The question of what percentage of space humanity has explored is difficult to answer with a single number. This difficulty arises because the term “space” encompasses everything from low Earth orbit to the edge of the observable universe. Furthermore, “explored” can mean either physical presence or observational knowledge. A single percentage is misleading because the scale of exploration changes drastically depending on the distance from Earth.
Why Calculating a Single Percentage is Misleading
Measuring the extent of human exploration requires establishing two distinct definitions: physical visitation and observational mapping. Physical exploration involves the reach of human-made objects, such as probes or landers, representing a direct, in-situ study of a location. Observational exploration, in contrast, involves collecting data via telescopes and other instruments, allowing us to characterize distant objects without visiting them.
The vastness of space makes any calculation based on volume nearly meaningless. Even within the solar system, the volume occupied by celestial bodies is negligible compared to the sheer volume of empty vacuum. Exploration is better measured by the number of unique celestial bodies studied or the fraction of the night sky mapped in detail, rather than by a volumetric percentage.
The majority of the universe’s content remains a mystery, consisting of dark matter and dark energy, which are not directly observable. Visible matter—stars, galaxies, and planets—comprises only about five percent of the total mass-energy of the cosmos. Our exploration efforts focus primarily on this five percent, meaning any percentage calculation excludes 95 percent of the universe’s ultimate composition.
The Extent of Physical Exploration in Our Solar System
Within the solar system, physical exploration is defined by the destinations of robotic probes and human missions. Low Earth Orbit (LEO) is the most thoroughly explored and utilized area, containing tens of thousands of tracked objects, including satellites and debris. Beyond LEO, the Moon remains the only other celestial body where humans have physically set foot.
Robotic probes have dramatically extended our physical reach across the major bodies of the solar system. Every one of the eight major planets has been visited by at least one spacecraft, from Mercury to Neptune. Specific planets like Mars and Venus have been the targets of successful landers, with rovers currently operating on the surface of Mars.
The outer planets have seen less direct visitation. Uranus and Neptune were only observed during the single flyby of the Voyager 2 spacecraft decades ago. The hundreds of moons of the gas giants have been selectively targeted, with missions studying bodies like Titan and Europa. Of the hundreds of thousands of cataloged asteroids and comets, only about 19 minor planets have been visited up close by probes.
The furthest physical extension of humanity’s presence is marked by the Voyager 1 and Voyager 2 spacecraft, which have crossed the heliopause. The heliopause is the boundary where the Sun’s solar wind and magnetic influence give way to the interstellar medium. While these probes have left the Sun’s magnetic bubble, they are still within the greater gravitational sphere of influence of the solar system, which extends far beyond them, encompassing the vast, theoretical Oort cloud.
Mapping the Observable Universe through Data
Beyond the physical reach of our spacecraft, exploration relies entirely on light and other forms of electromagnetic radiation. This observational exploration is limited to the observable universe, the spherical region from which light has had time to reach Earth since the Big Bang. This cosmic horizon means we are fundamentally limited by the speed of light and the age of the universe.
Astronomical surveys have cataloged millions of celestial objects, providing a three-dimensional map of our local galactic neighborhood and beyond. The Sloan Digital Sky Survey (SDSS) has mapped over a quarter of the sky and collected spectra for more than three million galaxies and quasars. This data gives us a detailed understanding of the structure and distribution of matter in a significant portion of the sky.
Newer, more powerful instruments like the James Webb Space Telescope (JWST) are pushing the limits of observational exploration by looking deeper into space and further back in time. JWST images have revealed tens of thousands of galaxies in a single deep-field view, identifying objects too faint or distant for previous telescopes. Although these deep-field surveys cover a tiny fraction of the night sky, their data are extrapolated to estimate the total population of the cosmos.
Current estimates suggest the observable universe contains between 200 billion and two trillion galaxies. The number of galaxies we have cataloged and studied in detail represents a minuscule fraction of this estimated total, perhaps less than one percent. If exploration is defined as knowing the location, composition, and distance of every major celestial body, then the explored percentage of the observable universe remains essentially zero. We have mapped the brightest and nearest structures, but 90 percent of the galaxies theorized to exist are too faint and distant for our current technology to see.