Robert H. Goddard
Robert Hutchings Goddard was an American engineer, professor and inventor, credited with creating and building the world's first liquid-fueled rocket. Goddard launched his model on March 16, 1926, ushering in an era of space flight and innovation, he and his team launched 34 rockets between 1926 and 1941, achieving altitudes as high as 2.6 km and speeds as fast as 885 km/h. Goddard's work as both theorist and engineer anticipated many of the developments that were to make spaceflight possible, he has been called the man. Two of Goddard's 214 patented inventions—a multi-stage rocket, a liquid-fuel rocket —were important milestones toward spaceflight, his 1919 monograph A Method of Reaching Extreme Altitudes is considered one of the classic texts of 20th-century rocket science. Goddard applied three-axis control and steerable thrust to rockets to control their flight. Although his work in the field was revolutionary, Goddard received little public support for his research and development work; the press sometimes ridiculed his theories of spaceflight.
As a result, he became protective of his work. Years after his death, at the dawn of the Space Age, he came to be recognized as one of the founding fathers of modern rocketry, along with Robert Esnault-Pelterie, Konstantin Tsiolkovsky, Hermann Oberth, he not only recognized the potential of rockets for atmospheric research, ballistic missiles and space travel but was the first to scientifically study and construct the rockets needed to implement those ideas. NASA's Goddard Space Flight Center was named in Goddard's honor in 1959. Goddard was born in Worcester, Massachusetts, to Nahum Danford Goddard, a farmer, Fannie Louise Hoyt. Robert was their only child to survive. Goddard's family had roots in New England dating to the late 1600s. Shortly after his birth, the family moved to Boston. With a curiosity about nature, he studied the heavens using a telescope from his father and observed the birds flying. A country boy, he loved the outdoors and hiking with his father on trips to Worcester and became an excellent marksman with a rifle.
In 1898, his mother contracted. On Sundays, the family attended the Episcopal church, Robert sang in the choir. With the electrification of American cities in the 1880s, the young Goddard became interested in science—specifically and technology; when his father showed him how to generate static electricity on the family's carpet, the five-year-old's imagination was sparked. Robert experimented, believing he could jump higher if the zinc from a battery could be charged by scuffing his feet on the gravel walk. But, holding the zinc, he could jump no higher than usual. Goddard halted the experiments after a warning from his mother that if he succeeded, he could "go sailing away and might not be able to come back." He created a cloud of smoke and an explosion in the house. Goddard's father further encouraged Robert's scientific interest by providing him with a telescope, a microscope, a subscription to Scientific American. Robert developed a fascination with flight, first with kites and with balloons.
He became a thorough diarist and documenter of his work—a skill that would benefit his career. These interests merged at age 16, when Goddard attempted to construct a balloon out of aluminum, shaping the raw metal in his home workshop, filling it with hydrogen. After nearly five weeks of methodical, documented efforts, he abandoned the project, remarking, "... balloon will not go up.... Aluminum is too heavy. Failior crowns enterprise." However, the lesson of this failure did not restrain Goddard's growing determination and confidence in his work. He became interested in space when he read H. G. Wells' science fiction classic The War of the Worlds at 16 years old, his dedication to pursuing space flight became fixed on October 19, 1899. The 17-year-old Goddard climbed a cherry tree to cut off dead limbs, he was transfixed by the sky, his imagination grew. He wrote: On this day I climbed a tall cherry tree at the back of the barn... and as I looked toward the fields at the east, I imagined how wonderful it would be to make some device which had the possibility of ascending to Mars, how it would look on a small scale, if sent up from the meadow at my feet.
I have several photographs of the tree, taken since, with the little ladder I made to climb it, leaning against it. It seemed to me that a weight whirling around a horizontal shaft, moving more above than below, could furnish lift by virtue of the greater centrifugal force at the top of the path. I was a different boy. Existence at last seemed purposive. For the rest of his life, he observed October 19 as "Anniversary Day", a private commemoration of the day of his greatest inspiration; the young Goddard was a thin and frail boy always in fragile health. He suffered from stomach problems, pleurisy and bronchitis, fell two years behind his classmates, he became a voracious reader visiting the local public library to borrow books on the physical sciences. Goddard's interest in aerodynamics led him to study some of Samuel Langley's scientific papers in the periodical Smithsonian. In these papers, Langley wrote that birds flap their wings with different force on each side to turn in the air. Inspired by these articles, the teenage Goddard watched swallows and chimney swifts
Sergei Pavlovich Korolev (Russian: Серге́й Па́влович Королёв, IPA: transliterated as Sergey Pavlovich Korolyov, Ukrainian: Сергій Павлович Корольов / Serhiy Pavlovych Korolyov. He is regarded by many as the father of practical astronautics, he was involved in the development of the R-7 Rocket, Sputnik 1, launching Laika and the first human being into space. Although Korolev trained as an aircraft designer, his greatest strengths proved to be in design integration and strategic planning. Arrested on a false official charge as a "member of an anti-Soviet counter-revolutionary organization", he was imprisoned in 1938 for six years, including some months in a Kolyma labour camp. Following his release he became a recognized rocket designer and a key figure in the development of the Soviet Intercontinental ballistic missile program, he directed the Soviet space program and was made a Member of Soviet Academy of Sciences, overseeing the early successes of the Sputnik and Vostok projects including the first human Earth orbit mission by Yuri Alexeyvich Gagarin on 12 April 1961.
Korolev's unexpected death in 1966 interrupted implementation of his plans for a Soviet manned Moon landing before the United States 1969 mission. Before his death he was identified only as Glavny Konstruktor, or the Chief Designer, to protect him from possible cold war assassination attempts by the United States; some of the cosmonauts who worked with him were unaware of his last name. Only following his death in 1966 was his identity revealed and he received the appropriate public recognition as the driving force behind Soviet accomplishments in space exploration during and following the International Geophysical Year. Korolev was born in Zhytomyr, the capital of Volhynian Governorate of the Russian Empire now located in Ukraine, his father, Pavel Yakovlevich Korolev, was born in Mogilev to a Russian soldier and a Belarusian mother. His mother, Maria Nikolaevna Koroleva, was a daughter of a wealthy merchant in the Ukrainian city of Nizhyn with Cossack heritage. On his maternal side, in addition to Ukrainian Cossacks, he had Greek and Polish ancestry.
His father moved to Zhytomyr to be a teacher of the Russian language. Three years after his birth the couple separated due to financial difficulties. Although Pavel wrote to Maria requesting a meeting with his son, Sergei was informed by his mother that his father had died. Sergei never saw his father after the family break-up, Pavel died in 1929 before his son learned the truth. Korolev grew up in Nizhyn, under the care of his maternal grandparents Mykola Yakovych Moskalenko, a trader of the Second Guild and Maria Matviivna Moskalenko, a daughter of a local cossack. Korolev's mother had a sister Anna and two brothers Yuri and Vasily. Maria Koroleva was away attending Women's higher education courses in Kiev; as a child, Korolev was stubborn and argumentative. Sergei grew up a lonely child with few friends. Korolev began reading at an early age, his abilities in mathematics and other subjects made him a favorite student of his teachers, but caused jealousy from his peers, he stated in an interview, the torment of classmates bullying and teasing him as a small child encouraged his focus on academic work.
His mother divorced Pavel in 1915 and in 1916 married Grigory Mikhailovich Balanin, an electrical engineer educated in Germany but attending the Kiev Polytechnic University because German engineering diplomas were unrecognized in Russia. After getting a job with the regional railway, Grigory moved the family to Odessa in 1917, where they endured hardships with many other families through the tumultuous years following the Russian Revolution and continuing internecine struggles until the Bolsheviks assumed unchallenged power in 1920. Local schools were closed and young Korolev had to continue his studies at home. Grigory proved a good influence on his step-son, who suffered from a bout of typhus during the severe food shortages of 1919. Korolev received vocational training in carpentry and in various academics at the Odessa Building Trades School. Enjoyment of a 1913 air show inspired interest in aeronautical engineering. Korolev began designing a glider as a diversion while studying for his graduation exams at the vocational school.
He made an independent study of flight theory, worked in the local glider club. A detachment of military seaplanes had been stationed in Odessa, Korolev took a keen interest in their operations. In 1923 he joined Aerial Navigation of Ukraine and the Crimea, he had his first flying lesson after joining the Odessa hydroplane squadron and had many opportunities to fly as a passenger. In 1924 he designed an OAVUK construction project glider called the K-5, he trained in gymnastics until his academic work suffered from this distraction. Korolev hoped to attend the Zhukovsky Academy in Moscow, but his qualifications did not meet the academy's standards, he attended the Kiev Polytechnic Institute's aviation branch in 1924 while living with his uncle Yuri, earning money to pay for his courses by performing odd jobs. His curriculum was technically oriented, included various engineering and mathematics classes, he met and became attracted to a classmate, Xenia Vincentini, who would become his first wife.
In 1925 he was accepted into a
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the time varying conditions within the Solar System, including the solar wind, emphasizing the space surrounding the Earth, including conditions in the magnetosphere, ionosphere and exosphere. Space weather is distinct from but conceptually related to the terrestrial weather of the atmosphere of Earth; the term space weather was first used in the 1950s and came into common usage in the 1990s.. For many centuries, the effects of space weather were noticed but not understood. Displays of auroral light have long been observed at high latitudes. In 1724, George Graham reported that the needle of a magnetic compass was deflected from magnetic north over the course of each day; this effect was attributed to overhead electric currents flowing in the ionosphere and magnetosphere by Balfour Stewart in 1882, confirmed by Arthur Schuster in 1889 from analysis of magnetic observatory data. In 1852, astronomer and British major general Edward Sabine showed that the probability of the occurrence of magnetic storms on Earth was correlated with the number of sunspots, thus demonstrating a novel solar-terrestrial interaction.
In 1859, a great magnetic storm caused brilliant auroral displays and disrupted global telegraph operations. Richard Carrington connected the storm with a solar flare that he had observed the day before in the vicinity of a large sunspot group—thus demonstrating that specific solar events could affect the Earth. Kristian Birkeland explained the physics of aurora by creating artificial aurora in his laboratory and predicted the solar wind; the introduction of radio revealed that periods of extreme noise occurred. Severe radar jamming during a large solar event in 1942 led to the discovery of solar radio bursts, another aspect of space weather. In the 20th century the interest in space weather expanded as military and commercial systems came to depend on systems affected by space weather. Communications satellites are a vital part of global commerce. Weather satellite systems provide information about terrestrial weather; the signals from satellites of the Global Positioning System are used in a wide variety of applications.
Space weather phenomena can interfere with or damage these satellites or interfere with the radio signals with which they operate. Space weather phenomena can cause damaging surges in long distance transmission lines and expose passengers and crew of aircraft travel to radiation on polar routes; the International Geophysical Year increased research into space weather. Ground-based data obtained during IGY demonstrated that the aurora occurred in an auroral oval, a permanent region of luminescence 15 to 25 degrees in latitude from the magnetic poles and 5 to 20 degrees wide. In 1958, the Explorer I satellite discovered the Van Allen belts, regions of radiation particles trapped by the Earth's magnetic field. In January 1959, the Soviet satellite Luna 1 first directly observed the solar wind and measured its strength. A smaller International Heliophysical Year occurred in 2007-2008. In 1969, INJUN-5 made the first direct observation of the electric field impressed on the Earth's high latitude ionosphere by the solar wind.
In the early 1970s, Triad data demonstrated that permanent electric currents flowed between the auroral oval and the magnetosphere. The term space weather came into usage in the late 1950s as the space age began and satellites began to measure the space environment; the term regained popularity in the 1990s along with the belief that space's impact on human systems demanded a more coordinated research and application framework. The purpose of the US National Space Weather Program is to focus research on the needs of the affected commercial and military communities, to connect the research and user communities, to create coordination between operational data centers and to better define user community needs; the concept was turned into an action plan in 2000, an implementation plan in 2002, an assessment in 2006 and a revised strategic plan in 2010. A revised action plan was scheduled to be released in 2011 followed by a revised implementation plan in 2012. One part of the National Space Weather Program is to show users that space weather affects their business.
Private companies now acknowledge space weather "is a real risk for today's businesses". Within the solar system, space weather is influenced by the solar wind and the interplanetary magnetic field carried by the solar wind plasma. A variety of physical phenomena are associated with space weather, including geomagnetic storms and substorms, energization of the Van Allen radiation belts, ionospheric disturbances and scintillation of satellite-to-ground radio signals and long-range radar signals and geomagnetically induced currents at Earth's surface. Coronal mass ejections, their associated shock waves and coronal clouds are important drivers of space weather as they can compress the magnetosphere and trigger geomagnetic storms. Solar energetic particles accelerated by coronal mass ejections or solar flares can trigger solar particle events, a critical driver of human impact space weather as they can damage electronics onboard spacecraft, threaten the lives of astronauts as well as increase radiation hazards to high-altitude, high-latitude aviation.
Some spacecraft failures can be directly attributed to space weather. For example, 46 of the 70 failures reported in 2003 occurred during the October 2003 geomagnetic storm; the two most common adverse space weather effects
A launch vehicle or carrier rocket is a rocket used to carry a payload from Earth's surface through outer space, either to another surface point, or into space. A launch system includes the launch vehicle, launch pad, vehicle assembly and fuelling systems, range safety, other related infrastructure. Suborbital launch vehicles include ballistic missiles, sounding rockets, various crewed systems designed for space tourism or high-speed transport. Orbital or escape launch vehicles must be much more powerful and incorporate two to four rocket stages to provide sufficient delta-v performance. Various rocket fuels are used, including solid rocket boosters and cryogenic fuels fed to rocket engines. Most launch vehicles are expendable i.e. used only once and destroyed or abandoned during the flight. Attempts to reduce per-launch costs have led to reusable launch systems, in which part of the launch vehicle is recovered and reused for another flight. Multiple classes of launch vehicle exist for use with differing launch sites, payload mass, target orbits, price points, etc.
Numerous countries have sought to develop indigenous launch vehicles for use in national space programs. Expendable launch vehicles are designed for one-time use, they separate from their payload and disintegrate during atmospheric reentry. In contrast, reusable launch vehicles are designed to be launched again; the Space Shuttle was a part of a launch vehicle with components used for multiple orbital spaceflights. SpaceX has developed a reusable rocket launching system to bring back a part—the first stage—of their Falcon 9 and launch it again, With B1046 having flown a total of three flights making it the most flown orbital class booster, Falcon Heavy launch vehicles. A reusable VTVL design is planned for all parts of the ITS launch vehicle; the low-altitude flight test program of an experimental technology-demonstrator launch vehicle began in 2012, with more extensive high-altitude over-water flight testing planned to begin in mid-2013, continue on each subsequent Falcon 9 flight. Non-rocket spacelaunch alternatives are progressing.
In June 2017, Stratolaunch Systems began ground testing the carrier aircraft component of its air launch to orbit system. The Stratolaunch is the world's largest aircraft, weighing 500,000 pounds and composed of twin fuselages with an overall wingspan of 385 feet; the Spanish company Zero 2 Infinity is developing another launch system concept, the Bloostar, a balloon-borne launcher based on rockoon technology. Launch vehicles are classified by the amount of mass they can carry into a particular orbit. For example, a Proton rocket can lift 22,000 kilograms into low Earth orbit. Launch vehicles are characterized by their number of stages. Rockets with as many as five stages have been launched, there have been designs for several single-stage-to-orbit vehicles. Additionally, launch vehicles are often supplied with boosters supplying high early thrust burning with other engines. Boosters allow the remaining engines to be smaller, reducing the burnout mass of stages to allow larger payloads. Other reported characteristics of launch vehicles are the launching nation or space agency and the company or consortium manufacturing and launching the vehicle.
For example, the European Space Agency is responsible for the Ariane V, the United Launch Alliance manufactures and launches the Delta IV and Atlas V rockets. Many launch vehicles are considered part of a historical line of vehicles of the same or similar name. Land: spaceport and fixed missile silo for converted ICBMs Sea: fixed platform, mobile platform, submarine for converted SLBMs Air: aircraft, balloon, JP Aerospace Orbital Ascender, proposal for permanent Buoyant space port. There are many ways to classify the sizes of launch vehicles; the US civilian space agency, NASA, uses a classification scheme, articulated by the Augustine Commission created to review plans for replacing the Space Shuttle: A sounding rocket, used to study the atmosphere or perform brief experiments, is only capable of sub-orbital spaceflight and cannot reach orbit. A small-lift launch vehicle is capable of lifting up to 2,000 kg of payload into low Earth orbit. A medium-lift launch vehicle is capable of lifting 2,000 to 20,000 kg of payload into LEO.
A heavy-lift launch vehicle is capable of lifting 20,000 to 50,000 kg of payload into LEO. A super-heavy lift vehicle is capable of lifting more than 50,000 kg of payload into LEO; the leading European launch service provider, Arianespace uses the "heavy-lift" designation for its >20,000 kg -to-LEO Ariane 5 launch vehicle and "medium-lift" for its array of launch vehicles that lift 2,000 to 20,000 kg to LEO, including the Starsem/Arianespace Soyuz ST and pre-1999 versions of the Ariane 5. It refers to its 1,500 kg to LEO Vega launch vehicle as "light lift". Suborbital launch vehicles are not capable of taking their payloads to the minimum horizontal speed necessary to achieve low Earth orbit with a perigee less than the Earth's mean radius, which speed is about 7,800 m/s. Sounding rockets have long been used for brief, inexpensive unmanne
In mathematics, an equation is a statement that asserts the equality of two expressions. The word equation and its cognates in other languages may have subtly different meanings. Solving an equation containing variables consists of determining which values of the variables make the equality true. Variables are called unknowns and the values of the unknowns that satisfy the equality are called solutions of the equation. There are two kinds of equations: conditional equations. An identity is true for all values of the variable. A conditional equation is only true for particular values of the variables. An equation is written as two expressions, connected by a equals sign; the expressions on the two sides of the equals sign are called the "left-hand side" and "right-hand side" of the equation. The most common type of equation is an algebraic equation, in which the two sides are algebraic expressions; each side of an algebraic equation will contain one or more terms. For example, the equation A x 2 + B x + C = y has left-hand side A x 2 + B x + C, which has three terms, right-hand side y, consisting of just one term.
The unknowns are x and y and the parameters are A, B, C. An equation is analogous to a scale; when equal weights of something are placed into the two pans, the two weights cause the scale to be in balance and are said to be equal. If a quantity of grain is removed from one pan of the balance, an equal amount of grain must be removed from the other pan to keep the scale in balance. To keep an equation in balance, the same operations of addition, subtraction and division must be performed on both sides of an equation for it to remain true. In geometry, equations are used to describe geometric figures; as the equations that are considered, such as implicit equations or parametric equations, have infinitely many solutions, the objective is now different: instead of giving the solutions explicitly or counting them, impossible, one uses equations for studying properties of figures. This is the starting idea of an important area of mathematics. Algebra studies two main families of equations: polynomial equations and, among them, the special case of linear equations.
When there is only one variable, polynomial equations have the form P = 0, where P is a polynomial, linear equations have the form ax + b = 0, where a and b are parameters. To solve equations from either family, one uses algorithmic or geometric techniques that originate from linear algebra or mathematical analysis. Algebra studies Diophantine equations where the coefficients and solutions are integers; the techniques used are different and come from number theory. These equations are difficult in general. Differential equations are equations that involve their derivatives, they are solved by finding an expression for the function. Differential equations are used to model processes that involve the rates of change of the variable, are used in areas such as physics, chemistry and economics; the "=" symbol, which appears in every equation, was invented in 1557 by Robert Recorde, who considered that nothing could be more equal than parallel straight lines with the same length. An equation is analogous to balance, or seesaw.
Each side of the equation corresponds to one side of the balance. Different quantities can be placed on each side: if the weights on the two sides are equal, the scale balances, in analogy the equality that represents the balance is balanced. In the illustration, x, y and z are all different quantities represented as circular weights, each of x, y, z has a different weight. Addition corresponds to adding weight, while subtraction corresponds to removing weight from what is there; when equality holds, the total weight on each side is the same. Equations contain terms other than the unknowns; these other terms, which are assumed to be known, are called constants, coefficients or parameters. An example of an equation involving x and y as unknowns and the parameter R is x 2 + y 2 = R 2; when R is chosen to have the value of 2, this equation would be recognized, when sketched in Cartesian coordinates, as the equation for a particular circle with a radius of 2. Hence, the equation with R unspecified is the general equation for the circle.
The unknowns are denoted by letters at the end of the alphabet, x, y, z, w, …, while coefficients are denoted by letters at the beginning, a, b, c, d, …. For example, the general quadratic equation is written ax2 + bx + c = 0; the process of finding the solutions, or, in case of parameters, expressing the unknowns in terms of the parameters is called solving the equation. Such expressions of the solutions in terms of the parameters are called solutions. A system of equations is a set of simultaneous equations in several unknowns, for which the common solutions are sought, thus a solution to the
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