This is a picture of asteroid Ida and its moon Dactyl (taken by Galileo).
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This is a picture of asteroid Ida and its moon Dactyl (taken by Galileo).
There's a great deal of fuss around asteroids, these days - why should we care about these small chunks of rock? There are 2 very good reasons: (i) they tell us about solar system evolution, and (ii) they might hit us! In this session we can consider the more than 12,000 small objects that orbit the Sun as well as the fragments of these objects, meteorites, that land on the surface of the Earth. We shall look at:
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(1) Compare the 6 largest asteroids with the size of the Moon. Or go to the asteroid fact page Using the rule-of-thumb we used for moons in Session 15 for whether an object is round or remains "potato-shaped", how many asteroids do you think are close to spherical? This means that all the rest - thousands of them - are irregular in shape. |
Here is a very nice diagram of asteroid sizes and colors from Clark Chapman's chapter on asteroids in The New Solar System. The diagram shows many asteroids with their size given relative to the disk of Mars (on the left) and with a rough indication of the radial distance of their orbit. Each asteroid is named (there is a weird convention for naming asteroids - see the IAU page) and the number below the name gives the spin period in hours.
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(2) HOW can we determine the composition of other asteroids--at least their surface - using spectroscopy? Hint: Remember Session 13 on Light - or chapter 7 . |
There are 3 main types of asteroids:
This is a very wide range of compositions.
In the past few years we have gained the capability of imaging asteroids - either by spacecraft flybys (by Galileo and NEAR) or with large telescopes. These images are all collected at NASA's Small Bodies Data Center. Particularly important was Galileo's observations of the asteroid Ida. Click on the image to see Ida and Dactyl in their full glory.
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| This image was taken by the Galileo spacecraft, as it made a close approach to the asteroid Ida. Off to the right of Ida is its small moon, Dactyl. This is the first known case of a moon around an asteroid. |
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(3) (a) WHY is detecting a
moon around an object so very important? What does it tell us about the
orbitEE? Hint: see Session 8 on Gravity. |
Look at this plot of asteroid locations in the inner solar system. It shows The instant location of asteroids. The orbit of a "typical" asteroid is quite circular, though a couple of more eccentric orbits (of Apollo and Amor asteroids) cross the Earth's orbit. The orbits of asteroids lie mostly between Mars and Jupiter. But they are not uniformly distributed between Mars and Jupiter. There are radial distances where there are gaps without asteroids--the Kirkwood Gaps - see Figure 12.2 or here. These gaps are places of orbital resonances with Jupiter--if an asteroid landed in one of these resonances, Jupiter's gravity would perturb the asteroid, forcing it into a non-resonant orbit--perhaps sending it on a trajectory headed for Earth!! For example, the gap at 3.3 A.U. (labeled 2:1) is where an asteroid would orbit the Sun twice for every once that Jupiter orbited the Sun.
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(4) Describe the orbital resonance at 2.5 A.U. |
Here is a more complete plot of asteroid semi-major axes from the Minor Planets Center which collects and publishes information about the thousands of small objects whizzing about in the solar system.
With many objects orbiting the Sun within a relatively small range, collisions are likely to happen. Moreover, there were probably many more asteroids in the early history of the solar system. If collisions are with small relative velocities the net result is to coax each other into similar orbits (remember the "Why a disk" diagram?). On the other hand, if collisions are between objects moving at high relative velocity, damage is going to be done. The net result will be fragments. If one of the parent bodies was large enough to have been differentiated, then we are going to get chunks of different types of materials.
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(5) Now can you see where the S-type and M-type asteroids come from? What part of the parent body did each type originate? |
C-type asteroids are a little harder to explain. These carbon-rich asteroids are believed to have not been significantly heated since they formed and hence provide clues about the earliest period of formation of the solar system.
With the orbits of some asteroids crossing the Earth's orbit, it is perhaps to be expected that occasionally pieces of asteroids, and even whole asteroids hit the Earth (more about this later). Most of the material that approaches the Earth gets vaporized as it passes through the Earth's atmosphere - usually this is described as "burning up" in the atmosphere, but it is really not combustion - just glow due to excitation of the air as the object moves through, slowed down by the friction with the air. Check out this movie of the Peekskill Fireball!
Both cometary and asteroid material enters the atmosphere. The difference is that the volatile chunks of comets (mostly dust and ice, remember) vaporize completely before hitting the ground. Chunks of asteroid, on the other hand, particularly the metal ones, often survive and hit the ground - making a meteorite. Meteorites are therefore vital clues about the asteroids. Here are some pages showing meteorite types and more meteorite types. The most exciting meteorites are probably the ones that have been picked up in the past few years that are believed to have come from Mars. There was such a hullaballoo about the possibility of martian meteorites containing evidence of life that you would have had to have been on Mars for the past 2 years not to have heard about it. Here is a good place to start if you are interested in reading more about meteorites from Mars.
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(6) So, we have been discussing a variety of different objects - let's see if you have them sorted out. Distinguish between the following (provide a definition in each case): |
Hitting Earth is not just a remote possibility. Impacts have happened in the past - the Tunguska impact which occured in 1908 and flattened vast forests in Siberia (good thing the area is very sparsely populated - see here) --and don't forget the dinosaurs!
Figure 12.24 shows the chances of a really big impact happening. Note that the x- and y-axes are in powers of 10. Impacts like the Siberian Tunguska event are expected to occur about once every century. The type of impact that wipes out species, such as dinosaurs, probably occurs only every hundred of millions of years--not often enough to lose sleep about.
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(7)
(a)Reading from the graph,
how often does an object about 1 meter across run into the earth?
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Links about asteroids, meteorites and the potential hazards of an impact:
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This is a picture of Comet Hale Bopp.
Comets are perhaps the most dramatic astronomical objects that humans can observe with their naked eyes. Yet, they are just the debris left over from forming the solar system - cosmic junk. But very valuable junk - comets retain vital clues about the early history of the solar system. We shall study the following aspects of comets:
Did you manage to see one of the recent comets? Hale Bopp or Hyakutake? A comet can be an awesome sight on a dark night - spread across the sky - returning night after night for months.
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(8) (a) Comet tails can be on the
order of 20° in angular size across the sky. How many Moons
placed side by side would stretch 20° in the sky? |
Here we see the different parts of a comet - the nucleus, coma and tail. When we see a comet in the sky we are seeing the coma - gases and dust that have been vaporized from the body of the comet as it is heated up by the warmth of the Sun. We are seeing the gases and dust, that have expanded far away from the tiny nucleus, scatter sunlight. Large comets are ejecting as much as 30 tons of material every second when they pass closest to the Sun. This material is composed of volatile compounds - material that is a solid at the low temperatures of the outer solar system (about 50K) but turns into gas at temperatures in the inner solar system (about 300K) - materials like water, carbon dioxide, ammonia, etc.; Mixed in with these volatiles are dust particles that are carried away from the comet by the jets of gas.
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(9) How do we know what comet are made of? Hint: What does a spectrum of a comet tell us?. |
The tail of a comet has two components: a dust tail (which trails behind the comet in its orbit around the Sun) and an ion tail (which is pushed radially away from the Sun by the solar wind).
Comets orbit the Sun, just like all the other solar system objects, but their orbits have very high eccentricity (remind yourself of orbital properties in Session 8). Here is a plot of the orbit of Comet Hyakutake. This and other comet orbits are on the Minor Planets website.
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(10) (a) Comet Halley has an orbital
period of 76 years. What is the semi-major axis of comet Halley's orbit?
(Hint: Kepler's 3rd Law) |
Where do comets come from? Dutch astronomers seem to like comets. Remember
the Kuiper Disk from Session 16? The Kuiper Disk is the region between
50 and 150 A.U. where there are about 1 billion icy bodies - between
the size of comets (10km) and Pluto (2000 km). Another Dutch astronomer, Oort,
proposed that there is a cloud of 100 billion or so comets extending
out to 100,000 A.U.. Or more--this is the spherical Oort cloud.
Here we see the shapes and sizes of these two clouds of comets. Notice that comets from the Oort cloud can have any orbital inclination but comets from the Kuiper Belt will tend to have orbits that are closer to the eccliptic plane, with low inclinations.
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(11) (a) Comets out at these distances
are still orbiting the Sun and still obey Kepler's 3rd Law. What will
be their orbital periods? |
Occasionally, these billions of comets bump into each other. extremely occasionally, one gets too close to a Pluto-sized object and is "kicked" into the inner solar system where it follows the stages described above. We will only be able to see the comet when it gets inside about 4-5 AU and is sufficiently warmed by the Sun that volatile materials evaporate from the comet's surface. The Oort Cloud is the source of long period comets while the Kuiper Belt is the source of the short period comets.
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(12) To obtain a sample of the most primative material from the earliest period of solar system formation, which type of comet do we want to look at? |
What happens to the surface of the comet after the volatiles are removed? We really do not know what the surface of a nucleus of a comet looks like. By following comets with large telescopes after they have passed the Sun and are retreating back out into the solar system, we can tell that the surface is very dark - in fact, as black as coal (albedo ~ 0.04). This suggests that after the volatiles are removed either a pile of "rubble" remains or the exposure of the outside of the nucleus to energetic particles and sunlight produces a dark "crust". After several passages past the Sun much of the volatiles near the surface gets removed and the comet becomes "dead" - sometimes looking more like an asteroid than a comet.
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(13) Go to this comet catalogue (or the links below) and look at images of several of the famous comets. Which show separate ion and dust tails? Which are long period comets and which are short period comets? |
If a comet passes close to a planet (particularly Jupiter, of course) then the tidal forces - the difference in gravity across the object - can pull the comet apart. Remember Comet Shoemaker-Levy-9? See here. When it first passed Jupiter SL9 was broken into many fragments. The second time it passed Jupiter - the fragments hit Jupiter in a dramatic display of explosions. We are learning that this was not perhaps such an unusual event - comets often break up. Evidence of such comet distruction is provided by meteor showers. Material from the break up of a comet continues along the orbit of the comet. If the Earth happens to cross the orbit of a comet that has broken up then material will bombard the Earth's atmosphere - and we see a shower of material vaporizing in the atmosphere.
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(14) Look here for information on the dates of prominent meteor showers and their parent comets. When is the next meteor shower going to happen? |
Links:
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