Space

Space, or outer space, is the boundless three-dimensional extent in which objects and events occur beyond Earth’s atmosphere. It begins at the upper edge of the atmosphere, commonly defined as the Kármán line at about 100 kilometers above sea level, though there is no physical boundary in the fabric of space.

It is a near-vacuum, with extremely low particle density. Heat transfer occurs mainly by radiation, and temperatures can vary widely between scorching and freezing depending on exposure to solar radiation and shading. The environment contains dust, gas, magnetic fields, cosmic rays, and micrometeoroids, which present challenges to spacecraft and humans.

Space hosts a hierarchy of structures: planets and their moons orbit stars; stars form galaxies; galaxies cluster into larger assemblies. Most of the universe’s mass-energy is in forms not directly visible, such as dark matter and dark energy, while ordinary matter makes up a small fraction.

Physical laws describe motion in space through gravitation and relativity. The speed of light is the universal speed limit, and the universe is expanding on large scales. Distances are measured with units such as the astronomical unit (AU), the light-year, and the parsec.

Exploration and observation in space are conducted with spacecraft, satellites, and ground- and space-based telescopes. Human spaceflight, robotic probes, and space stations contribute to science, communication, and navigation. Space research advances astronomy, planetary science, cosmology, and Earth science, while expanding humanity’s practical capabilities.