Since Galileo spotted mountains on the moon in the seventeenth century, humans have itched to go there.
When the Apollo missions made that a reality in 1969, the US space program began envisioning an elaborate future on the moon. In 1984, a NASA symposium on the topic was convened after its organizers “realized the space transportation technology of the year 2000 would be capable of routinely carrying payloads to the moon” and thought they should start planning ahead.
Unfortunately, no nation on Earth possessed space transportation technology capable of routine visits to the moon by 2000. Moreover, we had little idea of the true nature of the lunar surface.
The problem with sending people to the moon (or anywhere in space) is twofold: Humans need to bring everything they need to survive—air, water, food, shelter—and bringing everything from the surface of the earth to space requires incredibly costly vehicles to do it. This effectively put a kibosh on sending humans back to the moon after Apollo.
But, between 1994 and 2009, a series of space probes confirmed what scientists had long suspected: There is water on the moon, specifically ice, in dark polar craters.
Water is not just the stuff of life; it’s also the stuff of rocket fuel. If that ice can be utilized, it can create air to breathe, water to drink and grow food, and perhaps most importantly, common spacecraft propellants: Liquid hydrogen and liquid oxygen.
Lunar water has become both a reason to go to the moon—it could make what we are already doing in space cheaper and more efficient—but also an enabler to make far more ambitious space missions possible.
How Reagan’s SDI helped set up a return to the moon
In 1984, the lunar visionaries imagined using nuclear reactors to bake oxygen out of metal oxides in the lunar dust, and hauling up vast quantities of hydrogen. For many obvious reasons, this was all cost prohibitive. Furthermore, the Reagan administration was more interested in funding space technology to defeat Soviet nuclear weapons than putting nuclear power on the moon. And to be fair to the administration, the immediate benefits of a permanent moon mission appeared diffuse compared to the costs.
Reagan’s space weapons program, the Strategic Defense Initiative, also known to its critics as “Star Wars,” was intended to use space-based assets to detect and destroy nuclear missiles before they could hit the US, replacing mutually-assured destruction with a high-tech shield.
The shield itself never quite came to fruition, thanks to the cost, technological, and political challenges to deploying SDI. But one of SDI’s projects was to develop a cheap spacecraft with a variety of sensors. Called Clementine, it became a partnership between the defense organization and NASA, which could use those sensors to map the moon.
Clementine launched in 1994, and its principle contribution was creating the first complete map of the lunar surface with radar, infrared and other imaging sensors. Toward the end of its trip around the moon, its controllers improvised another experiment: They beamed energy from their communications radar at the lunar pole, and tracked their reflections with large antennae on earth. When the data was analyzed, researchers found that some of the echoes at the poles were consistent with energy bouncing off ice, not rock.
This was a revelation. The Apollo missions had not discovered much evidence of water, though that is largely because they chose to go to the well-lit equatorial regions of the moon that could be scouted by telescopes from earth. When lunar astronauts performed geologic experiments on the surface of the moon, they didn’t find evidence of large underground reservoirs of water, as there are on Earth. Still, Soviet robotic missions had returned about 300 grams of lunar soil to earth, and the Russian researchers reported in 1978 that they had discovered a trace amount of water.
In 1996, one of Clementine’s architects predicted that the ability to harvest water on the moon would end up “making transport to and from the Moon more economical by at least a factor of ten.” That’s because so many critical supplies could be harvested on the moon instead of expensively brought up into space out of earth’s deep gravity well.
But the scientific community didn’t agree with the Clementine results immediately; remote sensing is error prone and the instruments weren’t even designed to hunt for water. But the findings helped inspire an effort to confirm them.
How scientists proved there’s water on the moon
After Clementine, a series of missions added to the picture. In 1998 NASA’s Lunar Prospector found concentrations of hydrogen in the same areas where Clementine detected ice. In 2008, the Indian space probe Chandarayaan-1 detected evidence for water with sensors designed by the Indian Space Research Organization and NASA, even prompting a minor war of words over which space agency found the volatile compound first.
The same year, new research techniques applied to old samples gathered during the Apollo missions also revealed volatile elements trapped in volcanic glass beads, more evidence that water could exist on the lunar surface.
That evidence was followed in 2009 by NASA’s Lunar Reconnaissance Orbiter and a companion mission called LCROSS (Lunar Crater Observation and Sensing Satellite) that fired a rocket into a crater near the moon’s south pole. Scanning the debris ejected by its impact, NASA’s two latest probes found more evidence of water.
“It wasn’t until LCROSS that we knew there was water and other volatiles in these permanently shaded regions,” Clive Neal, a Notre Dame planetary geologist, says.
Only this year, however, did a team of researchers led by the University of Hawaii’s Shuai Li analyze this data together to find “direct and definitive evidence for surface-exposed water ice in the lunar polar regions.” But we don’t know how deep that ice goes, how much is there, or what it may be contaminated with.
“The next step is to get to the surface and do good old fashioned geologic prospecting,” Neal tells Quartz.