Category Astronomy

How do astronauts walk in space?

          It appears strange but true that astronauts can walk in the space. This is so because in ordinary walking we rest our feet on the surface of the earth and the force of earth’s gravity pulls us towards it. But when there is nothing in the empty space – neither any surface to walk on nor any gravitational force to pull the feet down onto the ground – how does an astronaut walk in the space?

           Space walking by astronauts is quite different from the normal walking. To walk in the space the astronauts take the help of hand rockets which provide them the force to move. The hand rockets follow the principle of rocket propulsion. In rockets, the ejection of gas with a great force from the backside pushes the rocket forward with an equal thrust. This working principle is based on Newton’s third law of motion which states, ‘To every action there is always an equal and opposite reaction’. Similarly in hand rockets when the engine is powered the exhaust thrust pushes the rocket in the opposite direction and the astronaut walks along with this force as he carries the hand rocket with him. In fact, it is not ‘walking’ in the strict sense as there is no surface in the space to rest the feet but rather ‘floating’ – to express more accurately.

          But why do the astronauts walk in the space? Apart from experimental reasons, sometimes they are required to shift from one spacecraft to another or need to carry out a repairing work on the outer surface of the craft. During such operations they use the specially designed hand rockets and the direction of the exhaust outlet is pointed opposite to the desired direction of walking.

          The first spacewalk was made in March 1965 by a Soviet astronaut, A. Leonov, who stayed outside the aircraft for 24 minutes. Another important walk was made in 1973 when the American satellite Skylab was to be repaired for damage in the heat shield that made the craft dangerously hot. 

How can the temperature of stars be measured?

           We can measure our body temperature with a thermometer. Thermocouples and other devices are used to measure the temperature of furnaces. But how can we measure the temperature of stars?

          The surface temperature of stars is determined by various techniques. The most conventional and fairly accurate estimate can be made by colour alone. Red-coloured stars are cool while blue ones are extremely hot. On the basis of colour, stars have been classified in the table given below.

          A more accurate determination of the temperature is made by the comparison of spectra of stars. Light, which comes from the sun and other stars, is made up of many different wavelengths. It can be separated into different wavelengths by a spectrograph (an instrument used to record spectrum). From the spectroscopic studies, it has been observed that stars are largely composed of hydrogen (about 75% on the average). Next in abundance is helium followed by various other metals. In the cooler stars, some compounds are present but at high temperatures, they disintegrate into atoms. In order to know the temperature, the spectra of stars are recorded. It will be different for different stars, depending upon their temperature.

          Moreover, the intensity of spectral lines, bright or dark, varies with the temperature. It has been found that blue stars have O-type spectra; our sun has G-type spectra and so on. Blue stars emit 20 or more times the radiation per unit area than that of our sun does, whereas a red type may emit as little as 1/20 as much per unit area.

          From these spectra, by measuring and comparing the intensity of different lines and using Wien’s Displacement Law, the temperature may be determined. Intensity of emitted light is plotted against wavelength and the curve is drawn. The temperature of the star is directly proportional to the frequency at which most of its radiation is given off, i.e. to the highest point of the curve. 

                                                                                                                                                                                            

Is there life on other planets?

          Earth is perhaps the only planet in the solar system where life exists. This is because the conditions favourable for the existence of life are available only on the Earth. Conditions present in other planets make life almost impossible.

          Mercury is the planet closest to the Sun. It is difficult to see it even with a powerful telescope. It does not have any atmosphere. The temperature during the daytime may even go beyond 400°C. The lack of oxygen and the extreme temperature make life on Mercury impossible.

          Venus, which comes next in terms of closeness to the Sun, is often described as the sister planet of the Earth. Venus and the Earth are almost identical in size, mass and density. Its diameter is 0.95 times and mass is 0.815 times as compared to the mass and diameter of the Earth. It is surrounded by thick clouds of carbon dioxide (95%). The temperature on its surface is about 95 times more (480°C) than that on the Earth. This makes Venus the hottest planet in the solar system. Under these conditions life is not possible here.

           Mars comes after the Earth and is much colder than the Earth. Its average temperature is about – 62°C. At night it may drop to – 101°C. It has an atmosphere much thinner than that of the Earth. It has been found that it contains 1 to 2% argon, 2 to 3% nitrogen, 95% carbon dioxide and 0.3% oxygen. These conditions suggest the possibility of existence of life on it but so far no traces have been detected.

          All other planets beyond Mars (Jupiter, Saturn, Uranus, Neptune and Pluto) are farther away from the Sun. The surface temperature of these planets is so low that no living organism can survive there. Besides, their atmospheres contain gases like methane and ammonia which are not favourable to the evolution of living organisms.

          Thus our Earth is the only known planet in the solar system where life exists. 

Why do stars twinkle?

          We see millions of stars twinkling in the sky during night. They radiate their light in all the directions. Even though they appear very small yet in reality they are very big. Most of the stars are many times bigger than our earth. They look smaller only because of their great distance from the earth. Do you know why the stars twinkle?

          Our earth is surrounded by a thick cover of air known as the atmosphere. There is vacuum beyond the atmosphere. The gases present in the atmosphere are in constant motion. Because of the movement of gases the density of air in the atmosphere is not uniform everywhere. As such the refractive index of air varies from place to place. When the light from a star enters our atmosphere, it gets deviated from its path several times before reaching our eyes because of the changing density and the consequent change in the refractive index of the air.

          The deviation in the path of light in its passage from one medium to another is called ‘refraction’. Because of this refraction, the light reaching our eyes from the star varies. Due to this variation the stars appear to be twinkling.

          Now the question arises: why don’t moon, sun and other planets twinkle like the stars? This is so because compared to the stars, sun, moon and the planets are very near to earth and as such they appear bigger than the stars. Hence the angles subtended by the moon, sun and the planets at our eyes are larger than the angles subtended by the stars. Because of the larger angles, our eyes are not able to detect the deviation in the path of light from the sun, moon and the planets and hence they do not appear to be twinkling. 

Why does the size of the moon appear to change?

          On the full moon day, the moon appears like a shining disc but it goes on waning till it disappears on the new moon night. And then it again starts increasing in size and becomes a full bright disc, on the full moon day. Changes in the moon’s size are called the ‘phases’ of the moon. Do you know why it occurs? 

          The fact is that the moon neither increases nor decreases in size. Its size simply appears to be changing because it changes its position with respect to the sun. We all know that the moon is the only satellite of the earth and revolves round the earth. It travels around the earth about 384,400 km away. This orbit takes 27 days and eight hours. It is illuminated by the sunlight. Only one side of the moon faces the earth. We do not see its other side. When the moon comes in between the earth and the sun, its bright side is not visible to people on the earth and only its dark side faces the earth. As a result the moon cannot be seen. This is the new moon day. As the position of the moon changes due to its motion from east to west with respect to earth, some illuminated part of its surface is visible to us. A week after the new moon, almost half of the illuminated lunar disc becomes visible to us. The bright part of the moon’s surface goes on increasing gradually and on the full moon day the whole disc, seen from the earth, is fully illuminated. On this day the earth is in between the sun and the moon. In the next fifteen days the bright portion of the lunar surface goes on decreasing and on the fifteenth day it disappears completely. In short, the changes in the apparent shape of the moon depend upon how much of its illuminated surface is visible to us.

          The cycle continues like this and we see the different phases of the moon. On the full moon day it appears just after the sunset and disappears only at the sunrise. The time between new moon phases is called the synodic month.

 

Which planets have satellites?

          The heavenly bodies that revolve round the sun are called planets. There are nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The bodies revolving round these planets are called their ‘satellites’ or ‘moons’.

          Scientific investigations made so far have revealed that all planets do not have satellites. For example, Mercury and Venus do not have any satellite. Earth has 1 satellite – the moon. Mars has 2 satellites and the Jupiter has 16. The number of the moons revolving round Saturn is 24. The Uranus and Neptune have 15 and 6 satellites respectively. Pluto has 1 satellite.

          The size of different satellites is different. There are some satellites which are bigger than moon. The diameter of two satellites of the Mars, Deimos and Phobos, and the outer satellites of the Jupiter, Ganymede and Callisto are as big as Mercury and Mars. The diameters of Titan and Triton – the satellites of Saturn and Neptune are 5150 kms and 2700 kms respectively and more than the diameter of our moon.

          Except Titan, all the satellites have small force of gravity. As such none of them has any atmosphere. Because of low temperature at Titan, it has an atmosphere consisting of methane and hydrogen. But there is no life on this satellite.

          As yet we have not come across any satisfactory theory regarding the origin of the satellites. However, it is believed that their origin is similar to that of our solar system.