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.