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The fastest a human has ever traveled is about 39,897 km/h (24,791 mph), the speed Apollo 10 hit on its 1969 return from the Moon. Uncrewed probes go far faster — NASA’s Parker Solar Probe reached roughly 692,000 km/h in 2024 — but bodies impose hard limits. Special relativity rules out reaching the speed of light at all, and sustained g-forces above about 5 g for long periods cause loss of consciousness, so any realistic crewed mission would top out well below light speed.
How often do you gaze up at the night sky? When is the last time you stared into the vastness of space and were mesmerized by the faint white stretch of the Milky Way and the countless other stars that light up the darkness? While observing the night sky, have you ever dreamed of reaching for those same stars? Or sending ships all the way up into the cosmos to explore places beyond our solar system?
To get answers, we need to look at the speed records we have actually set, the propulsion ideas we have on the drawing board, and the relativistic wall that nothing with mass can ever cross.
How Fast Have Humans Actually Gone?
The current crewed speed record is held by the three-man crew of Apollo 10 (Thomas Stafford, John Young, and Eugene Cernan), who hit roughly 39,897 km/h (24,791 mph) on their way back to Earth in May 1969. No human has flown faster than that.
The fastest uncrewed object we have built is NASA’s Parker Solar Probe, which on Christmas Eve 2024 swung past the Sun at about 692,000 km/h (430,000 mph), roughly 0.064% the speed of light. That is the speed at which a machine can use the Sun’s gravity well as a slingshot; no person has come close to it.
Crazy Propulsion Ideas!
If we ever want to do better than Apollo 10, we need new engines. A few real concepts are on the drawing board:
- Nuclear pulse propulsion (Project Orion). A 1950s–60s NASA/DARPA concept that proposed using small atomic detonations behind a heavily shielded pusher plate. Theoretical top speeds: up to 10% of the speed of light.
- Ion drives. Already used on missions like Dawn and BepiColombo. They sip xenon and produce a tiny but constant thrust that, over months and years, builds up to very high speeds — far better than chemical rockets, though still well below c.
- Solar sails. Giant reflective sheets pushed by sunlight (and, with lasers, much farther). NASA’s Sunjammer demonstrator and the more recent Breakthrough Starshot proposal (laser-pushed gram-scale chips aimed at Alpha Centauri at ~20% of c) are the headline concepts here.
- Fusion and antimatter drives. Still firmly hypothetical, but the only known physics that could in principle push a crewed ship to a meaningful fraction of c.
How Far Can We Get?
Suppose we somehow build a starship that could approach the speed of light. The interesting bit then is not just the speed itself but what happens to time. Imagine boarding a train that, from the passenger’s point of view, accelerates close to the speed of light.
The train starts off with a few brave passengers on board. After that, things start to get a little tricky. From the perspective of the passengers, the train travels at the speed of light for an entire week. However, this train isn’t just traveling through space; it’s also traveling through time. Allow me to explain: once you approach the speed of light, the universe slows time down around you just enough to ensure that you never attain that speed.
The difference in time is very significant. What the passengers felt as just one week would actually be an entire century for the rest of the planet. The passengers literally bought “A ticket to the Future”. And unfortunately, it was only a one-way!
This phenomenon may seem a bit confusing at first, and may take a while to settle in. However, once it does, we realize the true ramifications of space exploration and traveling at the speed of light. Even if the human race advances to create rockets that travel at the speed of light, the astronauts onboard those ships would be taking a one-way journey to the future. A one-year journey aboard such a ship would send the astronauts thousands of years into the future. Even then, they’d still be just one-fourth the distance to Proxima Centauri!
It Doesn’t End There
The other problem with traveling near the speed of light is that accelerating to that speed takes a great deal of time and energy. Even at a steady 1 g of acceleration (a comfortable Earth-gravity push that doubles as artificial gravity), it would take about a year of ship-time to get close to the speed of light, and the same amount of time to slow back down. This is because humans can only tolerate a limited number of g’s. Trained fighter pilots in g-suits can briefly handle around 9 g before greying out, and only the very best survive 12 g for a moment before losing consciousness. The astronauts aboard such a fast ship would have to constantly bear the G force over a period of months, both while speeding up and slowing down.
Traveling through space is lonely. Furthermore, the fact that traveling through space at high speeds means that you’d also be traveling through time makes it even scarier. To add one more level of terror, the planet that those travelers return to after their journey would be completely different compared to the way it was when they left. Generations will have passed. The planet they return to would be very different from what they called home.
Fun Thought: Ever thought about how it would feel to live your entire life at that speed? It sure would be lonely, right? Maybe that’s how the Flash feels. His entire world might just be too slow for him. Maybe that’s why the Flash needs friends.
The one thing we know for sure is that the speed of light is the ultimate speed. However hard we try, we’re always going to be restricted by that universal constant.
