The Tribune, Chandigarh, India

Space is polluted
Radhakrishna Rao

Outer space is no longer the tranquil and pollution-free canopy of the “overcrowded and overheated” planet earth. A phenomenal increase in the pace of the space activities over the last three decades has turned the final frontier into an area of rapidly piling up pollutants and debris. In fact, a study by the European Space Operations Centre of the European Space Agency (ESA) at Dermstadt goes to show that the amount of space debris keeps increasing at the rate of 5% per year.

Meanwhile, ESA is weighing the option of building smaller satellites that would on completion of their mission life burn upon re-entry. ESA also plans to position such satellites farther from the earth, beyond the most congested orbits.

As per a study carried out by a team of space scientists from Canary Islands Institute of Astrophysics, hundreds of thousands of fragments of space rubbish continue to remain in the near earth orbit. And there is a growing concern over the possibility of these rubbish hitting and damaging more than 600 operational satellites in orbit. Among the components of space debris are broken pieces of satellites, spent rocket stages, bolts, nuts, spanners and screw drivers left behind by the astronauts. According to Miquel Seera of Canary Islands Institute of Astrophysics, even the smallest piece, on account of its velocity, could pose a serious threat to the spacecraft.

According to the French space scientist Dr F. Alby, a major challenge before the global space community is to develop techniques for the detection of objects smaller than 10-cm which are capable of inflicting mission degrading effects on satellites. In a major breakthrough Marshall Space Flight Centre of the National Aeronautics and Space Administration (NASA) has found that weak laser beams can be used to remove minor space debris from the orbit of active satellites.

Researchers say that many of the pieces of satellites continue to stay in orbit indefinitely if they attain what is called the first cosmic speed. Only objects that dip below an altitude of 130-150 km re-enter the atmosphere to blaze down as unidentifiable pieces on earth. The collision hazard posed by the debris is mainly due to the high energy impact caused by rapid acceleration. The first confirmed collision between two catalogued objects occurred in July 1996 when the gravity gradient attitude control boom of the French satellite Cerise was damaged by fragment from a European Ariane rocket

In order to prevent the accumulation of debris in designated mission orbits due to collisions, disintegrated satellite and other useless objects must be removed from the mission orbit when lives end, before collision occurs. Studies indicate that for limiting orbital debris an object should not remain in orbit for more than 25 years.

While much of the space debris results from launch activities, garbage also results from satellite and meteorite collisions. The anti-satellite tests conducted by Russians are known to have contributed at least 500 pieces of junk to the space pollution belt.

The hazards involved in space exploration were highlighted in the melodrama that accompanied the uncontrollable descent of the American Skylab. Luckily to the great relief of the panic-stricken earthlings the Skylab made a soft landing in the waters off the Australian coast in July 1979. Overcrowding is the other problem that plagues the outer space.

The geostationary orbit — 36,000 km above the equator where a satellite appears stationary in relation to earth — often referred to as the real estate of the final frontier is experiencing congestion. The number of satellites in this orbit is growing by 10 per cent per year. Eminent science fiction writer and space-age prophet Arthur C. Clarke had not long back warned about the dangers posed by the overcrowding in the earth orbit.

Self-repairing car bodies
Shirish Joshi

Ever car owner knows how easily cars get dented. All it takes is a hit on a roadside pole or a reckless jerk while parking in a narrow spot.

The problem would be solved if scientists could make plastic composites, which could repair themselves the way the human body attends to its own injuries.

Dr Scott White, a professor at the University of Illinois Urbana Champaign, USA, and his colleagues asked themselves this question when they first thought of developing self-healing composites.

Our body heals itself continuously over our lifetime. If we get a cut, the repair process begins automatically. The local effects of the cut itself send a signal to nearby platelet cells and white blood cells to get to work.

They have developed a material, which contains millions of liquid-filled microcapsules. When a crack forms, the shells of the microcapsules (which measures roughly 0.1 of a millimetre, releasing the liquid “healing agent” dicyclopentadiene (DCPD).

Capillary action draws the liquid into the crack, where it comes into contact with embedded catalyst particles, causing the liquid to harden in a matter of minutes.

By catching and patching the cracks while they are still very small, a car’s body could retain up to 90 per cent of its original strength, extending its life span considerably.

However, once all the capsules have been broken, the material can no longer heal itself. Dr Scott proposes to make the material even more like human body. The human body has a highly developed circulatory system that transports not only the building blocks for regeneration and healing but also nutrients and everything else in your body.

Dr White’s team is working on a technology to make material with circulatory networks embedded within them, which could refill the supply of healing agents indefinitely throughout the lifetime of car body.

This would involve a “heart” to pump the fluids around the network and a tank that could be filled up periodically with a supply of healing agent. While self-healing composites could provide years of crack-free driving, they would not be able to repair damage incurred in a major accident.

The scientists have developed “bubbloy” (from “bubble’ and “alloy’) could come into play immediately. The bubbloy is made from a mixture of palladium, nickle, copper and phosphorus.

Understanding the Universe
With Prof Yash Pal

How does a radio catch signals of different frequencies when the tuning dial is rotated? Also, why can’t the radio catch the frequencies of TV signals?

When you turn the knob on a radio, you are changing the characteristic frequency of the circuit inside the radio. In most radios, varying the capacitance of the tuning circuit does this. This characteristic frequency is the natural frequency at which the current in a circuit, consisting of condensers and resistance elements, for example, can oscillate. Turning the knob can change this frequency. When the frequency of the incoming signal matches that of the circuit the two resonate and the gain is large. You have then picked up a desired channel.

Resonance is an important concept in the physical world. When you pluck the strings of a “sitar” or any other string instrument, you get a note of a specific pitch. If another string instrument tuned to exactly the same pitch is placed nearby, you may find that the string of that instrument also begins to vibrate giving an identical note. This is easily seen with tuning forks, often used for tuning pianos; tuning forks are often found in school laboratories. You would have heard the story of Tansen’s singing, where his voice was able to crack glass lamps through vibrations, excited by the volume and purity of his voice. This might be only a story, but it is not physically impossible. A resonating element can pick up an enormous amount of energy from an amorphous energy source. Moving to another example, a swing can be raised to a great height if a little push is given in step with its natural frequency. There is a story of a large hanging bridge that collapsed in gusts of wind because the gusts happened to contain frequencies of pressure change that were in resonance with the natural vibration period of the bridge structure. Modern architects who design tall bridges and other buildings have to keep such factors in mind.

Tuning of the TV channels is done using the same principles as tuning of radios. In general radios cannot catch a TV signal because the frequency range is different and the manner in which the signals are modulated with information is also not the same. But the sound channels of TV transmitters can often be picked up by FM radios.

How do we define a dimension? How many dimensions are possible and can all of them affect our life in some way?

Let us first consider our ordinary world. If the earth were a perfect sphere, with no mountains or valleys, not even tides in the oceans, then we would be able to give the position of any point on it by giving just two coordinates, the longitude and the latitude. This is because the third coordinate, namely the distance from the centre of the earth, is assumed to be given as a constant. In reality, we do have mountains and valleys, we have tides and we have buildings and things that have height, besides the fact that we also want to go to the bottom of the sea, drill holes for oil and water. So we do need the third dimension. Thus, run-of-the-mill space and all objects therein are very well located by giving just three coordinates. The three axes with respect to which the coordinates are given are orthogonal (at right angles) to each other.

But we also use another dimension to describe happenings in the world. This is the dimension of time. We want an answer to both — where and when! Till the beginning of last century, time was considered like a flowing river, the same for everything in the universe. And then Einstein happened. The theory of special relativity showed that it is best to think of time as a dimension of a four-dimensional space-time reality. “Now” of the Sun, for example, is separated from the “now” of the earth by 8 minutes, the time it takes any effect of the sun to be felt on the earth, which is the time it takes for light to travel from the sun to the earth.

But the story of dimensions does not end here. Mathematicians can construct spaces that have any number of dimensions. You have only to ensure that all the axes are orthogonal to each other in this mathematical space. For example, the present day mathematicians and physicists are working on “string” theories with ten dimensions in an attempt to describe all forces of nature and achieve a unification that has remained the dream of the most adventurous of scientists. The claim is that while theory demands ten dimensions, only four of these become manifest in the real universe in which we live!

I know that I have gone much beyond what you expected in reply to your simple question. And, as I have often stated in my answers, I do not claim any originality, not even a full understanding. But you asked for it!

Why do waves have crests and troughs?

I think your question has come from your observation of waves in a pond or at the seashore. You would get the same impression if you were to shake a string tied to a tree on the other end - you would see a crest travelling to the other end and then, perhaps be reflected back. Like the particles of the rope, the molecules of water in the pond also move up and down; they do not travel in the direction in which the wave travels. The word “wave” implies that at a given point, some parameter increases and decreases with some periodicity, and this tendency propagates. So crests and troughs are implied but they need not be always normal to the direction of propagation. This is so for water waves, waves in a string and also for light and radio waves. But this is not so for sound waves, for which the crests and troughs are in the density change of the medium through which the wave travels.

But let me come back to the exact wording of your question. When you throw a stone in a pond, you depress a bit of water; you do form a bit of a trough. The disturbed molecules of water also drag their neighbours down with them. But they cannot stay down permanently. This is because physical media have inertia and elasticity. The particles oscillate almost in the same location while this disturbance propagates.

The direction of propagation of the wave is normal to direction of oscillation for waves on the surface of a pond, those in a string or even radio or light waves. These waves are called transverse waves. On the other hand, sound is an example of longitudinal waves; here the movement of molecules is longitudinal, or back and forth. What propagate are the density fluctuations in the medium.