Why explore the Moon
On our celestial companion's immense scientific and exploration value.
Many people think that the Moon is just a gray ball of rock in the sky. Surprisingly, many scientists seem to share this notion too. I once had a physicist tell me the Moon is boring. So I’m writing this article to summarize the immense scientific and exploration value of our Moon.
The bootprints of Apollo astronauts who walked on the Moon are still there, and can be seen from orbit. That’s because the Moon lacks an Earth-like atmosphere, and so things stay unchanged for years. This single fact makes the Moon a geological time capsule.
During the early Solar System roughly 4 billion years ago, asteroids and comets have been bombarding planets and their moons at a vigorous pace. Most craters formed from such impacts on Earth are no longer visible due to erosion by wind and water. But the Moon being void of such processes has preserved most of its craters in roughly the same condition for millions and millions of years. By studying these craters, scientists infer what must have happened in the solar system’s past. Scientists use the Moon as an age reference to determine how old or young other features on worlds like Mars, Mercury and moons across the solar system are.
We know that life arose on Earth around the same time as when asteroids and comets were bombarding the Earth-Moon system, possibly bringing water and organics to Earth. Studying the still-intact craters on the Moon from that time is key to understanding what really happened during this period of Solar System history, making it a critical piece of our origin story.
Unlike Earth and Venus, the Moon lacks tectonic activity so its internal structure is well preserved since its formation. This gives scientists an opportunity to understand how insides of planets form. Studying lunar mountains, which are formed by asteroid or cometary impacts, offer scientists a glimpse into the Moon’s interior, as do its darker regions, which are solidified lava plains from the time of active volcanism on the Moon.
It’s not just science, the Moon also has technological and economic value. In the last two decades, NASA and ISRO spacecraft have discovered water ice on the Moon’s poles. Future human habitats on the Moon could tap into this water ice for drinkable water, breathable air and rocket fuel.
Further, scientists study the Moon to understand how space radiation and micrometeorite bombardment can affect astronauts living in deep space for long periods, such as on missions to Mars. The Moon’s proximity and access to resources make it a great testbed of technologies required for deep space exploration, including putting humans on Mars. Long term, the Moon’s low gravity barrier allows it to be an efficient rocket platform to sustainably expand humans across the solar system.
A brief history of lunar exploration and the road ahead
For centuries, telescopes were the most cutting-edge means to study the Moon, starting with Galileo’s observations of its craters, mountains and valleys. The advent of the Space Age in 1957 meant we could now send orbiters around the Moon to image and map those features at high resolution, and have landers & rovers take an even closer look from the surface.
In the 1960s, NASA sent the Ranger series of spacecraft to intentionally go smash into the Moon! They sent back over 15,000 photographs during their fall which taught us that the Moon was cratered all the way down, even on small scales. This allowed NASA to select safe landing spots for Apollo astronauts.
Around the same time, the Soviet orbiter Luna 10 revealed that the Moon’s gravity field has strong variations. These anomalies have caused spacecraft to unintentionally crash onto the lunar surface, and had to be avoided for having safe Apollo landings. Later, NASA launched the Lunar Prospector and the twin GRAIL orbiters in 1998 and 2011 respectively to make detailed gravity maps of the Moon. These maps have helped scientists see features beyond the surface and better understand the Moon’s structure.
Between 1969 to 1972, 12 astronauts walked on the Moon as part of NASA’s Apollo program. Apollo science experiments provided insights on how the Moon formed, whether it had a magnetic field, the nature of its volcanism, and more.
Humans have not been back since, and the Moon remains the only world besides Earth we’ve ever visited. In 2013, NASA launched the LADEE orbiter to probe the dust environment around the Moon to help assess its threats to future astronauts and human settlements.
Chinese lunar program
China has successfully landed three spacecraft on the Moon, Chang’e 3 in 2013, Chang’e 4 in 2019, and Chang’e 5 in 2020. It’s noteworthy because China is the only country to have landed anything on the Moon in this century while efforts from Israel and India unfortunately failed. Chang’e 4 is the world’s first mission to land on the Moon’s farside, where there is potential for transformative radio astronomy because of lack of interference from Earth.
The Chang’e 4 rover, Yutu 2, is scouting the landed region for clues of the Moon’s past. That’s because the region lies within the largest and deepest lunar crater, the South Pole-Aitken basin. Since this ancient crater holds clues to what was happening in the solar system about four billion years ago, a sample return mission from here has been assessed as a top priority in the last two Planetary Decadal Surveys, a report produced every 10 years by the U.S. scientific community to guide future NASA missions.
With Chang’e 5, China successfully brought samples from a geologically young and unique region on the Moon, which are now being studied to reveal details about the Moon’s thermal history.
Water on the Moon
In the 21st century, we’ve focused on steps towards sustainable presence on the Moon. NASA launched the Lunar Reconnaissance Orbiter in 2009, which produced the highest resolution Moon maps, including images, topography, temperatures, etc. Its extensive dataset has helped plan nearly all modern-day Moon landing missions, and will help future ones like those part of NASA’s Artemis program, which envisions landing humans on the Moon’s south pole where the water ice is.
Data from ISRO’s Chandrayaan 1 orbiter has shown the water ice on the Moon’s poles is inside the frigid, permanently dark craters. Based on remote observations by radars onboard Chandrayaan 1 and the Lunar Reconnaissance Orbiter, scientists estimate the Moon’s poles to host at least 600 billion kilograms of water ice, enough to fill at least 240,000 Olympic-sized swimming pools. As the next logical step, the Chandrayaan 2 orbiter, launched in 2019, is quantifying the amount of water ice on the lunar poles and mapping it.
The lunar science and exploration communities agree that we can harness water ice on the Moon’s poles for consumption needs of future habitats. But before we plan lunar habitats, we need technologies that enable us to explore the Moon’s poles. This includes an ability to land on and navigate extremely rocky terrain on the lunar poles, and function in the frigid water-hosting regions without access to sunlight or Earth communication. This is where NASA’s next lunar mission, VIPER, comes in. Slated for launch in 2023, VIPER will explore and probe permanently shadowed regions on the Moon’s poles to make high-resolution maps of water ice and other resource there.
The Japanese and Indian space agencies, JAXA and ISRO, will be launching a joint mission in 2024 or later called Lunar Polar Exploration (LUPEX) to explore the Moon’s south pole using a rover as well.
The Moon’s origin
Where did the Moon come from? The origin of our cosmic companion is a fundamental question in planetary science.
NASA Apollo astronauts returned a total of 382 kilograms of lunar soil, rock and core samples to Earth. The Soviet Union, using three robotic sample missions in the period 1970-1976, returned about 300 grams of material to Earth. These samples are analyzed even today in laboratories around the world and continue to provide insights on how the Moon formed. In fact, scientists think the Earth and the Moon may have a shared origin.
While the Apollo and Luna samples revolutionized our understanding of the Moon’s origin, they were from largely similar geological areas and thus not representative of the entire Moon. To continue piecing together the complex origin and history of the Earth-Moon system, we need samples from new locations, including access to pristine rocks below the lunar surface and from the farside.
To that end, NASA has initiated the CLPS program in which commercial landers will carry the agency’s science instruments to geologically diverse sites on the Moon. Missions like China’s Chang’e 6 will collect samples from the Moon’s south pole in 2024, possibly from the highly desired South Pole-Aitken basin. Further, China and Russia will be collaborating to build a permanent scientific base on the Moon’s south pole.
In addition to studying geologically diverse sites on the Moon, we also need to assess the nature of water at its poles to understand how it got there and how is it related to Earth’s water. That is where NASA’s Artemis program centered on the Moon’s south pole is most intriguing. Artemis is expected to offer abundant scientific opportunities, including bringing samples from pristine places on the Moon.
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This article is a variant of the one I wrote for The Planetary Society’s Moon page. It has been modified and published here with their permission. The article, first published in April 2020, has been updated in April 2021 to keep it current.