The comparison between space and the ocean is one of the oldest arguments in science policy. It usually goes like this: we know more about the surface of the Moon or Mars than about the bottom of our own ocean, yet we spend vastly more money looking up than looking down. There is truth in that, though the details are more interesting than the slogan. This page compares the two fairly, looks at what each has given us, and explains why I think the answer is not “either–or” but “much more of the ocean”.

My bias is obvious. I have spent decades diving reefs from the Caribbean to the South Pacific and growing corals in my own systems, and I have watched reefs change in that time. But I also think space exploration is one of humanity’s great achievements. The case for the ocean does not need anyone to give up on the stars.

The money: how the budgets compare

The funding gap is real, and it is large.

In the United States, NASA’s budget has been around $25 billion a year in recent years. The comparable figure for ocean exploration is harder to pin down because ocean science is spread across many agencies, including NOAA, the National Science Foundation, the Navy, the US Geological Survey and others. NOAA as a whole receives several billion dollars, but most of that goes to weather forecasting, fisheries management, charting and satellites. Its Office of Ocean Exploration and Research, the part specifically dedicated to exploring unknown ocean, has typically received tens of millions of dollars a year, often less than one percent of NASA’s budget. Other countries show similar patterns, though some, such as Japan and China, invest heavily in deep-sea technology.

It is fair to say that NASA’s budget also funds Earth science, including satellites that observe the ocean, so not every dollar spent on “space” is spent away from Earth. But dedicated ocean exploration, the kind that sends ships and robots to places no one has seen, remains very small by comparison.

What we have mapped

The statement that we have better maps of Mars than of our own seafloor is broadly true, with an important caveat.

Mars and the Moon have been mapped completely from orbit, because there is no water in the way and spacecraft can image or laser-range the whole surface. The seafloor, hidden under an average of about 3.7 kilometres of water, cannot be photographed from space. Satellites can estimate its shape indirectly by measuring tiny bumps in the sea surface caused by the gravity of seamounts and ridges, and that gives a complete but coarse global map with a resolution of a few kilometres. To get detailed maps, ships have to travel back and forth with multibeam sonar. As of the mid-2020s, the international Seabed 2030 project estimated that only about a quarter of the seafloor had been mapped at modern resolution. Our page on why we have better maps of Mars than of the ocean explains the comparison in detail.

People who have been there

For decades, the classic statistic was that more people had walked on the Moon than had visited the deepest point in the ocean. That is no longer true.

Twelve astronauts walked on the Moon between 1969 and 1972. The Challenger Deep in the Mariana Trench, nearly 11 kilometres down, was visited first by Jacques Piccard and Don Walsh in the bathyscaphe Trieste in 1960, and then not again by a person until James Cameron’s solo dive in 2012. Since 2019, the explorer Victor Vescovo’s submersible Limiting Factor has made repeated dives to the bottom, carrying scientists and paying passengers, and the total number of people who have reached the Challenger Deep now exceeds the number who have walked on the Moon. Our pages on the Moon and the deepest sea and James Cameron’s trench dive tell those stories. Even so, a handful of visits to a single trench is a tiny sample of the deep ocean, most of which no human has ever seen directly.

What space exploration has given us

It would be unfair to treat space spending as money thrown at the sky.

Space programmes have produced the satellite systems we now depend on every day: GPS and other navigation systems, weather satellites, communications and Earth observation. Many of those satellites watch the ocean, measuring sea surface temperature, sea level, ocean colour, winds and sea ice; without them, modern oceanography and hurricane forecasting would be impossible. Space science has also produced huge advances in materials, computing and medicine, and it has inspired generations of scientists and engineers. Missions to Jupiter’s moon Europa and Saturn’s moon Enceladus, both thought to have liquid oceans beneath ice, are partly ocean exploration in their own right.

What ocean exploration has given us

Ocean exploration has an equally strong record, often with far less money.

Exploring the seafloor led to the theory of plate tectonics and the discovery of hydrothermal vents, which revealed whole ecosystems powered by chemical energy and changed ideas about where life might exist, including on other worlds. Marine organisms have provided important medicines: the anticancer drug cytarabine was developed from compounds first found in a Caribbean sea sponge, and the painkiller ziconotide is based on a cone snail toxin. Ocean exploration underpins fisheries that feed billions of people, the undersea cables that carry most international internet traffic, offshore energy, tsunami warnings and the climate science that tells us how fast the planet is warming. And it keeps finding new species; see how many ocean species remain undiscovered.

Where the two meet

Space and ocean exploration are more connected than the rivalry suggests.

NASA has trained astronauts underwater for decades, including in the NEEMO missions at the Aquarius undersea laboratory off Key Largo, because living and working in a sealed habitat on the seafloor is a good analogue for spaceflight; Fabien Cousteau’s Mission 31 used the same station. Deep-sea robots and space rovers face similar engineering challenges: extreme pressure or vacuum, communication delays, power limits and the need for autonomy. Scientists studying life at hydrothermal vents help NASA plan how to search for life in the oceans of Europa and Enceladus. And Earth-observing satellites are among the most important tools in oceanography. In practice, the best ocean science already depends on space technology, and some of the most exciting space science is about oceans.

Why space wins the public’s attention

Part of the funding gap is simply about stories and pictures.

Spaceflight has astronauts, countdowns, launches and images of distant planets that anyone can understand at a glance. Ocean exploration has historically happened out of sight, on ships far from shore, with results published in technical journals. That has started to change. Expeditions such as those of the research vessels Nautilus and Okeanos Explorer now stream live video from remotely operated vehicles, and clips of strange deep-sea animals regularly go viral. Each one reminds people how much is still unknown a few kilometres beneath the surface. In my experience, the moment someone sees a reef or a deep-sea animal with their own eyes, or even on a live feed, the abstract argument about funding becomes personal very quickly. The more of the ocean people can see, the easier it will be to make the case for exploring it.

There is also a question of urgency. The Moon and Mars will look much the same in fifty years. Coral reefs, fish populations and polar seas may not. Exploring and documenting the ocean now gives us a baseline we can never recover later.

So which should we fund?

My answer is both, but with a much bigger slice for the ocean.

The ocean covers about 71 percent of the planet, holds most of its living space, produces much of its oxygen, absorbs most of the excess heat from climate change and supports the livelihoods of hundreds of millions of people. Yet we are still discovering basic facts about it, from the shape of its seafloor to the animals living in its middle depths. A modest increase in ocean exploration funding, even a few percent of what the world spends on space, could map the entire seafloor, put many more robots in the deep and transform our understanding of the planet we actually live on. For more on why the ocean deserves that investment, see why ocean exploration is important and how ocean exploration has changed our understanding of aquatic ecosystems.