Many people think that the Moon is just a gray ball of rock in the sky. Surprisingly, this notion is shared by many scientists as well. So I am writing this article as an effort to summarize the immense scientific and exploratory value of our Moon.

The Orientale basin on the Moon, reconstructed from LRO data. Credit: NASA

Why do we explore the Moon?

It is said that the boot prints of Apollo astronauts who walked on the Moon are still there. That is because the Moon is airless and so things stay as is for years. This single fact makes the Moon a geological time capsule.

Bootprints of astronauts who walked on the Moon will stay preserved for decades thanks to the Moon being airless. Credit: NASA

For most of the solar system’s existence, asteroids and comets have been bombarding planets and their moons at a vigorous pace. Most craters formed from such impacts are no longer visible on Earth due to processes like wind and water erosion. But the Moon being void of such processes has preserved most of its craters in roughly the same condition for millions of years. By studying these craters, scientists infer what must have happened in the solar system’s past.

We also know that life arose on Earth around the same time as when asteroids and comets were bombarding the Earth-Moon system four billion years ago. So studying the still-visible craters on the Moon from that time is key to understanding what happened during this period of solar system history, making it a critical piece of our origin story.

Unlike Earth, the Moon lacks tectonic activity. So its internal structure is well preserved since its formation giving scientists an opportunity to understand how interiors of planets form. Studying lunar mountains, which are formed by impacts, offer scientists a glimpse into the Moon’s interior, as do its dark 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. It is possible that future human habitats on the Moon tap into this water ice for consumption and fuel needs.

Further, scientists study the Moon to understand how space radiation and micrometeorite bombardment can affect astronauts living for long periods in deep space, 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 effective rocket platform to build a sustainable human presence 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 lunar 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 has craters all over the place, 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 local variations in the Moon’s gravity field. 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 lunar surface and better understand the Moon’s structure.

From 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 and thus help assess its threats to future astronauts and human settlements.

Chinese lunar program

China has successfully landed two spacecraft on the Moon, Chang’e 3 in 2013 and Chang’e 4 in 2019. It is noteworthy because China is the only country to have landed anything on the Moon in this century while Israel and India failed.

Chang’e 4 is the world’s first mission to land on the Moon’s farside, where there is a lot of potential for radio astronomy because of lack of interference from Earth. To that end, the Chang’e 4 lander is using three 5-meter antennas to create a high-resolution radio map of the sky.

Chang’e 4 lander on the far side of the Moon, as imaged by the Yutu 2 rover. Credit: CNSA Yutu 2 rover on the far side of the Moon, as imaged by the Chang’e 4 lander. Credit: CNSA

The Chang’e 4 rover, Yutu 2, is scouting the landed region for clues of the Moon’s past. That’s because it resides within the largest and deepest lunar crater, the South Pole-Aitken basin. This ancient crater holds clues to what was happening in the early solar system, about four billion years ago! This is why a sample return mission from the South Pole-Aitken basin has been assessed as a top priority in the last two Planetary Decadal Surveys, a report produced every 10 years by the scientific community to guide future NASA missions.

The Chinese lunar program is progressing quite well. Its next launch is the Chang’e 5 in 2020, a sample return mission from a geologically young and unique region on the Moon.

Water on the Moon

In the 21st century, the focus has been more 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 human landing program. The Artemis program envisions landing humans on the Moon’s south pole, where the water ice is.

Data from ISRO’s Chandrayaan 1 orbiter shows the water ice is in the frigid, permanently dark craters on the Moon’s poles. As the next logical step, Chandrayaan 2’s orbiter, launched in 2019, is quantifying the amount of water ice on the lunar poles and mapping it.

How much water does the Moon have?

Based on remote observations by radars onboard ISRO’s Chandrayaan 1 orbiter and NASA’s Lunar Reconnaissance Orbiter, scientists estimate the Moon’s poles to host more than 600 billion kg of water ice, enough to fill at least 240,000 Olympic-sized swimming pools.

The lunar science and exploration communities agree that we can harness water ice on the Moon’s poles to power 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 rough, mountainous 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, NASA’s VIPER will explore permanently dark craters on the Moon’s poles to make high-resolution maps of water ice and probe it.

An artist’s impression of NASA’s VIPER rover exploring the Moon’s south pole. Credit: NASA

The Moon’s origin

Where did the Moon come from? The origin of the Moon is a fundamental question in planetary science.

NASA astronauts returned a total of 382 kilograms of lunar soil and rocks samples to Earth. The Soviet Union, using three robotic sample missions in the period 1970-1976, also 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 origins, 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. We also need to assess the nature of water at the lunar poles to understand how it got there and how it is related to Earth’s water.

That is where NASA’s Artemis program to land astronauts on the Moon’s south pole is most intriguing. Like the Apollo program, Artemis is expected to offer abundant scientific opportunities, including lunar sample return.

This article is a variant of the one I wrote for The Planetary Society’s Moon page. It is modified and published here with their permission.