Once on the moon, Apollo astronauts had two major goals: get themselves and the moon rocks home safe.
To make space on the cramped lunar modules for the hundreds of kilograms of moon samples, the astronauts had to go full Marie Kondo. Anything that wasnt essential for the ride home got tossed: cameras, hammocks, boots and trash. Downsizing also meant abandoning big stuff, like moon buggies and the descent stage that served as a launchpad for a modules lunar liftoff.
But the astronauts left more than castoffs. Starting with the Apollo 11 mission, which touched down on July 20, 1969, astronauts left six American flags and plenty of personal and political mementos. Importantly, the crews also left behind instruments for about a dozen experiments to keep tabs on lunar conditions (SN: 8/2/69, p. 95); one is still running today.
These devices “were really important parts of Apollo,” says Noah Petro, project scientist for the Lunar Reconnaissance Orbiter mission. Back then, the experiments didnt get much time in the limelight, “because humans on the surface are obviously the big story,” says Petro, who is based at NASAs Goddard Space Flight Center in Greenbelt, Md.
Special Report: Moonstruck[hhmc]
50 years after Apollo 11, lunar science still surprises and delights
This story is part of a special report celebrating humans enduring fascination with the moon and exploring the many ways it affects life on Earth. More articles will be published in the coming weeks. See all the articles, plus our 1969 coverage of Apollo 11, here.
When we think of Apollos 50-year legacy, most of us probably arent picturing the scattered remnants of astronaut outposts gathering space dust. But as nations plan new ventures to the moon, preservationists are fighting to protect these historic sites so that future lunar visitors dont erase the marks of humans first steps beyond Earth.
Solving old mysteries [hhmc]
By December 1972, six Apollo crews had collectively spent nearly 80 hours exploring the moons surface (SN: 12/23/72, p. 404). They gathered rocks, photographed the landscape and performed all manner of experiments — from unfurling metal foil to catch solar wind particles to setting off explosives and measuring the resulting seismic tremors.
Apollo 11 left behind solar-powered seismometers and a reflector array that could be paired with lasers on Earth to precisely measure the distance between Earth and the moon. On five later missions, Apollo 12 through 17 (Apollo 13 returned home without landing on the moon), astronauts left more elaborate setups powered by nuclear batteries that generated electricity through radioactive decay (SN: 11/8/69, p. 434). Some of those instruments collected data through 1977, when NASA decided to focus on other projects and pulled the plug on the whole operation (SN: 10/1/77, p. 213).
“There was this period of time where the data languished,” Petro says. But within the last decade or so, a new generation of scientists has taken up the torch, analyzing Apollo observations to answer questions lingering from early studies. Unfortunately, this isnt nearly as simple as picking up where 1970s scientists left off, as geophysicist Seiichi Nagihara discovered when he set out to solve a decades-old puzzle about the moons underground temperature.
On Apollo 15 and 17, astronauts installed thermometers in the lunar surface, which took the moons temperature at various depths and sent the data back to Earth (SN: 9/11/71, p. 167). When Apollo-era scientists reviewed data collected through 1974, the results revealed something odd: The moons temperature just beneath the surface appeared to be slowly rising.
“Were talking about very minor warming,” just a couple degrees, says Nagihara, of Texas Tech University in Lubbock. But researchers at the time couldnt figure out why. Nagihara decided to examine all the temperature data collected through 1977 to figure out what was going on. Unfortunately, the tapes that recorded these measurements were missing. This is a common problem, because during the Apollo era, data were housed at the individual labs of scientists working on each experiment and many measurements were never properly archived.
“A group of us decided to … try to hunt down the tapes,” Nagihara says. After scouring thousands of documents at NASAs Johnson Space Flight Center in Houston, the researchers traced 440 tapes to an archive in Suitland, Md. But even those covered only about three months of observations. At the Lunar and Planetary Institute in Houston, Nagihara and colleagues discovered more temperature measurements noted by Apollo-era scientists in weekly memos. Between the recovered tapes and the memos, Nagiharas team pieced together a picture of the moons temperature from 1971 through 1977.
The slow warming under the surface continued through the end of data collection, the researchers reported in April 2018 in the Journal of Geophysical Research: Planets. In search of a source for the heat, Nagihara and colleagues turned to pictures taken by the Lunar Reconnaissance Orbiter, which has been orbiting the moon since 2009 (SN: 6/11/16, p. 10). The images showed that soil stirred up by astronaut activity was slightly darker than other lunar terrain. Perhaps it was dark enough to absorb more sunlight and warm the underlying ground.
Computer simulations confirmed that the moon wasnt heating up from internal processes. Astronauts trekking around the Apollo sites probably caused an increase in surface temperature of about 2 to 3 degrees Celsius, and the extra heat slowly spread more than a meter into the ground — causing the gradual warming detected by Apollo instruments. Turns out that astronaut footsteps left marks on the moon far deeper than those iconic boot prints.
Keeping vigil over gravity[hhmc]
While Nagihara and other researchers are digging up old Apollo data for new analyses, one lone project is still in full swing: the laser ranging retroreflector experiment.
This experiment uses arrays of reflectors placed on the moon by Apollo 11, 14 and 15 astronauts and anchored on two rovers left behind by the Soviets (SN: 5/20/78, p. 326). These arrays consist of special mirrors, each with three sides in the shape of a cubes corner, which always reflect light in the exact direction from which it came. By shooting a laser beam at a corner-cube array from a telescope on Earth and clocking the time it takes for the light to return, researchers can measure the exact distance between different spots on the moon and Earth.
To measure the Earth-moon distance, arrays of “corner-cube” mirrors were set up at Apollo sites (top). Inside each circle (bottom left) is a corner cube that reflects laser light back to Earth in the exact direction it came from (illustrated, bottom right).
Laser ranging retroreflector measurements have offered several insights — like the fact that the moon is withdrawing from Earth at about 3.8 centimeters per year. Plus, slight variations in the moons rotation suggest that the orb has a relatively small core.
Physicist Tom Murphy of the University of California, San Diego is using the corner-cube arrays to probe a question much bigger than the moon. Hes testing whether a key part of Einsteins general theory of relativity, called the equivalence principle, holds up.
The equivalence principle states that any two objects in the same gravitational field should fall at the same rate (SN: 1/20/18, p. 9). Just like a bowling ball and a golf ball should hit the ground simultaneously, the Earth and moon should fall around the sun (that is, orbit the sun) at exactly the same rate. “Youre sensitive to any difference in how theyre [orbiting] the sun by measuring the distance between the Earth and moon as they weave around each other,” Murphy says. If the Earth-moon distance ultimately breaks with the equivalence principle, that would reveal a shortcoming of general relativity. And that, in turn, could inform the creation of a theory of quantum gravity that resolves the tension between general relativity and quantum mechanics (SN: 10/17/15, p. 28).
So far, laser ranging retroreflector measurements with centimeter-level precision havent shown any difference in how quickly the Earth and moon are falling around the sun. But in 2006, Murphy started collecting data with millimeter-scale precision using improved laser technology and a larger telescope at the Apache Point Observatory in New Mexico.
Amassing enough data will require several more years of observation and researchers will need more sophisticated computer models to analyze the observations, Murphy says. Luckily, since the reflectors on the moon dont require any power, he can collect data into the foreseeable future. Eventually, those observations — at the millimeter level or even smaller scales — could reveal a crack in the equivalence principle.
Since general relativity is fundamentally incompatible with quantum mechanics, something eventually has to give. The equivalence principle might be one of those things, Murphy says. “We have to turn over every rock and see where the bugs are.”
One astronauts trash[hhmc]
Thermometers and reflectors were among about a dozen types of instruments installed on the moon. Other devices measured the moons magnetic field and sniffed out chemical components of the moons tenuous atmosphere. NASAs Lunar Data Project is restoring data from these and other Apollo experiments, so that scientists can continue to pore over the observations for years to come.
“When you have this incredibly rare resource, you cant not keep working on it,” says planetary scientist Renee Weber of NASAs Marshall Space Flight Center in Huntsville, Ala., who studies lunar seismic data. “There are always new techniques to try” and better computer processing to tease out previously missed signals.
Based on moonquakes sensed by Apollo seismometers, Weber and colleagues reported in May that the moon may still be tectonically active, as revealed by young faults on the lunar surface called lobate scarps (SN: 6/8/19, p. 7). Understanding moonquakes could help NASA and other agencies decide where to land future spacecraft or construct buildings on the moon, Weber says. If these lobate scarps truly mark sites of tectonic aRead More – Source