My Science School

Why does a satellite constantly require orbital corrections?

A satellite is subjected to various forces of nature. The Sun and the Moon constantly exert pulls on it. Above 1600 km, solar radiation can, known as solar wind, affects it. The Equator’s bulge causes gravitational perturbations. The Sun and the Moon exert gravitational pulls that keep on varying. Another factor is the Earth’s magnetic field which will reduce the spin rate of a satellite and affect its stability. Changes in the volume of on-board gas and liquid propellants can also tilt a satellite. Also the air molecules in the atmosphere of the Earth stretch far into space and when a satellite moves at 8 km a second, collisions with air molecules are frequent enough.

 

 

What is orbital decay of a satellite?

So long as the Earth’s gravity is unable to pull a satellite closer to the atmosphere, it can remains in orbit for years. However, its useful life depends on its stability in orbit and availability of on-board fuel for correction of any instability. If a satellite’s perigee is about 100 km, its orbit begins to ‘decay’ and it begins its final plunge into the lower atmosphere. The orbit will decay if its period is about 87 minutes or less.

 

 

 

 

Can artificial space satellites and other objects fall out of orbit?

Yes. It has been estimated that in the first 25 years of artificial satellites, over six thousand artificial objects have fallen out of space orbit. In fact, at any time, there are over 50,000 small and large objects in orbit. About 10 per week are slowed down to the point of re-entry. Most of them get burnt up in the heat of re-entry, while a very few such as Skylab have burnt out incompletely and fragments have hit the Earth.

 

 

 

 

What is meant by inertial guidance system of a spacecraft?

It is a system comprising gyroscopes and accelerometers that enables automatic plotting of the position of a moving spacecraft. This is because movement of the spacecraft and the distance travelled by it can be calculated if its acceleration and velocity are known. A specified star is usually taken as the reference point for this purpose. The inertial guidance system also takes corrective action when the errors in speed and position accumulate to unacceptable levels.

What are the relative merits of three-axis body stabilisation over other types of stabilisation?

Three-axis body stabilisation is the method of satellite stabilisation preferred nowadays over older methods like spin stabilisation. In a three-axis body stabilized spacecraft, the entire structure is available for mounting antennas and other devices which need high pointing accuracy. Earth-viewing area is much larger than in a pointing accuracy. Earth-viewing area is much larger than in a spinning satellite. Solar panels can be rotated so as to keep them the north-south direction and can be rotated so as to keep them perpendicular to the Sun-satellite line. Moreover, as the spacecraft does not rotate at high rates, light weight panels initially folded and kept on board can be deployed in space. A given area of the solar panel will be three times more efficient in power generation compared with a spinning satellite. It would thus be possible to generate larger quantities of power needed by advanced communication satellites.

 

 

 How does three-axis stabilisation work?

In three-axis stabilisation, the controlling mechanism is based on a, momentum wheel, essentially a fly wheel actuated by an electric motor and rotated at a predetermined speed. This system is known as zero momentum stabilisation where unwanted momentum accumulated by the wheel is reduced to zero. Correction is effected by accelerating or decelerating the momentum wheel on the basis of error signals provided by the satellite’s Earth sensors which point towards the Earth and lock on to the Earth. The accuracy of alignment to the Earth is of a high order of 0.1 to 0.2 degree. This position is required to be maintained for 5 to 7 years, which nowadays is the operational lifetime of a satellite.

 

 

 

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What are transfer orbits?

Transfer orbit is the path pursued by a spacecraft in moving from one orbit to another, e.g., from the orbit of the Earth to the orbit of Mars. Generally speaking, such an orbit will be an ellipse which intersects the orbit of the target planet. If the spacecraft is to enter an orbit around the target planet, or affect a landing, then the engines must be fired to achieve the correct trajectory.

 

 

 

 

 

 

What is the Hohmann transfer orbit?

The transfer orbit requiring the minimum expenditure of energy is an ellipse which just touches the orbits of the Earth as well as the target planet and is known as the Hohmann transfer orbit.

 

 

 

 

 

 

 

What are high velocity transfer orbits?

High velocity transfer orbits require much more energy than Hohmann transfer orbits but less time is required to reach the target planet. Such an orbit may be in the shape of a highly eccentric ellipse or a hyperbola.

 

 

 

 

 What are multiple transfer orbits?

The synchronous orbit is attained in stages and not in one step from the launch. Initially, the satellite is put in what is known as the transfer orbit where the perigee is at about 250 km, and the apogee about 35,800 km. The motor on board the satellite is then switched on to impart the incremental velocity required to circularize the orbit at 36,000 km. A 50-second burn of the motor will then increase the velocity by 1780 m/sec and take a satellite in a near synchronous orbit. Further corrections, if needed, are      done by micro-thrusters on board the satellite.

What is the relationship between the orbiting period of a satellite and its distance from Earth?

If an object such as a satellite moves at a velocity of about 8 km a second above the atmosphere it starts orbiting around the Earth. When the centrifugal force is equal and opposite to the Earth’s gravity there is a constant balance keeping the satellite in orbit. That implies that there is a definite speed for a satellite for a given altitude. At 7.91km a second, the satellite will go into a circular orbit. If the velocity is increased, the orbit becomes elliptical. If an object is sent at an initial velocity of 8.04 km/sec it goes around the Earth at a height of 235 km. Only if an object attains a speed of 11.26 km/sec or 40,000kmh, would it attain escape velocity to escape the pull of the Earth and remain as its satellite.

The period of a satellite in Earth’s orbit increases with the distance from the Earth. A satellite about 250 km from the Earth completes its orbit in 90 minutes, whereas the Moon at about 400,000 km takes almost a month to go round. At 1.6 million km from the Earth, the orbital period becomes 8 months, and beyond this point, the Sun’s gravitational field captures the satellite and Earth orbits are not practical.

What is a Sun-synchronous orbit?

In a Sun-synchronous orbit, the satellite appears over an area at the same Sun angle in successive orbits, which is ideal for remote sensing. The orbital plane is always at the same angle relative to the Sun-Earth line during all seasons. It also implies that the satellite will cross given latitude at the same local time. The solar illumination angle desired for remote sensing or weather study can be fixed for the whole year in such orbits. Any numbers of sun-synchronous orbits are possible if some conditions are met. One vital condition is that the satellite has to go in an east-to-west direction i.e., in a retrograde orbit. The angle of inclination of the orbital plane to the Equator will vary, depending on the altitude of the orbit. Earth’s equatorial bulge will cause an eastward shift of the plane of the orbit. This eastward regression will nullify the orbital plane of a satellite caused by the Earth’s movement relative to the Sun during the annual revolution.

Thus, the Sun-synchronous orbit becomes possible by a combination of natural forces and choice of orbits. Satellites at altitudes of 565 km, 893 km and 1261 km, having inclinations of 97.60, 990 and 100.70 would complete 15, 14 and 13 revolutions per day respectively. It is obvious that but for the equatorial bulge of the Earth, there can be no Sun-synchronous orbits.

Continue reading "Satellites and Space Travel"

 

What are space probes?

Space probes are spacecraft sent on missions to study various bodies in the Solar System and even beyond.

 

 

 

 

 

What are equatorial and polar orbits of satellites?

An orbit on or near the plane of the Equator is called an equatorial orbit. An orbit on or near the plane of the poles is called a polar orbit.

What is retrograde orbit of a satellite?

In a direct orbit, the satellite goes eastwards as it crosses the Equator from the southern to be northern hemisphere. In a retrograde orbit the motion gets reversed.

What is the optimum condition for a satellite launch?

A satellite is generally launched towards the east possibly from a point on the Equator, if launching facilities are available there.

 

 

 

 

What is regression of the nodes in the orbit of a satellite?

A satellite cannot go round the Earth at a fixed altitude closer to Earth, as the Earth is not a perfect sphere. The orbit of a satellite is determined by the shape of the Earth which has an educational bulge. The earth is also slightly pear-shaped. All these variations produce gravitational anomalies that affect the shape of the orbit.

As a satellite crosses the equatorial region, the plane of its orbit will change or shift because of the Earth’s equatorial bulge. This is known as regression of the nodes. When a satellite passes from the southern to the northern hemisphere, in a west to east direction, the satellite will have a westward regression. If the satellite passes from east to west, from the southern to the northern hemisphere, it will have an eastward regression. The rate at which the regression takes place depends on the altitude of the orbit and its inclination to the Equator. No regression takes place in a true polar orbit or in a true equatorial orbit.

 

 

 

 

What is a space shuttle?

A space shuttle is a reusable spacecraft that can carry heavy payloads of upto 30 tonnes. It is launched like a rocket along with fuel and oxygen tanks. After launching, the tanks are jettisoned and splashdown to Earth. The space shuttle remains in space to complete its mission and there after comes back to Earth and lands like an aircraft on a runway.

 

 

 

 

 

 

 

 

What is a hyper plane?

A hyper plane takes off like a jet plane, with only fuel and without any oxygen of board. But once it is airborne, it draws oxygen from the atmosphere to fill its onboard oxygen tanks. Once the oxygen tanks are full, the hyper plane can travel into space like a rocket. A hyper plane is less hyper plane can travel into space like a rocket or space shuttle since it carries no oxygen during launching and is therefore lighter.

Which organization in India is developing a hyper plane?

Bharat Dynamics Ltd., a defense undertaking in Hyderabad.

 

 

 

 

 

 

What are artificial satellites?

Modern technology has enabled man to launch man-made artificial objects from the Earth and them in orbit. Such objects are called artificial satellites.

What are nodes with reference to the orbit of artificial satellites?

The inclination of an Earth orbit is measured by the angle formed by the orbital plane of the satellite with that of the Equatorial plane. Nodes are the points of inter-section between the orbit and the equatorial plane of the Earth.

 

 

What are the functions of an artificial satellite?

An artificial satellite is an outpost in space and can be used for communication, navigation, meteorology, remote sensing of Earth resources, astronomy, military applications etc.

How is a satellite useful as an astronomical tool?

A satellite can be equipped with telescopes and instruments for carrying out astronomical observations. Observations from satellites are better than those made on the Earth since the distortions due to the Earth’s atmosphere are eliminated.

 

What are the vehicles used for space travel called?

Spacecraft or rockets.

What is re-entry?

The return of a spacecraft to Earth from space through the atmosphere is called re-entry.

What is a spacecraft heat shield?

During re-entry, some parts of the spacecraft may get heated up or even melt due to air friction, if they are unprotected. Vulnerable parts of a spacecraft are therefore protected by a heat shield made of special materials that can withstand the heat.

 

 

 

What is the difference between a jet plane and a rocket?

A jet plane carries only fuel onboard and no oxygen, since it absorbs oxygen from the air for burning its fuel. A rocket carries both fuel and oxygen onboard and is capable of travelling outside the atmosphere.

 

 

 

 

How can rockets fly in space?

A rocket moves due to the reaction created by the gases escaping from its engines and not due to any aerodynamics. The exhaust gases ejected by a rocket engine cause the rocket to move in a direction opposite to that of the exhaust gases.

What is the countdown period?

The countdown period is the time taken from the time the order is given for launching the rocket to the time it actually gets launched. During the period, an elaborate prescribed procedure is followed for checking and putting into operation various mechanisms connected with the launching of the rocket and may even last for a week. The last twenty-four hours of countdown is referred to as the final phase of countdown. The last few seconds of the final countdown period are usually announced on the public address system as“....... five, four, three, two, one, ZERO”. Zero refers to the moment of launching.

 

 

 

What are the different types of fuels used in rockets?

Rockets use solid or liquid propellants. However, for sustained propulsion over very long periods in space, nuclear propulsion is preferable.

What is lift off?

Lift off refers to the point, a few seconds after launching when the thrust from the rocket engines attains a level where it exceeds the total weight of the rocket. At this stage, the rocket literally begins to lift off the ground.

 

 

 

What are the liquid fuels used in a rocket?

The commonly used liquid rocket fuels are liquid are liquid hydrogen, nitrogen tetroxide, kerosene etc. Rockets also carry liquid oxygen required for combustion of the fuel. Liquid fuel rockets are commonly used for launching rockets from Earth into space which calls for larger initial thrust. Liquid hydrogen and liquid oxygen are also called cryogenic fuels since they arre stored at extremely low temperatures.

 

How much fuel does a rocket need to carry on board?

The speed of the rocket will increase if its engine is kept running for a longer period. Hence, the amount of fuel carried by the rocket will depend on the speed which it is required to achieve. For example, a rocket with an empty weight of 1 tonne will need to carry about 1.75 tonnes of fuel to reach the speed of its exhaust gases and about 19 tonnes of fuel to reach three times the speed of its exhaust gases.

What are splashdown and touchdown?

Splashdown is the moment a space vehicle or capsule lands in the ocean after completing its mission in space. If it comes down on land it is called touchdown.

 

 

 

 

 

 

What is a multi-stage rocket?

In a multi-stage rocket specific sections or stages of the rocket are jettisoned or discarded in flight to lighten the load. The rocket is therefore specially constructed in detachable parts to enable such multi-stage operation. A multi-stage rocket is more efficient than a single stage one and can reach much higher speeds. Depending on the number of stages, they may be designated as two-stage, three-stage, etc.

            The birthday of Gandhiji is celebrated in India as Gandhi Jayanti that is on 2nd October. It is one of the national holidays of the country.

            This day is also celebrated internationally as the Day of Non-Violence by the UN. Gandhiji was a man who believed in the power of universal brotherhood and harmony.

            Gandhi Jayanti is marked by prayer services and tributes all over India, and at Gandhiji's memorial in New Delhi where he was cremated.

            Popular activities include prayer meetings, commemorative ceremonies in different cities by colleges, local government institutions and socio-political institutions.

            Schools celebrate the day of complete cleaning and conduct various competitions on Gandhiji and Gandhian philosophies.

            The day of Gandhiji’s assassination, 30th January is observed as the Martyrs’ Day. 

          The Gandhi Peace Award is an award and cash prize presented annually since 1960. The Gandhi Peace Award was conceived by Promoting Enduring Peace’s founder, Yale professor Jerome Davis. Its main purpose is to promote international peace and goodwill. It is named in honour of Mahatma Gandhi. The award is presented to peace heroes. The award is also intended to recognise individuals for having made significant contributions, through cooperative and non-violent means in the true spirit of Gandhi. It recognizes the struggle to achieve a sustainable world civilization founded on enduring international peace.

          The Award consists of a medallion and a certificate with an inscription summing up the recipient’s work. The medallion features Gandhi’s profile and his words “Love Ever Suffers/Never Revenges Itself” cast in bronze. Some notable winners are Medea Benjamin, U Thant, Bill McKibben, Amy Goodman etc.

 

          The International Gandhi Peace Prize, named after Mahatma Gandhi, is awarded annually by the Government of India.

          The Government of India launched this prize in 1995, on the occasion of the 125th birth anniversary of Mahatma Gandhi. This is an annual award given to individuals and institutions. This award is open to all individuals, regardless of their nationality, creed or race.

          A jury consisting of the Prime Minister of India, the Leader of the Opposition in the Lok Sabha, the Chief Justice of India, and two other eminent persons decides the awardee each year. The awardee gets an amount of 1 crore in cash, a plaque, and a citation.

          The first recipient of the award was Julius Nyerere, who was a political leader of Tanzania. Nelson Mandela received this award in the year 2000. 

            There are numerous documentaries and feature films centred on Gandhiji’s life. There are films that also have Gandhiji as a character. One such example is the film ‘Sardar’, where it fundamentally highlights how both Patel and Gandhiji together strived for Independence. 

            Jahnu Barua’s Maine Gandhi Ko Nahin Mara (I did not kill Gandhi), places contemporary society as a backdrop with its vanishing memory of Gandhi’s values. This film was released in 2005.

            Besides, there were many documentaries based on Gandhiji. One such famous documentary was ‘Mahatma: ‘Life of Gandhi’. The documentary was released by Vithalbhai Jhaveri in 1968.

            Another celebrated documentary on Gandhiji is ‘Mahatma Gandhi: 20th Century Prophet’. It is directed by Stanley Neal, and written by Quentin Reynolds. It features the life of the Mahatma. 

             The Making of the Mahatma, Shyam Benegal’s film, portrays Gandhiji’s transformation from an introverted lawyer to a freedom fighter.

             This film was about Gandhi’s experiments with truth and non-violence in colonial South Africa and was produced by India and South Africa jointly.

             It was based on the book ‘Apprenticeship of a Mahatma’ by Fatima Meer, and was directed by one of India’s most respected directors, Shyam Benegal.

             ‘The Making of the Mahatma’ premiered in November at New York’s Guild Theatre. The film deliberately lacks the panoramic proportions and epic scale of Attenborough’s “Gandhi”.

               The film documents Gandhi’s 21 years in South Africa, from age 19, and the changes which came over this Anglicized, London-trained advocate as he encountered the racial discrimination and bias of the colonists in South Africa.

               Rajit Kapur gives a solid performance as Gandhi, and Pallavi Joshi portrays his wife Kasturba. The Hindi title is ‘Gandhi se Mahatma Tak’. 

          ‘Gandhi’, released in 1982, was directed by Richard Attenborough and is the story of the life of Mahatma Gandhi. It is an epic movie, covering Gandhiji’s life from his days as a struggling lawyer in South Africa, fighting against racism, to becoming the leader of the non-violence movement that won India freedom from British rule.

          ‘Gandhi’ is without doubt, one of the finest biographical epics ever made. The film shows that Gandhi was not just a simple, humble man in a loincloth, but a shrewd, practical man as well. Ben Kingsley’s portrayal of Mohandas K. Gandhi is amazing.

          The film won six Academy Awards, including Best Picture, Best Actor, and Best Director. ‘Gandhi’ is not just an outstanding film. Its importance lies in the fact that it brings a deeper message to its viewers-that peace, justice, and equality for all people can best be achieved through non-violent means. 

            Mahatma Gandhi’s autobiography, ‘The Story of My Experiments with Truth’ is indeed a great work written with a mighty pen that was his life.

            It was not first published in book form, and appeared in weekly installments, in his journal ‘Navjivan’ from 1925 to 1929.

            This was written originally in Gujarati, and later translated to English by his notable disciple, Mahadev Desai.

            It has chapters on his childhood, experiences in England, and in South Africa. It records the events of his life from his childhood till 1921. He melted the hearts of thousands of people with his story about his principles of Satyagraha. He regarded it as a tale of experiments with life, and with truth.

            In the present era of chaos and falsehoods, this work stands out as a symbolic representation of truth and order. He talked about his own philosophy of life that stood the test of time. 

 

 

              Gandhiji was a good writer and a voracious reader. Gandhiji wrote many articles on various topics from hygiene to the development of villages. But he wrote only three books.

               Most prominent among them are Gandhiji’s autobiography ‘The Story of My Experiments with Truth’, which covers Gandhiji’s life from his early childhood to 1921. 

 

 

 

 

 

 

                  The second book was ‘Hind Swaraj or Indian Home Rule’. This was written in 1909. In this book, Gandhiji expresses his views on various subjects such as Swaraj, modern civilization, mechanization etc. The book was originally written in Gujarati, and it was banned by the British. Gandhiji later translated it into English, but the translated version was not banned by the British. 

 

 

 

 

 

 

 

 

                       The third book was ‘The Key to Health’ which was written by Gandhiji while he was in the Aga Khan Palace at Pune during 1942-1944. The book was written in Gujarati, and later translated to English by Dr. Sushila Nayyar under Gandhiji’s own guidance.