Water as Rocket Fuel on the Moon or Mars

Avi Loeb
4 min readApr 24, 2023


Ice on the floor of a crater near the Martian north pole (Credit: ESA)

Water is essential for the chemistry of life as we know it. Nearly two thirds of the mass budget of the human body is water, implying that we are mostly bags of water and our differences are `the icing on the cake.’

The phase diagram of water allows for liquid water under atmospheric pressure and a temperature of order 300 degrees Kelvin above absolute zero. This happens to be the surface temperature of objects warmed by sunlight at the Earth-Sun separation, the so-called habitable zone. However, most of the water-rich objects in the solar system, like comets, asteroids or Moons, are much smaller than the Earth and cannot retain an atmosphere by their reduced self-gravity.

The message comes close to our home planet. Even though the surface gravity on Mars is 38% of the Earth’s g=9.8 meters per second squared, Mars lost its atmosphere and the ability to support life-as-we-know-it in liquid water at around the middle of its lifespan. Today, the remaining water on Mars is locked into ice embedded in rock.

Water ice on solar system rocks sublimates to gas when it is warmed up by sunlight, creating the coma of gas and dust around comets. The molecular vapor can be destroyed by absorbing ultraviolet light from the Sun through a process called photodissociation.

The water molecule contains three atoms, two hydrogen and one oxygen. Both hydrogen and oxygen can be used to fuel a rocket. This resource could prove handy as soon as sustainable human bases are established on the Moon or Mars, where large subsurface reservoirs of water ice are likely to exist.

One way to find water ice in permanently shadowed craters of the Moon or Mars is to crash a projectile on the surface and detect the ejected water vapor and ice spectroscopically. This was done with the Lunar Crater Observation and Sensing Satellite (LCROSS) mission. In October 2009, a Centaur rocket struck the persistently shadowed region within the lunar south pole crater Cabeus, ejecting debris of dust and water. About 155 kilogram of water vapor and ice was observed by a second “shepherding” spacecraft, which carried nine instruments, including cameras, spectrometers, and a radiometer. Icy debris was also identified in a fresh meteor crater, whose impact was noticed by NASA’s InSight lander mission in December 2021.

Once the water reservoirs on the Moon and Mars are identified, they can be harvested to propel cargos to Earth and beyond. How can rocket fuel be produced out of water?

One approach for creating fuel is to use electricity from solar panels to split the water into hydrogen and oxygen through a process called electrolysis. When the separate hydrogen and oxygen gases are recombined and ignited, they will burn or explode — giving out the energy that they took in during the splitting process.

Solar panels have no moving parts and are ideally suited to operate in low gravity and the extreme environments of space, producing an electric current out of sunlight. Electrolysis is achieved by running the current through water with some soluble electrolyte. This breaks down the water into oxygen and hydrogen, which are released separately at the two electrodes. On the Moon or Mars, gravity or a special centrifuge could then be used to separate the gases. Electrolysis had been use in space before to provide oxygen supplies for humans in space missions without the need for high-pressure oxygen storage tanks, for example on the International Space Station.

Solid ice is far more difficult to harvest and process into rocket fuel through a compact mobile facility. Therefore, an advanced technological civilization that wishes to refuel its interstellar spacecraft along their journey, might target Earth-mass planets in the habitable zone of stars where oceans of liquid water can be harvested. Once on site, the probes might dive into the ocean to refuel.

The reports by the Office of the Director of National Intelligence that some Unidentified Aerial Phenomena (UAPs) are associated with objects that go in and out of the Earth’s oceans, led to the establishment of the All-Domain Anomaly Resolution Office (AARO) in the US Department of Defense. The study of UAP is also at the focus of the Galileo Project, which has an operating observatory at Harvard University and plans to assemble copies of it at other locations in the coming months. It will be interesting to see what the `fishing expeditions’ of AARO and the Galileo Project observatories find.


Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, is scheduled for publication in August 2023.



Avi Loeb

Avi Loeb is the Baird Professor of Science and Institute director at Harvard University and the bestselling author of “Extraterrestrial” and "Interstellar".