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Telescopes and space missions

Telescopes and space missions

Giant leaps for knowledge

16 Jul 2019
Taken from the July 2019 issue of Physics World. Members of the Institute of Physics can enjoy the full issue via the Physics World app.

NASA’s Apollo 11 mission will forever be remembered as the first to land humans on the Moon. But the mission and the rest of the Apollo programme should also be remembered for the science it achieved and enabled. Sue Nelson explores Apollo’s scientific legacy

Buzz Aldrin
Lunar luggage: Buzz Aldrin carrying two of Apollo 11’s science experiments – the Passive Seismic Experiment (in his left hand) and the Lunar Laser Ranging Experiment (in his right) – on 21 July 1969. (Courtesy: Neil Armstrong/NASA)

The world has changed since 1969. Home computers did not exist, colour televisions were up-and-coming, and telephones only came attached to a wall. Indeed, it is often said that a single modern smartphone contains more processing power than the computers that sent Neil Armstrong, Michael Collins and Buzz Aldrin to the Moon in July 1969. Yet, despite 50 years of advancing technology here on Earth, the scientific legacy of NASA’s Apollo missions is far from over.

Take the three laser ranging retroreflectors that were put on the Moon by astronauts from Apollo 11, Apollo 14 (January–February 1971) and Apollo 15 (July–August 1971). Despite being on the Moon for almost half a century, the devices are still in operation. Comprising an array of corner-cube reflectors, each device is around the size of a small suitcase – the Apollo 15 array being the largest with 65 × 105 cm of reflectors. Using Earth’s telescopes, scientists can aim a laser beam at the arrays and detect the reflected photons to gain accurate measurements of the distance between the Earth and the Moon (see “How high the Moon”).

This experiment has contributed to our understanding of the Moon’s orbit, the variation in its rotation related to distribution of mass, the Earth’s rotation rate and the Earth’s precession of its spin axis. It’s even been used to test Einstein’s general theory of relativity. It’s also revealed the rate the Moon is receding from the Earth (3.8 cm per year and counting) – a rate that contributes to theories about the Moon’s origin since you can extrapolate in the other direction.

“The Moon has moved since we put the laser reflectors on it and it’s consistent,” says NASA’s chief scientist Jim Green, a physicist who has worked at the agency for the last 40 years. “If it’s constantly moving away, it must have been closer [in the past]. And so we’ve now done the analysis to determine the Moon was created very close to the Earth and from an impact. That helps support one of our theories about the Moon and how it was created.”

An origin story

This lunar-creation theory – the giant impact hypothesis – involves another planet called Theia, which is thought to have been a similar size to Mars and travelled in the same orbit as an early Earth. Gravity attracted Theia and Earth together, producing an enormous collision that created the Earth as we know it and our Moon.

There is also a rival theory, which says that the Moon formed from debris orbiting the Earth. But whatever exactly happened, one thing is clear. “When [things] settled out, the Moon was just above a place we call the Roche limit,” says Green, referring to the minimum distance at which a satellite, such as the Moon, can exist while orbiting a larger body. “It formed outside an area that’s three Earth radii away. Right now, the Moon is 60 Earth radii away,” Green continues. “So that means if you were standing on the Earth at that time, which I wouldn’t recommend because it was just a mess, the Moon would have been 16 times larger in the sky than it is today.”

Given the closeness of the Moon, it follows that at that time, one Earth day was a mere five hours long. Green uses the classic analogy of a spinning ballet dancer who spins faster when her arms are closer to her body. “As the Moon moved away, the Earth had to slow down – just like the ballet dancer slows down when she moves her arms away from her – to conserve angular momentum,” he says.

“We had the basic concepts for theories about how the Earth and the Moon are put together,” Green explains, “but now we’re putting the next big steps on it.” In other words, this one experiment, placed on the lunar surface half a century ago, has not only given us continual measurements relating to the physics of the Earth and the Moon, but also continues to influence and inform scientific ideas.

Short but extraordinary

The Apollo 11 mission was a relatively short affair. After Armstrong’s famous first step onto the Moon’s surface, he and Aldrin ventured no more than 60 m from the lunar module and travelled just 1 km in total. Within three hours they were safely back inside the Eagle. The mission was after all a proof of concept and, apart from the political statement, its primary aim was to pilot a crewed landing on the Moon and return home safely.

Apollo 11 was a proof of concept and its primary aim was to pilot a crewed landing on the Moon and return home safely

The scientific experiments performed during Apollo 11 were therefore limited. Alongside the retroreflectors, Armstrong and Aldrin deployed a suite of experiments that each took only 10 minutes to set up and were designed to send data to Earth after the astronauts had left. The package included a Passive Seismic Experiment, which was sensitive enough to pick up the astronauts’ footsteps and even Armstrong tossing and turning in his hammock. It ran for three weeks after deployment, registering “moonquakes” and meteoric impacts, and consequently provided information about the Moon’s interior.

Alsep

The mission also had a detector that measured the accumulation of lunar dust and the radiation damage to the solar cells that were powering the experiment and communication devices. In addition, Armstrong and Aldrin collected samples of the solar wind using sheets of aluminium foil on a pole (switched to platinum foil on Apollo 16). Facing the Sun, the foils were exposed for different periods of time to check for variations in the make-up of the wind. They were then returned to Earth where scientists found isotopes of noble gases, which varied in quantity according to the wind intensity. The astronauts’ helmets also had cosmic-ray detectors that were analysed after they returned to Earth.

The mission was a huge achievement, but anyone hoping for evidence of life was disappointed. After returning to Earth, the astronauts, spacecraft and samples underwent a 21-day quarantine while the Lunar Receiving Laboratory sterilized the containers of samples and analysed sections of lunar rock using spectrometers and microscopes for gases, chemical and physical properties as well as testing for radioactivity and biological life. The lab reported that: “No micro-organisms of extraterrestrial origin were recovered from either the crew or the spacecraft.”

Upscaling the science

Science on the first crewed landing may have been a secondary objective, but in November 1969 it took centre stage with Apollo 12. The primary objectives this time included inspecting, surveying and sampling the Moon, and deploying the first full Apollo Lunar Surface Experiments Package (ALSEP). Excluding investigations that took place from lunar orbit and on the journeys to and from the Moon, the six Apollo missions that landed on the lunar surface performed a total of 53 experiments. However, the number and type of experiments varied from mission to mission.

For instance, Apollos 12, 14 and 15 had a Suprathermal Ion Detector Experiment to measure the flux, mass and relative energy of positive ions with energies less than 50 eV. The sources of these ions included the solar wind and atmospheric gases that were ionized by ultraviolet radiation. Increased levels were also recorded shortly after the Passive Seismic Experiments (deployed by missions 11, 12, 14, 15 and 16) registered meteoroid impacts. Higher-energy particles were studied using Apollo 14’s Charged Particle Lunar Environment Experiment, and the particle-track cosmic-ray detectors of Apollo 16 (April 1972) and Apollo 17 (December 1972).

Apollos 12, 14 and 15 also performed a Cold Cathode Ion Gauge Experiment to measure the pressure of the Moon’s tenuous atmosphere. While Apollo 12’s experiment lasted just a few weeks, those for 14 and 15 continued returning data to Earth from 1971 to 1975. Apollo 17 went on to measure the constituents of the atmosphere with the Lunar Atmospheric Composition Experiment, revealing that the primary gases present are neon, helium and hydrogen. When analysed in conjunction with seismic measurements, researchers found that levels of argon-40 increased during periods of high seismic activity. It is thought that the gas is produced by the decay of radioactive potassium-40 beneath the surface and is released when moonquakes create fractures.

Apollos 12, 15 and 16 included a lunar surface magnetometer, which measured the Moon’s magnetic field and contributions from external sources, namely the Earth and Sun. By measuring over several months, the fluctuating external fields could be excluded, revealing that the Moon’s own magnetic field varies over time in certain regions. Using the magnetometer observations as the Moon passes in and out of Earth’s magnetic field, it is also possible to estimate how the Moon’s electrical conductivity varies with depth via a technique called electromagnetic sounding.

The variety of science experiments deployed by the Apollo astronauts has provided clues about the lunar interior – and the data are still revealing the Moon’s secrets decades later. Take the lunar core, for example. While the Apollo magnetometer measurements did not require the Moon to have an iron-rich core, the results limited its radius to a maximum of 450 km – a figure supported by the seismic data. Meanwhile, the variations in the Moon’s rotation, observed by the laser ranging retroreflectors, constrained it further, implying that the radius is less than 350 km. But it wasn’t until 2011, when researchers re-analysed data from the Passive Seismic Experiments, that a more detailed picture of the core was achieved (Science 331 309). Using seismic array-processing to enhance faint signals in the Apollo data, the scientists were able to deduce that the lunar core is iron-rich, and consists of a solid inner ball of about 240 km in radius and a 90 km-thick fluid shell. The re-analysis also suggests that the core contains a small percentage of light elements such as sulphur, which is similar to Earth’s core.

Moon dust and spacesuits

Not all of the Apollo missions’ science objectives went as planned. On Apollo 11, Armstrong and Aldrin collected 21.5 kg of soil and rocks but did not take a lunar environmental sample or a gas analysis sample. Nor did they complete all the planned sample documentation due to the mission’s time constraints.

All the Apollo astronauts were plagued by very fine Moon dust, which coats the lunar surface and is particularly “sticky” because solar radiation strips away electrons, making it static. The dust clogged up, wore down and interfered with spacesuits and experiments alike. Apollo 11’s Passive Seismic Experiment, for example, overheated and failed because of Moon dust, while Apollo 17 astronaut Harrison Schmitt not only reported it wearing through three layers of Kevlar-like material on his boot, but also complained of “lunar dust hay fever”.

The bulky spacesuits themselves also caused problems. Apollo 16 commander John Young tripped over a cable and broke the heat-flow experiment, reminding NASA that all spacecraft hardware needed to be more durable when its operators had limited mobility and were wearing spacesuits and helmets.

Despite Young’s accident, the rest of Apollo 16 was a success. Its scientific findings included the discovery of two new auroral belts around Earth and the first photograph of the geocorona in the hydrogen wavelength. The mysterious geocorona – part of the Earth’s outer atmosphere – is a gaseous cloud of hydrogen atoms that are luminous in far-ultraviolet light. The images confirmed that the hydrogen geocorona could be detected above the interplanetary Lyman alpha background – radiation emitted by distant galaxies from neutral hydrogen. Intriguingly, it was discovered in 2019 that this atmosphere extends beyond the Moon (J. Geophys. Res. Space Phys. 10.1029/2018JA026136), which means that the astronauts were actually within the geocorona too.

Members of the Apollo 16 crew also measured the composition of the lunar surface while they were in orbit, using X-ray fluorescence spectrometers (to look at magnesium, aluminium and silicon) and gamma-ray spectrometers (to identify thorium, iron and titanium). An alpha-particle spectrometer detected radioactive radon emission from the Moon and identified regions, such as the Aristarchus Crater, with higher activity, indicating areas where there is more uranium in the Moon’s crust.

Bigger steps

As the Apollo missions progressed, the time the astronauts spent on the surface during extravehicular activities (EVAs) – excursions out of the lunar module – increased and so did the science. From Apollo 15 they even took a battery-powered buggy – the Lunar Roving Vehicle – that let them explore further. By Apollo 17, NASA moonwalkers spent a record 22 hours performing EVAs, travelling a cumulative distance of about 35.9 km, performing 10 science experiments and collecting an impressive 741 rock and soil samples, including some from as deep as 3 m below the surface.

As the Apollo missions progressed, the time the astronauts spent on the surface increased and so did the science

In total, the Apollo astronauts brought back 382 kg of lunar rocks and soil. These pieces of lunar regolith ranged from volcanic basalt and ancient rocks from the Lunar Highlands, to breccia – sedimentary rocks containing fragments of other rocks or minerals – formed from the impacts that produced the distinctive craters visible from Earth. Soil samples from both Apollo 11 and 12 were even used on Earth to grow plants or exposed to seeds. Tests were done on 35 species and the conclusion was that lunar material was not harmful to plants.

Harrison Schmitt EVA

Apollo 12’s cache included the Bench Crater meteorite – the first meteorite to be found on another world. But the largest sample of them all was from Apollo 14. “It was roughly the size of a water melon,” says Katherine Joy, a lunar geologist at the University of Manchester in the UK. She studies samples from the Apollo missions, supplied by NASA, and was part of a team that analysed the Bench Crater meteorite in 2013.

“From the Apollo samples we’ve learnt how the Moon’s crust formed,” Joy says. “The white areas of the Moon probably formed in a magma ocean. The early Moon was very hot and then it formed a crystalline crust out of that magma ocean – and that’s useful as it tells us how other planets have generated a crust.”

Considering images from the Moon show astronauts on a monotonously grey surface, it is the colours beneath the dust that Joy finds surprising while working with Apollo samples. “Some of the rocks are beautiful and crystalline,” she explains. “Some are brilliant white, some are brown, some are green. Some have large crystals that you can see with your eyes.”

A grey Moon full of colours

Despite all the science experiments, the only astronaut on the Apollo programme to have trained as a scientist is Harrison Schmitt, who got a PhD in geology at Harvard University in 1964. As part of the Apollo 17 mission, he and fellow crew member Gene Cernan were the last human beings to walk on the Moon. During the mission, Schmitt got extremely excited when he spotted some soil that was not grey, famously shouting: “It’s orange. It’s orange!” The orange colour was due to volcanic glass formed within the soil, though it can take on other hues too. “Some of the lunar soil is black, some is grey,” Joy explains. “There’s this beautiful bottled green soil from Apollo 15 that’s also volcanic glass. Ancient volcanoes covered the lunar surface with their pyroclastic products.”

Lunar sample

Once the lunar samples were back on Earth, NASA allowed teams of scientists around the world to study them, which led them to deduce that the Moon once had active volcanoes on its near side during a period 3–3.8 billion years ago. “The black stuff is basalt like you get in Hawaii or Iceland,” Joy continues. “The white areas are made up of anorthosite, which is similar to granite. It’s a rock that’s dominated by a mineral called feldspar and the significance of that rock is that most of the Moon is made of it and it was formed in that earliest phase of the magma ocean. Then everything got bashed by impact so it’s all a big mix.”

Watery Moon

The lunar samples, when first analysed, registered small concentrations of volatiles – elements and compounds with low boiling points that vaporize easily. This includes nitrogen, hydrogen, carbon dioxide, methane and water, so it was assumed that the Moon was dry.

However, in 2010 this assumption came into question when a team led by Jeremy Boyce, at Caltech in the US, studied grains of the calcium phosphate mineral, apatite, within lunar sample 14053 – a chunk of basalt collected by Apollo 14 (Nature 466 466). Using an ion microprobe, the team discovered hydrogen in the form of structurally bound hydroxyl groups (OH) – implying that water could be present beneath the lunar surface. Indeed, in terms of the hydrogen, sulphur and chlorine content, the Apollo 14 apatite was indistinguishable from volcanic rock found on Earth, suggesting they were made through similar processes. Although this doesn’t mean the Moon is rich in water like Earth, it demonstrates that lunar geological processes can make at least one hydrous mineral.

The hypothesis that the Moon might not be as dry as we originally thought was confirmed in perhaps one of the greatest recent discoveries about our neighbour. In 2018 Shuai Li at the University of Hawaii and colleagues found direct evidence of water ice in regions of permanently shadowed craters at the lunar poles (PNAS 115 8907). The finding was accomplished through optical methods and has implications for any future human presence or base on the Moon – especially for producing energy with hydrogen fuel cells using in situ resources.

Those trips to the Moon will depend a lot on the information and experience gained by the Apollo programme. And, as we develop and improve our analysis techniques, the Apollo samples and data could reveal more secrets. We may this year be celebrating the 50th anniversary of that famous “giant leap for mankind”, but the legacy of the Apollo missions is living on.

Astronaut hindsight

Harrison Schmitt

In April 2019 I attended a symposium at the US National Academy of Sciences in Washington to celebrate the 50th anniversary of the Universities Space Research Association, which had been set up in 1969 for scientists to study the lunar rock and soil brought back by the Apollo missions. One of the speakers was a former senator from New Mexico: Apollo 17 astronaut and trained geologist Harrison Schmitt. He showed a recent image of the Moon taken of his craft’s landing site by NASA’s Lunar Reconnaissance Orbiter – a mission that has been continuously mapping the Moon for the last 10 years. It revealed the dark grey dune tracks left by the lunar rover wheels while travelling for samples, which are still visible due to the lack of wind or atmosphere on the Moon.

Apollo 17’s Active Seismic Experiment deployed at the site showed that the regolith under Schmitt’s space boots was about 10–12 m deep. “Quite a bit deeper than elsewhere,” he pointed out.

When talking about deploying the scientific suite of instruments that made up his mission’s Apollo Lunar Surface Experiments Package (ALSEP), Schmitt was surprisingly harsh on himself. “Why didn’t I deploy the ALSEP right here,” he said, pointing to a region near the lunar lander that was obviously more exciting to a geologist like himself, “instead of walking 50 to 80 metres and putting it in a not-quite-so-good spot? And I can’t tell you.” Waiting for the laughter to die down, Schmitt, then 83, added: “Things look very different when you’re down in the craters.”

During that final mission, in late 1972, Schmitt and his fellow astronaut Gene Cernan also collected a sample core from the Taurus-Littrow Valley. It was sealed and has been left untouched since then. Earlier this year, it was announced that geologist Chip Shearer, from the University of New Mexico, will soon be unsealing the core to study it using the latest available techniques. The result is likely to further our scientific understanding of the Moon.

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