Light leaves the surface of Proxima Centauri and arrives at Earth approximately 4.24 years later. Light is fast. Light is, by every available measure, the fastest thing in the universe. Light makes the trip from the Sun’s surface to Earth in just over eight minutes. Light makes the trip from one side of the Pacific Ocean to the other in approximately seventy milliseconds. Light makes the trip from Proxima Centauri to Earth in 4.24 years, which is roughly the time between US presidential elections, or the length of a typical undergraduate degree, or the gap between two successive World Cup tournaments. This is, on cosmic scales, an unusually short trip. Proxima Centauri is, in every meaningful sense, our nearest neighbour. And it is still 40 trillion kilometres away.

According to Britannica’s overview of Proxima Centauri’s basic stellar properties, the star itself is a small, cool, low-luminosity red dwarf — approximately 12.5 percent the mass of the Sun, approximately 14 percent the Sun’s diameter, only about 50 percent larger than the planet Jupiter, and approximately 33 times denser than the Sun. Its surface temperature is approximately 3,100 Kelvin, about half the Sun’s. Its total energy output across all wavelengths is approximately 0.17 percent of the Sun’s. Its visible-light output is even lower — about 0.0056 percent of the Sun’s — because most of its radiation comes out in the infrared, invisible to the human eye. The star was discovered in 1915 by the Scottish astronomer Robert Innes, working at the Union Observatory in Johannesburg, who noticed that a small, faint star a few degrees from the bright Alpha Centauri binary system was moving across the sky at the same rate as the bright pair — meaning it must be at roughly the same distance, and gravitationally associated with them. Proxima Centauri turned out to be the third star in the Alpha Centauri system, orbiting the central binary pair at approximately 13,000 astronomical units with an orbital period of roughly 550,000 years.

How fast humans can currently travel

The single most useful number for thinking about the practical distance to Proxima Centauri is the speed of the fastest spacecraft humans have ever sustained on a long outbound trajectory. As reported by a detailed analysis of interstellar transit times by former astronaut Tom Jones, the Voyager 1 spacecraft — launched in 1977 and currently the most distant human-made object in existence, at approximately 24 billion kilometres from Earth — is travelling at approximately 17 kilometres per second relative to the Sun. This is, in the context of typical Earth-based experience, fast: it is approximately 50 times the speed of a commercial airliner, approximately 1,500 times the speed of a passenger car on a motorway, approximately 4,000 times the speed of a human running. It is also, in the context of the distance to Proxima Centauri, extremely slow. At Voyager 1’s speed, sustained continuously without acceleration or deceleration, a journey to Proxima would take approximately 74,000 years. The Voyager 1 spacecraft is not actually heading toward Proxima Centauri — it is moving on a trajectory that will take it past nothing in particular for the foreseeable future — but the sustained cruise speed gives a reasonable approximation of what current chemical-propulsion technology can deliver for a long outbound interstellar trip.

The Parker Solar Probe, launched in 2018, has at moments achieved speeds approximately ten times faster than Voyager — peaking at approximately 192 kilometres per second, or 692,000 kilometres per hour, at its closest approach to the Sun. This is the fastest speed any human-built object has ever travelled. It is, however, achieved only briefly, at perihelion, through gravitational acceleration toward the Sun, and cannot be sustained on a trajectory away from the solar system. Per SYFY’s coverage of the Parker Solar Probe’s record-breaking solar approaches, the probe’s peak speed is a function of its specific orbital geometry, not a cruise speed available for interstellar travel. The interstellar-relevant speed for assessing realistic travel times to Proxima Centauri is the sustained-cruise speed of probes actually heading out of the solar system — which is roughly Voyager-class, in the neighbourhood of 14 to 17 kilometres per second. At those speeds, the trip to Proxima takes 73,000 to 80,000 years.

How long 73,000 years actually is

The intuitive impact of the 73,000-year figure is best assessed by comparison with the durations of various events in the recent history of the species. Modern humans, Homo sapiens, evolved approximately 300,000 years ago. The migration of modern humans out of Africa, by the best current archaeological evidence, occurred approximately 70,000 to 75,000 years ago — roughly the same span as a Voyager-speed journey to Proxima Centauri. The Toba supervolcano eruption in Indonesia, which is hypothesised to have produced a several-thousand-year volcanic winter and to have temporarily reduced the global human population to a small group of survivors, occurred approximately 74,000 years ago. The last ice age peaked approximately 22,000 years ago and ended approximately 12,000 years ago. Agriculture was invented approximately 10,000 years ago. Writing was invented approximately 5,500 years ago. The entire span of recorded human civilisation — from the first cuneiform tablets in Mesopotamia to the present moment — fits comfortably in less than 8 percent of the time a Voyager-class spacecraft would take to reach the nearest star.

The implication is that a spacecraft launched today, at the speed of the fastest probes currently in flight, would arrive at Proxima Centauri at a time so distant from the present that essentially no aspect of contemporary human civilisation would have survived to receive it. The English language would have changed beyond recognition. The political entities that currently exist on Earth would have ceased to exist, replaced by entities that do not currently exist and that no person currently alive will ever observe. The Holocene — the geological epoch we are living in, which began with the end of the last ice age — would itself have ended. The crew of the spacecraft, if any human crew were aboard, would have lived and died approximately a thousand generations earlier, with their remains arriving at the destination as an essentially anonymous human-made artefact. The signal from the arrival back to Earth would, even at light speed, take an additional 4.24 years to reach a planet whose inhabitants would have no living memory of having launched the mission.

What this implies about the rest of the galaxy

As detailed in the National Radio Astronomy Observatory’s analysis of the practical constraints on interstellar travel with current technology, the 73,000-year trip to Proxima Centauri represents the easiest possible interstellar journey. Every other star in the galaxy is further away. The next-nearest stars after Proxima are Barnard’s Star at 5.96 light-years, Wolf 359 at 7.86 light-years, and Sirius at 8.66 light-years — each requiring a trip of approximately 100,000 to 150,000 years at sustained chemical-rocket cruise speeds. The galactic centre is approximately 26,000 light-years away. The Andromeda galaxy, the nearest large galaxy to our own, is approximately 2.5 million light-years away — a Voyager-speed trip of approximately 43 billion years, which is more than three times the current age of the universe. The numbers do not get easier from here. The 40 trillion kilometres separating us from the nearest star are, by a substantial margin, the closest the rest of the galaxy ever comes. Everything else is farther.