Category Astronomy

 

 

Why are instruments necessary for astronomical observations?

With our sense we can feel only light and heat from the heavenly bodies and for that too, we need instruments to accurately measure them. Other forms of energy such as X-rays, microwaves, gamma rays, etc., present in the Universe cannot be detected without instruments.

What instruments are used to detect the faint light from heavenly bodies?

Heavenly bodies faintly visible to the naked eye can be seen through telescopes. For observing very faint objects, telescopes of very high resolving power are used which can collect more light than the unaided eye.

 

 

 

What are the different designs of telescopes?

Mirrors that use lenses are known as refractor telescopes and those of reflector telescopes are Newtonian, Cassegrain, Herschelian, Gregorian, Conde and Schmidt designs. The refractor telescope has no major design variations.

 

 

 

 

 

 

 

 

 

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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.

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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.