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&7Issues in Space Exploration: Hazards and Criticisms There are several concerns and criticisms regarding the exploration of space. A few of these will be discussed in this section. There are many hazards associated with travelling through space. The extreme cold and near vacuum of space make it an unforgiving environment. Human errors or technical failure could be catastrophic. There is very little gravity in space and there are high-energy ionising cosmic rays (HZE). These could have severe health implications for astronauts on a long voyage such as to Mars or beyond. Damage could occur to cells, tissues, and the hormonal and immune systems. Other concerns include alterations in microbes due to exposure to space flight conditions. This may result in previously harmless microorganisms endangering the crew and life support systems. Spaceflight conditions may alter the immune system in astronauts. This results in an increased risk of infections or cancers. Work at the International Space Station (ISS) could play an important role in assessing the health dangers in space and in the development of potential countermeasures. In observing astronauts travelling on the Space Shuttle and Russian cosmonauts' long-term visits to the Mir space station, indications are that time spent in Og has serious effects on bone and muscle physiology and the cardiovascular system. For instance, the return from Og to 1g leads to an inability to maintain an appropriate blood pressure when in an upright position-orthostatic intolerance-and insufficient blood flow to the brain. Astronauts returning from orbit therefore have to rest for several minutes, and the time needed to normalize their blood pressure increases with the time spent in Og. This could mean that astronauts travelling to Mars-which would take at least one year in 0g-would need considerable time to readapt to gravity after landing there or after their return to Earth, unless we find a technological solution to the creation of artificial gravity on a spaceship. Moreover, there are other cardiovascular effects, such as cardiac arrhythmia and atrophy, that need to be studied in more detail before we can ensure the safety of astronauts on a Mars mission. Other effects of extended time in low gravity are loss of bone mass and muscle deterioration. Without adequate countermeasures, these could impair the ability of astronauts to perform necessary functions on a spacecraft or on the surface of Mars. The cost of space research and exploration is very high. For example, the cost of sending a crew of six or seven astronauts on a three-year trip to Mars and back is certainly not feasible without significant physical and financial collaboration and cooperation among many countries. An even longer voyage in a manned spacecraft outside the solar system is not feasible at the present time. A possible solution is to construct new lift-off capabilities and a much faster spacecraft to drastically reduce the time being spent in space and thus the radiation exposure and other stresses on astronauts. However, if nuclear-powered spacecraft are used to accomplish this goal, it is hard to envisage take-off and landing scenarios that would satisfy environmental concerns. There are many categories of risks that must be researched further and for which countermeasures need to be developed. &7 Kepler's First Law Johannes Kepler (1571-1630) was born in southwest Germany. He had been greatly influenced by the work of the Greeks and spent a great deal of time trying to devise systems with which to explain the motion of the planets about the sun. The great Danish astronomer Tycho Brahe (1546-1601) had prepared very accurate tables showing the positions of the planets in the skies. When Brahe died, he left this invaluable data for Kepler. No matter how hard he tried, Kepler could not find a system of perfect circles to describe the motion of the planets. Finally in 1595 he stopped investigations focused on the celestial spheres and instead searched for some non-circular curve that would suit the data. First, he tried an egg-shaped oval, and then he settled on the ellipse. Using Tycho's observations of the planet Mars, Kepler found that its motion fitted an elliptical orbit with a high degree of accuracy. He later found that the orbits of the other known planets could be drawn as ellipses, with the sun at one of the foci. This he announced in a book published in 1609. The motion of the planets in ellipses became known as Kepler's First Law. An exaggerated diagram of an ellipse and two foci is shown below. The sun is at one of the foci, Fy. sun An ellipse can be defined as a closed curve such that the sum of the distances from any point P on the curve to two fixed points (called the foci F4 and F;) remains constant. That is, the sum of the distances F;P + F,P is the same for all points on the curve. A circle is a special case of an ellipse in which the two foci coincide at the centre of the circle. We can state Kepler's First Law as follows. The planets move about the sun in elliptical orbits, with the sun at one focus of the ellipses. &7 Kepler's Second Law Kepler's Second Law further describes the motion of a planet around the sun. This law examined the areas swept out by the planets as they moved around the sun. area B area A Aty Aty4 4 sun The diagram shows two equal time intervals Aty and At 4. For these time intervals, the areas swept out by the line joining the sun and the planet are equal, that is area A = area B. This means that the closer a planet is to the sun, the faster it will move, and it will move in a predictable way. Kepler's ellipses put an end to Greek astronomy. The long established concept of celestial spheres and perfectly circular motion had been destroyed. Kepler's scheme of the solar system has been followed by astronomers and scientists ever since, without significant change. Kepler's Second Law can be stated as follows. The straight line joining the sun and a given planet sweeps out equal areas in equal intervals of time. @ Kepler's Ihird Law In 1619, Kepler published a second book. It contained what is now known as Kepler's Third Law. It stated that the square of the orbital period of a planet is proportional to the cube of its mean distance from the sun. It can be expressed mathematically as where R is the mean radius of the orbit of the planet, T is the period of revolution about the sun, and K is a constant, usually referred to as Kepler's constant. The following table provides some accurate information about our solar system. Using the values for the mean orbital radius and the period of each planet, the ratio R%/T2 has been calculated in the last column. Note that using modern measurements, the value is a constant for the solar orbits, as predicted by Kepler in his Third Law. Object Mass Radius of Period of Mean Radius Period of Kepler (kg) object (m) rotation of orbit (m) revolution constant on axis (s) of orbit (s) R/T? (m?/s?) sun 198 x 10 6.95 x 10 2.14 x 10 - - - Mercury 328 x10% 257 x10 5.05x10 579 x10" 760 x10 3.35x 108 Venus 4.83 x 10 631x10 210x10" 1.08 x 10" 1.94 x 107 3.35 x 10' Earth 5.98 x 10 6.38 x10 8.61 x10* 149 x10" 3.16 x 10" 3.35 x 10" Mars 6.37 x10 343 x10 8.85x10* 228 x10' 594 x10" 3.35 x 10" Jupiter 1.90 x 10?7 718 x 107 3.54 x10* 7.78 x 10" 3.74 x10 3.35 x 10' Saturn 567 x10% 6.03 x 107 3.60x10* 143 x10'2? 930 x10 3.35x 10' Uranus 8.80 x 10 267 x 107 3.88 x 10* 287 x 102 266 x10 3.34 x 10'8 Neptune 1.03 x 1026 248 x 107 569 x10 4.50 x 10" 520 x 10 3.37 x 108 Pluto 6 x 1022 3x10 551x10 59x10'? 7.82x10 3.36x10' moon 7.35x102 174x10 236x10 3.85x10 236x10 - On the basis of this data, we can say that for objects in orbit around the sun, the value of Kepler's constant is K = 3.35 x 108 m3/s2. By using Kepler's Third Law, it is possible to determine the period of revolution, or the mean radius of orbit for any object in orbit around the sun. Kepler's constant can also be determined for objects in orbit around other central bodies. So for example, the motion of the moon around Earth could be used to determine K for our earth. Once we know what the value of K is, then we can also determine the period of revolution or the mean radius of orbit for that object. This will be further explored in the assignment. In this lesson, we studied the benefits, and the hazards and criticisms of contributions of space travel and space research and the contributions of Johannes Kepler to our understanding of planetary motion. Kepler's First Law states that the planets move about the sun in elliptical orbits, with the sun at one focus of the ellipses. e sun Kepler's Second Law states that the straight line joining the sun and a given planet sweeps out equal areas in equal intervals of time. area B area A At Atz 4 4 sun Kepler's Third Law states that the square of the orbital period of a planet is proportional to the cube of its mean distance from the sun. It can be expressed mathematically as R3 K where R is the mean radius of the orbit of the planet, T is the period of revolution about the sun, and K is a constant, usually referred to as Kepler's constant. Kepler's constant can also be determined for objects in orbit around other central bodies. In this case, a new value of K would have to be determined. We are now ready to move on to the next lesson where we will learn about the contributions of Newton and how his work allows us to further understand the motion of satellites. Question(s): 1. The physics of Mars and its two moons. Mars has two moons, Phobos and Deimos (Fear and Panic, the companions of Mars, the god of war). Deimos has a period of 30 h 18 min and a mean distance from the centre of Mars of 2.3 x 104 km. If the period of Phobos is 7 h 39 min, what is the mean distance (in km) from the centre of Mars? 2. The physics of orbital radius and orbital period of a planet. If a small planet were discovered whose orbital period was twice that of Earth, how many times farther from the sun would this planet be? 3. The physics of Kepler's constant for Earth satellites. Using the data presented in the lesson, determine Kepler's constant for any satellite of Earth