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59-397: The term LEO region is used for the area of space below an altitude of 2,000 km (1,200 mi) (about one-third of Earth's radius). Objects in orbits that pass through this zone, even if they have an apogee further out or are sub-orbital , are carefully tracked since they present a collision risk to the many LEO satellites. No human spaceflights other than the lunar missions of

118-568: A domino effect known as Kessler syndrome . NASA's Orbital Debris Program tracks over 25,000 objects larger than 10 cm diameter in LEO, while the estimated number between 1 and 10 cm is 500,000, and the number of particles bigger than 1 mm exceeds 100 million. The particles travel at speeds up to 7.8 km/s (28,000 km/h; 17,500 mph), so even a small impact can severely damage a spacecraft. [REDACTED]  This article incorporates public domain material from websites or documents of

177-406: A hydrazine thruster) the capacity of the reaction control system is Δ v = 2100   ln ⁡ ( 1 0.8 ) m/s = 460 m/s . {\displaystyle \Delta {v}=2100\ \ln \left({\frac {1}{0.8}}\right)\,{\text{m/s}}=460\,{\text{m/s}}.} If v exh {\displaystyle v_{\text{exh}}}

236-623: A characteristic pressure-temperature curve. As the pressure gets lower, the temperature decreases. The rate of decrease of temperature with elevation is known as the adiabatic lapse rate , which is approximately 9.8 °C per kilometer (or 5.4 °F [3.0 °C] per 1000 feet) of altitude. The presence of water in the atmosphere complicates the process of convection. Water vapor contains latent heat of vaporization . As air rises and cools, it eventually becomes saturated and cannot hold its quantity of water vapor. The water vapor condenses (forming clouds ), and releases heat, which changes

295-447: A higher heart rate, and adjusting its blood chemistry. It can take days or weeks to adapt to high altitude. However, above 8,000 metres (26,000 ft), (in the " death zone "), altitude acclimatization becomes impossible. There is a significantly lower overall mortality rate for permanent residents at higher altitudes. Additionally, there is a dose response relationship between increasing elevation and decreasing obesity prevalence in

354-543: A satellite into a LEO, and a satellite there needs less powerful amplifiers for successful transmission, LEO is used for many communication applications, such as the Iridium phone system . Some communication satellites use much higher geostationary orbits and move at the same angular velocity as the Earth as to appear stationary above one location on the planet. Unlike geosynchronous satellites , satellites in low orbit have

413-441: A small field of view and can only observe and communicate with a fraction of the Earth at a given time. This means that a large network (or constellation ) of satellites is required to provide continuous coverage. Satellites at lower altitudes of orbit are in the atmosphere and suffer from rapid orbital decay , requiring either periodic re-boosting to maintain stable orbits, or the launching of replacements for those that re-enter

472-541: A spacecraft's delta- v , then, would be the change in velocity that spacecraft can achieve by burning its entire fuel load. Delta- v is produced by reaction engines , such as rocket engines , and is proportional to the thrust per unit mass and the burn time. It is used to determine the mass of propellant required for the given maneuver through the Tsiolkovsky rocket equation . For multiple maneuvers, delta- v sums linearly. For interplanetary missions, delta- v

531-433: A subset of LEO. These orbits, with low orbital inclination , allow rapid revisit times over low-latitude locations on Earth. Prograde equatorial LEOs also have lower delta-v launch requirements because they take advantage of the Earth's rotation. Other useful LEO orbits including polar orbits and Sun-synchronous orbits have a higher inclinations to the equator and provide coverage for higher latitudes on Earth. Some of

590-462: Is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or landing on a planet or moon, or an in-space orbital maneuver . It is a scalar that has the units of speed . As used in this context, it is not the same as the physical change in velocity of said spacecraft. A simple example might be the case of a conventional rocket-propelled spacecraft, which achieves thrust by burning fuel. Such

649-428: Is a non-constant function of the amount of fuel left v exh = v exh ( m ) {\displaystyle v_{\text{exh}}=v_{\text{exh}}(m)} the capacity of the reaction control system is computed by the integral ( 5 ). The acceleration ( 2 ) caused by the thruster force is just an additional acceleration to be added to the other accelerations (force per unit mass) affecting

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708-422: Is also notable that large thrust can reduce gravity drag . Delta- v is also required to keep satellites in orbit and is expended in propulsive orbital stationkeeping maneuvers. Since the propellant load on most satellites cannot be replenished, the amount of propellant initially loaded on a satellite may well determine its useful lifetime. From power considerations, it turns out that when applying delta- v in

767-445: Is an exponential function of delta- v in accordance with the rocket equation , it will also depend on the exhaust velocity. It is not possible to determine delta- v requirements from conservation of energy by considering only the total energy of the vehicle in the initial and final orbits since energy is carried away in the exhaust (see also below). For example, most spacecraft are launched in an orbit with inclination fairly near to

826-428: Is even more so when the planet is a large one with a deep gravity field, such as Jupiter. Due to the relative positions of planets changing over time, different delta-vs are required at different launch dates. A diagram that shows the required delta- v plotted against time is sometimes called a porkchop plot . Such a diagram is useful since it enables calculation of a launch window , since launch should only occur when

885-406: Is hot, it tends to expand, which lowers its density. Thus, hot air tends to rise and transfer heat upward. This is the process of convection . Convection comes to equilibrium when a parcel of air at a given altitude has the same density as its surroundings. Air is a poor conductor of heat, so a parcel of air will rise and fall without exchanging heat. This is known as an adiabatic process , which has

944-465: Is just the rocket equation applied to the sum of the two maneuvers. This is convenient since it means that delta- v can be calculated and simply added and the mass ratio calculated only for the overall vehicle for the entire mission. Thus delta- v is commonly quoted rather than mass ratios which would require multiplication. When designing a trajectory, delta- v budget is used as a good indicator of how much propellant will be required. Propellant usage

1003-415: Is often plotted on a porkchop plot , which displays the required mission delta- v as a function of launch date. Δ v = ∫ t 0 t 1 | T ( t ) | m ( t ) d t {\displaystyle \Delta {v}=\int _{t_{0}}^{t_{1}}{\frac {|T(t)|}{m(t)}}\,dt} where Change in velocity

1062-495: Is only slightly less than on the Earth's surface. This is because the distance to LEO from the Earth's surface is much less than the Earth's radius. However, an object in orbit is in a permanent free fall around Earth, because in orbit the gravitational force and the centrifugal force balance each other out. As a result, spacecraft in orbit continue to stay in orbit, and people inside or outside such craft continuously experience weightlessness . Objects in LEO orbit Earth between

1121-403: Is sometimes defined to begin at 2,400 meters (8,000 ft) above sea level. At high altitude, atmospheric pressure is lower than that at sea level. This is due to two competing physical effects: gravity, which causes the air to be as close as possible to the ground; and the heat content of the air, which causes the molecules to bounce off each other and expand. The temperature profile of

1180-407: Is the coordinate acceleration. When thrust is applied in a constant direction ( ⁠ v / | v | ⁠ is constant) this simplifies to: Δ v = | v 1 − v 0 | {\displaystyle \Delta {v}=|v_{1}-v_{0}|} which is simply the magnitude of the change in velocity . However, this relation does not hold in

1239-677: Is useful in many cases, such as determining the change in momentum ( impulse ), where: Δ p = m Δ v {\displaystyle \Delta \mathbf {p} =m\Delta \mathbf {v} } , where p {\displaystyle \mathbf {p} } is momentum and m is mass. In the absence of external forces: Δ v = ∫ t 0 t 1 | v ˙ | d t {\displaystyle \Delta {v}=\int _{t_{0}}^{t_{1}}\left|{\dot {v}}\right|\,dt} where v ˙ {\displaystyle {\dot {v}}}

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1298-417: Is vital for performance and safety. Higher altitudes mean reduced oxygen levels, which can lead to altitude sickness if proper acclimatization measures are not taken. Vertical distance measurements in the "down" direction are commonly referred to as depth . The term altitude can have several meanings, and is always qualified by explicitly adding a modifier (e.g. "true altitude"), or implicitly through

1357-501: The Apollo program (1968-1972) and the 2024 Polaris Dawn have taken place beyond LEO. All space stations to date have operated geocentric within LEO. A wide variety of sources define LEO in terms of altitude . The altitude of an object in an elliptic orbit can vary significantly along the orbit. Even for circular orbits , the altitude above ground can vary by as much as 30 km (19 mi) (especially for polar orbits ) due to

1416-487: The National Aeronautics and Space Administration . Altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometry, geographical survey, sport, or atmospheric pressure). Although the term altitude is commonly used to mean

1475-456: The height above sea level of a location, in geography the term elevation is often preferred for this usage. In aviation, altitude is typically measured relative to mean sea level or above ground level to ensure safe navigation and flight operations. In geometry and geographical surveys, altitude helps create accurate topographic maps and understand the terrain's elevation. For high-altitude trekking and sports, knowing and adapting to altitude

1534-429: The oblateness of Earth's spheroid figure and local topography . While definitions based on altitude are inherently ambiguous, most of them fall within the range specified by an orbit period of 128 minutes because, according to Kepler's third law , this corresponds to a semi-major axis of 8,413 km (5,228 mi). For circular orbits, this in turn corresponds to an altitude of 2,042 km (1,269 mi) above

1593-435: The troposphere (up to approximately 11 kilometres (36,000 ft) of altitude) in the Earth's atmosphere undergoes notable convection; in the stratosphere , there is little vertical convection. Medicine recognizes that altitudes above 1,500 metres (4,900 ft) start to affect humans, and there is no record of humans living at extreme altitudes above 5,500–6,000 metres (18,000–19,700 ft) for more than two years. As

1652-508: The vacuum I sp is used for calculating the vehicle's delta- v capacity via the rocket equation . In addition, the costs for atmospheric losses and gravity drag are added into the delta- v budget when dealing with launches from a planetary surface. Orbit maneuvers are made by firing a thruster to produce a reaction force acting on the spacecraft. The size of this force will be where The acceleration v ˙ {\displaystyle {\dot {v}}} of

1711-498: The LEO region but are not in a LEO orbit because they re-enter the atmosphere . The distinction between LEO orbits and the LEO region is especially important for analysis of possible collisions between objects which may not themselves be in LEO but could collide with satellites or debris in LEO orbits. The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s (4.8 mi/s), which translates to 28,000 km/h (17,000 mph). However, this depends on

1770-458: The US, but may be as low as 3,000 feet (910 m) in other jurisdictions). So when the altimeter reads the country-specific flight level on the standard pressure setting the aircraft is said to be at "Flight level XXX/100" (where XXX is the transition altitude). When flying at a flight level, the altimeter is always set to standard pressure (29.92  inHg or 1013.25  hPa ). On the flight deck,

1829-597: The United States. In addition, the recent hypothesis suggests that high altitude could be protective against Alzheimer's disease via action of erythropoietin, a hormone released by kidney in response to hypoxia. However, people living at higher elevations have a statistically significant higher rate of suicide. The cause for the increased suicide risk is unknown so far. For athletes, high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres, long jump , triple jump )

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1888-441: The altitude increases, atmospheric pressure decreases, which affects humans by reducing the partial pressure of oxygen . The lack of oxygen above 2,400 metres (8,000 ft) can cause serious illnesses such as altitude sickness , high altitude pulmonary edema , and high altitude cerebral edema . The higher the altitude, the more likely are serious effects. The human body can adapt to high altitude by breathing faster, having

1947-471: The atmosphere is a result of an interaction between radiation and convection . Sunlight in the visible spectrum hits the ground and heats it. The ground then heats the air at the surface. If radiation were the only way to transfer heat from the ground to space, the greenhouse effect of gases in the atmosphere would keep the ground at roughly 333 K (60 °C; 140 °F), and the temperature would decay exponentially with height. However, when air

2006-431: The atmosphere. The effects of adding such quantities of vaporized metals to Earth's stratosphere are potentially of concern but currently unknown. The LEO environment is becoming congested with space debris because of the frequency of object launches. This has caused growing concern in recent years, since collisions at orbital velocities can be dangerous or deadly. Collisions can produce additional space debris, creating

2065-915: The challenge of maintaining body heat in cold temperatures, due to their small volume to surface area ratio. As oxygen is used as a source of metabolic heat production, the hypobaric hypoxia at high altitudes is problematic. There is also a general trend of smaller body sizes and lower species richness at high altitudes, likely due to lower oxygen partial pressures. These factors may decrease productivity in high altitude habitats, meaning there will be less energy available for consumption, growth, and activity. However, some species, such as birds, thrive at high altitude. Birds thrive because of physiological features that are advantageous for high-altitude flight. Delta-v Delta- v (also known as " change in velocity "), symbolized as Δ v {\textstyle {\Delta v}} and pronounced /dɛltə viː/ , as used in spacecraft flight dynamics ,

2124-414: The context of the communication. Parties exchanging altitude information must be clear which definition is being used. Aviation altitude is measured using either mean sea level (MSL) or local ground level (above ground level, or AGL) as the reference datum. Pressure altitude divided by 100 feet (30 m) is the flight level , and is used above the transition altitude (18,000 feet (5,500 m) in

2183-505: The definitive instrument for measuring altitude is the pressure altimeter , which is an aneroid barometer with a front face indicating distance (feet or metres) instead of atmospheric pressure . There are several types of altitude in aviation: These types of altitude can be explained more simply as various ways of measuring the altitude: The Earth's atmosphere is divided into several altitude regions. These regions start and finish at varying heights depending on season and distance from

2242-455: The denser part of the atmosphere and below the inner Van Allen radiation belt . They encounter atmospheric drag from gases in the thermosphere (approximately 80–600 km above the surface) or exosphere (approximately 600 km or 400 mi and higher), depending on orbit height. Satellites in orbits that reach altitudes below 300 km (190 mi) decay quickly due to atmospheric drag. Equatorial low Earth orbits ( ELEO ) are

2301-558: The direction of the velocity the specific orbital energy gained per unit delta- v is equal to the instantaneous speed. This is called the Oberth effect. For example, a satellite in an elliptical orbit is boosted more efficiently at high speed (that is, small altitude) than at low speed (that is, high altitude). Another example is that when a vehicle is making a pass of a planet, burning the propellant at closest approach rather than further out gives significantly higher final speed, and this

2360-402: The exact altitude of the orbit. Calculated for a circular orbit of 200 km (120 mi) the orbital velocity is 7.79 km/s (4.84 mi/s), but for a higher 1,500 km (930 mi) orbit the velocity is reduced to 7.12 km/s (4.42 mi/s). The launch vehicle's delta-v needed to achieve low Earth orbit starts around 9.4 km/s (5.8 mi/s). The pull of gravity in LEO

2419-647: The first generation of Starlink satellites used polar orbits which provide coverage everywhere on Earth. Later Starlink constellations orbit at a lower inclination and provide more coverage for populated areas. Higher orbits include medium Earth orbit (MEO), sometimes called intermediate circular orbit (ICO), and further above, geostationary orbit (GEO). Orbits higher than low orbit can lead to early failure of electronic components due to intense radiation and charge accumulation. In 2017, " very low Earth orbits " ( VLEO ) began to be seen in regulatory filings. These orbits, below about 450 km (280 mi), require

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2478-403: The general case: if, for instance, a constant, unidirectional acceleration is reversed after ( t 1 − t 0 )/2 then the velocity difference is 0, but delta- v is the same as for the non-reversed thrust. For rockets, "absence of external forces" is taken to mean the absence of gravity drag and atmospheric drag, as well as the absence of aerostatic back pressure on the nozzle, and hence

2537-482: The integration variable from time t to the spacecraft mass m one gets Assuming v exh {\displaystyle v_{\text{exh}}\,} to be a constant not depending on the amount of fuel left this relation is integrated to which is the Tsiolkovsky rocket equation . If for example 20% of the launch mass is fuel giving a constant v exh {\displaystyle v_{\text{exh}}} of 2100 m/s (a typical value for

2596-563: The lapse rate from the dry adiabatic lapse rate to the moist adiabatic lapse rate (5.5 °C per kilometer or 3 °F [1.7 °C] per 1000 feet). As an average, the International Civil Aviation Organization (ICAO) defines an international standard atmosphere (ISA) with a temperature lapse rate of 6.49 °C per kilometer (3.56 °F per 1,000 feet). The actual lapse rate can vary by altitude and by location. Finally, only

2655-447: The latitude at the launch site, to take advantage of the Earth's rotational surface speed. If it is necessary, for mission-based reasons, to put the spacecraft in an orbit of different inclination , a substantial delta- v is required, though the specific kinetic and potential energies in the final orbit and the initial orbit are equal. When rocket thrust is applied in short bursts the other sources of acceleration may be negligible, and

2714-907: The letter "A". Athletes also can take advantage of altitude acclimatization to increase their performance. The same changes that help the body cope with high altitude increase performance back at sea level. These changes are the basis of altitude training which forms an integral part of the training of athletes in a number of endurance sports including track and field, distance running, triathlon, cycling and swimming. Decreased oxygen availability and decreased temperature make life at high altitude challenging. Despite these environmental conditions, many species have been successfully adapted at high altitudes . Animals have developed physiological adaptations to enhance oxygen uptake and delivery to tissues which can be used to sustain metabolism. The strategies used by animals to adapt to high altitude depend on their morphology and phylogeny . For example, small mammals face

2773-487: The magnitude of the velocity change of one burst may be simply approximated by the delta- v . The total delta- v to be applied can then simply be found by addition of each of the delta- v' s needed at the discrete burns, even though between bursts the magnitude and direction of the velocity changes due to gravity, e.g. in an elliptic orbit . For examples of calculating delta- v , see Hohmann transfer orbit , gravitational slingshot , and Interplanetary Transport Network . It

2832-488: The maneuver as a shift from one Kepler orbit to another by an instantaneous change of the velocity vector. This approximation with impulsive maneuvers is in most cases very accurate, at least when chemical propulsion is used. For low thrust systems, typically electrical propulsion systems, this approximation is less accurate. But even for geostationary spacecraft using electrical propulsion for out-of-plane control with thruster burn periods extending over several hours around

2891-721: The mass ratios of the maneuvers, and v 1 , v 2 are the delta- v of the first and second maneuvers m 1 m 2 = e V 1 / V e e V 2 / V e = e V 1 + V 2 V e = e V / V e = M {\displaystyle {\begin{aligned}m_{1}m_{2}&=e^{V_{1}/V_{e}}e^{V_{2}/V_{e}}\\&=e^{\frac {V_{1}+V_{2}}{V_{e}}}\\&=e^{V/V_{e}}=M\end{aligned}}} where V = v 1 + v 2 and M = m 1 m 2 . This

2950-452: The mean radius of Earth, which is consistent with some of the upper altitude limits in some LEO definitions. The LEO region is defined by some sources as a region in space that LEO orbits occupy. Some highly elliptical orbits may pass through the LEO region near their lowest altitude (or perigee ) but are not in a LEO orbit because their highest altitude (or apogee ) exceeds 2,000 km (1,243 mi). Sub-orbital objects can also reach

3009-533: The mission is within the capabilities of the vehicle to be employed. Delta- v needed for various orbital manoeuvers using conventional rockets; red arrows show where optional aerobraking can be performed in that particular direction, black numbers give delta- v in km/s that apply in either direction. Lower-delta- v transfers than shown can often be achieved, but involve rare transfer windows or take significantly longer, see: Orbital mechanics § Interplanetary Transport Network and fuzzy orbits . For example

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3068-439: The nodes this approximation is fair. Delta- v is typically provided by the thrust of a rocket engine , but can be created by other engines. The time-rate of change of delta- v is the magnitude of the acceleration caused by the engines , i.e., the thrust per total vehicle mass. The actual acceleration vector would be found by adding thrust per mass on to the gravity vector and the vectors representing any other forces acting on

3127-399: The object. The total delta- v needed is a good starting point for early design decisions since consideration of the added complexities are deferred to later times in the design process. The rocket equation shows that the required amount of propellant dramatically increases with increasing delta- v . Therefore, in modern spacecraft propulsion systems considerable study is put into reducing

3186-571: The poles. The altitudes stated below are averages: The Kármán line , at an altitude of 100 kilometres (62 mi) above sea level , by convention defines represents the demarcation between the atmosphere and space . The thermosphere and exosphere (along with the higher parts of the mesosphere) are regions of the atmosphere that are conventionally defined as space. Regions on the Earth 's surface (or in its atmosphere) that are high above mean sea level are referred to as high altitude . High altitude

3245-630: The reduction in atmospheric pressure signifies less atmospheric resistance, which generally results in improved athletic performance. For endurance events (races of 5,000 metres or more) the predominant effect is the reduction in oxygen which generally reduces the athlete's performance at high altitude. Sports organizations acknowledge the effects of altitude on performance: the International Association of Athletic Federations (IAAF), for example, marks record performances achieved at an altitude greater than 1,000 metres (3,300 ft) with

3304-435: The spacecraft and the orbit can easily be propagated with a numerical algorithm including also this thruster force. But for many purposes, typically for studies or for maneuver optimization, they are approximated by impulsive maneuvers as illustrated in figure 1 with a Δ v {\displaystyle \Delta {v}} as given by ( 4 ). Like this one can for example use a "patched conics" approach modeling

3363-492: The spacecraft caused by this force will be where m is the mass of the spacecraft During the burn the mass of the spacecraft will decrease due to use of fuel, the time derivative of the mass being If now the direction of the force, i.e. the direction of the nozzle , is fixed during the burn one gets the velocity increase from the thruster force of a burn starting at time t 0 {\displaystyle t_{0}\,} and ending at t 1 as Changing

3422-483: The total delta- v needed for a given spaceflight, as well as designing spacecraft that are capable of producing larger delta- v . Increasing the delta- v provided by a propulsion system can be achieved by: Because the mass ratios apply to any given burn, when multiple maneuvers are performed in sequence, the mass ratios multiply. Thus it can be shown that, provided the exhaust velocity is fixed, this means that delta- v can be summed: When m 1 , m 2 are

3481-410: The use of novel technologies for orbit raising because they operate in orbits that would ordinarily decay too soon to be economically useful. A low Earth orbit requires the lowest amount of energy for satellite placement. It provides high bandwidth and low communication latency . Satellites and space stations in LEO are more accessible for crew and servicing. Since it requires less energy to place

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