Space weather

Lore & Background

For many centuries, the effects of space weather were noticed but not understood. Displays of auroral light have long been observed at high latitudes.

Reader's Guide

Space weather matters because it affects modern technological systems. Communications satellites, weather satellites, and GPS signals can be interfered with or damaged by space weather phenomena. Geomagnetic storms can cause damaging surges in long-distance transmission lines and expose aircraft passengers and crew to radiation, especially on polar routes. Space weather is listed as a natural hazard in risk assessments in Ireland, the United Kingdom, and New Zealand.

Did You Know?

• The term 'space weather' was first used in the 1950s and popularized in the 1990s.

The Physical Fabric of the Void

Outer space is, as far as we can measure, the closest thing nature offers to a perfect vacuum. Rather than being truly empty, it is threaded with an ultra-thin plasma of hydrogen and helium, scattered electromagnetic radiation, cosmic rays, neutrinos, magnetic fields, and fine dust. The baseline temperature, set by the residual glow of the Big Bang, sits at just 2.7 kelvins—roughly minus 270 degrees Celsius. Between galaxies, that plasma is so sparse that fewer than one hydrogen atom occupies a cubic metre, yet its kinetic temperature reaches millions of kelvins. This intergalactic medium is believed to hold about half of all ordinary baryonic matter in the observable universe. Local clumps of matter have condensed into stars and galaxies, but even those structures are overwhelmingly empty. The remaining mass-energy budget is dominated by two mysterious components—dark matter and dark energy—that resist direct detection and reshape our understanding of what fills the void.

From the First Expansion to the Observable Cosmos

According to the Big Bang model, the universe began roughly 13.8 billion years ago as an extraordinarily hot and dense state that expanded at a tremendous rate. For the first 380,000 years, matter and radiation were locked together in a searing fog. When the temperature finally dropped enough for protons and electrons to bind into hydrogen—the recombination epoch—photons were released and have been streaming freely ever since. That ancient light, now redshifted into microwaves, is what satellites like the Wilkinson Microwave Anisotropy Probe have mapped to determine the universe's geometry. The result: the observable universe is spatially flat, meaning parallel light rays stay parallel across cosmic distances unless bent by local gravity. Combined with measured mass density and the observed accelerating expansion, this flatness points to a non-zero vacuum energy we call dark energy. On average, the universe contains the equivalent of about 5.9 protons per cubic metre, yet ordinary atoms make up only 4.6 percent of that total, and density varies enormously from the packed cores of galaxies to the near-emptiness of vast intergalactic voids.

Drawing the Line Between Sky and Space

Unlike a mountain range or an ocean trench, outer space has no sharp physical boundary. The convention used in international treaties and aerospace record-keeping is the Kármán line, set at 100 kilometres above sea level, named after Theodore von Kármán, who described the altitude at which atmospheric drag becomes negligible and spacecraft can operate free of it. Below that threshold, portions of the upper stratosphere and mesosphere are sometimes called near space. The legal architecture governing the region above was codified in the Outer Space Treaty, which took effect on 10 October 1967. That treaty bars any nation from claiming sovereignty over celestial territory and instead grants every state the right to explore freely. The terminology itself has a long literary pedigree: John Milton used the bare word space in Paradise Lost in 1667, Alexander von Humboldt applied outer space to astronomy in 1845, and H. G. Wells popularised the phrase after 1901. Yet despite UN resolutions calling for peaceful use, anti-satellite weapons have still been tested in Earth orbit, reminding us that the legal line is not always the practical one.

The Human Reckoning with the Void

The idea that the gap between Earth and the Moon is a vacuum emerged in the 17th century once scientists demonstrated that air pressure falls with altitude. It was not until the 20th century that the true scale of that void became measurable, when the distance to the Andromeda Galaxy was first determined. Physical exploration followed in stages: high-altitude balloon flights opened the door, then crewed rocket flights, and finally Yuri Gagarin's orbit of Earth in 1961 for the Soviet Union. The economic price of launching mass into space remains so steep that human missions have been confined largely to low Earth orbit and the Moon, while uncrewed probes have visited every known planet in the Solar System. The environment itself is hostile: the vacuum and pervasive radiation pose constant threats, and microgravity steadily erodes the human body, causing muscle atrophy and bone loss. Space is, in every measurable sense, a place that resists human presence, and our foothold there remains fragile and expensive.