Low Earth orbit
A Low Earth Orbit is an Earth-centered orbit with an altitude of 2,000 km or less, or with at least 11.25 periods per day and an eccentricity less than 0.25. Most of the manmade objects in space are in LEO. A histogram of the mean motion of the cataloged objects shows that the number of objects drops beyond 11.25. There is a large variety of other sources; the altitude of an object in an elliptic orbit can vary along the orbit. For circular orbits, the altitude above ground can vary by as much as 30 km due to the oblateness of Earth's spheroid figure and local topography. While definitions in terms of 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. For circular orbits, this in turn corresponds to an altitude of 2,042 km above the mean radius of Earth, consistent with some of the upper limits in the LEO definitions in terms of altitude; the LEO region is defined by some sources as the region in space.
Some elliptical orbits may pass through the LEO region near their lowest altitude but are not in an LEO Orbit because their highest altitude exceeds 2,000 km. Sub-orbital objects can reach the LEO region but are not in an LEO orbit because they re-enter the atmosphere; the distinction between LEO orbits and the LEO region is 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 International Space Station conducts operations in LEO. All crewed space stations to date, as well as the majority of satellites, have been in LEO; the altitude record for human spaceflights in LEO was Gemini 11 with an apogee of 1,374.1 km. Apollo 8 was the first mission to carry humans beyond LEO on December 21–27, 1968; the Apollo program continued during the four-year period spanning 1968 through 1972 with 24 astronauts who flew lunar flights but since there have been no human spaceflights beyond LEO. The mean orbital velocity needed to maintain a stable low Earth orbit is about 7.8 km/s, but reduces with increased orbital altitude.
Calculated for circular orbit of 200 km it is 7.79 km/s and for 1500 km it is 7.12 km/s. The delta-v needed to achieve low Earth orbit starts around 9.4 km/s. Atmospheric and gravity drag associated with launch adds 1.3–1.8 km/s to the launch vehicle delta-v required to reach normal LEO orbital velocity of around 7.8 km/s. The pull of gravity in LEO is only less than on the earth's surface; this is. However, an object in orbit is, in free fall, since there is no force holding it up; as a result objects in orbit, including people, experience a sense of weightlessness though they are not without weight. Objects in LEO encounter atmospheric drag from gases in the thermosphere or exosphere, depending on orbit height. Due to atmospheric drag, satellites do not orbit below 300 km. Objects in LEO orbit Earth between the denser part of the atmosphere and below the inner Van Allen radiation belt. Equatorial low Earth orbits are a subset of LEO; these orbits, with low inclination to the Equator, allow rapid revisit times and have the lowest delta-v requirement of any orbit.
Orbits with a high inclination angle to the equator are called polar orbits. Higher orbits include medium Earth orbit, sometimes called intermediate circular orbit, further above, geostationary orbit. Orbits higher than low orbit can lead to early failure of electronic components due to intense radiation and charge accumulation. In 2017, a very-low LEO orbit began to be seen in regulatory filings; this orbit, referred to as "VLEO", requires 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 low communication latency. Satellites and space stations in LEO are more accessible for servicing. Since it requires less energy to place a satellite into a LEO, 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, move at the same angular velocity as the Earth as to appear stationary above one location on the planet.
Satellites in LEO have a small momentary field of view, only able to observe and communicate with a fraction of the Earth at a time, meaning a network of satellites is required to in order to provide continuous coverage. Satellites in lower regions of LEO suffer from fast orbital decay, requiring either periodic reboosting to maintain a stable orbit, or launching replacement satellites when old ones re-enter. Earth observation satellites and spy satellites use LEO as they are able to see the surface of the Earth by being close to it, they are able to traverse the surface of the Earth. A majority of artificial satellites are placed in LEO, making one complete revolution around the Earth in about 90 minutes; the International Space Station is in a LEO about 330 km to 420 km above Earth's surfac
Venera 2 known as 3MV-4 No.4 was a Soviet spacecraft intended to explore Venus. A 3MV-4 spacecraft launched as part of the Venera programme, it failed to return data after flying past Venus. Venera 2 was launched by a Molniya carrier rocket; the launch occurred at 05:02 UTC on 12 November 1965, with the first three stages placing the spacecraft and Blok-L upper stage into a low Earth parking orbit before the Blok-L fired to propel Venera 2 into heliocentric orbit bound for Venus, with perihelion of 0.716 AU, aphelion of 1.197 AU, eccentricity of 0.252, inclination of 4.29 degrees and orbital period of 341 days. The Venera 2 spacecraft was equipped with cameras, as well as a magnetometer and cosmic x-ray detectors, piezoelectric detectors, ion traps, a Geiger counter and receivers to measure cosmic radio emissions; the spacecraft made its closest approach to Venus at 02:52 UTC on 27 February 1966, at a distance of 23,810 kilometres. During the flyby, all of Venera 2's instruments were activated, requiring that radio contact with the spacecraft be suspended.
The probe was to have stored data using onboard recorders, transmitted it to Earth once contact was restored. Following the flyby the spacecraft failed to reestablish communications with the ground, it was declared lost on 4 March. An investigation into the failure determined that the spacecraft had overheated due to a radiator malfunction. List of missions to Venus
The Venera 11 was a Soviet unmanned space mission part of the Venera program to explore the planet Venus. Venera 11 was launched on 9 September 1978 at 03:25:39 UTC. Separating from its flight platform on December 23, 1978 the lander entered the Venus atmosphere two days on December 25 at 11.2 km/s. During the descent, it employed aerodynamic braking followed by parachute braking and ending with atmospheric braking, it made a soft landing on the surface at 06:24 Moscow time on 25 December after a descent time of 1 hour. The touchdown speed was 7 to 8 m/s. Information was transmitted to the flight platform for retransmittal to earth until it moved out of range 95 minutes after touchdown. Landing coordinates are 14°S 299°E. After ejection of the lander probe, the flight platform continued on past Venus in a heliocentric orbit. Near encounter with Venus occurred on December 25, 1978, at 35,000 km altitude; the flight platform acted as a data relay for the descent craft for 95 minutes until it flew out of range and returned its own measurements on interplanetary space.
Venera 11 flight platform carried solar wind detectors, ionosphere electron instruments and two gamma ray burst detectors – the Soviet-built KONUS and the French-built SIGNE 2. The SIGNE 2 detectors were flown on Venera 12 and Prognoz 7 to allow triangulation of gamma ray sources. Before and after Venus flyby, Venera 11 and Venera 12 yielded detailed time-profiles for 143 gamma-ray bursts, resulting in the first catalog of such events; the last gamma-ray burst reported by Venera 11 occurred on January 27, 1980 List of flight platform instruments and experiments: 30–166 nm Extreme UV spectrometer Compound plasma spectrometer KONUS Gamma-ray burst detector SNEG Gamma-ray Burst detector Magnetometer 4 Semiconductor counters 2 Gas-discharge counters 4 Scintillation counters Hemispherical proton telescopeThe mission ended in February, 1980. Venera 11 is in heliocentric orbit, with perihelion of 0.69 AU, aphelion of 1.01 AU, eccentricity of 0.19, inclination of 2.3 degrees and orbital period of 284 days.
The lander carried instruments to study the temperature and atmospheric and soil chemical composition. A device called. Both Venera 11 and Venera 12 had landers with two cameras, each designed for color imaging, though Soviet literature does not mention them; each failed to return images when the lens covers did not separate after landing due to a design flaw. The soil analyzer failed. A gas chromatograph was on board to measure the composition of the Venus atmosphere, as well as instruments to study scattered solar radiation. Results reported included evidence of lightning and thunder, a high Ar36/Ar40 ratio, the discovery of carbon monoxide at low altitudes. List of lander experiments and instruments: List of missions to Venus Timeline of artificial satellites and space probes Venera 11 & Venera 12 Experiments on Venera 11 Has detail on each experiment/instrument. Drilling into the Surface of Venus
Venera 15 was a spacecraft sent to Venus by the Soviet Union. This unmanned orbiter was to map the surface of Venus using high resolution imaging systems; the spacecraft was identical to Venera 16 and based on modifications to the earlier Venera space probes. Venera 15 was launched on June 2, 1983 at 02:38:39 UTC and reached Venus' orbit on October 10, 1983; the spacecraft was inserted into Venus orbit a day apart from Venera 16, with its orbital plane shifted by an angle of 4° relative to the other probe. This made it possible to reimage an area; the spacecraft was in a nearly polar orbit with a periapsis ~1000 km, at 62°N latitude, apoapsis ~65000 km, with an inclination ~90°, the orbital period being ~24 hours. Together with Venera 16, the spacecraft imaged the area from the north pole down to about 30°N latitude over the 8 months of mapping operations; the Venera 15 and 16 spacecraft were identical and were based on modifications to the orbiter portions of the Venera 9 and Venera 14 probes.
Each spacecraft consisted of a 5 m long cylinder with a 0.6 m diameter, 1.4 m tall parabolic dish antenna for the synthetic aperture radar at one end. A 1-meter diameter parabolic dish antenna for the radio altimeter was located at this end; the electrical axis of the radio altimeter antenna was lined up with the axis of the cylinder. The electrical axis of the SAR deviated from the spacecraft axis by 10 degrees. During imaging, the radio altimeter would be lined up with the center of the planet and the SAR would be looking off to the side at 10 degrees. A bulge at the opposite end of the cylinder held fuel tanks and propulsion units. Two square solar arrays extended like wings from the sides of the cylinder. A 2.6 m radio dish antenna for communications was attached to the side of the cylinder. The spacecraft each massed 4,000 kg. Both Venera 15 and 16 were equipped with a Synthetic Aperture Radar. A radar was necessary in this mission because nothing else would be able to penetrate the dense clouds of Venus.
The probes were equipped with on board computers that saved the images until the entire image was complete. This radar system replaced the normal landers. List of spacecraft instruments and experiments: Polyus-V Synthetic Aperture Radar Omega Radar altimeter Infrared Fourier Spectrometer Cosmic-Ray Detectors Solar-Plasma DetectorsTo get to Venus, Venera 15 was placed in a heliocentric orbit with perihelion of 0.71 AU, apohelion of 1.01 AU, eccentricity of 0.17, orbital inclination of 2.3 degrees and orbital period of 293 days. List of missions to Venus The Soviet Exploration of Venus Catalog of Soviet Venus images Venera 15 Venera 15/16 Radar Mosaic Browser
The term apsis refers to an extreme point in the orbit of an object. It denotes either the respective distance of the bodies; the word comes via Latin from Greek, there denoting a whole orbit, is cognate with apse. Except for the theoretical possibility of one common circular orbit for two bodies of equal mass at diametral positions, there are two apsides for any elliptic orbit, named with the prefixes peri- and ap-/apo-, added in reference to the body being orbited. All periodic orbits are, according to Newton's Laws of motion, ellipses: either the two individual ellipses of both bodies, with the center of mass of this two-body system at the one common focus of the ellipses, or the orbital ellipses, with one body taken as fixed at one focus, the other body orbiting this focus. All these ellipses share a straight line, the line of apsides, that contains their major axes, the foci, the vertices, thus the periapsis and the apoapsis; the major axis of the orbital ellipse is the distance of the apsides, when taken as points on the orbit, or their sum, when taken as distances.
The major axes of the individual ellipses around the barycenter the contributions to the major axis of the orbital ellipses are inverse proportional to the masses of the bodies, i.e. a bigger mass implies a smaller axis/contribution. Only when one mass is sufficiently larger than the other, the individual ellipse of the smaller body around the barycenter comprises the individual ellipse of the larger body as shown in the second figure. For remarkable asymmetry, the barycenter of the two bodies may lie well within the bigger body, e.g. the Earth–Moon barycenter is about 75% of the way from Earth's center to its surface. If the smaller mass is negligible compared to the larger the orbital parameters are independent of the smaller mass. For general orbits, the terms periapsis and apoapsis are used. Pericenter and apocenter are equivalent alternatives, referring explicitly to the respective points on the orbits, whereas periapsis and apoapsis may refer to the smallest and largest distances of the orbiter and its host.
For a body orbiting the Sun, the point of least distance is the perihelion, the point of greatest distance is the aphelion. The terms become apastron when discussing orbits around other stars. For any satellite of Earth, including the Moon, the point of least distance is the perigee and greatest distance the apogee, from Ancient Greek Γῆ, "land" or "earth". For objects in lunar orbit, the point of least distance is sometimes called the pericynthion and the greatest distance the apocynthion. Perilune and apolune are used. In orbital mechanics, the apsides technically refer to the distance measured between the barycenters of the central body and orbiting body. However, in the case of a spacecraft, the terms are used to refer to the orbital altitude of the spacecraft above the surface of the central body; these formulae characterize the pericenter and apocenter of an orbit: Pericenter Maximum speed, v per = μ a, at minimum distance, r per = a. Apocenter Minimum speed, v ap = μ a, at maximum distance, r ap = a.
While, in accordance with Kepler's laws of planetary motion and the conservation of energy, these two quantities are constant for a given orbit: Specific relative angular momentum h = μ a Specific orbital energy ε = − μ 2 a where: a is the semi-major axis: a = r per + r ap 2 μ is the standard gravitational parameter e is the eccentricity, defined as e = r ap − r per r ap + r per = 1 − 2 r ap r per + 1 Note t
Kosmos 96, or 3MV-4 No.6, was a Soviet spacecraft intended to explore Venus. A 3MV-4 spacecraft launched as part of the Venera programme, Kosmos 96 was to have made a flyby of Venus; the 3MV-4 No.6 spacecraft was built for a mission to Mars, with launch scheduled for late 1964. After it was not launched by the end of its launch window, the spacecraft was repurposed, along with two other spacecraft which were launched as Venera 2 and Venera 3, to explore Venus. A Molniya carrier rocket was used to launch 3MV-4 No.6. The launch occurred from Site 31/6 at the Baikonur Cosmodrome at 03:21 UTC on 23 November 1965. Late in third stage flight, a fuel line ruptured, causing one of the engine's combustion chambers to explode; the rocket tumbled out of control, as a result the fourth stage, a Blok-L, failed to ignite. The spacecraft was deployed into a low Earth orbit with a perigee of 209 kilometres, an apogee of 261 kilometres, 51.9 degrees of inclination to the equator. The spacecraft was named Kosmos 96, part of a series used for military and experimental satellites in order to cover up the failure.
Had it departed Earth's orbit, it would have received the next designation in the Venera series, at the time Venera 4. Kosmos 96 was destroyed when it reentered the Earth's atmosphere on 9 December 1965, its reentry has been suggested as a possible explanation of UFO sightings over the United States and Canada, centred on Kecksburg, Pennsylvania. List of missions to Venus