The distance humanity has traveled in space depends on the type of traveler considered. Crewed missions have a limited reach, confined to the immediate vicinity of Earth and the Moon. However, robotic probes have extended our distance billions of miles, well past the traditional boundaries of our solar system and into interstellar space. Distinguishing between these two forms of exploration—crewed and uncrewed—helps illustrate the different scales of achievement in humanity’s journey outward.
The Maximum Distance Traveled by Humans
The record for the farthest distance ever achieved by human travelers belongs to the three-person crew of the Apollo 13 mission in 1970. Following an explosion in an oxygen tank, the astronauts were forced to swing around the Moon in a free-return trajectory to return to Earth. This unplanned maneuver took them to the far side of the Moon, where they reached their maximum distance.
At its farthest point, the Apollo 13 spacecraft reached approximately 248,655 miles (400,171 kilometers) from Earth while passing over the lunar far side. This record remains unbroken, as subsequent human spaceflight has been confined to low Earth orbit or closer lunar trajectories. Even successful Apollo missions, like Apollo 11, did not travel as far from Earth as the emergency slingshot maneuver of Apollo 13.
The Farthest Reach of Robotic Exploration
The ultimate distance record for any human-built object is held by the uncrewed Voyager probes. These twin spacecraft, launched in 1977, completed their initial mission studying the outer planets and are now exploring the space between stars.
Voyager 1, the more distant of the two, is currently over 15.8 billion miles (24.4 billion kilometers) from Earth. Voyager 2 trails behind at approximately 13.2 billion miles (20.4 billion kilometers). Both probes have crossed the heliopause, the point where the outward pressure of the solar wind is balanced by the pressure of the interstellar medium.
Crossing the heliopause means the probes have entered interstellar space, a region dominated by plasma and magnetic fields originating from the galaxy rather than the Sun. Voyager 1 crossed this boundary in August 2012, and Voyager 2 followed in November 2018.
Despite the immense distances, the probes continue to communicate with Earth using radio waves transmitted from their small onboard antennas to massive receiving dishes. The signals, traveling at the speed of light, take over 23 hours to reach Voyager 1 and nearly 20 hours to reach Voyager 2. These long communication delays highlight the vastness of the distance traveled as the probes transmit scientific data about the interstellar environment.
Navigating the Cosmos: Units of Measurement
Scientists use specialized units of measurement to manage the enormous distances in space. For distances within our solar system, the Astronomical Unit (AU) is the preferred measure.
One AU is defined as the average distance between the Earth and the Sun, approximately 93 million miles (150 million kilometers). Using AU simplifies solar system distances, providing a practical yardstick for our local neighborhood where using miles or kilometers would result in unwieldy numbers. For example, Jupiter orbits at about 5.2 AU, and Neptune is roughly 30 AU away.
For the extreme distances of the Voyager probes and anything beyond our solar system, the light-year is used. A light-year is the distance light travels in one Earth year, nearly six trillion miles (9.46 trillion kilometers). This unit is significantly larger than the AU, making it suitable for interstellar measurements.
Even the Voyager 1 probe, at over 170 AU from the Sun, is still only a tiny fraction of a light-year away. Using the light-year or related time-based units like light-hours provides context for the vast gulfs of space separating star systems.
Pushing the Boundaries: Future Space Travel Goals
Future missions are designed to dramatically extend both crewed and uncrewed distance records. For human travel, the Artemis program aims to return astronauts to the Moon and establish a sustainable lunar presence. These missions will break the Apollo-era distance record and test the technologies needed for longer voyages.
The ultimate goal for crewed exploration is sending humans to Mars, a journey millions of miles farther than any previous trip. For uncrewed exploration, scientists are planning interstellar probes that will travel much faster than the Voyagers. These proposed spacecraft would aim to reach the boundary of the solar system in years rather than decades.
The next generation of probes will conduct detailed studies of the interstellar medium and carry new technologies to travel faster toward other star systems. These efforts are designed to accelerate the pace of exploration, further expanding humanity’s reach into the cosmos.