The Effects of the Sun on Comets
By Ruth Bannerman, Jack Cay, Ivan Wittmeier, Mackenzie Little, Pourya Pourhaj
Heat, radiation, solar wind, and gravity are four major influences and effects between comets and the Sun. Comets were first discovered with a telescope in 1680 by Gottfried Kirch and have two origins, the Kuiper belt and Oort Cloud. Depending on the comet’s age and their interaction with the Sun, the surface of each comet differs. We see this in the different variations between the Wild 2 and Temple 1; with the Wild 2 comet maintaining its surface due to lack of interaction with the Sun, and the Temple 1 comet losing its surface because of its constant orbiting of the Sun. Gottfried’s discovery of comets has led to other discoveries including how comets interact with heat, radiation, solar wind and gravity. The Sun’s solar heat causes the formation of the comet’s coma. Comets that get close to the Sun warm up and develop this coma, which causes the ice on comets to turn into gas. The more exposed comets are on their elliptical pathway, the more gases are produced. This gas trail is known as a plasma tail. Due to the charge-exchange reactions in the coma from the Sun, it creates the distinctive twists and knots. In addition, radiation from the Sun is wave energy that directs the dust and gas tails of a comet. The radiation is coming directly from the Sun, so as a result the tails are pushed away from the Sun as well. Solar wind is particles shot out from the Sun that knock loose material from a comet’s surface, causing it to erode. Both radiation and solar wind from the Sun play important roles in a comet’s display and life. Finally, gravity is the force that powers the orbits of comets, and an understanding of gravitational force allows us to interpret the elliptical motion of these bodies in our pursuit of astronomical understanding. The comet’s constant speed changes as a result of distance changes in relation to the Sun help to explain why the orbits are so elliptical in contrast to the planets, we are more familiar with. Comets are one of the smaller bodies in our solar system and help show us how much the Sun influences everything around it. This paper will examine four major influences and effects that occur between comets and the Sun’s heat, radiation, solar wind and gravity.
The Sun and Comets
The Sun is a “hot ball of glowing gases” while comets are “cosmic snowballs of frozen gases, rock and dust” that orbit the Sun.1 As comets get closer to the Sun, they heat up, discharging gas and dust.2 Prior to discussing the effect of the Sun on Comets, outlined below is an overview of Comets. Gottfried Kirch (Figure 1), a German, discovered comets in 1680. Kirch was first to view a comet with a telescope, an important discovery.3 Comets have two origins: first, long-period comets that come from the Oort Cloud. Second, short-period comets originate from the Kuiper Belt. In general, comets are too faint and must be viewed through a telescope. However, when comets pass close to the Sun, their tails and coma reflect sunlight absorbed from the sun, making them visible without a telescope.4 There are four parts to a comet: the coma, the ion tail, the dust tail and the nucleus (Figure 2).5
Figure 2: Halley’s Comet © B. Nath, https://www.needpix.com/photo/664458/space-stars-comet-astronomy-universe-science
The coma made of comet dust and ice are particles and gases that form clouds around the nucleus while the ion tail is created from gases from the coma. The dust tail is formed from dust particles of the coma.6 The nucleus, referred to as the heart of the comet, is a mixture of dust and ice particles along with carbon dioxide, methane and ammonia and is the solid part of a comet.7, 8 The surfaces of a comet nuclei (the nucleus of the comet) differs (Figure 3) depending on Sun’s alteration of it. As we see in Figure 3: Comet Wild 2, the comets are covered in craters: these surface holes are “consistent with high-velocity collisions between the comet and smaller meteorite like objects” while the surface of Temple 1 reveals an older comet.9 Referenced as a “marshmallow dipped in sugar,” every five and a half years, Temple 1 “is moved nearer the fire and withdrawn to a cooler place. With each approach to the Sun, a bit more of its constituent dust and volatile material is driven off into interplanetary space; so it evolves.”9 Using the marshmallow analogy, as marshmallows dipped in sugar get closer to the fire, they start to lose their surface like with the Temple 1 comet which also starts to lose parts of its surface (i.e., its constituent dust and volatile material) as it nears heat; in this case, the Sun. Unlike Temple 1, Wild 2 is a newer comet and has not orbited the Sun as much at a close distance.10
Comet: “Wild 2” Comet: “Temple 1”
Figure 3: Comparing Comets © NASA, https://www.nasa.gov/pdf/518711main_Comparing_Comets_SA.pdf
Solar Heat and Comets
Solar heat is important to consider while examining the effects of the Sun on comets. As theorized by astronomer Gerard Kuiperin in 1951, comets come from a disc-like belt of icy bodies.11 The extreme solar heating and plasma environments that comets encounter can explain to us the aspects of their behavior.12 These icy objects are often pushed by gravity into the orbit of the Sun. Each comet carries a part called the nucleus which contains icy lumps and frozen gases. When comets come into orbit with the Sun, they warm up and develop an atmosphere. The solar heat from the Sun affects the comet’s ice and changes them into gases, therefor the atmosphere or coma will become larger and can extend hundreds of thousands of kilometres.11 Comets are completely frozen when they are far away from the Sun, and as they move closer, their appearance changes aggressively. By the time a comet reaches 5 AU within the Sun, the solar heat begins to form the coma, while the escaping atmosphere drags away the dust particles which were mixing with the sublimating ice. The gas tail is also known as a plasma tail, and due to the charge-exchange reactions and photo ionization of atoms in the coma from the Sun, they create the distinctive twists and knots.13 The closer the comet reaches the Sun, the more ice turns into a gas and the intense heat vaporizes the gas and releases the dust which creates the two distinct tails behind the comet.14 The striking feature of the comet’s nuclei is that they can show unexpected behavior. Some will outburst when they are close to the Sun and others will when they are far away.15 That is why while examining comets it is important to consider other factors such as the electromagnetic radiation released from the Sun.
Radiation, Solar Wind and Comets
Radiation is the release of energy in the form of waves or particles. We are exposed to radiation every day through man-made material or natural things like the sun. There are two different kinds of radiation: particulate and electromagnetic radiation. Particulate radiation is when an unsettled atom disintegrates. Electromagnetic radiation is energy that has no mass and travels in various wave-like patterns. These assorted patterns of waves make up what is called the electromagnetic spectrum. The higher the energy level of the electromagnetic radiation the shorter the wavelength.16 The Sun releases electromagnetic radiation in the form of light. The Sun emits various wavelengths, but the shorter wavelengths are in the form of sunlight and ultraviolet rays.17 Solar winds are particles that also come from the Sun and disperse into the universe. Among many other things in the solar system, comets are affected by the radiation and solar wind that the Sun gives off.
Comets fly through our universe, but interesting things start to happen when they get close to our Sun. The closer the comet comes to the Sun, the warmer they become. When this happens, the ice on the comet turns into a gas and the radiation from the Sun drives dust away from the coma. This results in the dust tail. Charged particles from the Sun can also make gasses from the comet turn into an ion. This produces a second tail called the ion tail. These two tails are always pointing away from the Sun as the comet orbits it. The reason this happens is that radiation is driven away from the Sun in all directions. When the comet comes within the Sun’s radiation, they push the two tails away from the Sun because radiation is pushing the particles of the tail away.4, 18 Solar wind also plays a role in directing the comet’s tail away from the Sun. Solar wind is particles that are shot out by the Sun. These particles bombard comets and knock off loose material, causing the comet to erode while also directing the tails. The solar wind also plays an important part in the shaping of a comet. This is how radiation and solar wind affect a comet. For example, let us say you put a lit lantern in a dark room, then take an object and place it anywhere around the lantern. Wherever that object is in relation to the lantern, the shadow cast from the object is always pointing away from the lantern. This is how radiation and solar wind affect a comet. The lantern is the Sun, the light from the lantern is the radiation and solar wind, the object is the comet, and its shadow is the tails of the comet. Radiation and solar wind play a role in the direction of both the dust and ion tail on the comet, but also on the erosion of the comet, making them smaller as they shed the dust off it.
Figure 4: Comet’s tails as they go around the Sun. Sun© NASA
Gravity and Comets
Gravity is a force that attracts any two bodies toward each other and is the driving force behind orbits in the universe. This gravitational force was first defined by Isaac Newton and is described in Figure 5 below.
m1: mass of object 1
m2: mass of object 2
d: distance between objects 1 and 2
Figure 5: Newtons Universal Gravitation Equation
One of the interesting properties of comets is the shape and behavior of their orbits caused by this gravitational force. Planetary orbits around the Sun are nearly circular in nature, meaning that they have an almost constant speed and trajectory as they orbit around the sun. Comet orbits around the Sun are more elliptical in nature, and the speed of their orbit is constantly changing as a result. This can be easily explained by the equation outlined in Figure 5. While the masses and gravitation constant stay about the same, the distance is constantly changing, causing the pulling force between the two objects to be constantly changing .19 As a comet approaches the Sun for instance, the distance is decreasing, causing the force between the objects to increase at a rate proportional to (1/d2), thus causing acceleration of the comet towards the Sun. It is worth noting that the reason that the comet is accelerating, and the Sun is staying relatively stationary, is because of Newtons second law(force=mass*acceleration) which essentially states that the force required to accelerate an object is proportional to its mass meaning that in order to move the Sun’s very large mass, a very large force would be required. The gravitational force acting on the Sun from the comet is negligible from the Sun’s perspective, thus allowing the Sun to win the tug of war battle between the two bodies. Getting back to the acceleration of the comet towards the Sun, once the comet passes the Sun, the distance between the two objects is so small, that it causes the comet to take a pretty hard turn and start to travel away from the Sun, following it’s elliptical path. However, once they have passed the Sun, the distance between the two is increasing, causing the pulling force between the two objects to decrease at a rate of (1/d2). This causes the comet to begin decelerating. Once the comets speed has decreased enough, they begin to turn around and follow its elliptical orbit back to the Sun and the whole process starts over again. This orbit is demonstrated by Figure 6. It should be noted that the gravitational forces of other astronomical objects. Planets for instance, can exert enough gravitational force on comets to deflect them from their solar orbit and onto new trajectories.
Figure 6: Diagram demonstrating the orbits of a comet and Earth around the Sun http://cse.ssl.berkeley.edu/SegwayEd/lessons/cometstale/frame_orbits.html
In conclusion, comets are beautiful, dazzling objects in the night sky. Discovered in 1680, these chunks of ice and rock tell us a lot about heat, radiation, solar wind and gravitation. Comets come from either the Kuiper belt between Mars and Jupiter or the Oort cloud past Neptune. When comets enter our solar system, the heat from our Sun melts the comet’s ice. The ice turns into a gas, creating a coma and making the comet look like a hairy star. Radiation is energy that is emitted from the Sun and always directs a comet’s tails away from our star; while solar wind comes from the Sun and pelts comets with many little particles that remove dust and other loose material from the comet resulting in erosion. Gravity from the Sun, as well as other bodies, shape the behavior of a comet’s orbit. Newton’s second law and his law of Universal Gravitation are connected broadly to how the Sun affects comets. Many of a comet’s traits are majorly affected by the Sun. Comets are little bodies in the universe that the Sun affects by heat, radiation, solar wind and gravity.
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