Outer space

Outer space is the vast expanse existing beyond Earth's atmosphere and between all celestial bodies. It is characterized by ultra-low particle densities, making it a near-perfect vacuum. This vacuum is primarily composed of hydrogen and helium plasma.

Space is also permeated by electromagnetic radiation, cosmic rays, neutrinos, and magnetic fields. While it seems empty, it contains various forms of energy and matter. Much of the mass-energy in the observable universe consists of unknown forms called dark matter and dark energy.

Understanding space is essential for grasping the scale and history of our entire universe.

The state of the universe began with the Big Bang approximately 13.8 billion years ago. During this era, the universe was in an extremely hot and dense state that expanded rapidly. About 380,000 years after this expansion began, the universe cooled enough for protons and electrons to combine. This event is known as the recombination epoch, which allowed for the formation of hydrogen.

Once this occurred, matter and energy decoupled, allowing photons to travel freely through expanding space. Following this initial expansion, matter underwent gravitational collapse. This process created the stars, galaxies, and other astronomical objects we observe today. The remaining vast areas between these objects became the deep vacuum we call outer space.

Space can be categorized by its density and composition. Intergalactic space occupies most of the universe's volume. It is incredibly sparse, with a density of less than one hydrogen atom per cubic metre. In contrast, local concentrations of matter have condensed into stars and galaxies. Within these galaxies, density is much higher due to planets, stars, and black holes.

There is also a distinction between baryonic matter and dark components. Baryonic matter refers to ordinary matter composed of atoms. This makes up only 4.6% of the total energy density of the universe. The rest is comprised of dark matter and dark energy, which influence the universe's expansion.

Human understanding of space has evolved through centuries of scientific discovery. In the 17th century, scientists realized that air pressure decreases with altitude. This led to the concept that the space between Earth and the Moon must be a vacuum. The term "outer space" was first applied in astronomy by Alexander von Humboldt in 1845. Later, the writings of H. G. Wells helped popularize the term after 1901. In the 20th century, scientists grasped the immense scale of the universe by measuring the distance to the Andromeda Galaxy.

Physical exploration began with high-altitude balloons, followed by crewed rocket flights. In 1961, Yuri Gagarin of the Soviet Union achieved the first crewed Earth orbit.

The boundary of space is not defined by a single altitude, but a convention exists. The Kármán line is an altitude of 100 kilometers above sea level. It is used in space treaties and for aerospace record-keeping to mark the start of outer space.

International space law was established by the Outer Space Treaty, which entered into force on 10 October 1967. This treaty prevents any nation from claiming sovereignty over space. It also allows all states to explore space freely. Despite these peaceful goals, anti-satellite weapons have been tested in Earth's orbit.

The environment of space presents extreme physical challenges. Because there is almost no friction, planets and moons move freely in their orbits. However, the vacuum and radiation pose significant risks to humans.

Space is filled with cosmic rays, which are energetic subatomic particles. These particles can damage electronic components and threaten the health of travelers. Furthermore, microgravity causes negative physiological effects. Specifically, it can lead to muscle atrophy and bone loss in humans. Even the scent of space is unique; astronauts returning from extravehicular activity report a burned, metallic odor.

Despite these hazards, life shows remarkable resilience in space. Certain species can withstand extreme conditions for various periods. For example, lichen carried on the ESA BIOPAN facility survived ten days in space in 2007. Some seeds, such as Arabidopsis thaliana, germinated after 1.5 years of exposure. Additionally, a strain of Bacillus subtilis survived for 559 days in low Earth orbit or simulated Martian environments. These findings help scientists understand how life might exist or travel through the cosmos.

Space remains a frontier that connects biology, physics, and the history of the universe.