by Mikhail PANASYUK, Dr. Sc. (Phys. & Math.), Director of the Skobeltsyn Institute of Nuclear Physics, Moscow State University
Space studies in the wake of the first artificial satellites of the earth enabled scientists to make a signal discovery by detecting the radiation belts of our planet. That was something one had never suspected before-as it turned out, powerful fluxes of particles envelop our planet.
Carrying gigantic energies, these fluxes may pose a grave threat to the safety of space flights. Neither did we have data on the earth's magnetosphere formed under the effect of solar plasma fluxes and interplanetary magnetic fields. And something else just as fantastic: our close celestial neighbors - the Moon, Mars and Venus-do not have structures like that. Today we know: the giant planets - Jupiter, Saturn, Neptune and Pluto-do have them. Thus direct experiments carried out in outer space in the 1950s and 1960s gave birth to a new discipline, and this is cosmic, or space, physics. One of the men who stood at the cradle of this science was the eminent Russian physicist Academician Sergei Nikolayevich Vernov.
Sergei Vernov was born in 1910 at Sestroretsk, a small town near St. Petersburg. Upon his graduation from the Leningrad Polytechnic, he got a job at the famous Radiation Institute, a research center named after Vitaly Khlopin. With the outbreak of the Great Patriotic War against nazi Germany Sergei Vernov moved to Moscow and joined the research staff of the Lebedev Physics Institute set up by the Academy of Sciences of the USSR.
Meanwhile still before the war, in 1940, a new academic chair was set up at Moscow State University on the initiative of another outstanding physicist Academician Sergei Vavilov. It was involved with the physics of the atomic nucleus and radioactive emissions. It was there, at this department, that Sergei Vernov began as a lecturer. In 1944 he became a full- fledged professor. Dr. Vernov continued as a Moscow University lecturer to the end of his days.
Soon after the Second World War a new research center was established at Moscow University on Academician Dmitry Skobeltsyn's initiative-the Research Institute of Nuclear Physics, NIIYAF for short in Russian. In 1946 Professor Vernov became its scientific director, and in 1960-its full director, and he continued as head of this research institute until his death in 1982.
Outer space was a natural extension of Dr. Vernov's research activities. He was a pupil of Academician Dmitry Skobeltsyn, a prominent Russian physicist and founder of the Russian school on the physics of the atomic nucleus and cosmic rays. Dr. Vernov's research interests lay in the study of cosmic rays, which are superhigh energy particles reaching our planet from distant regions of the universe. The enigma of these fluxes-where in particular they are generated, their composition and how they gain such immense energies-was on the mind of many research scientists of the first half of the twentieth century. Dr. Vernov began by making use of ground stations and airborne instruments carried aloft by airships and blimps. Afterwards space satellites could be employed for the purpose as well-tha's what physicists needed for studying cosmic particles in their natural environment, far beyond the terrestrial atmosphere distorting the initial characteristics.
The dramatic story of these experiments is typical enough. In fact, all pioneering discoveries share this kind of fate. In 1956 a meeting held by the USSR Academy of Sciences decided to detail plans for experiments on board the first Russian sputniks, the artificial satellites of the earth. The designers of space vehicles set rigid limitations on the weight and energy consumption of onboard instruments. Finally, after a good deal of picking and choosing, one opted for gas-discharge counters-the compact and reliable instruments that had performed nicely on the ground and on balloons alike. These instruments were in the kit by October 1957, when the Soviet Union launched the world's first artificial satellite, unmanned yet. But the first sputnik was off without them. More than that, our scientists learned about the launching from the news media. Their disappointment knew no bounds indeed.
Then Dr. Vernov had an audience with Konstantin Bushuyev, who was deputy of Academician Sergei Korolyov, the man directly responsible for the Soviet space project. Vernov talked Bushuyev into having the NIIYAF-built instruments installed aboard the second Soviet sputnik, unmanned too, orbited in November 1957. The space vehicle continued in orbit for ten days only...
But we are wise now-we know that the radiation belts surround our planet at rather high altitudes where they follow the form of the lines of force of the magnetic field of the earth; this field is close to a dipole one. That is why space satellites orbiting the earth at an altitude of several hundred kilometers are outside the zone of intensive radiation, skirting it now and then over South Atlantic near Brazil. This region is known as the South Atlantic Anomaly, where radiation belts come closest to the terrestrial surface and "hover" above it, giving off powerful energy fluxes down to an altitude of 200-300 km-within a limited area, though.
Conducting their first experiment on the second Soviet sputnik (unmanned, as we remember), our researchers aimed to have data on the anomaly radioed back to earth. But they could access this information only from orbital sections above the Soviet territory-the South Atlantic was beyond their field of vision. The readings of the detectors on board the orbital
vehicle deviated from the anticipated characteristics of space rays: on the seventh of November 1957 an intensity burst was registered, a phenomenon that was due, as it came out later, to a "shower" of particles from the radiation belts during a magnetic storm. Yet at that time research scientists attributed that outburst to solar particles intruding into the terrestrial atmosphere.
The research team under Dr. G. Van-Alien (State University of Iowa, USA) had better luck. They learned about the launching of the first Soviet sputnik on board a research vessel in the Pacific while studying cosmic rays. The news spurred the Americans to undertake such studies in circumterrestrial space as well. The United States orbited its first artificial satellite, Explorer-7, on February 1, 1958. (We might as well note here in passing: the rocket designer Werner von Braun was all set to have the satellite launched in 1956, but President Dwight Eisenhower prohibited using the third stage of the booster rocket Jupiter-S to avoid further military confrontation with the Soviet Union.) Experiments carried out by Explorer-1 and Explorer-3 enabled the Americans to detect radiation fluxes over our planet, as it was announced at a session of the US Physical Society on 1st May 1958.
Meanwhile Sergei Vernov and coworkers had a big kit of instruments taken aboard the third Soviet sputnik that was launched on May 15, 1958. In that experiment Soviet physicists confirmed the presence of the radiation belt discovered by Dr. Van-Alien's team and even detected a new one at higher altitudes. Besides, the Soviet physicists studied the composition of these two belts: the one closer to the earth was found to consist of protons by and large, while the other, outer belt-of electrons.
The radiation belts over our planet are giant toroidal structures composed mainly of electrons and protons. Ions and other elements are also present there, though in far smaller quantities. It is the sun and the ionosphere of the earth that are the source of all these particles. The plasma of the solar wind and of the ionosphere, getting inside the magnetosphere, is accelerated; as a result the constituent particles can gain energy millions of times as high as the initial one. The most powerful acceleration processes are induced by magnetic storms, and these are caused as a rule by a gain in the solar wind velocity during active processes occurring on the sun.
Prof. Vernov and his colleagues were the first to postulate the mechanism underlying radiation belt formation: cosmic rays, by interacting with the atmosphere of the earth, trigger nuclear reactions whereby electrons and protons are formed; these are trapped by the magnetic field of the earth.
This bold suggestion came as a surprise-so much so that most physicists would not accept this hypothesis at first. The point is that space studies were launched at the time of intensive nuclear weapon tests. Nuclear charges were exploded both on the terrestrial surface and in the atmosphere. And so the verdict of American experts was like this: radiation belts could appear as a consequence of nuclear weapon tests carried out by the Soviet Union from high-altitude bombers. And when some of those boffins came to the Soviet Union in 1959, they heard the same story from their Soviet counterparts: the belts resulted from the nuclear tests carried out by... the United States! But in the long run all physicists agreed that radiation belts were a unique natural phenomenon.
As of mid-1958 some of the nuclear tests were being carried out in near space as well. The United States and the Soviet Union blasted several bombs there, and that became a source of man-induced radiation. On one hand, those blasts hampered physicists for many years in studying the natural radiation belts for nuclear explosions interfered with the ecology of circumterrestrial space; but on the other, physicists obtained a unique chance to verify their hypotheses.
Both the Vernov and Van-Alien teams worked in secrecy for security considerations. Sure enough, the military had a finger in the pie too. And yet from the standpoint of basic physics, nuclear tests in near space came pat as an experimental proof validating the theoretical premise about the nature of the
stable motion of charged particles in the radiation belts: these particles not only gyrate around magnetic field lines and bounce along them, they also drift around the earth.
On Dr. Vernov's initiative twin satellites were designed and built in the early 1960s for comprehensive studies of the radiation belts. And so in 1964 two duple vehicles like that, the Elektrons, were put in orbit. One had an elongated orbit to ensure the measurement of trapped particles at the periphery of the belts and radiation beyond them, while the other, low-flying, probed within. In this comprehensive experiment our physicists studied the structure and dynamics of particles against the background of geomagnetic activity and its changes. The data thus obtained helped NIIYAF physicists design a model of ambient radiation. These data likewise confirmed the postulated mechanism of radiation belt formation (radial diffusion)- namely, the transport of particles across the magnetic field as a result of random changes in the solar plasma pressure on the magneto-sphere of the earth. This process, alongside the generation of particles trapped by cosmic rays, turned out to be a major formative mechanism for radiation belts.
In 1958 Dr. Vernov headed a large-scale program of research into the levels of radiation and cosmic rays near the Moon, Venus and Mars. The next year, in 1959, three lunar vehicles-Lunas 1, 2 and 3 - were launched. They carried
gasdischarge and scintillation detectors (counters used for particle registration and spectrometry). The lunar probes crossed the zone of radiation belts around the earth and supplied a wealth of data. The Luna-2 probe reached the surface of our planet's natural satellite and furnished evidence on the absence of any detectable magnetic field and radiation fluxes around the Moon. At a later date Prof. Vernov was behind such innovative experiments on spacecraft launched toward Venus and Mars. These planets were found to be devoid of strong magnetic fields and radiation belts too. But while the absence of a magnetic field next to Venus was obvious, physicists were not as sure about Mars and postulated the presence of a weak magnetic field there. Had it been really in existence, trapped particles would have been detected in the vicinity of Mars. I can well remember the interest with which Sergei Nikolayevich Vernov scrutinized in his office the data transmitted from the Soviet Mars probe that reached the Martian surface. Alas, there was no visible increase in radiation fluxes until the very moment of the touchdown on the Red Planet. In a nutshell, until the 1970s, when the American space probe Pioneer-10 approached Jupiter, the earth had been the only planet of the solar system with a clearly identified magnetosphere and radiation belts. In the meantime our country was getting ready for manned space flights. This work was well underway from the late 1950s. Needless to say, radiation safety came first. Well before Yuri Gagarin's pioneering space flight a team of NIIYAF researchers under Dr. Vernov took up this problem. Dr. Vernov and coworkers designed special-purpose radiation counters, they drew up maps showing in much detail the distribution of radiation fields along different orbits. Their conclusion was that the radiation risk was insignificant in orbits not higher than 360 km above the earth's surface and at a slope (inclination) of 60 degrees, except during short periods when the South Atlantic Anomaly had to be crossed. Yet subsequent studies carried out on board the Russian orbital station Mir showed the radiation fluxes in that anomalous region to be dependent on cyclic variations of solar activity, i.e. radiation maxima occurred in the years of a solar activity minimum. But radiation fluxes become more intense in orbits higher than 400 km, and the radiation hazard increases.
And thus a new applied discipline, cosmic dosimetry, or radiation monitoring, was born in our country on Dr. Vernov's initiative. Ever since dosemeters (radiation monitors) have become part and parcel of the safety kit of both piloted and unpiloted automatic spacecraft. Outer space, you see, is an aggressive medium for space vehicles and their hardware (electronic devices too). Radiation can induce changes on the surface of a solid body and within. As a consequence some structural materials may change their physical properties, and electronic elements may go out of commission.
We know of many examples of adverse effects of radiation on space hardware elements. For instance, the degradation of solar batteries that lose their output capacity; or the fogging of optical elements; or radiation breakdowns of every kind. Now and then satellites employed for aerial photography delivered just black films.
Another important step in the studies of cosmic radiation was made in 1967. It involved many "associated" experiments on board the spacecraft on the Molniya and Kosmos space vehicles. The idea of installing research instruments on special-purpose satellites (military, communication and so on) proved exceptionally fruitful for it enabled scientists to obtain desired data at minimum expense. For instance, between 1967 and 1975 ten experiments were conducted in studying the acceleration of protons and electrons in the radiation belts of the earth. All that helped us broaden our knowledge on the structure and dynamics of particles in a geomagnetic trap. Here's what we found out: during magnetic storms when the magnetosphere of the earth is deformed under the action of advancing solar plasma fluxes, particles within the magnetosphere are accelerated to energies of dozens and hundreds of megaelectronvolts. This occurs within very short intervals of time compared with regular acceleration processes taking place in quiet periods. The problem of pinpointing the sources of these particles came to the fore in the 1970s and 1980s as the terrestrial ionosphere, alongside the solar wind, was found to be a major supplier of plasma into the magnetosphere. Whereas weak and moderate magnetic storms are generated by a plasma flux ring rotating around the earth at a distance equal to three or four of its radii and composed mainly of protons, the picture is different where strong magnetic storms are concerned: here the basic component of the rings are oxygen ions injected from the ionosphere. A significant role in the study of these processes was played by "associated" experiments aboard the Gorizont satellites put into geostationary orbit in the 1980s and 1990s.
But not only radiation belt particles fill circumterrestrial space. Present side by side with them are also cosmic rays, the giant-energy particles reaching us from distant regions of the universe. Most likely they are generated by the explosions of supernovae and gain speed while propagating in interstellar space. In 1958 NIIYAF physicists, working on a ground-based installation, made an important discovery: energies equal to about 10(15) eV induce changes in the key characteristics of the space rays-their energy spectrum. It develops a "bend"-a jump from a gently sloping to a steep pattern. This happens because the energies of particles do not accelerate higher than 10(15) eV at supernova blasts. But on the other hand, this phenomenon may be caused by their transport in the magnetic fields of the universe. This problem is still among the central ones in the physics of cosmic rays.
A particle flux at energies ca. 10(15) eV is all too small, and therefore instruments possessing very high "light-gathering power" are needed for its registration. Say, a device with a weight of several tons might be quite good in such data collection. Sergei Vernov and his colleagues detailed plans for experiments with a kit of instruments two to three tons in weight as the Soviet Union began testing Proton rockets in the mid-1960s. These plans were realized in due course. Instead of the ballast (sand), the rockets carried aloft giant calorimeters which, covering a wide range of space radiation energies from 10(12) eV to 10(15) eV, also assayed their chemical composition for the first time ever. The energy spectrum then obtained still serves as a standard for comparing the results of other experiments.
Cosmic rays are also generated by the sun, not only by the distant supernovae. The solar particles carry far less energy than do galactic cosmic rays so-called. However, solar flares and corona discharges spew forth enormous amounts of matter into interplanetary space and give rise to superhigh-energy particles in the 10 to 10 eV range. The acceleration of such particles is not yet determined in full. They may gain speed in the solar atmosphere too, in its active regions, and during transportation in interplanetary space-in the process of interaction with the shock-waves produced by high-velocity (up to 1,000 km/s and more) plasma fluxes of the solar wind.
Solar radiation monitors, designed at NIIYAF and installed on as good as all probes launched toward the Moon and some planets of the solar system, became the basis of a long-term program for studying the energized solar particles. Sergei Vernov was the man who masterminded this program way back in the early 1960s. All this research has added much to our knowledge about the acceleration of these particles and, more, has made it possible to develop standard models for radiation safety of space missions to distant planets.
Sergei Vernov would have turned 90 in the year 2000. Marking his birth anniversary, we may justly say: his name is closely associated with the breakthroughs of our science in space research.
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