--- title: "Environment" id: "environment" project: "Interstellar Travel Safety Procedures Publication2" url: "https://hedronlabs.clickhelp.co/articles/interstellar-travel-safety-procedures-publication2/environment" updated: 2026-10-01T08:39:33Z --- # Environment ## Environment ![](/resources/Storage/interstellar-travel-safety-procedures-publication2/Imported/environment/img0001.png) Outer space is the closest known approximation to a [perfect vacuum](https://en.wikipedia.org/wiki/Perfect_vacuum). It has effectively no [friction](https://en.wikipedia.org/wiki/Friction), allowing stars, [planets](https://en.wikipedia.org/wiki/Planets), and [moons](https://en.wikipedia.org/wiki/Moons) to move freely along their ideal [orbits](https://en.wikipedia.org/wiki/Orbit), following the [initial formation](https://en.wikipedia.org/wiki/Planetary_formation) stage. The deep vacuum of [intergalactic space](https://en.wikipedia.org/wiki/Outer_space#Intergalactic_space) is not devoid of [matter](https://en.wikipedia.org/wiki/Matter), as it contains a few [hydrogen atoms](https://en.wikipedia.org/wiki/Hydrogen_atoms) per cubic meter. By comparison, the air humans breathe contains about 10^25^ molecules per cubic meter. The low density of matter in outer space means that [electromagnetic radiation](https://en.wikipedia.org/wiki/Electromagnetic_radiation) can travel great distances without being scattered: the [mean free path](https://en.wikipedia.org/wiki/Mean_free_path) of a [photon](https://en.wikipedia.org/wiki/Photon) in intergalactic space is about 10^23^ km, or 10 billion light years. In spite of this, [extinction](https://en.wikipedia.org/wiki/Extinction_%28astronomy%29), which is the [absorption](https://en.wikipedia.org/wiki/Absorption_%28electromagnetic_radiation%29) and [scattering](https://en.wikipedia.org/wiki/Scattering) of photons by dust and gas, is an important factor in galactic and intergalactic [astronomy](https://en.wikipedia.org/wiki/Astronomy). Stars, planets, and moons retain their [atmospheres](https://en.wikipedia.org/wiki/Atmosphere) by gravitational attraction. Atmospheres have no clearly delineated upper boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from outer space. The Earth's atmospheric [pressure](https://en.wikipedia.org/wiki/Pressure) drops to about 0.032 [Pa](https://en.wikipedia.org/wiki/Pascal_%28unit%29) at 100 kilometres (62 miles) of altitude, compared to 100,000 Pa for the [International Union of Pure and Applied Chemistry](https://en.wikipedia.org/wiki/International_Union_of_Pure_and_Applied_Chemistry) (IUPAC) definition of [standard pressure](https://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure). Above this altitude, isotropic gas pressure rapidly becomes insignificant when compared to [radiation pressure](https://en.wikipedia.org/wiki/Radiation_pressure) from the [Sun](https://en.wikipedia.org/wiki/Sun) and the [dynamic pressure](https://en.wikipedia.org/wiki/Dynamic_pressure) of the [solar wind](https://en.wikipedia.org/wiki/Solar_wind). The [thermosphere](https://en.wikipedia.org/wiki/Thermosphere) in this range has large gradients of pressure, temperature and composition, and varies greatly due to [space weather](https://en.wikipedia.org/wiki/Space_weather). The [temperature](https://en.wikipedia.org/wiki/Temperature) of outer space is measured in terms of the [kinetic](https://en.wikipedia.org/wiki/Kinetic_theory_of_gases) activity of the gas, as it is on Earth. The radiation of outer space has a different temperature than the [kinetic temperature](https://en.wikipedia.org/wiki/Kinetic_temperature) of the gas, meaning that the gas and radiation are not in [thermodynamic equilibrium](https://en.wikipedia.org/wiki/Thermodynamic_equilibrium). All of the observable universe is filled with photons that were created during the [Big Bang](https://en.wikipedia.org/wiki/Big_Bang), which is known as the [cosmic microwave background radiation](https://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation) (CMB). (There is quite likely a correspondingly large number of [neutrinos](https://en.wikipedia.org/wiki/Neutrino) called the [cosmic neutrino background](https://en.wikipedia.org/wiki/Cosmic_neutrino_background).) The current [black body](https://en.wikipedia.org/wiki/Black_body) temperature of the background radiation is about 3 [K](https://en.wikipedia.org/wiki/Kelvin) (−270 [°C](https://en.wikipedia.org/wiki/Celsius); −454 [°F](https://en.wikipedia.org/wiki/Fahrenheit)). The gas temperatures in outer space can vary widely. For example, the temperature in the [Boomerang Nebula](https://en.wikipedia.org/wiki/Boomerang_Nebula) is 1 K, while the [solar corona](https://en.wikipedia.org/wiki/Solar_corona) reaches temperatures over 1.2–2.6 million K. Magnetic fields have been detected in the space around just about every class of celestial object. Star formation in spiral galaxies can generate small-scale [dynamos](https://en.wikipedia.org/wiki/Dynamo), creating turbulent magnetic field strengths of around 5–10 μ[G](https://en.wikipedia.org/wiki/Gauss_%28unit%29). The [Davis–Greenstein effect](https://en.wikipedia.org/wiki/Davis%E2%80%93Greenstein_effect) causes elongated [dust grains](https://en.wikipedia.org/wiki/Cosmic_dust) to align themselves with a galaxy's magnetic field, resulting in weak optical [polarization](https://en.wikipedia.org/wiki/Polarization_%28waves%29). This has been used to show ordered magnetic fields exist in several nearby galaxies. [Magneto-hydrodynamic](https://en.wikipedia.org/wiki/Magnetohydrodynamics) processes in [active](https://en.wikipedia.org/wiki/Active_galactic_nucleus) [elliptical galaxies](https://en.wikipedia.org/wiki/Elliptical_galaxy) produce their characteristic [jets](https://en.wikipedia.org/wiki/Astrophysical_jet) and [radio lobes](https://en.wikipedia.org/wiki/Radio_lobe). Non-thermal [radio sources](https://en.wikipedia.org/wiki/Astronomical_radio_source) have been detected even among the most distant, [high-z](https://en.wikipedia.org/wiki/Redshift) sources, indicating the presence of magnetic fields. Outside a protective atmosphere and magnetic field, there are few obstacles to the passage through space of energetic [subatomic particles](https://en.wikipedia.org/wiki/Subatomic_particle) known as cosmic rays. These particles have energies ranging from about 10^6^ [eV](https://en.wikipedia.org/wiki/Electronvolt) up to an extreme 10^20^ eV of [ultra-high-energy cosmic rays](https://en.wikipedia.org/wiki/Ultra-high-energy_cosmic_ray). The peak flux of cosmic rays occurs at energies of about 10^9^ eV, with approximately 87% protons, 12% helium nuclei and 1% heavier nuclei. In the high energy range, the flux of [electrons](https://en.wikipedia.org/wiki/Electron) is only about 1% of that of protons. Cosmic rays can damage electronic components and pose a [health threat](https://en.wikipedia.org/wiki/Health_threat_from_cosmic_rays) to space travelers. According to astronauts, like [Don Pettit](https://en.wikipedia.org/wiki/Don_Pettit), space has a burned/metallic odor that clings to their suits and equipment, similar to the scent of an [arc welding](https://en.wikipedia.org/wiki/Arc_welding) torch.