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Orbit by Tega Jessa Everything in the universe circles or “orbits” something else. It’s true for meteors and planets, stars and galaxies. They’re all under the influence of each other, moving in an ageless and eternal dance. So what exactly is is orbit? What causes this phenomenon? And why is it so important to spaceflight and space exploration? In this article I’ll try to briefly answer these questions. First of all, orbit is the circular – or more accurately – ovoid path of an object around a center of mass. As we all learn in high school physics, Force equals mass times acceleration. When a mass changes speed and direction (which is acceleration), it produces a force. The main expression of this force in space is gravity. Gravity is the pull of every object in existence on each other. The bigger the difference between two objects in proximity to each other, the greater the force of gravity between them. This is why a person is held to the ground by the gravitational force of the much more massive Earth. This is true for any two objects with a similar mass ratio. The only way to escape the gravitational pull of a massive object is to have considerable mass yourself and or move at a high speed. If you were to give a short explanation of why orbits happen it would be that an object isn’t massive enough and or fast enough to most escape the gravitational pull of its center of mass. Orbit happens as an object is able to normally have enough mass and speed to avoid the pull of gravity from the much larger object near it. If we are to use the Earth as example we find out why. Say you launched a baseball into the air. Under normal circumstances the ball will quickly fall back to the ground. But what happens if you were to throw the ball higher? It would still fall but take longer depending on how high it is. However if you were to throw the ball fast enough and high enough it would go into orbit. That is because as an object goes in a straight line at a fast enough speed past a planet, the gravity of that planet pulls on the object. This create the object’s orbit. Another good example is a ball on a string being swung by a child. As long as the child pulls on the string while swinging the ball it goes in a circle, but when the child lets go the ball goes in a straight line. Orbit is important to space flight for several reasons. First, the observation of orbit gave aeronautics the principles that helps space craft escape the earth’s atmosphere. Second, several instruments and craft used by spaceflight rely on the use of orbits in one way or another. So as you can see orbit are very important for a lot of reasons. Gravity in Space by Tega Jessa One of the big challenges to long term travel in space is gravity. We now know that other than radiation and the cold vacuum of space, the biggest threat to a person’s health is lack of gravity. The human body was developed to thrive under the force of gravity. This is why our muscular and skeletal systems are shaped the way they are. Over time, in a zero g or weightless environment, the bones of the body become brittle and certain muscles, like those in the thigh actually weaken. There are also worries about its effect on the circulatory system as well Planetary Orbits by John Carl Villanueva So if a voyage with the latest propulsion technology to Mars takes 11 months or more coming and going, there is a concern that the long amount of time in space might have even more detrimental effects, like making it impossible for astronauts to safely return to Earth. So scientist and even science fiction writers have been thinking about how to create gravity in space. There are several proposals some already proven and others that are still theory. The first is to us the centripetal force of a rotating hull. This is method is the most trustworthy as the effect has been observed on Earth. Every object in the universe wants to go in a straight line. In the case of planets, its the pull of gravity from an object with great mass that curves most celestial object’s paths. In the case of a space craft it would be the hull itself. Since you can’t walk through solid walls the force of your body wanting to go in a straight line would resemble gravity. This theory has drawbacks. First, the amount of gravity you feel would vary depending on how close you are to the center of the craft. Scientists believe that the longer the radius of the spacecraft the less the effect will be felt. The other challenge is nausea. You are still basically on something like a huge merry go round. Like on its namesake, some people will be able to handle it and others will experience nausea. But in most cases its believed that if if the space craft rotates at 1 rotation per minute the nausea won’t be an issue. The other method of maintaining gravity is to constantly accelerate a spacecraft at 9.8 m/s, Earth’s acceleration due to gravity. The problem with this is that due to limitations imposed by fuel there is no spacecraft that can maintain this rate of acceleration more than 7 minutes. This idea might become possible if a more reliable source of propulsion is found. Planets in a star system move around their mother star in their planetary orbits. Most of these orbits are elliptical, i.e., shaped as an ellipse. That means, while it may be possible for some planets to follow a circular path (since a circle is just a special kind of ellipse) around their mother star, most of them do not. The deviation of the planets‘orbital path from a perfect circle, which is measured in terms of a parameter known as eccentricity, is normally not large though. Eccentricity values of elliptical orbital paths range from zero (the eccentricity of a perfect circle) to 1. In our Solar System, Mercury has the largest eccentricity (most deviated from a perfect circle) at 0.2056. Interestingly, its adjacent neighbor, Venus, has the lowest eccentricity at 0.0068. Had scientists not demoted Pluto from being planet, it would have had the highest eccentricity at 0.249. In case you’re interested, Earth‘s eccentricity is 0.0167. Eccentricity plays a role in the determining the amount of heat a planet gets from its mother star. A planet gets its lowest amount of sunlight at its aphelion. There was a time when the Earth, not the Sun, was believed to be the center of the Solar System (in fact, also of the Universe). This idea was known as the geocentric model. It was so well-embraced, that many ancient Greek astronomers, including Plato, Aristotle and Ptolemy, subscribed to it. The idea existed as far back as 6th century BC, and prevailed in teachings of astronomy for a long time. In fact, it was not until the mid 1500′s that a well presented argument against the geocentric model was introduced. The opposing theory was proposed by Nicolaus Copernicus and gave mankind its first taste of heliocentric cosmology, the view that the Sun was the center about which other celestial bodies orbited. About half a century later, Johannes Kepler composed his three laws of planetary motion. The first stated that, “The orbit of every planet is an ellipse, with the Sun at a focus.” The second stated that, “A line joining a planet and the Sun sweeps out equal areas in equal times.” Finally, the third stated that, “The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.” On that same century, in 1687, Isaac Newton formulated his law of universal gravitation, which talked about the force that held the planets in their planetary orbits. The most recent prominent contribution to the theories governing planetary orbits is Einstein’s general theory of relativity, which explains gravity in the context of spatio-temporal curvature. Eccentricity by Jean Tate When it comes to space, the word eccentricity nearly always refers to orbital eccentricity, or the eccentricity of the orbit of an astronomical body, like a planet, star, or moon. In turn, this relies on a mathematical description, or summary, of the body’s orbit, assuming Newtonian gravity (or something very close to it). Such orbits are approximately elliptical in shape, and a key parameter describing the ellipse is its eccentricity. In simple terms, a circular orbit has an eccentricity of zero, and a parabolic or radial orbit an eccentricity of 1 (if the orbit is hyperbolic, its eccentricity is greater than 1); of course, if the eccentricity is 1 or greater, the ‘orbit’ is a bit of a misnomer! In a planetary system with more than one planet (or for a planet with more than one moon, or a multiple star system other than a binary), orbits are only approximately elliptical, because each planet has a gravitational pull on every other one, and these accelerations produce nonelliptical orbits. And modeling orbits assuming the theory of general relativity describes gravity also leads to orbits which are only approximately elliptical (this is particular so for binary pulsars). Nonetheless, orbits are nearly always summarized as ellipses, with eccentricity as one of the key orbital parameters. Why? Because this is very convenient, and because deviations from ellipses can be easily described by small perturbations. The formula for eccentricity, in a two-body system under Newtonian gravity, is relatively easy to write, but, unfortunately, beyond the capabilities of the HTML coding of this webpage. However, if you know the maximum distance of a body, from the center of mass – the apoapsis (apohelion, for solar system planets), ra – and the minimum such distance – the periapsis (perihelion), rp – then the eccentricity, e, of the orbit is just: E = (ra – rp)/( ra+ rp) Weight on Other Planets by Abby Cessna Many children, and even adults, dream of visiting other planets and wonder what it would be like to stand on another planet. For one thing, your weight would be different on another planet, depending on a number of factors including the mass of the planet and how far you are away from the center of the planet. Before we start, it’s important to understand that the kilogram is actually a measurement of your mass. And your mass doesn’t change when you go anywhere in the Universe and experience different amounts of gravity. Your weight is best measured in newtons. But since your bathroom doesn’t measure your weight in newtons, we’ll use kilograms. This is what your bathroom scale would say if you stepped on another world. Mercury is the smallest planet in our Solar System, but it is dense. Because Mercury is so small, it has very little gravity. If you weighed 68 kg on Earth, you would only weigh 25.7 kg on Mercury. Venus is very close to Earth in size and mass. Venus’ mass is roughly 90% of the mass of the Earth. Thus, it is no surprise that someone would weigh a similar amount on Venus. Someone who weighed 68 kg on Earth would weigh 61.6 kg on Venus. Mars is quite a bit smaller than Earth with only 11% of our planet‘s mass. Mars is larger than Mercury, but it is not as dense as the smaller planet. If you weighed 68 kg on Earth then you would weigh 25.6 kg on Mars. Since Pluto was demoted to a dwarf planet, Mars became the planet where you would weigh the least. Jupiter is the largest planet in our Solar System with the most mass. Because of Jupiter’s mass, you would weigh more on that planet than on any other one in our Solar System. If you weighed 68 kg on Earth then you would weigh 160.7 kg on Jupiter, over twice your normal weight. That is if you could actually stand on Jupiter’s surface, which is impossible because it is a gas giant, and gas giants do not have solid surfaces. Saturn is a gas giant best known for its planetary rings system. It is also the second biggest planet in our Solar System. Despite its mass though, the planet has a very low density and a lower gravity than Earth. If you weighed 68 kg on Earth, you would weigh 72.3 kg on Saturn. Uranus is a gas giant without a solid surface. Although Uranus is larger in size than Neptune, it has less mass and therefore less gravity. You would only weigh 60.4 kg on Uranus, if you weighed 68 kg on Earth. Neptune, the last planet in our Solar System, is a gas giant. If you weighed 68 kg on Earth, then you would weigh 76.5 kg on Neptune if you could stand on the planet’s surface. Although the Moon is not a planet, it is one of the few objects that astronauts have actually visited. Because the Moon is so small, it has a low density and low gravity. If you weighed 68 kg on Earth, then you would only weigh 11.2 kg on the Moon.