29:50 Modern Astronomy
Fall 1999
Lecture 40 ...December 6, 1999
Mars (Concluded)
Watch the skies! Watch the skies!
The aliens got Mars Polar Lander too!
Another meteor shower; The Geminids. Usually 2nd best of year.
Peaks December 13-14. Roof will be open if clear, starting at 8PM. Description of
shower from textbook: ``rates increase for several days before maximum, then drop off
quickly.'' The Geminids are debris from the asteroid Phaethon.
Asteroids and BIG Impacts
1. Major planets have a big gap between the Terrestrial planets and Jovian planets, from 1.52 au to 5.20 au.
2. That space is not empty, in 1801 the first of the minor planets was discovered, Ceres. After that, Pallas, Vesta, Juno etc. These are small planets. Scientific term is minor planet or asteroid.
3. Large asteroids are substantial objects. See Appendix 7, pA-8 in book. Diameters:
Ceres 910 km, Pallas, 520, Vesta 500 km.
``Famous transparency''.
These are big rocks, but too small for atmospheres.
4. There are lots of them.
Figure 15.10.
7000 plotted in this figure and known as of 1991.
4500 well enough observed to have accurate orbit known.
Thousands with diameters > 10km.
5. What do we think they are?
(a) planetesimals
(b) presumed to have had extensive modifications from collisions (see p341 of text).
6. What do they look like?
Look at nice pictures on p338,339.
There are three for which we have close up pictures from spacecraft. We also have space
telescope images for Vesta.
Gaspra: 19 X 12 X 11 km.
Ida: 54 X 24 X 21 km.
Mathilde: 60 km.
Web pictures of asteroids.
In January the NEAR spacecraft will rendezvous with comet Eros.
7. Classes of asteroids (This is science, there have to be classes!)
Most are C type (darkish. Reflectivity of a few percent. Some as black as cartridge toner).
Example is Mathilde.
Next most common, S type. Reflectance spectra show evidence of olivine, a common type
of mineral. Gaspra and Ida are of this type.
V type (only one) Vesta; shows reflectance spectrum of mineral pyroxine. This uniqueness has
allowed us to identify meteorites that came from Vesta!
8. Earth Orbit Crossing Asteroids.
Asteroids are big rocks. You wouldn't want the Earth to get hit by one.
Fortunately, most asteroids are in the Main Asteroid Belt, with
au.
In 1932 was the discovery of the asteroid Apollo, with 
. This was disturbing in that it moves from outside the orbit of the Earth to inside.
Perihelion is at less than 1au. Aphelion is at greater than 1 au.
More were found, and grouped together as the Apollo asteroids. For real nit-pickers, there are two classes, the real Apollos and a subgroup, the Aten asteroids.
9. How many of these are there, anyway?
As of June 1994, 152 Apollos and 17 Atens.
Look at Figure 15.14 for sizes.
10. BIG IMPACTS
We have seen that the Moon, Mars, etc have craters caused by impacts of meteors. Larger
craters were caused by asteroids. This happened on Earth too, but the craters are
obliterated after a few tens or hundreds of thousands of years by erosion.
Cases where we can see evidence of impacts on Earth:
(a) Barringer crater in Arizona
(b) From geological evidence, see paleocraters (including Manson crater here in Iowa).
Transparency with old impacts.
What is the likelihood we'll get nailed again? Tune in next time.