The informal boundary between the Earth and outer space lies within the atmosphere

The Kármán line is a suggested boundary between Earth and outer space

Beyond this line, approximately 100 kilometers above Earth's surface, aerodynamic lift is insufficient to keep aircraft aloft without special propulsion systems. The United Nations has historically accepted the line as delineating the boundary of space.

Why the edge of Earth's atmosphere varies depending on which property is considered

As the distance from Earth's center increases, the planet's gravitational force weakens, causing it to gradually lose its ability to retain gases at the farthest edge of its atmosphere—the exosphere. At a height of approximately 200,000 kilometers (124,000 miles)—halfway to the Moon—the exosphere has so little density that it is indistinguishable from interstellar space.

View a diagram of Earth's atmosphere, which is composed of five layers

Starting from the surface, the troposphere is home to Earth's weather, followed by the stratosphere, which houses the ozone layer, and the mesosphere, which burns up most debris from space. Above these layers lies the thermosphere, where auroras are produced, and the exosphere, which lacks a clear outer boundary as it fades into outer space.

The weight of Earth's atmosphere is equivalent to a 33-foot deep, global ocean

Because air is made up of gas molecules with mass, it is not weightless. On the planet's surface, we are effectively living in what is the equivalent of the bottom of an invisible ocean of these particles pressing against us, just as water molecules do when we are submerged.

Understanding why the thermosphere feels cold, despite exceeding 2,000 degrees Celsius

High-energy solar radiation can raise temperatures above 2,000 degrees Celsius (3,632 degrees Fahrenheit), but the air density in this layer is so low that there would be insufficient collisions between energized molecules and a person's skin to cause them to experience this warmth. Variations in the level of solar radiation reaching the layer cause it to swell and shrink.

Who’s in space right now?

Hundreds of people (and a few animals) have been to space since the first creatures flew past the Kármán line in 1947. There are a handful of astronauts aboard the International Space Station at any time, while others may be cresting space in...

Understanding why suction cups rely on Earth's atmosphere to work

When pressed against a surface, air is squeezed out of a sealed volume, causing the internal pressure to drop below the surrounding atmospheric pressure, and this imbalance creates the "sticking." For this reason, astronauts do not use suction cups or related technologies to stay on spacecraft during spacewalks.

Orbital spaceflight requires reaching 23 times the speed of sound

After the spacecraft accelerates up to at least 17,500 miles per hour, space tourists can travel in elliptical orbits hundreds of miles above Earth for days or weeks at a time. Suborbital flights instead max out at about 2,200 miles per hour and travel only 50 to 70 miles above Earth's surface for a few minutes.

Differences in air pressure above and below an airplane's wing allow it to fly

The bending of air around a surface causes it to flow more quickly, reducing the pressure exerted by the airflow. In a commercial airplane, the pressure difference between air diverted above the wing and air flowing undiverted below it creates enough lift to overcome gravity.

High-altitude kites can perform airborne collection of energy from high-speed winds

Winds at high altitude are stronger and more consistent than those near Earth's surface, where natural and artificial barriers disrupt airflow. Kites pulled by the wind can extend a tether like a yo-yo, creating electricity in an attached ground generator.

The boiling temperature of water decreases with increasing elevation because the molecules need less energy to overcome the lower atmospheric pressure associated with thinner air.

An overview of atomic composition, scale, and structure, which is almost all 'empty' space

It takes about 500,000 carbon atoms lined up next to one another to span the thickness of human hair, and over 99.9994% of the mass of each is contained in the nucleus. About 99.999999999999% of an atom's volume lies outside this nucleus, where electrons exist in clouds of probability. Removing these clouds, the Empire State Building would be the size of a rice grain.

In a closed space, acceleration cannot be distinguished from gravity

Just as pressing into a door when in a car makes a turn resembles the feeling of being pushed from the side, Albert Einstein’s equivalence principle states that being in a gravitational field feels the same as when a floor accelerates up against you, as in a rocket.

The inhabitability of the Martian environment presents significant colonization risks

Terraforming the thin atmosphere, toxic dust, extreme cold, and lack of magnetic field is infeasible, given limitations to the size of what can be sent to Mars. NASA and SpaceX are instead exploring creating underground shelters.

Great circles provide the shortest path between two points on a sphere

On Earth, a great circle is formed by slicing through the planet, such that one cuts through the center alongside the two locations from which the shortest distance needs to be found. The resulting arc along the Earth's surface is responsible for curved routes on flight maps, rather than straight trajectories.

As of 2025, over 131 million pieces of debris orbit Earth

These pieces range in size from a grain of sand to the size of a bus and orbit with an average speed of 10 kilometers per second.

How SpaceX Made Space Affordable

SpaceX has cut launch costs by over 95%—from $54,000 per kilogram to around $2,000—reshaping how we access space.

The most mysterious (known) star in the universe

Something massive, with roughly 1,000 times the area of Earth, is blocking the light coming from a distant star known as KIC 8462852. One of astronomers' more intriguing theories: Alien technology. Discovered via NASA's Kepler space telescope and a...

Instabilities in the polar jet stream can bring cold Arctic air southward

The polar jet stream marks the boundary between mid-latitude air and the much colder polar air, which are normally contained by the polar vortex—a ring of strong winds that circles the Arctic around 16 to 50 kilometers (10 to 30 miles) above the Earth's surface. Climate change may be responsible for "blocking patterns" that distort the jet stream, allowing polar air to leak into lower latitudes.

The Earth orbits the sun within the Orion Spur arm of the Milky Way galaxy

The solar system is roughly halfway between the galaxy's outer rim and its center, which is located about 25,000 light years away. The Milky Way is itself one of hundreds of thousands of galaxies in the Virgo Supercluster.

Stratospheric balloons: a visual guide

In late January 2023, a Chinese high-altitude balloon, believed to be for surveillance purposes, traversed the US and caused a diplomatic stir. The technology itself is established—though typically used for atmospheric measurements. This visual...

NASA uses a specialized plasma wind tunnel to test heat shield technology

The ArcJet Complex ionizes compressed air that fires onto shields at hypersonic speeds, replicating the conditions of high-speed, planetary reentry. An enhanced facility incorporates lasers to mimic the radiant heating experienced during high-energy atmospheric traversal.

The construction of LIGO needed to compensate for Earth's curvature

Across the 4-kilometer (2.5-mile) arms, the height of Earth's surface falls by almost a meter, preventing the lasers traveling within the arms from meeting the mirror at the end of each. Concrete slabs built beneath the arms help keep them level and provide structural support against seismic vibrations.

The event horizon is an inescapable sphere around a black hole singularity

The horizon is often surrounded by an accretion disc of superheated material that orbits the black hole while emitting radiation. Light emitted behind a black hole can be bent around it to create the appearance of a ring of material known as a photon sphere.

Into the Mariana Trench

Located in the western Pacific Ocean, the Mariana Trench is the deepest known point on Earth. Its lowest point, known as Challenger Deep, is farther down than the height of Mount Everest by more than a mile. This 13-minute video discusses how the trench was formed, and the extreme conditions at the sea floor.

While the sun's gravity keeps the planets in orbit, their gravity also pulls on the sun, causing it and everything else in the solar system to orbit around a mutual balance point called the barycenter—the true center of the solar system.

Atmospheric rivers are the largest freshwater transport mechanism on Earth

Serving as highways that carry the water vapor equivalent of up to 15 times the average flow of water at the mouth of the Mississippi River, atmospheric rivers are approximately 400 to 600 kilometers (250 to 375 miles) wide and provide 30-50% of the annual precipitation of the US West Coast.

Track the location of the Voyager probes, currently between 13 and 15 billion miles away from Earth.

Most of the observable universe's volume consists of voids and supervoids—enormous regions of space with little to no galaxies, stars, or light.

All rainbows are 'raincircles' caused by water droplets separating white sunlight into a spectrum of colors, but Earth's surface blocks the bottom half of the circle from our line of sight.

Shooting stars, or meteors, are fragments of asteroids and comets that burn up as they enter Earth's atmosphere, creating streaks in the sky called meteor showers.

Chaos theory—informally known as the butterfly effect—is the reason why weather forecasts are presented with probabilities rather than certainties.

Before humans, the Soviet Union and the United States sent animals—including apes, dogs, and cats—to space during the Space Race.

Power lines are dotted with large, brightly colored balls to improve the visibility of these thin wires to helicopters and small planes and help prevent potential crashes.

The syncytiotrophoblast is the outer layer of the placenta and the largest human cell, spanning an area about twice that of a king-sized bedsheet.

Waves off the coast of Nazaré, Portugal, can reach over 100 feet and are caused by the area's deep underwater canyon geography

The deadliest aspect of volcanic eruptions is often not lava, but pyroclastic flows—avalanches of ash, gas, and rock that can move more than half the speed of sound.

After 25 people used virtual reality headsets to practice flying with virtual wings—flapping their arms—their visual cortex responded to the wings almost as if they were real arms, treating them as body parts.

Scientists can determine Earth's past climate conditions by analyzing air trapped in microscopic bubbles within ice cores—columns of compressed snow layers that build up over hundreds of thousands of years.

A 2019 Facebook event called on attendees to "Storm Area 51, They Can't Stop All of Us," resulting in countless memes, the creation of a music festival, and eventually the arrests of those who did try to approach the area.

Louisiana's 'Cancer Alley' is infamous for having some of the highest industrial pollution, toxic air emissions, and cancer risks in the US.

In 1984, Operation Smash Hit intentionally crashed a 239-ton train into a nuclear waste canister at 100 miles per hour to prove the canister's safety, which remained completely intact and lost just 0.29 of its 100 pounds of pressure.

The price of gas is a composite made up of several different factors, with the federal gas tax making up less than 20% of what drivers pay at the pump.

Existentialist writer Jean-Paul Sartre coined the famous phrase 'Hell is—other people' in his play 'No Exit'

Iran's Revolutionary Guard Corps is independent of Iran's regular army and is one of the most powerful and influential military-political organizations in the Middle East.

Massive 3D printers have been used to build homes by stacking layers of 'concrete toothpaste,' a fire-resistant material that effectively insulates against outside noise.

In 2009, the nation was gripped by the story of a missing young boy who was believed to be floating over Colorado in a homemade balloon, until it was revealed that he was at home the whole time.

The Russia-Ukraine conflict is the largest ground war in Europe since World War II in terms of casualties, forces mobilized, and displacement of civilians.

Take a 3D tour of the Hajj, Islam's annual pilgrimage to Mecca, undertaken by over 1.5 million Muslims.

The word 'jazz' evolved from 'jasm,' 19th-century American slang for 'energy'