Earth fact and figures

Equatorial radius 6,378 km

Equatorial inclination 23.5°

Mass 5.97×1024 kg

Average density 5.5 g/cm3

Rotational period 0.997 days

Orbital period 1 year

Average distance from the Sun 149.6 million km

Perihelion 147.1 million km

Aphelion 152.1 million km

Orbital eccentricity 0.0167

Orbital inclination 0.0003°

Moons 1

Parkinson

Parkinson disease develops as a part of the brain known as the substantia nigra degenerates. The substantia nigra is located in the midbrain, halfway between the cerebral cortex and the spinal cord. In healthy people, the substantia nigra contains certain nerve cells, called nigral cells, that produce the chemical dopamine. Dopamine travels along nerve cell pathways from the substantia nigra to another region of the brain, called the striatum. In the striatum, dopamine activates nerve cells that coordinate normal muscle activity. In people with Parkinson disease, nigral cells deteriorate and die at an accelerated rate, and the loss of these cells reduces the supply of dopamine to the striatum. Without adequate dopamine, nerve cells of the striatum activate improperly, impairing a person’s ability to control movement.

A study published in 2000 found that people with Parkinson disease have a decreased number of nerve fibers in the heart. These results suggest that the disease affects nerves in organs outside the brain and may explain symptoms common in people with Parkinson disease, such as a drop in blood pressure when a person stands up, constipation, and difficulty urinating.

Scientists do not understand the mechanisms underlying nerve cell death in Parkinson disease. Most researchers believe that Parkinson disease results from a combination of factors involving genetics, environmental agents, and abnormalities in cellular processes.

rocket

You hear a rumble and a roar. A blast of fire shoots out of a big rocket. The rocket heads up into the air. Maybe the rocket is carrying a satellite into orbit around Earth. Maybe the rocket is carrying a space probe to another planet!
Only a big rocket can make it into outer space. No other machine is as powerful as a rocket.
WHAT DO ROCKETS LOOK LIKE?
A rocket looks like a long tube. Most rockets have fins on the back end to help them fly straight.
Rockets that carry fireworks can be short, only a few inches long. They are usually made of cardboard.
Rockets that go into space are huge. They are made mostly of metal.
WHAT MAKES ROCKETS GO?
Rockets burn fuel. Many different chemicals can be used as rocket fuel. The burning fuel makes hot gases. The gases blow out of the bottom end of the rocket. The hot gases shooting downward make the rocket go upward.
You can see how a rocket moves by blowing up a balloon. Hold the end of the balloon tightly so the air cannot get out. Then let go. The air rushes out of the opening in the balloon. The air rushing out makes the balloon fly around.
A rocket, like a balloon, has a small opening. The opening in a rocket is called a nozzle. Hot gases blasting out of the nozzle make the rocket go.
HOW DO WE USE ROCKETS?
People use rockets to carry things through air and space. Different kinds of rockets carry different things.
Sounding rockets carry instruments to measure air pollution, rays from space, and weather. Lifesaving rockets carry ropes to ships stranded offshore. Distress rockets signal for help.
The most powerful rockets carry satellites and spacecraft into space. Many spacecraft use smaller rockets called thrusters to move around once they’re in space.
Rockets can also be used as weapons. The rocket weapons are called missiles. Most of the rockets made are missiles.
WHAT IS A MISSILE?
Missiles are rockets that carry bombs. The British used rockets carrying bombs against the United States in the War of 1812. The national anthem of the United States even has a line about the rockets: “And the rockets’ red glare ….”
Guided missiles have steering systems that guide them to destroy their targets. The smallest guided missiles can be carried by soldiers. The biggest guided missiles are huge. They can carry nuclear bombs around the world.
LAUNCHING A MISSILE
Missiles can be launched (fired into the air) from the ground, from airplanes, from ships, and even from submarines. Missiles can also be launched from bombproof underground tubes called silos.
Soldiers on battlefields launch small missiles out of tubes that they can carry on their shoulders. Special trucks carry ground-to-air missiles that aim at airplanes. Special racks underneath fighter planes carry air-to-air missiles.
HOW DO YOU LAUNCH A SPACE ROCKET?
Big rockets are launched from launch pads. A rocket stands on the pad next to a tall tower. The towers have elevators to take workers up and down. Gigantic tractors called crawler transporters bring big rockets or the space shuttle to a launch pad.
Controllers count the seconds before launch as they finish checking everything. “Five, four, three, two, one ….” Bridges that connect the tower to the rocket swing away. “Ignition!” The rocket engines fire. The spacecraft lifts off into the sky.
Firing one rocket does not always provide enough power to send a spacecraft far from Earth. The most powerful rockets often have different stages. Stages are separate rockets stacked on top of each other.
HOW DO ROCKET STAGES WORK?
Rockets headed for space must go really fast, about 25,000 miles per hour (40,000 kilometers per hour). They must go fast enough to overcome Earth’s gravity. Gravity is the force that pulls you back down to the ground when you jump up. Using more than one rocket stage is the best way to go really fast.
The bottom stage fires, uses up its fuel, and drops off. Then the next stage fires. Each stage takes the spacecraft faster and higher. The huge Saturn V rocket that sent Apollo astronauts to the Moon had four stages.
WHO INVENTED ROCKETS?
Chinese people probably invented rockets more than 1,000 years ago. By the end of the 13th century people in Asia and Europe also knew how to make rockets.
American physicist Robert H. Goddard researched new, more powerful kinds of rockets during the early 1900s. A German inventor named Wernher von Braun helped Germany make missiles during World War II (1939-1945). After World War II, von Braun helped the Americans make rockets. The Soviet Union also made rockets. Soviet scientists launched the first satellite into space in 1957.
The United States, Russia, and other countries made bigger and more powerful rockets. Rockets have launched spacecraft to the Moon and most of the planets.
Space engineers are working on better rockets. They are testing rockets that use nuclear power. They are trying to build rockets that get their power from beams of light.

Viruses That Cause Human Disease

Family

Virus

Disease

Adenovirus	Common cold
Bunyavirus	Hantaan La Crosse Sin Nombre	Kidney failure Encephalitis (brain infection) Lung syndrome
Calicivirus	Norwalk	Gastroenteritis (diarrhea, vomiting)
Coronavirus	Corona	Common cold
Filovirus	Ebola Marburg	Hemorrhagic fever Hemorrhagic fever
Flavivirus	Hepatitis C (non-A, non-B) Yellow fever	Hepatitis Hepatitis, hemorrhage
Hepadnavirus	Hepatitis B virus (HBV)	Hepatitis, liver carcinoma
Herpesvirus	Cytomegalovirus Epstein-Barr virus (EBV) Herpes simplex type 1 Herpes simplex type 2 Human herpesvirus 8 (HHV8) Varicella-zoster	Birth defects Mononucleosis, nasopharyngeal carcinoma Cold sores Genital lesions Kaposi's sarcoma Chicken pox, shingles
Orthomyxovirus	Influenza types A and B	Flu
Papovavirus	Human papillomavirus (HPV)	Warts, cervical carcinoma
Picornavirus	Coxsackie virus Echovirus Hepatitis A Poliovirus Rhinovirus	Myocarditis (heart muscle infection) Meningitis Infectious hepatitis Poliomyelitis Common cold
Paramyxovirus	Measles Mumps Parainfluenza	Measles Mumps Common cold, ear infections
Parvovirus	B19	Fifth disease, chronic anemia
Poxvirus	Orthopoxvirus	Smallpox (eradicated)
Reovirus	Rotavirus	Diarrhea
Retrovirus	Human immunodeficiency virus (HIV) Human T-cell leukemia virus (HTLV-I)	Acquired immunodeficiency syndrome (AIDS) Adult T-cell leukemia, lymphoma, neurologic disease
Rhabdovirus	Rabies	Rabies
Togavirus	Eastern equine encephalomyelitis Rubella	Encephalitis Rubella, birth defects

The Life Cycle Of A Virus

Close your eyes and look. What you saw at first is there no more; and what you will see next has not yet come to life. —Leonardo da Vinci
We can apply these words very aptly to a virus—of the bacteria-infecting kind known as bacteriophage. When a phage particle enters a cell, it loses its infective power and its identity as a particle. Generally its entrance into the cell is followed within 15 minutes to an hour by the emergence of a new generation of infectious virus particles. Sometimes, however, there is no immediate pathological event. The genetic material of the virus that has passed into the cell combines with the genetic material of the cell itself. In doing so it is converted into something that has been named a 'provirus,' meaning before virus. Days or years afterward the provirus may suddenly develop into virus and the bacterium give rise to a group of virus particles.
The term provirus needs some explanation. The expression 'proman' would certainly not evoke the idea of a human egg, from which Homo sapiens always develops, but rather that of an evolutionary ancestor of man which would have to undergo a genetic transformation to become man. A provirus may perhaps correspond to an evolutionary ancestor of a virus. But it is also much more than that.
Before attacking the question of the nature of proviruses, we must know something about viruses themselves. What is a virus? We shall leave out of the discussion the much debated issue as to whether viruses are living organisms or not; our concern is to find out how they differ from 'normal' organisms of the microbiologist's world. The two attributes that are usually thought to define viruses are their very small size and the fact that they can multiply only inside living cells—usually requiring a specific kind of cell host. But to learn more about their peculiarities let us go beyond this definition and compare viruses with other small biological units.
First of all, how does a virus differ from a cell? Most cells are capable of reproducing themselves: they possess the genetic material which is the basis of heredity and the tools necessary to synthesize the essential building blocks and to organize these into a structure just like themselves. We can see three important differences between a cell and a virus, taking bacteriophage as a typical virus: (1) whereas cells contain both desoxyribonucleic acid (DNA) and ribonucleic acid (RNA), the phage contains only DNA; (2) whereas cells are reproduced from essentially all their constituents, bacteriophage is reproduced from its nucleic acid; (3) whereas cells are able to grow and to divide, the virus particle as such is unable to grow or to undergo fission. Bacterial viruses are never produced directly by division of an existing virus; invariably they are formed by organization of material produced in the host cell.
Next we must consider whether viruses bear any likeness to the particles within a cell, particularly the particles called plasmagenes. Here differences are less easy to find. The theory has been proposed that viruses may originate as mutated plasmagenes. But we know that some plasmagenes (e.g., the chloroplasts of green plants) can grow and divide. Furthermore, plasmagenes are not pathogenic or lethal to the cell, as virus particles are. Let us just note, for the time being, that nothing which resembles a bacteriophage in its properties, life cycle, shape or organization has been found in normal cells.
Let us now consider the peculiar behavior of the virus. The virus particle as such is only the beginning and the end of a life cycle. Its only physiological function is to obtain entry of the virus' genetic material into the host cell. After this occurs, what remains of the virus is devoid of infectious power. There follows a vegetative phase in which the specific constituents for new viruses are produced. Finally these constituents are organized into virus particles, which are liberated by lysis (dissolution) of the cell. The whole process usually takes from 15 to 60 minutes.
But a bacterial virus may multiply in another way, and this is where the provirus enters the picture. Ordinarily the virus nucleic acid passed into a cell proceeds promptly to multiply and to synthesize specific protein material for new phage particles. Sometimes, however, the nucleic acid may anchor onto a bacterial chromosome and act as if it were a normal constituent of the cell. It behaves as a bacterial gene, being replicated at each bacterial division and transmitted to each daughter bacterium. This is what we call a provirus. It is a potential virus; it may eventually give rise to virus particles. In the meantime the bacterial offspring go on growing and dividing as normal bacteria, and each daughter bacterium yields progeny capable of producing viruses. In other words, the ability to produce viruses is perpetuated inside the bacterium; no new infection from outside is needed.
Bacteria containing proviruses are called lysogenic. When a small number of such bacteria are broken down, no infectious particles can be found. This means that the provirus is not infectious. And yet in every large population of these bacteria some mature bacteriophage particles appear. From time to time a bacterium in such a culture suddenly disappears, and about 100 phage particles emerge. The probability of a lysogenic bacterium spontaneously giving rise to viruses varies from 1/100 to 1/100,000. In some systems the probability is apparently independent of external factors; it cannot be modified. In other lysogenic systems phage production may be initiated at will by inducing agents, such as X-rays, ultraviolet rays, nitrogen mustard and other substances—all of which are known to be capable of producing mutation. Within 30 to 60 minutes after exposure to one of these agents, practically all of the bacteria produce viruses and lyse.
How do lysogenic bacteria produce viruses? Before discussing this question we must know more about the proviruses. We are inclined to think that proviruses originally arose as mutants of normal bacterial genes. Whatever their origin, the reservoir of bacterial viruses seems to be the provirus-carrying bacteria. These bacteria may have perpetuated provirus, that is to say, the hereditary ability to produce virus, for many thousands of years.
The study of lysogenic bacteria has led to a clear picture of the provirus. Apparently it does not contain virus protein, for lysogenic bacteria do not cause the production of specific antibodies to phage protein in experimental animals. It is therefore tempting to visualize the provirus as a large molecule of nucleic acid. Secondly, the provirus is associated with a certain genetic character of bacteria and is located at a specific site on a bacterial chromosome. Thirdly, two genetically related proviruses in a bacterium may cross over and recombine. Fourth, a lysogenic bacterium is immune to infection by a phage particle related genetically to its provirus, though it can be killed by an unrelated phage. As long as the provirus remains in that state, a genetically related superinfecting phage is unable to develop into phage. Finally, the mere presence of the provirus may modify the properties of a bacterium: it may endow certain bacteria with the ability to produce a toxin they could not otherwise make, or it may change the typical appearance of bacterial colonies. Things happen as if the provirus either carries a specific gene or modifies the neighboring bacterial genes. From all these data it may be concluded that provirus is the bacterial virus' genetic material, bound to a specific site in the bacterium and responsible for a specific bacterial immunity.
Now it is difficult to imagine that this immunity is due only to the presence of the provirus. A particle cannot exert a specific action by its mere presence. The only way the provirus can make the bacterium immune—that is, prevent multiplication of a virus invader—is by modifying or blocking a specific activity of the bacterium necessary for that reproduction. And the provirus can do this only if it is present at a specific site. As a matter of fact, we can account for all the properties of proviruses and of lysogenic bacteria by the hypothesis that the provirus is the genetic material of the virus anchored at a given site in the bacterium. The genetic material of an infecting virus becomes a provirus when and because it becomes bound at that site to a specific receptor, which modifies the material. It then gives the bacterium immunity against genetically related infecting particles. An inducing agent such as ultraviolet rays destroys the immunity because it displaces the genetic material of the virus from its specific site.
For a long time virologists have concentrated on the virus particle itself. Yet the particle is only a prelude to the infection. During the longest and most important part of the life cycle, the pathogenic phase in a cell, no virus particle is present. As a matter of fact, disappearance of the virus particle is the sine qua non for the development of the cellular lesion. Indeed, there are cases in which all the bacteria in a lysogenic population die although very few of them produce bacteriophage particles; the cells are killed by a defective development of proviruses initiated by an inducing agent. One could even conceive of a condition in which the probability of the virus ever appearing would be infinitely small, that is to say, practically absent. Some bacteria actually carry a gene which can initiate the synthesis of a protein lethal to themselves. But that is another story.
Biologists have long been accustomed to think of death in terms of the destruction or alteration of some vital structure. We have been less inclined to think of living cells as carrying the seeds of their own destruction, or of the possibility that lethal agents may kill in more than one way. For example, X-rays sometimes kill by destroying essential structures, but they may also destroy a cell by inducing a gene to express its lethal potentiality. This potentiality is sometimes the power to start a new synthesis which may or may not end in virus particles.
To what extent are the phenomena disclosed in bacteria valid for higher organisms? May animal or plant cells perpetuate proviruses? Are some viral diseases of man the result of the activation of a provirus? May immunity to virus diseases be explained in terms of proviruses? Do the findings concerning lysogeny have any bearing on cancer? Let us recall that the inducing agents which can trigger proviruses to give rise to viruses are all not only mutagenic but also carcinogenic—radiations, nitrogen mustard and so on. It is indeed tempting to theorize that carcinogens may induce malignancy by initiating the formation of a pathological structure from a provirus-like material. Many facts are in favor of the hypothesis that proviruses originate some animal diseases, but the problem cannot be discussed within the limits of this article. Suffice it to say that this is, at any rate, a working hypothesis.
I have tried to outline the concept of the provirus, to analyze its relations with the concept of the cell and of the virus and to show the impact of the newly acquired knowledge on our conceptions of cellular disease. The common denominator of the various phases of the life cycle of a virus is the genetic material—the nucleic acid—which may exist in three states: infectious, proviral and vegetative. Throughout these three states the genetic material apparently remains essentially the same in structure, but it changes radically in dynamic potentialities and behavior. The virus particle, the end product of the vegetative phase, is a quiescent nucleoprotein particle, unable to grow or to divide. The provirus is an integrated nucleic acid, which behaves like a gene and is replicated like the host genes. Neither the virus particle nor the provirus is pathogenic per se; their pathogenic property is only potential. The only pathogenic phase of the virus is the vegetative phase, during which the specific viral nucleic acid multiplies and during which the specific viral protein is synthesized. Things happen as if the synthesis of the protein is responsible for pathogenicity.
The provirus produces provirus; it is order. The vegetative particle produces virus particles and a disease of the host; it is disorder. The virus particle does not produce anything; it is an extremely conservative particle—the absence of any activity, that is to say, a kind of order. Thus the virus is an alternation of order and disorder.
As a result, my presentation of the subject may seem somewhat disordered. For this I had decided to apologize, when I came across an unpublished letter which Martin de Barcos, Abbot of Saint-Cyran, wrote to Mother Angélique in 1652: 'Allow me to tell you that you would be wrong to apologize for the disorder of your discourse and of your thoughts, because, if they were otherwise, things would not be in order, especially for a person belonging to your profession. As there is a wisdom which is folly before God, there is also an order which is disorder, and in consequence, there is a folly which is wisdom and a disorder which is the true rule.' This being exactly the case of the virus, I decided not to apologize.