|SCIENCE & TECHNOLOGY||Thursday, August 28, 2003, Chandigarh, India|
that could fuel future spacecraft
marvel of opto-electronics
that could fuel future spacecraft
The word "fuel" fuels the states of mind to think of traversing through the sea of stars and galaxies. Everybody desires to go on fascinating sojourn to stars. With the use of conventional chemical fuels, astronauts have made it possible to land on the moon and some unmanned missions to Mars.
The space journeys to other planets were not successful because such gigantic trips involve much more propulsion power and time. To overcome these barriers, the researchers at National Aeronautics and Space Administration (NASA) are investigating the possible alternate fuels for spacecraft propulsion so as to reduce the payload and shorten the time for space trip. It is envisaged that antimatter has tremendous potential to be used as the ultimate fuel for future spacecraft.
What is antimatter?
The word "antimatter" sounds as exotic as the black holes and, nowadays, it has become the subject matter of intense debate at scientific deliberations. The antimatter is opposite to matter. Everybody in our daily use, is made of matter. It consists of atoms, molecules and ions which further have constituent particles like protons, neutrons, electrons, etc, called fundamental particles. There are large numbers of such fundamental particles in nature and to every such particle; there is a corresponding anti-particle. The protons, neutrons, electrons etc have antiparticles like anti-protons, anti-neutrons, anti-electrons (positrons, e+) respectively. The particles and their anti-particles have nearly identical properties except some key characteristics which are opposite e.g. protons and anti-protons have all physical properties the same except opposite charge on them. The proton carries one unit of +ve charge whereas anti-proton carries one unit of —ve charge (1 unit of charge = 1.6 x 10-19 coulomb). The neutral particle like neutron also has its antiparticle that differs with regard to its magnetic moment.
How the antimatter as fuel can help humankind? The answer lies in Einstein’s mass energy equivalence, E=mc2. When a matter particle annihilates with its antimatter counterpart, their entire mass is converted into pure energy. The energy released per unit mass or particle exceeds many a time the efficiency of any other reaction in physics such as burning of H2 and O2 in the Space Shuttle’s main engine. For example, the electron-positron annihilation produces gamma ray of energy 5.11 x 105 electronvolt (eV).
The gamma rays have the greatest penetration power amongst the radiation. Thus, the gamma rays from a perfect reaction of matter and antimatter annihilation would immediately escape from the antimatter engine unless the spacecraft had thick shielding. The thick shielding would make the craft bulky and the very purpose is not served. Dr George Schmidt and others at NASA have conducted experiments on the annihilation of heavier particles like protons and anti-protons. The process, somewhat different from perfect annihilation, emits gamma rays along with showers of secondary particles, which eventually decay into neutrinos, and low-energy gamma rays. The charged debris in proton anti-proton annihilation can propel a spacecraft deep into space. Further, it is desired to evolve a process as close as possible to the perfect annihilation event. The pertinent point here is to intercept some pions and other charged particles so that their energy is used as thrust on a spacecraft.
An antimatter engine will still be a Newtonian type of rocket zooming through the space and working on the principle of action and reaction. These would not be the vehicles moving faster than the speed of light. The reaction in such engines would be amazing. The efficiency of an engine is measured by the specific impulse imparted to it. The Space Shuttle Main Engine, working on conventional fuel, has specific impulse of 455 seconds. With the advent of fuels based on fission and fusion, the specific impulse is enhanced to 104 seconds and 105 seconds, respectively. Surprisingly, with antimatter as fuel, the specific impulse of 106 seconds could be achieved.
Source and storage
The question now is that from where to get antimatter? It has to be created. Its production process is complex and very expensive. The very first antimatter particle i.e. positron (e+) was observed by Carl Anderson in 1932. In 1955, the anti-proton was produced at the Berkeley Bevatron. In 1995, the scientists at the European Centre for Nuclear Research (CERN), could create anti-hydrogen atom by combination of an anti-proton and positron. But these anti-hydrogen atoms could not last too long. During the production of anti-protons, about 50% of them get wasted, thus, reducing the production efficiency to 50%. The anti-protons are produced, though in modest quantity, from high energy particle accelerators where protons collide with targets made of Ni. These anti-protons are collected and held in a magnetic bottle.
At present, there are two main producers of antimatter viz.., the Fermilab in Batavia, Illinois and the CERN, a European Consortium. These two together produce only 10-15 nanograms (1 nanogram = 10-9 gram) of antimatter annually. The aim is to obtain about a few micrograms of anti-protons for the technology existing at NASA to start working. Right now, the most expensive substance on earth is antimatter costing about $ 62.5 trillion per gram.
The conventional fuels can be stored easily in any form of matter in metallic containers. The storage technique for antimatter is very tedious as antimatter annihilates when it comes in contract with matter. It is, therefore, stored in highly evacuated chambers (vacuum), where the anti-protons are hold by powerful magnetic fields. Dr Gerald Smith at Pennsylvania State University has suggested a magnetic bottle called Penning Trap, to trap the anti-protons. The cloud of anti-protons is hold by keeping the bottle cold and quiet with liquid nitrogen and helium. This chamber when completely designed, will weigh about 100 kg, most of it will have liquid nitrogen and helium just to keep a trillion anti-protons quiescent (motionless) across a zone of ~ Imm.
Scientists envisage the following uses of antimatter fuel.
(i) Physicist suggest that 1 gm of anti-hydrogen would release energy about 23 times that of delivered by Space Shuttle’s external fuel tanks.
(ii) It is reported in the Journal of Propulsion and Power that as little as one millionth of a gram i.e. 10-6 gm would generate energy sufficient for one-year manned mission to Jupiter.
(iii) The technique called antimatter-initiated micro-fusion would also be the novel fuel for engines. These engines would use anti-protons to superheat the tiny pellets of material, initiating tiny nuclear explosions and thus propelling the spacecraft forward. The energy from this technique would be able to propel a 100-kg payload on a 50-year trip outside the solar system just by burning 100 micrograms of antimatter.
(iv) The designs of antimatter propelled engines have already been analysed with computer models but what impedes yet, is to have enough quantity of antimatter.
(v) Today, the antimatter (low energy positron) is being used in medical imaging technique for diagnoses, called Positron Emission Tomography. The positrons are also used in the studies of important materials of relevance to electronic circuits. These low energy positrons are obtained as a result of natural decay of radioactive isotopes.
(vi) Scientists predict that there are more chances for antimatter to be used for medical treatment before it is used as fuel for travelling to Mars or other planets. The technique based on antimatter will be more effective than X-rays in killing the cancerous cells.
Finally, the day is not far when NASA would come out with a portable trap to store antimatter. It hopes to use this device to transport antimatter to a rocket-launching site. Thus, it is just a question of waiting for some years.
— marvel of opto-electronics
of information storage have come a long way since the ancient times of
our great rishis who wrote volumes of Holy books with ink on tree
leaves. The present technique is to burn thin film of special
materials deposited on a plastic disc by a laser beam. The quest of
mankind for fast storage and retireval of large amount of information
had to wait till the successful development of semiconductor lasers
and detectors, signal processing, vacuum deposition techniques,
plastic materials, moulded plastic micro-lenses, etc.
Compact Disc (CD) is a data storage medium that can be written to and read using a leser beam. Originally developed in the late 1960s by James T. Russell, it was called optical disc. However, the commercial CDs were produced by Philips and Sony in 1982. Since then, there has been a constant succession of optical discs formats like CDs, followed by a number of DVDs. And optical disc holds much more data compared to a floppy having 1.44 Megabytes (MB) only. Data stored on CD has a long archival life; the storage and retrieval of data is also very fast. Emerging technology such as Blue-ray, offers up to 27 Gigabytes (GB) on a single-sided 12-centimeter DVD.
The enlarged view of the part of a CD shown in the figure illustrate its construction. A clear optical grade polycarbonate plastic disc 12 cm diameter and 1.2 mm thick contains pits and lands into the upper surface of the disc. The surface has a thin reflecting aluminium layer coated by vacuum deposition technique. The reflecting layer which is less than hundred nanometers (billionths of a meter) is covered with a 5 to 10 microns (millionths of a meter) thick lacquer layer to protect it from mechanical and environmental damages. The disc label is printed on the lacquer.
CDs and CD-ROMs are manufactured by first spin-coating a dye polymer substance (recording layer) onto a glass master disc which has a microscopic grooved helical trackformed onto its surface by moulding process. This groove is used by the writing head drive to guide the laser beam during the writing process. The data is then written by momentarily exposing the recording layer to a high power (approximately 10 milliwatt) laser beam that is focused onto its surface. As the recording layer is heated, permanent microscopic marks called "pits" are formed on the helical track in the disc. The pits differ in length depending on how long the write laser is turned on, which is how information is stored on the master disc. The recorded track consisting of pits and lands (blank spaces between adjacent pits) has a total length of about 5,800 meters. The track is approximately 0.5 microns wide, with a spacing of 1.6 microns between the adjacent tracks. When the CD is played, the laser beam of lower power is permitted to pass through the CD’s polycarbonate material which reflects off the aluminum layer and hits an optoelectronic sensor in the optical head as shown in the figure. The pits reflect diffused light whereas "land" areas between the pits being smooth, reflect much more light. The optoelectronic sensor detects these changes in reflectivity, and the electronics in the CD-player interpret the modulated signal, which is then decoded into the original user data by the playback device. This data is read at the rate of 1.4 Mb/s.
It is important to note that the data is stored in the pits very close to the label side of the disc, and it is read by a laser beam focused on the pits by travelling through the thickness of the plastic disc from the bottom. As a result, scratches on the bottom may disrupt the paser beam, but they can often be removed by polishing. Scratches on the label side, however, may easily go through the thin top layers, permanently damaging the stored data. Therefore users should be careful in handling the CDs.
Future trends: Hard disk drive storage capacities in personal computers have grown to enormous dimensions at very low prices, making the need for reliable and convenient backups more important than ever. The solution of using stack of floppies as higher capacity backup lacks in convenience. Therefore, a latest innovation called micro-drive of a matchbox size having tiny optical discs that are encased in a plastic shell is commercially available now-a-days. Each disc is capable of holding 500 MB of information. The drive actually reads both sides of the disc, meaning that the disc stores 250 MB per side. Blue-Ray is a next generation large capacity disc format. Blue-ray technology enables the recording, rewriting and play back of up to 27 GB of data on a single sided single layer 12 cm DVD. Further research is being made into the development of a larger capacity, such as 30 GB on a single sided single layer disc and over 50 GB on a single sided double layer disc.
We know that friction does not depend on the area of contact. Why then do we have to apply larger force when we use broad tyres on a bicycle?
I feel there is some confusion in the application of your statement that friction does not depend on the area of contact. Consider a situation in which a horse is pulling over a rough road a plank of wood that has been loaded with a pile of bricks. In this case it can be said that the frictional force would be independent of the area of the plank. But I am not sure you are right, when you suggest that we have to use larger force when the tyres are broad. Since the broad tyres are heavy, just setting them to rotate needs a significant amount of energy. If the tyres are made broad and thick by just piling up rubber there is another sink for energy. This is due to the following reason. In order to drive the tyres need to push the ground back. This is possible only if there is enough friction between the ground and the tyre surface. The sticking out undulations in the rubber of the thick tread of the tyre bend and twist in the process. This produces heat and wearing out of these undulations. The friction is also essential for road holding and during turning and braking.
In any case the obstruction to the bicycle motion is not so much due to the friction between the tyre and the ground. Remember we are talking of rolling friction not a sliding friction for which your statement is valid. The energy loss in wheeled vehicles with tyres increases when the tyres are not properly inflated. This is because energy is continuously wasted in compression and expansion of tyres as they roll. Lot of this energy is converted into heat and in destruction of the tyres.
If we drink coffee or tea after eating sweets, they seem to be much less sweet. What is the reason?
I will hazard an answer even though I am not so certain about its accuracy. Taste buds are receptors that fit in the molecules that give us a sensation of sweetness. After eating things that are very sweet these receptors are saturated with molecules. There are few seats that are vacant. Therefore the signal of sweetness from things that are less sweet remains weak. Therefore things taste less sweet than they would if we had not taken the sweets beforehand.
In electrical wiring, what is the difference between the neutral and the ground?
The ground is connected to the ground and the neutral is not. The ground wire ensures that in case of a leakage a connected appliance does not acquire a voltage that might cause injury or malfunction. The live terminal is the one to which the voltage is connected. It appears that there should be no separate identification of the live and neutral terminals when you are dealing with AC current. However for reasons of safety and connecting electrical networks it becomes prudent to do so.
When we throw a stone while travelling in a car, it gets more force than the ordinary throw. Why?
The answer to this question should be obvious. If car is travelling at 120 kilometres per hour then a stone dropped out by a child would be travelling at the same speed; this is similar to that of a ball thrown by some of the best fast bowlers in cricket. So if the stone is thrown forward through the car window, the speed of the car gets added to the speed at which you throw. You also know that fast bowlers in cricket take a long fast run before throwing the ball. They are adding their running speed to that at which they can throw just standing still on the ground.