How Far Away Is Space? Closer Than You Think

How far away is space? A lot closer than you think. By the most common definition, space starts just 100 kilometers (62 miles) above sea level. That is about an hour on the freeway at 60 mph, only straight up. Earth is roughly 12,700 km (7,900 miles) across, so the trip to space is less than 1 percent of that. And here is the twist: there is no fixed border, and even experts argue whether it is 80 or 100 kilometers. We will show you how close space really is, who drew the line, and why it is so shaky.

How far away is space? About 100 kilometers

Space begins 100 km (62 miles) above sea level, according to the most widely used definition. That line is called the Kármán line, and the World Air Sports Federation (FAI) uses it. It is not a wall or a fence. It is an agreement, so that records and space rules have a clear starting point.

For context: if you could drive straight up at 60 mph, you would be in space in about an hour. Plenty of road trips between two cities take longer than that.

How high are the things you already know?

To get a feel for 100 km, here are a few markers to compare:

Mount Everest is 8,848.86 meters (29,032 feet) tall, based on the 2020 survey by China and Nepal. You could stack it more than 11 times before you reach the Kármán line.

A passenger jet usually cruises at around 10 to 12 km (6 to 7.5 miles). That is only about a tenth of the way.

Felix Baumgartner jumped from 38,969 meters (about 24 miles) on October 14, 2012. A record, but still under 40 percent of the way to space.

The International Space Station orbits at about 420 km (260 miles). That is more than 4 times higher than the Kármán line.

The Moon is 384,400 km (238,855 miles) away on average. That is more than 3,800 times farther than the Kármán line. Read more in our post How Far Is the Moon From Earth?

Why is there no real border to space?

Because the air does not stop suddenly. It gets thinner with every kilometer until almost no particles are left. There is no spot where you can say: here it ends. There is also still no international legal definition of where a country's airspace ends and outer space begins.

That is why the Kármán line is a compromise. In the 1950s, physicist Theodore von Kármán worked out the height where a plane would have to fly as fast as orbital speed just to stay up, because the air is too thin to carry it. His result was about 83.8 km. Later, that was rounded to 100 km.

Who says 80 and who says 100 kilometers?

Here is where it gets interesting: the US sees it differently than the FAI. The US Armed Forces count a person as an astronaut after flying higher than 50 miles. That is 80 km.

In 2018, space historian Jonathan McDowell studied satellite data. His finding: satellites with their lowest point below about 80 km are very unlikely to complete another orbit. The air slows them down too much. That is why he proposes roughly 80 km as the border.

So experts argue about a gap of 20 km. That is a fifth of the whole distance.

How long does it take to get to space?

With a rocket, it goes surprisingly fast. One flight of Blue Origin's New Shepard rocket reached about 106 km. The whole trip from launch to capsule landing took only around 10 minutes. The passengers were weightless for roughly 3 to 4 minutes.

The hard part is not the height. The hard part is the speed. To stay in orbit, a satellite has to move sideways extremely fast, or it falls back down. The ISS is constantly falling around the Earth. If you only go up, you come right back.

Test your sense of heights and distances

Want to see how well you can guess distances in space and on Earth? Try our science estimation questions. Free, runs in your browser, perfect for your next road trip.

Frequently asked questions about space

The takeaway

Space is only about 100 km away, but there is no fixed border. If numbers like this fascinate you, also read How Tall Is the Tallest Mountain in the World? and browse the Quiztimate homepage. Or jump straight into the science quiz and guess for yourself.