Scholars Online Astronomy - Chapter 15: Vagabonds of the Solar System: Comets and Asteroids
Reading: Astronomy, Chapter 15: Vagabonds of the Solar System: Comets and Asteroids
Study Notes: notes on your assigned reading from the text
15.1 Several chapters back, we discussed the Titius-Bode law, which describes a mathematical relationship between the distances of the planets from the sun. When Bode used this geometrical progression, he got a series of distances in astronomical units that matched the distances to the planets — except that it skipped one. There was no matching planet for the predicted distance of 2.8 astronomical units. By searching the skies for objects moving with the velocity that would match a distance of 2.8AU, astronomers were able to discover what is now known as the dwarf planet Ceres. Subsequent observations led to the discovery of over 300 000 asteroids in a band at this distance. This led to the realization that such "space junk" exist everywhere in the solar system, although not elsewhere with such density.
15.2 Two competing theories were raised to explain the existence of the asteroid belt. In the first, a planet formed at the expected distance, but was torn apart by gravitational tidal forces from both the Sun and Jupiter. In the second theory, matter collected in what is now the asteroid belt, but never coalesced into a planet because of the tidal forces from Jupiter. The modern view is a combination of the two: astronomers believe that several Mars-type bodies probably did form in the asteroid belt, but were unable to coalesce or to withstand the tidal disruptions from Jupiter's gravitational field. They never pulled in all of the materials, and all of them eventually succumbed to tidal disruption and broke apart. In the period since its breakup, many of the bits were accelerated by Jupiter's gravitational field and ejected form the solar system altogether.
15.3 Because of their size and relative low albedo (reflectivity), asteroids are very hard to observe from Earth-based telescopes. The dogged determination of amateur astronomers (who have the time to stand closely observing the sky over many years) have led to the discovery of individual asteroids, and the mapping of their orbits. Recent radar studies and even some spacecraft missions to asteroids have allowed us to more closely observe their surfaces and estimate their compositions.
15.4 Besides the asteroids in the asteroid belt itself, there are groups of asteroid type objects that have collected in different gravitational "pockets". One such group are the Trojan asteroids, which cluster at Lagrange points before and after Jupiter in its orbit. These two points are pockets where the gravitational tidal forces from the sun and the gravitational tidal forces from Jupiter cancel out. There is no net force to pull the group apart, so the mutual gravitational attraction of the members of the group keep them together.
There is also some evidence that asteroids have crossed portal boundaries and instruct the planets; asteroids in orbit near the Earth (NEOs or "Near-Earth Objects") are carefully tracked for any deviation that might threaten the planet. Some geologists believe that past asteroid collisions were responsible for graders like Barringer crater in Arizona, and for cataclysmic events which could be responsible for mass extinctions like the one the end of the Cretaceous period, dated 65 million years ago.
15.5 Asteroids are usually considered larger bodies, meteoroids smaller bodies, both of which orbit the sun, but none of which are large enough to collapse in the spheres or sweep their neighborhoods free of debris. When these fragments enter the earth's atmosphere and burn up, the burning object is a meteor; the fragments of rock which actually make it to the surface of the earth are meteorites. Meteorites fall into three categories:
- stony: account for 95% of all meteorite material; These are primarily rock similar to that of the Earth itself, and difficult to distinguish when discovered.
- stony-irons: account for about 1% of meteorites; these often resemble a popcorn ball of small rocky materials held together by once-melted iron.
- irons: account for about 4% of meteorites; these contain no stone, but may be 10-20% nickel.
15.6 Some carbonaceous chondrite meteorites appear to be made of the original collapsing material of the solar system, and have high water content, carbon carbon compounds, and complex organic molecules bound into them.
15.7 (9) In contrast to asteroids, comets have high parabolic orbits, and are generally composed primarily of ice with some rocky bits. When observed from Earth on approach to the sun, the head of the comet containing the nucleus appears brightest, and is called the coma. The comet will trail both gases and dust. Ions blown off by the solar wind form a straight bluish tail (blue because of the hydrogen content) pointing directly away from the sun at any moment. Small particles of rocky material or ice shed by the comet forms a curved white tail, marking the comet's passage along its parabolic orbit. If the comet's path crosses the Earth's orbit, the dust debris will give rise to a meteor showers each year when the Earth reaches the intersection point.
15.8 Because they're composed primarily of ice, most astronomers believe that comets originate from the most distant reaches of the solar system, either the Kuiper belt, or an even more distant collection of icy junk called the Oort cloud. If the Oort cloud theory is true, then there may be as many as 5 trillion "dirty ice balls" in a spherical cluster around the sun, each one a potential candidate to become a comet should some body with enough mass pass through the cloud and disturb its orbit, sending it plunging sunward.
15.9 As comets circle the sun, they evaporate and lose part of their mass, which includes small bits of rock. These continue to orbit the sun along the path of the comet. If the earth's orbit crosses this path, it may run into this trail of detritus and encounter many small meteors all at once — a meteor "shower", in fact. Most periodic meteor showers correspond to the earth crossing the path of a known comet.
Key Formulae to Know
- There are no key formulae for this chapter.
Read the following weblecture before chat: Comets and Asteroids
- Identify at least three dwarf planets (not in the asteroid belt) and use the planetarium to explore their surfaces and orbits. How do the orbits of dwarf planets compare to those of the major planets? Do all dwarf planets revolve about the Sun in the same direction as the major planets? Do all dwarf planets have prograde rotation?
- Identify at least three major asteroids, including Ceres and Pallas, and use the planetarium to explore their surfaces and orbits. How do the orbits of asteroids compare to those of the major planets? Do all asteroids revolve about the Sun in the same direction as the major planets? Do all asteroids have prograde rotation? How do the orbits of Ceres and Pallas compare in shape and orientation to the ecliptic plane?
- Use the planetarium program to locate and observe current comets (if any), along with Hale-Bopp and Hyakutake, if possible.
Website of the Week:Explore the NASA Kuiper Belt Site. Click on "Read More about the Kuiper Belt" for information on the larger objects we hae so far discovered.
Chat Preparation Activities
- Essay question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
- Go over the list of Key Words and Key Ideas at the end of the chapter. If you don't remember the definition of the key word, review its use (the page number on which it is explained is given).
- Read through the Review Questions and be prepared to discuss them in class. If any of them confuses you, ask about it!
- Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
© 2005 - 2018 This course is offered through Scholars Online, a non-profit organization supporting classical Christian education through online courses. Permission to copy course content (lessons and labs) for personal study is granted to students currently or formerly enrolled in the course through Scholars Online. Reproduction for any other purpose, without the express written consent of the author, is prohibited.