Characteristics and Origins of the Solar System
Lecture
26
November
15, 2000
First, an interesting additional fact about the Giotto spacecraft encounter with comet Halley. The nucleus was found to be extremely black, to the extent that the nucleus was called “the charcoal peanut”. Its reflectivity (or albedo) to use the technical term was 2.7 %. This is almost exactly the reflectivity of coal dust; the nucleus of Halley’s comet is obviously saturated with dark carbonaceous materials.
As may be seen from pictures of comets, as well as the
close-ups of Comet Halley, comets flake
off material (dust). The dust continues
to orbit around the Sun with the comet.
Over time, after many passages, this dust tends to get spread out along
the orbit of the comet. If the orbit of
the comet intersects the orbit of the Earth, we see an increase in the number
of meteors when we pass that point. This is referred to as a meteor shower.
>>>>>>>> diagram on blackboard.
On p281
there is a list of meteor showers, together with the names of the comets that
correspond to those meteor showers. The
two best known ones are the Perseids, associated with comet
Swift-Tuttle, and the Leonids, which are related to comet
Tempel-Tuttle. Before the end of the
year there will be yet another shower, the Geminids.
The Perseids come from an old comet (i.e. one that has been on its present orbit for a long time), so the comet debris has been uniformly spread around the orbit. The intensity of the shower (given by the hourly zenith rate) is pretty constant from year to year.
The Leonids come from a younger comet, so there tends to be a big concentration of dust and material near the comet. The period of comet Temple-Tuttle is 33 years, so there is a tendency to have extremely intense Leonid displays every 33 years. The last big display was in the mid sixties, so we are in a high-probability situation for an intense meteor shower in these years.
When you look at the meteors from the Leonids, you should keep in mind that you are looking at primitive pieces of a comet, representing primitive solar system material, entering the Earth’ atmosphere and burning up a few tens of miles over your head.
The Stardust Spacecraft
As mentioned before, we would very much like to get our hands on cometary matter, which probably represents the most pristine samples of the matter which formed at the beginning of the solar system. Right now, the Stardust is out in space, on its way to a comet for exactly this mission.
Stardust is a “sample return” mission, meaning it is intended to go to the comet, collect pieces of cometary material, then return to Earth. The Internet site for stardust is http://stardust.jpl.nasa.gov.
Here are some of the key features of Stardust.
1. It was launched in February 1999.
2. It gets out to the orbit of the comet by a gravitational boost from the Earth.
3. It is currently “inbound” to the Earth for a flyby and gravitational boost in January.
4. It will arrive at the comet in 2004, and fly close to the nucleus, within the coma.
5. It will collect samples via aerogels, blocks of porous material that act like 3D flypaper.
6. It will then return to Earth with its samples, arriving in 2006.
These samples will contain fascinating information on the formative events in the early solar system.