My Science School

Satellites must be launched into orbit with enough speed to prevent Earth’s gravity from pulling them back down to the ground. Imagine throwing a ball horizontally. Gravity pulls the ball back to Earth very quickly. If the ball could be thrown hard enough, however, then it would have enough force to keep on travelling horizontally forever. It would be in orbit. A satellite at an altitude of 200km (120 miles) must be travelling at 7.8km/s (4.8mp/s) to prevent it being pulled back down to Earth.

An artificial satellite is a marvel of technology and engineering. The only thing comparable to the feat in technological terms is the scientific know-how that goes into placing, and keeping, one in orbit around the Earth. Just consider what scientists need to understand in order to make this happen: first, there’s gravity, then a comprehensive knowledge of physics, and of course the nature of orbits themselves. So really, the question of How Satellites Stay in Orbit is a multidisciplinary one that involves a great of technical and academic knowledge.

First, to understand how a satellite orbits the Earth, it is important to understand what orbit entails. Johann Kepler was the first to accurately describe the mathematical shape of the orbits of planets. Whereas the orbits of planets about the Sun and the Moon about the Earth were thought to be perfectly circular, Kepler stumbled onto the concept of elliptical orbits. In order for an object to stay in orbit around the Earth, it must have enough speed to retrace its path. This is as true of a natural satellite as it is of an artificial one. From Kepler’s discovery, scientists were also able to infer that the closer a satellite is to an object, the stronger the force of attraction, hence it must travel faster in order to maintain orbit.

Next comes an understanding of gravity itself. All objects possess a gravitational field, but it is only in the case of particularly large objects (i.e. planets) that this force is felt. In Earth’s case, the gravitational pull is calculated to 9.8 m/s2. However, that is a specific case at the surface of the planet. When calculating objects in orbit about the Earth, the formula v=(GM/R)1/2 applies, where v is velocity of the satellite, G is the gravitational constant, M is the mass of the planet, and R is the distance from the center of the Earth. Relying on this formula, we are able to see that the velocity required for orbit is equal to the square root of the distance from the object to the center of the Earth times the acceleration due to gravity at that distance. So if we wanted to put a satellite in a circular orbit at 500 km above the surface (what scientists would call a Low Earth Orbit LEO), it would need a speed of ((6.67 x 10-11 * 6.0 x 1024)/(6900000))1/2 or 7615.77 m/s. The greater the altitude, the less velocity is needed to maintain the orbit.

So really, a satellites ability to maintain its orbit comes down to a balance between two factors: its velocity (or the speed at which it would travel in a straight line), and the gravitational pull between the satellite and the planet it orbits. The higher the orbit, the less velocity is required. The nearer the orbit, the faster it must move to ensure that it does not fall back to Earth.

Any object in orbit around a celestial body is called a satellite. Earth has had its own natural satellite — the Moon — for billions of years. Since 1957, however, hundreds of artificial satellites have been launched into orbit around Earth, each transmitting a cacophony of radio signals to locations across the planet. Satellites are now vital to modern life and are used in many areas of technology, including communications, entertainment and espionage.

But usually when someone says "satellite," they are talking about a "man-made" satellite. Man-made satellites are machines made by people. These machines are launched into space and orbit Earth or another body in space. There are thousands of man-made satellites. Some take pictures of our planet. Some take pictures of other planets, the sun and other objects. These pictures help scientists learn about Earth, the solar system and the universe. Other satellites send TV signals and phone calls around the world.

Satellites fly high in the sky, so they can see large areas of Earth at one time. Satellites also have a clear view of space. That's because they fly above Earth's clouds and air. Before satellites, TV signals didn't go very far. TV signals only travel in straight lines. So they would go off into space instead of following Earth's curve. Sometimes they would be blocked by mountains or tall buildings.

Phone calls to faraway places were also a problem. It costs a lot and it is hard to set up telephone wires over long distances or underwater. With satellites, TV signals and phone calls can be sent up to a satellite. The satellite can then send them back down to different spots on Earth.

There are dozens upon dozens of natural satellites in the solar system, with almost every planet having at least one moon. Saturn, for example, has at least 53 natural satellites, and between 2004 and 2017, it also had an artificial one — the Cassini spacecraft, which explored the ringed planet and its moons.

Artificial satellites, however, did not become a reality until the mid-20th century. The first artificial was Sputnik, a Russian beach-ball-size space probe that lifted off on Oct. 4, 1957. That act shocked much of the western world, as it was believed the Soviets did not have the capability to send satellites into space.

Scientists have sent probes to investigate many kinds of celestial objects. In 1995, the Ulysses probe was launched towards the Sun and took readings of the solar wind and the star’s magnetism. The Giotto probe, launched in 1986, battled its way past flying debris and gas into the heart of Halley’s Comet, taking incredible pictures of its nucleus. Asteroids have also been visited by space probes. The Near Earth Asteroid Rendezvous probe landed on the asteroid Eros in 2001.

The 1985-1986 emergence of Halley's Comet, the first since the advent of the space age, was explored by a variety of spacecraft. The Vega 1, launched by the USSR together with the Eastern-block alliance, passed 5523 miles from the Comet's nucleus at 7:20:06 Universal time. It indicated that the Comet was about 300 miles closer to the sun than had been predicted. The Japanese spacecraft, Suisei, was created to map the distribution of neutral hydrogen atoms outside Halley’s visible coma. Its pictures indicated that the Comet's output of water varied between 25 and 60 tons per second. Five days after the Vega 2's passage through the Comet, the Giotto (sponsored by the European Space Agency) probe appeared. Giotto's close approach took place 3.1 minutes after midnight UT on March 14th; the craft had passed 376 miles from its target. Giotto's data indicated that the nucleus was bigger than expected, and that the Comet was composed primarily of water, CO2 and N2. The Vegas and Giotto found that as the solar wind approaches Halley, it slows gradually and the solar magnetic lines embedded in the wind begin to pile up. Pick-up ions, from the Comet's halo of neutral hydrogen, were found in this solar wind. Sensors on the Vega spacecraft found a variety of plasma waves propagating inside the bow wave. In order to synthesize all the results, a conference on the exploration of Halley's Comet will be held this October

Data on the nitrogen-containing compounds that observed spectroscopically in the coma of Comet Halley are summarized, and the elemental abundance of nitrogen in the Comet Halley nucleus is derived. It is found that 90 percent of elemental nitrogen is in the dust fraction of the coma, while in the gas fraction, most of the nitrogen is contained in NH3 and CN. The elemental nitrogen abundance in the ice component of the nucleus was found to be deficient by a factor of about 75, relative to the solar photosphere, indicating that the chemical partitioning of N2 into NH3 and other nitrogen compounds during the evolution of the solar nebula cannot account completely for the low abundance ratio N2/NH3 = 0.1, observed in the Comet. It is suggested that the low N2/NH3 ratio in Comet Halley may be explained simply by physical fractionation and/or thermal diffusion.

A probe’s landing procedure is a complicated and dangerous one. Because scientists do not know everything about a target planet, they can never be sure what the conditions will be like when a probe lands. Mars, for example, suffers from enormous dust storms that could seriously damage a probe descending to the surface. The diagram below shows a procedure for a landing.

A Probe lander is a spacecraft which descends toward and comes to rest on the surface of an astronomical body. By contrast with an impact probe, which makes a hard landing and is damaged or destroyed so ceases to function after reaching the surface, a lander makes a soft landing after which the probe remains functional.

For bodies with atmospheres, the landing occurs after atmospheric entry. In these cases, landers may employ parachutes to slow down and to maintain a low terminal velocity. Sometimes small landing rockets are fired just before impact to reduce the impact velocity. Landing may be accomplished by controlled descent and set down on landing gear, with the possible addition of a post-landing attachment mechanism for celestial bodies with low gravity. Some missions (for example, Luna9 and Mars Pathfinder), used inflatable airbags to cushion the lander's impact rather than a more traditional landing gear.

When a high velocity impact is planned not for just achieving the surface but for study of consequences of impact, the spacecraft is called an impactor. Several terrestrial bodies have been subject of lander or impactor exploration: among them Earth's Moon, the planets Venus, Mars, and Mercury, the Saturn moon Titan, the asteroids and Comets.

Beginning with Luna 2 in 1959, the first few spacecraft to reach the lunar surface were impactors, not landers. They were part of the Soviet Luna program or the American Ranger program.

In the year 1966, the Soviet Luna 9 became the first spacecraft to achieve a lunar soft landing and to transmit photographic data to Earth. The American Surveyor program (since 1966) was designed to determine where Apollo could land safely. As a result, these robotic missions required soft landers to sample the lunar soil and determine the thickness of the dust layer, which was unknown before Surveyor.

Probes investigate as much of their target as they can. Cameras take an assortment of photographs from different angles and distances, while antennae detect magnetism and radio waves. Lander probes, such as the Vikings that touched down on Mars, can take soil samples and analyze the atmosphere.

NASA's Viking Project found a place in history when it became the first U.S. mission to land a spacecraft safely on the surface of Mars and return images of the surface. Two identical spacecraft, each consisting of a lander and an orbiter, were built. Each orbiter-lander pair flew together and entered Mars orbit; the landers then separated and descended to the planet's surface.

The Viking 1 lander touched down on the western slope of Chryse Planitia (the Plains of Gold), while the Viking 2 lander settled down at Utopia Planitia. Besides taking photographs and collecting other science data on the Martian surface, the two landers conducted three biology experiments designed to look for possible signs of life. These experiments discovered unexpected and enigmatic chemical activity in the Martian soil, but provided no clear evidence for the presence of living microorganisms in soil near the landing sites. According to scientists, Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil.

The Viking mission was planned to continue for 90 days after landing. Each orbiter and lander operated far beyond its design lifetime. Viking Orbiter 1 continued for four years and 1,489 orbits of Mars, concluding its mission August 7, 1980, while Viking Orbiter 2 functioned until July 25, 1978. Because of the variations in available sunlight, both landers were powered by radioisotope thermoelectric generators -- devices that create electricity from heat given off by the natural decay of plutonium. That power source allowed long-term science investigations that otherwise would not have been possible. Viking Lander 1 made its final transmission to Earth November 11, 1982. The last data from Viking Lander 2 arrived at Earth on April 11, 1980.

So far space probes have visited every planet in the Solar System except Pluto. Venus was the first planet to be investigated as Mariner 2 flew by in 1962. Mariner 10 orbited Mercury in 1975. Mars is the most visited planet in the Solar System, with over five probes landing on its surface. The Pioneer and Voyager probes, launched between 1979 and 1989, investigated the outer planets Jupiter, Saturn, Uranus and Neptune. The Pluto-Kuiper Express aims to visit Pluto between 2006 and 2008.

NASA launched the Voyager spacecraft in 1977 to take advantage of a rare alignment among the outer four planets (Jupiter, Saturn, Uranus and Neptune) that would not take place for another 175 years. A spacecraft visiting each planet could use a gravitational assist to fly on to the next one, saving on fuel. The original plan called for launching two spacecraft pairs — one pair to visit Jupiter, Saturn and Pluto, and the other to look at Jupiter, Uranus and Neptune. The plan was cut back due to budgetary reasons, resulting in two spacecraft: Voyager 1 and Voyager 2.

The primary five-year mission of the Voyagers included the close-up exploration of Jupiter and Saturn, Saturn's rings and the larger moons of the two planets. The mission was extended after a succession of discoveries. After passing by Saturn in 1980, Voyager 1 made a sharp turn out of the plane of the solar system. Voyager 2's trajectory, however, was planned to take it past Uranus and Neptune. While the initial budget for Voyager 2 didn't guarantee it would last long enough to transmit pictures from those two planets, it thrived and made successful flybys of Uranus in 1986 and Neptune in 1989.

Between them, the two spacecraft have explored all the giant outer planets of our solar system — Jupiter, Saturn, Uranus and Neptune — as well as 49 moons, and the systems of rings and magnetic fields those planets possess. 

The current mission, the Voyager Interstellar Mission, was planned to explore the outermost edge of our solar system and eventually leave our sun's sphere of influence to enter interstellar space — the space between the stars. Since passing the boundary of interstellar space in 2012, Voyager 1 is examining the intensity of cosmic radiation, and also looking at how the sun's charged particles are interacting with particles from other stars, according to Voyager project scientist Ed Stone. Voyager 2 is still traveling within the solar system, but is expected to breach interstellar space in the next few years.

The first space probe to complete its mission was Luna 2. It was launched by the USSR in 1959 and successfully landed on its destination — the Moon. Its predecessor, Luna 1, was launched towards the same target several months earlier but missed by 6000km (3730 miles).

Luna 1 (launched Jan. 2, 1959) was the first spacecraft to escape Earth’s gravity. It failed to impact the Moon as planned and became the first man-made object to go into orbit around the Sun. Luna 2 (launched Sept. 12, 1959) was the first spacecraft to strike the Moon, and Luna 3 (Oct. 4, 1959) made the first circumnavigation of the Moon and returned the first photographs of its far side. Luna 9 (Jan. 31, 1966) made the first successful lunar soft landing. Luna 16 (Sept. 12, 1970) was the first unmanned spacecraft to carry lunar soil samples back to Earth. Luna 17 (Nov. 10, 1970) soft-landed a robot vehicle, Lunokhod 1, for exploration. It also contained television equipment, by means of which it transmitted live pictures of several kilometres of the Moon’s surface. Luna 22 (May 29, 1974) orbited the Moon 2,842 times while conducting space research in its vicinity. Luna 24 (Aug. 9, 1976) returned with lunar soil samples taken from a depth of seven feet (about two metres) below the surface.

Luna 2 was the first object of human origin to make contact with another celestial body. The spacecraft scattered spherical emblems of the Soviet Union on the lunar surface. The spacecraft's sensors found no evidence of a lunar magnetic field or radiation belt.

After an aborted launch on Sept. 9, the Ye-1A probe, also known at the time as the Second Soviet Cosmic Rocket, successfully lifted off on Sept. 12 (Sept. 13 Moscow time). When the spacecraft reached about 97,000 miles (about 156,000 kilometers) from Earth, it released one kilogram of sodium gas on Sept. 12 in a cloud that expanded to about 400 miles (650 kilometers) in diameter and was clearly visible from the ground.

Three days later, Luna 2 achieved escape velocity (the speed and direction required to travel beyond Earth’s gravity). This sixth Soviet attempt at lunar impact was much more accurate than its predecessors, and the spacecraft reached the surface of the Moon at 23:02:23 UT on Sept. 14, 1959, becoming the first object of human origin to make contact with another celestial body.

The probe collided with the moon at approximately 30 degrees north latitude and 0 degrees longitude on the slope of the Autolycus crater, east of Mare Serenitatis.

Luna 2 (as it was renamed in 1963) deposited Soviet emblems on the lunar surface carried in 9 x 15-centimeter metallic spheres. The spacecraft’s magnetometer measured no significant lunar magnetic field as close as 55 kilometers to the lunar surface. The radiation detectors also found no hint of a radiation belt.

Space probes are highly advanced robotic craft, often the size of a large car, launched into space to investigate celestial objects. They use radio transmitters to communicate with mission specialists on Earth. All probes have highly sensitive electronic equipment on board.

The accurate navigation of space probes depends on four factors: First is the measurement system for determining the position and speed of a probe. Second is the location from which the measurements are taken. Third is an accurate model of the solar system, and fourth, models of the motion of a probe.

For all U.S. interplanetary probes, the antennas of the Deep Space Network (DSN) act as the measurement system. These antennas transmit radio signals to a probe, which receives these signals and, with a slight frequency shift, returns them to the ground station. By computing the difference between the transmitted and received signals, a probe's distance and speed along the line from the antenna can be determined with great accuracy, thanks to the high frequency of the signals and a very accurate atomic clock by which to measure the small frequency changes. By combining these elements, navigators can measure a probe's instantaneous line-of-sight velocity and range to an accuracy of 0.05 millimeter-per-second and three meters respectively, relative to the antenna.

Many probes also carry cameras that are used to image the destination, whether it be a moon, planet or other body. During the final approach, these images are used when the distance becomes small. For example, the Cassini spacecraft's camera provides an angular measurement with an accuracy of three microradians (three kilometers) at a distance of one million kilometers. The images complement the radio data and provide a direct tie to the target.

Calculation of the trajectory of a space probe requires the use of an inertial coordinate system as well, wherein a grid is laid over the solar system and fixed relative to the star background. For interplanetary missions, an inertial coordinate system with an origin at the center of mass of the solar system is used. Because the measurements provide information on the position of a probe relative to the antenna, knowledge of the antenna's location relative to this inertial coordinate system is used to convert the measurements into elements in the system. Where the antenna is depends not only on its geographic location on Earth's surface, but on Earth's position relative to the solar system center of mass (known as the Earth ephemeris). Measurements of this ephemeris have an accuracy of about 0.5 kilometer and the location of the antenna is known to an accuracy of better than five centimeters.

Airlocks are vital for protecting the crew of a space-craft. In a submerged submarine, if there was no airlock, the vessel would instantly be flooded with water as soon as the hatch was opened. In the same way, a space station with no airlock would depressurize the instant the door was opened, killing anybody on board who was not wearing a spacesuit. This is because air always tries to remain at a level pressure. If the pressure inside a spacecraft is greater than the pressure outside, as soon as the hull is breached, air will rush out into outer space.

Airlocks, typified by the International Space Station’s (ISS) primary Quest Joint Airlock, are designed to permit safe passage of people and objects between a pressurised vessel and its surrounding environment. Further, they are designed to minimise pressure and air-level changes within the host craft.

The Quest airlock of the ISS is split into two main sections: an equipment chamber and crew lock chamber. The former connects to the ISS and supplies an auxiliary holding bay for any essential equipment, such as Extravehicular Mobility Units (EMUs – or spacesuits), as well as other key gear. It also supplies a staging area where astronauts can prepare for a spacewalk -namely get in and out of their spacesuits.

Connected to the equipment compartment is the crew lock, a smaller cylindrical chamber into which astronauts enter prior to any spacewalk. Once inside this section, the interior hatch between the equipment lock and the crew lock is shut. This provides a sealed environment for the suited astronaut and allows depressurisation to proceed. When the crew lock is fully depressurised, an external hatch becomes operational, providing an exit for the astronaut to enter space.

Importantly, before any spacewalk is attempted, astronauts must ‘camp out’ within the equipment chamber of the airlock in a reduced-nitrogen environment in order to purge nitrogen from their blood stream. This ensures that astronauts avoid decompression sickness in the low pressure experienced within the pure-oxygen atmosphere of the spacesuit. Nitrogen and oxygen are supplied and replenished via four externally mounted gas tanks, which ensures that the lock does not need to draw upon the host ISS’s own gas supplies.