There is no firm boundary where space begins. However the Kármán line, at an altitude of 100 km (62 mi) above sea level, is conventionally used as the start of outer space for the purpose of space treaties and aerospace records keeping. The framework for international space law was established by theOuter Space Treaty, which was passed by the United Nations in 1967. This treaty precludes any claims of national sovereignty and permits all states to explore outer space freely. In 1979, the Moon Treaty made the surfaces of objects such as planets, as well as the orbital space around these bodies, the jurisdiction of the international community. Additional resolutions regarding the peaceful uses of outer space have been drafted by the United Nations, but these have not precluded the deployment of weapons into outer space, including the live testing of anti-satellite weapons.
Humans began the physical exploration of space during the twentieth century with the advent of high-altitude balloon flights, followed by the development of single and multi-stage rocket launchers. Earth orbit was achieved by Yuri Gagarin in 1961 and unmanned spacecraft have since reached all of the known planets in the Solar System. Achieving orbit requires a minimum velocity of 28,400 km/h (17,600 mph); much faster than any conventional aircraft. Outer space represents a challenging environment for human exploration because of the dual hazards of vacuum and radiation.Microgravity has a deleterious effect on human physiology, resulting in muscle atrophy and bone loss. As of yet space travel has been limited to low Earth orbit and the Moon for manned flight, and the vicinity of the Solar System for unmanned; the remainder of outer space remains inaccessible to humans other than by passive observation with telescopes.
Outer space is the closest natural approximation to a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along their ideal orbits. However, even the deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter. By comparison, the air we breathe contains about 1025 molecules per cubic meter. The sparse density of matter in outer space means that electromagnetic radiation can travel great distances without being scattered: the mean free path of a photon in intergalactic space is about 1023 km, or 10 billion light years. In spite of this, extinction, which is the absorption and scattering of photons by dust and gas, is an important factor in galactic and intergalactic astronomy.
Stars, planets and moons retain their atmospheres by gravitational attraction. Atmospheres have no clearly delineated boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from the surrounding environment. The Earth's atmospheric pressure drops to about 3.2 × 10−2 Pa at 100 kilometres (62 miles) of altitude, compared to 100 kPA for the International Union of Pure and Applied Chemistry (IUPAC) definition of standard pressure. Beyond this altitude, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the Sun and the dynamic pressure of the solar wind. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather.
On the Earth, temperature is defined in terms of the kinetic activity of the surrounding atmosphere. However the temperature of the vacuum cannot be measured in this way. Instead, the temperature is determined by measurement of the radiation. All of the observable Universe is filled with photons that were created during the Big Bang, which is known as the cosmic microwave background radiation (CMB). (There is quite likely a correspondingly large number of neutrinos called the cosmic neutrino background.) The current black body temperature of the background radiation is about 3 K (−270 °C; −454 °F). Some regions of outer space can contain highly energetic particles that have a much higher temperature than the CMB, such as the corona of the Sun where temperatures can range over 1.2–2.6 MK.
Outside of a protective atmosphere and magnetic field, there are few obstacles to the passage through space of energetic subatomic particles known as cosmic rays. These particles have energies ranging from about 106 eV up to an extreme 1020 eV of ultra-high-energy cosmic rays. The peak flux of cosmic rays occurs at energies of about 109 eV, with approximately 87% protons, 12% helium nuclei and 1% heavier nuclei. In the high energy range, the flux of electrons is only about 1% of that of protons. Cosmic rays can damage electronic components and pose a health threat to space travelers.
Contrary to popular belief, a person suddenly exposed to a vacuum would not explode, freeze to death or die from boiling blood. However, sudden exposure to very low pressure, such as during a rapid decompression, could cause pulmonary barotrauma—a rupture of the lungs, due to the large pressure differential between inside and outside of the chest. Even if the victim's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture. Rapid decompression can rupture eardrums and sinuses, bruising and blood seep can occur in soft tissues, and shock can cause an increase in oxygen consumption that leads to hypoxia.
As a consequence of rapid decompression, any oxygen dissolved in the blood would empty into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrived at the brain, humans and animals will lose consciousness after a few seconds and die of hypoxia within minutes. Blood and other body fluids boil when the pressure drops below 6.3 kPa, and this condition is called ebullism. The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid. Swelling and ebullism can be reduced by containment in a flight suit. Shuttle astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa. Space suits are needed at 8 km (5.0 mi) to provide enough oxygen for breathing and to prevent water loss, while above 20 km (12 mi) they are essential to prevent ebullism. Most space suits use around 30–39 kPa of pure oxygen, about the same as on the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of blood could still cause decompression sickness and gas embolisms if not managed.
Because humans are optimized for life in Earth gravity, exposure to weightlessness has been shown to have deleterious effects on the health. Initially, more than 50% of astronauts experience space motion sickness. This can cause nausea and vomiting, vertigo, headaches, lethargy, and overall malaise. The duration of space sickness varies, but it typically lasts for 1–3 days, after which the body adjusts to the new environment. Longer term exposure to weightlessness results in muscle atrophy and deterioration of the skeleton, or spaceflight osteopenia. These effects can be minimized through a regimen of exercise. Other effects include fluid redistribution, slowing of the cardiovascular system, decreased production of red blood cells, balance disorders, and a weakening of the immune system. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, and puffiness of the face.
For long duration space travel, radiation can pose an acute health hazard. Exposure to radiation sources such as high-energy, ionizing cosmic rays can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the white blood cell count. Over longer durations, symptoms include an increase in the risk of cancer, plus damage to the eyes, nervous system, lungs and the gastrointestinal tract. On a round-trip Mars mission lasting three years, nearly the entire body would be traversed by high energy nuclei, each of which can cause ionization damage to cells. Fortunately, most such particles are significantly attenuated by the shielding provided by the aluminum walls of a spacecraft, and can be further diminished by water containers and other barriers. However, the impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research will be needed to assess the radiation hazards and determine suitable countermeasures.
There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases. There are several standard boundary designations, namely:
- The Fédération Aéronautique Internationale has established the Kármán line at an altitude of 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of roughly 100 km (62 mi), as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself.
- The United States designates people who travel above an altitude of 50 miles (80 km) as astronauts.
- NASA's mission control uses 76 mi (122 km) as their re-entry altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag becomes noticeable (depending on the ballistic coefficient of the vehicle), thus leading shuttles to switch from steering with thrusters to maneuvering with air surfaces.
In 2009, scientists at the University of Calgary reported detailed measurements with an instrument called the Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at 118 km (73 mi) above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over 268 m/s (600 mph).
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