Wednesday 21 September 2011

Cheddar Ranch Observatory The Oklahoma City Astronomy Club is developing an observatory site about 80 miles west of OKC (about 100 miles by road from OU). The site is reasonably dark. It is very dark towards the north and west. Towards the southeast the "light dome" glow of OKC is far too evident. There is a nice "warmup building" with bathrooms, kitchen and lounge area. There are concrete pads for telescopes. There are a few private domes with telescopes. The club has a 12.5 inch classical Cassegrain telescope for general use (but it is getting a new mount and I am not sure if it is in working order now.)

If there is any serious interest in going out there to see the sky from a reasonably dark place, I will try to set up a "field trip".

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Comet orbit and origin of meteor showers.. Small pieces of material are shed from a comet as it orbits the Sun. (These pieces are the size of BBs to peanuts- MUCH larger than the micron-sized dust particles that produce the dust tail.) This debris trail is concentrated along the orbital path of the comet. If the Earth's orbit happens to intersect this trail, we can get a meteor shower, a time when there is an enhanced number of meteors seen in the Earth's sky. As the Earth orbits the Sun, it intersects a number of debris trails on about the same day each year, leading to well- know meteor showers, such as the Leonids in mid-November, the Geminids in mid-December, and the Perseids in mid-August. Very rarely, the Earth hits a particularly dense clump of debris, and there can be a spectacular meteor storm, with thousands of meteors visible for a relatively brief period.

There have been no meteorites known to be associated with meteor showers, indicating that the cometary debris particles are probably small enough to burn up completely in the Earth's atmosphere. The rocks that become meteorites that land on Earth come from asteroids, not comets.

Leonids from above Image of Leonid meteors from ABOVE! (Looking down from Space Shuttle.)

Legendary 1966 Leonid Meteor Storm In 1966, there was a meteor storm associated with the Leonids. Photographs just don't do this event justice. Quotes about this event from January 1967 issue of Sky and Telescope: "A rate of 150,000 meteors per hour was seen for about 20 minutes" "Meteors coming too rapidly to count". The rate of 150,000 / hour is equal to 40 per second!! WOW!

More spacecraft missions to comets:

(1) Deep Impact (artists conception). (2) Deep Impact encounter sequence (3) Best overall image of Tempel 1 NASA's Deep Impact mission (2005) was a smashing success! A copper impactor, with mass of about 3 moderate-mass football players, separated from its "mother ship" and smashed into Comet Tempel 1 at a speed of about 10 km/sec. The impact left a crater about the size of a football field (but unfortunately all the dust kicked up by the impact made it difficult for the mother ship to image the crater). Material heated by the impact continued to spew from the crater for several days (remember the gravity of the nucleus is very low). The comet was found to be dustier (and less icy) than expected. Spectroscopic signs of lots of organic (carbon containing) (NOT biological!) molecules were found in the ejecta. The nucleus (like that of Halley) was found to have a low density- about half that of water, perhaps indicating a "fluffly" composition and/or voids and cracks.

The nucleus of Temple 1 is about 8x5 km in size, this somewhat smaller than Halley.

Deep Impact - The Movies!. 2nd movie The first movie shows the impact of the copper "bullet" as imaged from the mother ship. The second movie shows the comet as imaged by a camera on the bullet.

Comet Wild 2 Comet Wild 2 was imaged by a NASA flyby mission named Stardust. The nucleus is about 5x4x3 km in size. Wild 2 is a "close in" comet- it has an orbital period of about 6 years. In 1974 Wild 2 passed withing 0.2 AU of Jupiter. This "gravitational assist" changed the comet orbit from a period of about 43 years to the current 6 years!

Rosetta trajectory Rosetta is an ambitious European comet mission now en route to comet 67P/Churimov-Gerasimenko.