Learning From Apollo 50 Years Later

Nearly fifty years ago, millions of Americans held their breath, watching in awe as a human being stepped on the moon, the black and white images brought into the intimacy of their living rooms. With the recent anniversary of Apollo, pictures of boot prints on the lunar surface and astronaut helmets reflecting the Earth below have found their place alongside headlines once again, reminding the country of one of its pinnacle achievements. 

While interest in lunar missions has ebbed and flowed over the last fifty years, the infamous boot prints remain on the lunar surface, frozen in time. The lack of atmosphere on the Moon has kept them safe from the erasure of erosion, preserving their story hundreds of thousands of miles from Earth. 

Tracks left by Apollo astronauts during the Apollo 17 mission. NASA.

For lunar scientists, these boot prints present more than a nostalgic event of the past. Recent investigations by members of the NASA Lunar Reconnaissance Orbiter (LRO) science team are using information from the Apollo missions to guide increasingly advanced thermal models. 

Launched in 2009, LRO is a robotic spacecraft about the size of a minivan, orbiting approximately fifty kilometers above the lunar surface. Its camera has taken high-resolution images of the Apollo landing sites, the boot prints looking like dark paint drizzled across the light surface.

One of seven instruments on board LRO is the Diviner radiometer – a device used to measure temperatures. Diviner is covered with a thin silver shield, giving it the appearance of a giant roll of aluminum foil. This small cylindrical tool rides along with LRO, measuring the night and day surface temperatures on the Moon. The instrument has collected almost one trillion measurements to date and has helped scientists identify polar ice deposits, map soil and rock variations across the surface, and even determine sites for future exploration. 

LRO orbiting the Moon. NASA.

So, why do today’s LRO scientists with access to advanced geophysical instruments in orbit care about decades old Apollo data? The answer is found in the unique nature of the Apollo experiments.

Sharing the Diviner goal of recording lunar temperatures, two Apollo missions carried a suite of experiments for emplacement by the Apollo astronauts, designed for minimum weight and maximum scientific return. The most important of these for LRO scientists is the Heat Flow Experiment, or HFE. The HFE’s core purpose was to measure temperatures at various depths below the surface.

“Temperature measurements made during the Apollo missions have been crucial to our understanding of the Moon’s thermophysical properties,” says Dr. Matthew Siegler, a scientist on the LRO Diviner team. The Diviner team has used the Apollo measurements in modeling lunar subsurface temperatures, providing an essential check on the scientists’ projections and Diviner’s measurements. Most recently, the team have used this combination of data to understand permanently-shadowed regions on the Moon which may store ice on the surface. After more than fifty years since humans first set foot on the Moon, we still have a lot to learn. 

The Apollo HFE data has not always been thus appreciated, however. Along with the US interest in lunar science, the completeness and availability of the data has varied through time, creating a complicated paper trail that investigators are just now piecing together.

The story of the data begins with the deployment of the HFE hardware, a process which required Apollo astronauts to drill two holes on the Moon a few meters apart, maneuvering the machinery in their cumbersome spacesuits. They then dropped slender temperature probes into the hallowed pits, beginning the collection of temperature data. 

Images of the HFE on the Moon present a scene of seemingly scattered junk to the untrained eye, the thin temperature probes peeping just above the surface with their soft tethers running along the ground, connecting the probes to a series of boxes. Craters fill the surrounding landscape, their walls inclining at various angles, their depths difficult to estimate through the visors of spacesuit helmets. 

The Apollo 17 HFE. NASA.

The scene reflects the difficulty of a challenging experiment emplacement: astronauts needed to transport sizeable components away from the immediate landing site, safe from disturbances caused by their entry and descent. Walking through the grey landscape, one can easily lose their sense of proportion and direction. Boot prints stamped into the surrounding soil mark the trails of the astronauts, their weight casting lines of shadow visible in LRO’s orbital images.

The first Apollo HFE began transmitting data in 1971 with the next mission following closely behind, operating together for the next five years. While another Apollo mission was also equipped with the HFE package, an unfortunate cable caught astronaut John Young’s foot and disconnected the experiment. Two missions alone therefore provide the only ground truth measurements of temperatures on the Moon to date. 

While physical and technical complications presented experiment challenges, the real issues with the HFE data started upon its arrival to Earth.

The history of the data since it was archived in the 1970s until its current use by LRO scientists presents a curious case of investigation, transportation, loss, and revival. Originally, NASA used sixteen separate tracking stations to downlink data, sending the tape recordings to Johnson Space Center in Houston, TX. The tapes were then sent to Lamont Observatory in Palisades, NY, where they appear to have been ambiguously renamed multiple times and subsequently lost. Lamont did, however, produce a separate copied set of tapes for the NASA Space Science Data Archive, which serves as a permanent storage center for mission data. 

Owing to budget cuts and focus on other missions, decades passed before a new hunt for reliable, unaltered Apollo HFE data ensued in 2005. With the original archives lost and the questionable set sent to the NASA Space Science Data Archive in the late 1970s, scientists and NASA investigators have identified two other sources for the data outside of the original, lost archives: Johnson Space Center and the University of Texas at Galveston. Many at Johnson Space Center have been lost or damaged and those at UT Galveston lack important calibration data. 

Since the rediscovery of these sources, community interpretive work has successfully recovered and restored much of the missing and damaged data. Studies have devoted their entire budget to cleaning up the data and making it available for public use. Owing to these efforts, teams like the LRO Diviner scientists can now effectively use it for new science, continuing to expand the Apollo legacy as the work toward new discoveries. 

The narrative of the Apollo HFE over the past fifty years has been far from straightforward. From requiring scientists to drill on the Moon to spurring a scientific community effort, the journey of the HFE data and its contribution to science is ever-evolving. In looking to the future, LRO scientists will continue to glance at the past through Apollo, using the HFE data in new models and keeping the images of boot prints on the Moon pinned to their office doors.