Effect on biology and human bodies while outside their spacecraft.|alt=The lower half shows a blue planet with patchy white clouds. The upper half has a man in a white spacesuit and maneuvering unit against a black background. Despite the harsh environment, several life forms have been found that can withstand extreme space conditions for extended periods. Species of
lichen carried on the ESA
BIOPAN facility survived exposure for ten days in 2007. Seeds of
Arabidopsis thaliana and
Nicotiana tabacum germinated after being exposed to space for 1.5 years. A strain of
Bacillus subtilis has survived 559 days when exposed to low Earth orbit or a simulated Martian environment. The
lithopanspermia hypothesis suggests that rocks ejected into outer space from life-harboring planets may successfully transport life forms to another habitable world. A conjecture is that just such a scenario occurred early in the history of the Solar System, with potentially
microorganism-bearing rocks being exchanged between Venus, Earth, and Mars. Because bacteria can survive for millions of years, it is at least theoretically possible for Galactic-scale panspermia to occur.
Vacuum The lack of pressure in space is the most immediate dangerous characteristic of space to humans. Pressure decreases above Earth, reaching a level at an altitude of around that matches the
vapor pressure of water at the
temperature of the human body. This pressure level is called the
Armstrong line, named after American physician
Harry G. Armstrong. At or above the Armstrong line, fluids in the throat and lungs boil away. More specifically, exposed bodily liquids such as saliva, tears, and liquids in the lungs boil away. Hence, at this altitude, human survival requires a pressure suit, or a pressurized capsule. Out in space, sudden exposure of an unprotected human to very low
pressure, such as during a rapid decompression, can cause
pulmonary barotrauma—a rupture of the lungs, due to the large pressure differential between inside and outside the chest. Even if the subject'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, oxygen dissolved in the blood empties into the lungs to try to equalize the
partial pressure gradient. Once the deoxygenated blood arrives at the brain, humans lose consciousness after a few seconds and die of hypoxia within minutes. Blood and other body fluids boil when the pressure drops below , 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
pressure suit. The Crew Altitude Protection Suit (CAPS), a fitted elastic garment designed in the 1960s for astronauts, prevents ebullism at pressures as low as . Supplemental oxygen is needed at to provide enough oxygen for breathing and to prevent water loss, while above pressure suits are essential to prevent ebullism. Most space suits use around of pure oxygen, about the same as the partial pressure of oxygen at the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of nitrogen dissolved in the blood could still cause
decompression sickness and
gas embolisms if not managed.
Weightlessness and radiation Humans evolved for life in Earth
gravity, and exposure to weightlessness has been shown to have deleterious effects on human 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. During long-duration space travel, radiation can pose an
acute health hazard. Exposure to 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 increased risk of cancer, plus damage to the eyes,
nervous system, lungs and the
gastrointestinal tract. On a round-trip
Mars mission lasting three years, a large fraction of the cells in an astronaut's body would be traversed and potentially damaged by high energy nuclei. The energy of such particles is significantly diminished by the shielding provided by the walls of a spacecraft and can be further diminished by water containers and other barriers. The impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research is needed to assess the radiation hazards and determine suitable countermeasures.
Boundary The transition between Earth's atmosphere and outer space lacks a well-defined physical boundary, with the air pressure steadily decreasing with altitude until it mixes with the
solar wind. Various definitions for a practical boundary have been proposed, ranging from out to . In 2009, measurements of the direction and speed of ions in the atmosphere were made from a
sounding rocket. The altitude of above Earth was the midpoint for charged particles transitioning from the gentle winds of the Earth's atmosphere to the more extreme flows of outer space. The latter can reach velocities well over . High-altitude
aircraft, such as
high-altitude balloons have reached altitudes above Earth of up to 50 km. Up until 2021, the United States designated people who travel above an altitude of as astronauts.
Astronaut wings are now only awarded to spacecraft crew members that "demonstrated activities during flight that were essential to public safety, or contributed to human space flight safety". The region between airspace and outer space is termed "near space". There is no legal definition for this extent, but typically this is the altitude range from . For safety reasons,
commercial aircraft are typically limited to altitudes of , and air navigation services only extend to . The upper limit of the range is the
Kármán line, where
astrodynamics must take over from
aerodynamics in order to achieve flight. This range includes the
stratosphere,
mesosphere and lower
thermosphere layers of the Earth's atmosphere. Larger ranges for
near space are used by some authors, such as . These extend to the altitudes where
orbital flight in
very low Earth orbits becomes practical. Spacecraft have entered into a highly elliptical
orbit with a perigee as low as , surviving for multiple orbits. At an altitude of , descending spacecraft begin
atmospheric entry as
atmospheric drag becomes noticeable. For
spaceplanes such as
NASA's
Space Shuttle, this begins the process of switching from steering with thrusters to maneuvering with
aerodynamic control surfaces. The Kármán line, established by the
Fédération Aéronautique Internationale, and used internationally by the
United Nations, is set at an altitude of as a working definition for the boundary between aeronautics and astronautics. This line is named after
Theodore von Kármán. Around this altitude a vehicle cannot generate sufficient
aerodynamic lift from the atmosphere to support itself. There is no internationally recognized legal altitude limit on national airspace, although the Kármán line is the most frequently used for this purpose. Objections have been made to setting this limit too high, as it could inhibit space activities due to concerns about airspace violations. It has been argued for setting no specified singular altitude in international law, instead applying different limits depending on the case, in particular based on the craft and its purpose. Increased commercial and military sub-orbital spaceflight has raised the issue of where to apply laws of airspace and outer space. Spacecraft have flown over foreign countries as low as , as in the example of the Space Shuttle.
Legal status remain legal under the
law of armed conflict, even though they create hazardous
space debris The
Outer Space Treaty provides the basic framework for international space law. It covers the legal use of outer space by nation states, and includes in its definition of
outer space, the Moon, and other celestial bodies. The treaty states that outer space is free for all nation states to explore and is not subject to claims of national sovereignty, calling outer space the "province of all mankind". This status as a
common heritage of mankind has been used, though not without opposition, to enforce the right to access and shared use of outer space for all nations equally, particularly non-spacefaring nations. It prohibits the deployment of
nuclear weapons in outer space. The treaty was passed by the
United Nations General Assembly in 1963 and signed in 1967 by the Union of Soviet Socialist Republics (USSR), the United States of America (USA), and the United Kingdom (UK). As of 2017, 105 state parties have either ratified or acceded to the treaty. An additional 25 states signed the treaty, without ratifying it. Since 1958, outer space has been the subject of multiple United Nations resolutions. Of these, more than 50 have been concerning the international co-operation in the peaceful uses of outer space and preventing an arms race in space. Four additional
space law treaties have been negotiated and drafted by the UN's
Committee on the Peaceful Uses of Outer Space. Still, there remains no legal prohibition against deploying conventional weapons in space, and
anti-satellite weapons have been successfully tested by the USA, USSR, China, and in 2019, India. The 1979
Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. The treaty has not been ratified by any nation that currently practices human spaceflight. In 1976, eight equatorial states (Ecuador, Colombia, Brazil, The Republic of the Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia: with their "Declaration of the First Meeting of Equatorial Countries", or the
Bogotá Declaration, they claimed control of the segment of the geosynchronous orbital path corresponding to each country. These claims are not internationally accepted. An increasing issue of international space law and regulation has been the dangers of the growing number of
space debris.
Earth orbit , an illustration of how objects can "fall" in a curve around the planet When a rocket is launched to achieve orbit, its thrust must both counter gravity and accelerate it to
orbital speed. After the rocket terminates its thrust, it follows an arc-like
trajectory back toward the ground under the influence of the Earth's
gravitational force. In a
closed orbit, this arc will turn into an
elliptical loop around the planet. That is, a spacecraft successfully enters Earth orbit when its
acceleration due to gravity pulls the craft down just enough to prevent its momentum from carrying it off into outer space. For a
low Earth orbit, orbital speed is about ; by contrast, the fastest piloted airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was in 1967 by the
North American X-15. The upper limit of orbital speed at is the
velocity required to pull free from Earth altogether and enter into a
heliocentric orbit. The energy required to reach Earth orbital speed at an altitude of is about 36
MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude. Very low Earth orbit (VLEO) has been defined as orbits that have a mean altitude below 450 km (280 mi), which can be better suited for Earth observation with small satellites. Low Earth orbits in general range in altitude from and are used for scientific satellites.
Medium Earth orbits extends from , which are favorable orbits for navigation and specialized satellites. Above are the
high Earth orbits used for weather and some communication satellites. Spacecraft in orbit with a
perigee below about (low Earth orbit) are subject to drag from the Earth's atmosphere, which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere, which is significantly affected by
space weather. At altitudes above , orbital lifetime is measured in centuries. Below about , decay becomes more rapid with lifetimes measured in days. Once a satellite descends to , it has only hours before it vaporizes in the atmosphere. Radiation in orbit around Earth is concentrated in
Van Allen radiation belts, which trap
solar and galactic radiation. Radiation is a threat to astronauts and space systems. It is difficult to shield against and space weather makes the radiation environment variable. The radiation belts are equatorial
toroidal regions, which are bent towards Earth's poles, with the
South Atlantic Anomaly being the region where charged particles approach Earth closest. The innermost radiation belt, the inner Van Allen belt, has its intensity peak at altitudes above the equator of half an Earth radius, centered at about 3000 km, increasing from the upper edge of low Earth orbit which it overlaps. == Regions ==