for that mission. On Apollo 11, Buzz Aldrin skipped the system altogether due to a shortage of time. Fueling the RTG on the surface required a specialized Dome Removal Tool, and a separate Fuel Transfer Tool was needed to extract the plutonium fuel element from its cask. On Apollo 12, thermal expansion caused the fuel element to stick, and Conrad pounded the side of the cask with a hammer while Bean worked it free. Once fueled, the astronauts loaded everything onto a carrybar, which they later repurposed as the antenna mast for the Central Station.

• 5. What the Instruments Did

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  Moonquakes were one of the central targets of the ALSEP science program. The Passive Seismic Experiment was sensitive enough that, during Apollo 11's limited deployment, the seismometer registered Neil Armstrong's movements while he slept. Later versions on the full ALSEP stations used seismology to probe the Moon's internal structure and detected signals both from natural events and from deliberate impacts. After Apollo 13's abort, mission planners deliberately crashed the mission's S-IVB rocket stage onto the lunar surface to produce a controlled signal for the Apollo 12 seismometer already in place. The Heat Flow Experiment drilled down to about 2.5 meters below the surface to measure how heat escapes from the Moon's interior, with data potentially revealing the concentration of radioisotopes deep inside. The Laser Ranging Retroreflectors, by contrast, required no power at all: they simply bounced laser pulses fired from Earth back to their source, with the round-trip travel time providing a precise measure of the Earth-Moon distance. The retroreflectors are the only ALSEP experiments still in use today. The Lunar Surface Gravimeter on Apollo 17 was intended to detect gravitational waves, but a design error prevented the instrument from doing what it was built for.

• 6. Mission by Mission

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  Apollo 11 left the Early Apollo Scientific Experiments Package rather than a full ALSEP, because geologists persuaded NASA to allow only experiments that could be set up within ten minutes. One EVA of two hours and forty minutes was planned, and deploying a full ALSEP typically required one to two hours. The Apollo 11 seismometer, the Passive Seismic Experiment Package, failed after twenty-one days. Apollo 14 deployed the Active Seismic Experiment, which included a Thumper device for detonating charges and a mortar for lobbing explosives. Thirteen of the twenty-two Thumper charges fired successfully. None of the four mortar explosives were ever fired: concerns during the mission kept them dormant, and when engineers later attempted to fire them at the end of the ALSEP's operational lifetime, the long dormancy had rendered them useless. On Apollo 15, the Heat Flow Experiment ran into unexpected soil resistance during drilling, and the probes could not reach their intended depth. Accurate data from that experiment had to wait until Apollo 17's version could be compared against it. Apollo 16 suffered a different kind of loss: Commander John Young accidentally caught his foot on the cable connecting the Heat Flow Experiment to the Central Station, pulling it free from its connector. Mission control decided that the time a repair would require was better spent on other work, and the experiment was abandoned. The Cold Cathode Ion Gauge on Apollo 12 failed after just fourteen hours.

• 7. After the Missions Ended

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  The five ALSEP stations kept transmitting long after the last Apollo crew departed in December 1972. Controllers on Earth managed the experiments remotely, adjusting settings and collecting data across the years that followed. By 1977, at least one station's power supply had degraded to the point where it could no longer run both its transmitter and any other instrument simultaneously. Budgetary considerations drove the decision to terminate support operations, and on the 30th of September 1977, the official mission ended. But the transmitters were never switched off. Between the 18th of October and the 28th of November 1977, after the shutdown had already been declared, the Soviet radio telescope RATAN-600 continued to observe all five ALSEP stations and confirmed that signals were still coming through. Decades later, the Lunar Reconnaissance Orbiter photographed the landing sites from orbit, and images from those passes show the ALSEP hardware still sitting where the astronauts left it.