Space exploration is a profound endeavor defined by two distinct approaches: the physical and the observational. Physical investigation involves launching crewed and robotic spacecraft to directly visit celestial bodies and measure the space between them. This method is constrained by the limits of engineering and the vastness of distance. Observational exploration relies on powerful telescopes and instruments to collect light and other radiation from distant objects, mapping the universe indirectly. These methods reveal a staggeringly large universe, where our physical presence is limited to a tiny fraction of our immediate neighborhood, while our observational reach extends to the very beginning of cosmic time.
The Local Reach: Exploring the Solar System
Humanity’s physical presence in space has so far been confined to a small fraction of the Solar System. The farthest distance humans have traveled is to the Moon, approximately 384,400 kilometers from Earth. This distance is a mere fraction of the Astronomical Unit (AU), the 150 million kilometer distance between the Earth and the Sun.
Our most extensive physical exploration has been accomplished by uncrewed robotic probes. The farthest human-made objects, the Voyager 1 and 2 spacecraft, have traveled far beyond the orbits of the planets. Voyager 1 is currently over 169 AU, or more than 25 billion kilometers, from the Sun.
These probes have successfully crossed the heliopause, the boundary where the Sun’s solar wind is pushed back by the interstellar medium. Voyager 1 passed this boundary at about 121 AU, marking our first entry into true interstellar space. Voyager 2 followed at 119 AU, confirming this region’s fluctuating nature.
Even at these distances, the probes have not yet reached the Solar System’s outermost boundary, the Oort Cloud. This vast, spherical shell of icy bodies is theorized to begin at an estimated 2,000 to 5,000 AU from the Sun. The outer edge of the Oort Cloud may extend as far as 200,000 AU, representing the gravitational edge of the Sun’s influence. Traveling at its current speed, Voyager 1 will take about 30,000 years to cross it entirely.
The Galactic Survey: Mapping the Milky Way
Shifting from physical presence to observational mapping increases the scale of our exploration to include the entire Milky Way galaxy. Telescopes and radio astronomy allow us to study the structure, composition, and content of our galaxy far beyond the range of any spacecraft. This indirect survey relies on measuring the light, motion, and distance of celestial objects.
The European Space Agency’s Gaia mission has precisely measured the positions and movements of nearly two billion stars. While this is an immense number, it represents a small percentage of the galaxy’s total population, estimated to contain between 100 and 400 billion stars. Our solar system is situated within the Milky Way’s disk, which is approximately 87,400 light-years in diameter.
The regions we have mapped in detail are concentrated in our immediate stellar neighborhood. Most of the galaxy remains conceptually explored but not truly mapped. Dusty regions toward the galactic center and outer spiral arms obscure our visible-light view, requiring infrared and radio telescopes to pierce the veil and complete the structural picture.
We have determined the general structure of the galaxy, identifying its spiral arms and the central bar-shaped structure. Despite cataloging billions of stars and thousands of exoplanets, the sheer volume of the Milky Way means that less than one percent of its stellar content has been individually cataloged and studied in detail.
The Cosmic Horizon: Understanding the Observable Universe
The final scale of exploration extends beyond our galaxy to the entire cosmos, where the limit is not a matter of distance but of time. The Observable Universe is the spherical region of space from which light has had time to reach Earth since the Big Bang. Due to the constant expansion of space, the objects whose light we see were once closer but are now estimated to be up to 46.5 billion light-years away in every direction, giving the Observable Universe a diameter of about 93 billion light-years.
The ultimate boundary of our observational exploration is the Cosmic Microwave Background (CMB), the oldest light we can detect. This radiation was emitted about 380,000 years after the Big Bang when the universe cooled enough for light to travel freely. Because the universe before this time was opaque to light, the CMB represents a time horizon beyond which we cannot directly see, serving as the effective edge of our explored universe.
Within this vast Observable Universe, we have mapped and cataloged billions of galaxies containing trillions of stars. However, the Observable Universe is not the entire universe; it is merely the portion we can currently interact with. The Actual Universe, the totality of all space and time, is hypothesized to be much larger, potentially 250 times bigger than our observable bubble, or even infinite.
When comparing the small, physically-explored region of our Solar System or the observationally-mapped fraction of the Milky Way to the sheer scale of the total, unobservable cosmos, the percentage of the universe we have explored is effectively zero. Our exploration continues to be a journey of ever-increasing knowledge constrained by the physical speed of light and the finite age of the universe.