Asteroids: Tiny Worlds

At this point, we have discussed the major planets, and one class of minor solar system object: the comets. Now we will go on to another, and very interesting class, the asteroids. We will also discuss the close connection between asteroids and the only space objects that we can hold in our hands, meteorites.

When I mention Pluto during astronomy lectures, I generally say that I don’t consider it a “major planet” because it is so much smaller than all of the rest of the planets, and that the solar system is filled with all kinds of other, smaller objects. We have seen comets as one such class of objects. Today we will discuss asteroids, which also go under the name of “minor planets”.

When discussing the major planets, whether terrestrial or jovian, I emphasized where they are in the solar system. A “map” of part of the solar system which I showed before is this:

Notice in particular the region between the orbits of Mars and Jupiter, which appears here as empty space.

In actuality, a map of this part of the solar system which showed all the objects we know of is this:

The blue circles indicate the orbits of the planets Mercury, Venus, Mars, and Jupiter, with the outermost circle being the orbit of Jupiter. Notice the swarm of “green things” between the orbits of Mars and Jupiter. These are the asteroids, or minor planets, and this part of the solar system is called the asteroid belt.

As we will discuss in a minute, asteroids are essentially huge rocks (up to 600 miles in diameter). The “red things” on this picture are also asteroids, a class called near-Earth asteroids. Since all objects in the solar system move around in obedience to Kepler’s Laws, these big rocks are moving in space close to the Earth.

Now let’s discuss asteroids in some depth.

Asteroids are intriguing little worlds. They are fascinating objects in their own right, and give us insight into the processes going on in the solar system since its formation.

First, a bit of the history of their discovery. There is a big gap in the solar system between the orbit of Mars (a=1.52 astronomical units) and Jupiter (a=5.20 au) (see diagram above). This was noted centuries ago, and astronomers expected to find a planet there. What they found (beginning in 1801) was a lot of little ones. After the first discovery, lots more were found.

Appendix 8 of your textbook gives data on selected asteroids, including the biggest ones. This table contains a lot of interesting information. For starters, see that the biggest one (Ceres) is almost 1000 kilometers in diameter (think about how this stacks up relative to the other objects we have talked about). All of the “top ten” in Appendix 8 are larger than 240 kilometers in diameter.

At the present time, there are about 10000 asteroids which have had their orbits determined.

The vast majority of known asteroids have semimajor axes between 2.2 and 3.3 au. This part of space is known as the asteroid belt.
There are some outside the Main Belt in an equilateral triangle with Jupiter. Since they were named by classically-educated astronomers of the 19^th century for heroes of the Trojan war, they are called the Trojan asteroids.
Notice that there are a few where they have no business being, inside the orbit of the Earth. These are the Apollo asteroids. Appendix 8 lists them as the “Earth Orbit Crossing Asteroids”.

Until 1993, we did not have any close-up pictures of asteroids. Since they are small, solid, airless worlds, we had an idea that these would show cratered surfaces, but it is nonetheless interesting to look and see if our ideas are right. Since then, there have been spacecraft flybys of the asteroids Gaspra, Ida, and Mathilde. These three are shown in Figure 15-12 of your textbook, which shows them to scale (i.e. the relative sizes are accurately shown.)

In addition, the NEAR spacecraft was in orbit around the asteroid Eros for about a year, after which it landed and is still there. The NEAR spacecraft returned a wealth of information on asteroids. (check out its website on the course homepage). A picture of Eros is shown in Figure 15-14 of your textbook.

We can check out the pictures of the following asteroids:

1. Gaspra (Type S) 16 km “diameter”

2. Ida (Type S) 56 km in length Ida also has a “moon”. About 1 km in diameter, named “Dactyl” which is seen in Figure 15-13.

3. Mathilde (Type C) 60 km in diameter.

4. Eros

5. Vesta (imaged by HST).Vesta is remarkable because it is the second largest of the asteroids, and because of its high albedo, it is usually the brightest. It is often an easy object to find with binoculars and a good star chart. It is large enough to show some detail in a telescope at the Earth. The Hubble Space Telescope was able to observe it over its rotation period, and generate a crude map of the surface (including elevation)

The upper left corner shows an image of the asteroid as it is seem with the HST. The upper right corner shows a computer model of the brightness and darkness variations on the surface of the asteroid. This model is consistent with the observations that have been made. Finally, the bottom shows a topographical map of the asteroid.

On the bottom is what appears to be a large impact crater (with central peak). We believe that pieces blown out from this impact have traveled to Earth. Some of them are in the Smithsonian Museum of Natural History in Washington.

With the HST observations, we have also generated a computer video of the asteroid as it rotates. This is given at the following internet address.

http://www.solarviews.com/cap/ast/vvesta1.htm

One final remark about Vesta is that this Fall is a good time for observing it. It is in the southeastern sky in the constellation of Aquarius in the early evening. All you need to find it is a dark sky, a pair of binoculars, and a good set of star charts. For a chart, check the Sky and Telescope magazine home page at http://skyandtelescope.com and click on “Observing Highlights”. Go to “Spot Vesta and Uranus”.

Summary of Asteroid Properties

Given what we have talked about this semester, you might expect that the surfaces of these objects would be cratered at a density consistent with that from the age of bombardment. Interestingly enough, this is not really what was found. The surface of Gaspra has been exposed to space for about 200 million years, that of Ida for about one billion. This would seem to be very odd, since these objects do not seem to be big enough for geological processes, hydrology, etc. The relative youth of the surfaces of these objects is a good hint as to their geological history.

One would expect that with these asteroids, a “you see one, you’ve seen them all situation would apply, but this is not the case. Much information has been obtained from reflectance spectra of asteroids; measuring the spectrum of sunlight reflected from the surfaces of these objects. For example, such reflectance spectra in the cases of many asteroids show an absorption feature due to the mineral Pyroxine, which is Mg₂(Si₂O₆).

On the basis of these reflectance spectra and the albedo, or reflectivity, of asteroids, there have been at least 9 categories or classes established. Some of the major ones are as follows. Let me describe clearly what albedo means. It is the amount of light that is reflected rather than absorbed. An albedo of 100% means a pure white object, that reflects all the light that falls on it, and absorbs none. An albedo of 0 % would be a completely black object; if you shine a light on it, no light would be reflected back at you. For purposes of reference, the Earth has an albedo of 36%.

Note: there is a good reason, which will become clear later, why we are going into all this detail on classification of asteroids.

1. S: these have albedos of 7 to 23. They are the main class of asteroid in the inner and central asteroid belt.

2. C: carbonaceous, these have albedos of 2 – 7 percent (about like coal). They become the dominant type of asteroid in the outer belt.

3. M: metallic. There surfaces seem to be metal, like iron and nickel. They are primarily found in the central belt.

4. V: for Vesta. Vesta has high reflectivity (albedo of 38%), and shows a reflectance signature characteristic of basalt, the type of mineral involved in lava flows.

From these characteristics, there are a couple of points worth making.

· It is interesting that there is a relation between the location in the asteroid belt and the type of asteroid. It is not a 100% correlation, but it is a strong one.

· The carbonaceous asteroids are interesting. They once again illustrate the trend in the solar system to coat things with a hydrocarbon substance, sometimes containing water incorporated in the mineral (water of hydration). It is speculated that some of these C asteroids might contain water bound in the form of frost under their surfaces. There is an even more extreme type of C asteroids called class D, which have been described as “D materials are probably en even lower temperature organic sludge that condensed just as the ices were beginning to condense” (It’s alive!!!).

These characteristics should be kept in mind when thinking about the different asteroids that have been photographed close up.

The characteristics of the asteroids seem much more varied than expected. There are several different mineralogical classes, and they are in different parts of the asteroid belt. Furthermore, they do not seem to have been unaltered in space since the age of bombardment. What is going on?

The answer to this is that, just as tides determined the geology of the Galilean satellites, collisions dominate the characteristics of the asteroids. Asteroids generally don’t have surfaces much older than a few hundred million years because this is the mean interval between collisions. At least some of the differences in types are due to the collisional disruption of a differentiated object, i.e. one where the metal had been able to settle to the core, leaving a silicate mantle. Finally, it has been found that there are certain families of asteroids that have similar reflectance spectra (thus composed of similar minerals) and are also on similar orbits.

Next time we will see that these processes, and other things going on in the asteroid belt, have implications for us Earthlings.