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Recall that the path of an object under the influence of gravity through space is called its orbit, whether that object is a spacecraft, planet, star, or galaxy. An orbit, once determined, allows the future positions of the object to be calculated. Two points in any orbit in our solar system have been given special names. The place where the planet is closest to the Sun (helios in Greek) and moves the fastest is called the perihelion of its orbit, and the place where it is farthest away and moves the most slowly is the aphelion. For the Moon or a satellite orbiting Earth (gee in Greek), the corresponding terms are perigee and apogee. (In this book, we use the word moon for a natural object that goes around a planet and the word satellite to mean a human-made object that revolves around a planet.) Orbits of the PlanetsToday, Newton’s work enables us to calculate and predict the orbits of the planets with marvelous precision. We know eight planets, beginning with Mercury closest to the Sun and extending outward to Neptune. The average orbital data for the planets are summarized in Table 1. (Ceres is the largest of the asteroids, now considered a dwarf planet.) According to Kepler’s laws, Mercury must have the shortest orbital period (88 Earth-days); thus, it has the highest orbital speed, averaging 48 kilometers per second. At the opposite extreme, Neptune has a period of 165 years and an average orbital speed of just 5 kilometers per second. All the planets have orbits of rather low eccentricity. The most eccentric orbit is that of Mercury (0.21); the rest have eccentricities smaller than 0.1. It is fortunate that among the rest, Mars has an eccentricity greater than that of many of the other planets. Otherwise the pre-telescopic observations of Brahe would not have been sufficient for Kepler to deduce that its orbit had the shape of an ellipse rather than a circle. The planetary orbits are also confined close to a common plane, which is near the plane of Earth’s orbit (called the ecliptic). The strange orbit of the dwarf planet Pluto is inclined about 17° to the ecliptic, and that of the dwarf planet Eris (orbiting even farther away from the Sun than Pluto) by 44°, but all the major planets lie within 10° of the common plane of the solar system. You can use an orbital simulator to design your own mini solar system with up to four bodies. Adjust masses, velocities, and positions of the planets, and see what happens to their orbits as a result. Orbits of Asteroids and CometsIn addition to the eight planets, there are many smaller objects in the solar system. Some of these are moons (natural satellites) that orbit all the planets except Mercury and Venus. In addition, there are two classes of smaller objects in heliocentric orbits: asteroids and comets. Both asteroids and comets are believed to be small chunks of material left over from the formation process of the solar system. In general, asteroids have orbits with smaller semimajor axes than do comets (Figure 1). The majority of them lie between 2.2 and 3.3 AU, in the region known as the asteroid belt (see Comets and Asteroids: Debris of the Solar System). As you can see in Table 1, the asteroid belt (represented by its largest member, Ceres) is in the middle of a gap between the orbits of Mars and Jupiter. It is because these two planets are so far apart that stable orbits of small bodies can exist in the region between them.  Figure 1: Solar System Orbits. We see the orbits of typical comets and asteroids compared with those of the planets Mercury, Venus, Earth, Mars, and Jupiter (black circles). Shown in red are three comets: Halley, Kopff, and Encke. In blue are the four largest asteroids: Ceres, Pallas, Vesta, and Hygeia.
Comets generally have orbits of larger size and greater eccentricity than those of the asteroids. Typically, the eccentricity of their orbits is 0.8 or higher. According to Kepler’s second law, therefore, they spend most of their time far from the Sun, moving very slowly. As they approach perihelion, the comets speed up and whip through the inner parts of their orbits more rapidly.
The closest point in a satellite orbit around Earth is its perigee, and the farthest point is its apogee (corresponding to perihelion and aphelion for an orbit around the Sun). The planets follow orbits around the Sun that are nearly circular and in the same plane. Most asteroids are found between Mars and Jupiter in the asteroid belt, whereas comets generally follow orbits of high eccentricity. Glossaryaphelion: the point in its orbit where a planet (or other orbiting object) is farthest from the Sun apogee: the point in its orbit where an Earth satellite is farthest from Earth asteroid belt: the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun perigee: the point in its orbit where an Earth satellite is closest to Earth perihelion: the point in its orbit where a planet (or other orbiting object) is nearest to the Sun satellite: an object that revolves around a planet
Credit: © 1997 Loke Kun Tan Comets are large balls of rock, ice, dust and gases. They contain frozen leftovers from the early Solar System. Material that is about 4,600 million years old! Scientists think that comets could have hit the Earth when the Solar System was very young. They may have brought water and organic compounds with them. These are the ingredients needed for life. During most of their orbit, comets are hard to see. This is because they are small and made of dusty ice which does not reflect much light from the Sun. As a comet get closer to the Sun, the ice in the comet heats up. Some of this ice turns into a gas. The gas gets lit up by the Sun's light, making it easier to spot with a telescope. Astronomers have discovered about 4,000 comets in our Solar System so far. Most comets come from beyond Pluto, in the Kuiper Belt and Oort Cloud. We have not fully explored these dark and distant parts of space yet. So there are probably billions more comets out there! What are the different parts of a comet?Credit: NSO The solid centre of a comet is called its nucleus. It is made of solid chunks of ice along with frozen gases, rocks and dust. You can think of it as giant, dirty snowball. The nucleus is usually only hundreds of metres across, but can be the size of a small town! Astronomers have seen comets releasing jets of gas and dust from their nucleus, like a geyser. As a comet get closer to the Sun, the ice in the comet heats up. Some of this ice turns into a gas. The gas (plus dust and some water) forms a cloud around the nucleus, called a coma. You can think of the coma as the comet’s atmosphere. The coma can be as large as a planet! Sunlight and super-fast particles from the Sun (solar wind) can push the coma into a large tail. This tail always points away from the Sun. The gas in the tail contains changed particles called ions, so this tail is sometimes called the ion tail. Some comet's ion tails are over a million kilometres long! Comets can also leave a trail of small rocks and dust in their path. This is known as the dust tail. When the Earth passes through a comet's dust tail, debris from the comet falls through our atmosphere. This creates meteor showers. Why do comets sometimes get close to the Sun?Credit: NASA Comets sometimes get close to the Sun because they have very elliptical orbits. This means that they go around the Sun in a large, stretched out oval shape, rather than a circle. The furthest point from the Sun in an orbit is called aphelion. The aphelion of most comets is way beyond the planet Neptune. The closest point from the Sun in an orbit is called perihelion. The perihelion of some comets can be closer to the Sun than the planet Mercury! These kinds of comets are called 'sungrazers'. These comets may end up crashing into the Sun. Or the heat from the Sun will cause them to break into pieces and evaporate away. What are some well-known comets?
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Occupation: Astronomer, Educator, Librarian Year born: 1818 Research Areas: Comets, Solar Eclipse, Planets, Double Stars, Nebulae
Credit: NASA Occupation: Astronomer & Planetary Scientist Year born: 1905 Research Areas: Atmospheres and Moons
Credit: R Phillips Occupation: Astronomer and Mathematician Year born: 1656 Research Areas: Comets, Transits, Stars, Maths, Geophysics
Credit: Jedimaster
Credit: NASA/JPL
Credit: © 1997 Loke Kun Tan Comets are large balls of rock, ice, dust and gases. They contain frozen leftovers from the early Solar System. Material that is about 4,600 million years old! |
What is the point in the orbit of a planet asteroid or comet at which it is closest to the Sun?
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