Class 23 - The 3 Swarms I

In these last 2 classes we tied up some loose ends associated with small bodies in the 3 main swarms: Asteroids, Kuiper Belt and the Oort Cloud of comets. This is material covered in Chapters 6 and 7.

Something to think about - we know a great deal about the asteroid belt and much less about the distant and more recently explored Kuiper Belt. As we discuss properties of the asteroid belt consider what we might expect for the Kuiper Belt - what will be similar, what will be different?

Reminder that this diagram summerized the formation of the different swarms

.

The Minor Planet Center at the Harvard-Smithsonian Institute for Astronomy has all sorts of information about these objects.

For these planetesimals to hang around over the age of the solar system, they need to be in "safe" orbits - orbits that do not get scattered by the planets. See Figure 6.2 on page 129 which shows the liftimes calculated for test particles' orbital distance. Not the wide graviational "sphere of influence" of the outer planets - particularly Neptune. Notice that objects need to be out in the Kuiper belt to last for longer than a million years.

Asteroids

Sizes

Asteroids are small compared with planets and moons. (But, as we shall discuss later, they are BIG when we think about the possibility of one hitting Earth!)

An interesting aspect of asteroid sizes is that their SHAPE depends on size - specifically, objects less than about 600 km in diameter are not spherical.

 

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.

Composition

Many texts say "Asteroids are classified in terms of their spectroscopic properties". That's easy to say, but what does it mean. The diagram of asteroid sizes and colors illustrates the range of colors - and, indeed, color is a crude indicator of composition. But spectroscopy reveals much more than color....

There are 3 main types of asteroids:

The S and Ms are relatively easy to explain, no? 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.

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.

 

Recent HST observations of Ceres

Asteroid moons - Bill Merline's discoveries

NEAR - including the landing...

 

So - remember, moons give us DENSITY - via NVK3L....

Note the big range in densities - from Vesta at 3600 +_ 305 vs Matilde at 1300 +-200. Yet these objects are basically ROCK. So, clearly, Matilde has lots of AIR - gaps - between the rocks - it is a rubble pile ("bean bag"). Vesta, on the other hand probably a differentiated object with iron at the cneter.

 

Orbits

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 -

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....

Note that for the asteroid belt the orbits with resonant orbits with Jupiter are unstable locations while in the Kuiper belt being in a resonance with Neptune makes a safe place to hang out - particularly if this means that whenever the object approaches Neptunes orbit, Neptune itself is the othe side of the solar system - as in the case of Pluto.

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.

So, there are several different classes of asteroids - see Table 7-2 on pages 158-9.:

 

Meteoroids, Meteors, Meteorites

Peices of comets, asteroids and "stuff" that impact the Earth - important for telling us about the properties of the swarms.

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.

see page 136 for a better version.

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.

Finally - so you can explain it to your friends when they ask - this is why you see shooting stars close to dawn:

Impact rate? Hartmann quotes 100 tons/day. That's loads o' stuff. While the abundance (and hence impact rate) of metals is much lower than rocky or carbonaceous material, the latter tends to vaporize on entry.

Hazards

Links about asteroids, meteorites and the potential hazards of an impact:

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)