Meteorites and Fireballs

By Mogens Winther and Anders Västerberg

We have all observed falling stars. Why not photograph these meteors together with your students? This article gives a number of suggestions on how an ordinary family camera may take scientific and interesting pictures of a physical and mathematical phenomenon.

There are two popular ways of photographing meteor trails:

1. The easiest way is to place your family camera on a rigid mounting. This is the most popular method, however, it has a disadvantage; star images become trailed.

2. The more complicated way is where the camera is placed on a 24 hour motor driven mounting.

The camera has to follow the stars, a bit complicated, but most rewarding. If correctly mounted, all stars get beautiful pinpoint images. The milky way may glow as a beautiful background. Zooming into such an image- we have obtained the image below.

A Perseid meteor passes close to the red star Epsilon Eridani on the picture below. This meteor enters the Earth's atmosphere with the tremendous speed of 210 000 km/h. Leonid meteors have an even higher speed - 250 000 km/h

Fig1

Figure 1: A bright meteor. Click to see larger image (GIF, 184k)

The light flash from this object probably started at an altitude of 120 km, and ended maybe 60-80 km above surface. Zooming this image, you get a picture like this:

fig2

Figure 2

Please observe how the object actually rotates (thick-thin-thick meteor trail) - until it explodes in a big flash down in the right corner. The light was so intense, that trees and bushes around us gave shadows. Meteors that intense are called "fireballs". This particular fireball was observed from several places in Denmark. Maybe a few fragments survived this fall, and meteors now lie unrecognised somewhere in northern Germany.

In teaching mathematics, many interesting meteorite projects would be possible.

Imagine, if your students could measure such a meteor trail themselves, by means of their own stereoscopic photos from two separate positions.

The trick is to apply the background stars as a reference system. Most planetary programs will tell you which stars are at what height on a specified time. So, all you have to do is to compare the star positions on your photo with altitude information from your planetary program.

Another approach is the distributed meteor trail software (basic-source text). If you do not download- you may order it for something like $25 on a general software disk by Sky Publishing Corp., P.O. Box 911, Belmont, Massachusetts 02178-9111 / fax price request : Int+1+617-864-6117.

What does a meteorite look like?

The dream of most astronomy interested students is of course to find a real and big meteorite.

fig3

Figure 3: The Willamette meteorite, 14.1 tons, New York.

History shows that both large and small meteorites may stay unrecognised for years.

In our local Danish area a farmer found a genuine iron meteorite in 1975 in his fields. The rare 13.5 kg object was taken into his farm house. He even tried to drill a 3/8" hole in order to mount a lamp-shade! Following years of bad treatment, and even (unsuccessful) efforts by his son cutting it into pieces, this very rare meteorite was 15 years later given to a local museum. Irrespective of the bad treatment, the meteorite finder was given a high reward.

Another similar case happened in Kalundborg, Denmark in the 1970's. During telephone cable work, a 40 kilo iron object had been recovered. This object was left unattended in the countryside for quite a while, and - only by accident - experts were called.

Where do these iron-nickel meteorites come from?

These objects - originally molten - have been part of some smaller, now vanished planet. These planets were large enough to have a molten interior, the heavier elements like iron and nickel sedimented into the planet center. The center of these warm planets slowly cooled down, probably as slowly as 1 oC pr. 10 000 years. During this time huge crystals up to 1 meter size could form, nearly undisturbed by gravity.

Collisions were numerous in the start of our solar system. You may observe this yourself when looking with a telescope (25-50x) to the numerous craters at the old, southern pole of our Moon (Figure 4: student CCD picture). Sooner or later the small planet mentioned collided with other objects, and the iron-nickel interior was slung into space, and - eventually - fell to the Earth's surface as huge fireballs.

The Kalundborg meteorite showed unusually large crystals after being cut:

Figure 5

The Danish meteorite expert, Vagn Buchwald, in this meteorite found a new mineral Roaldit, which had not been seen previously on Earth.

These two Danish stories, and many other similar cases from all over Europe, show that many meteorites still lie around, unrecognised and unattended. Some might even be placed in High School stone collections. Let us thus take a look on how to recognise a real meteor.

Meteorites commonly divide into 3 groups: stone, stone-iron , and iron-nickel. Usually all 3 meteorite types deflect a magnetic compass more or less strongly.

If you want to be sure, calculate the mass density. Stone-iron meteorites have a density of 4.3-5.8 g/cm3; iron nickel meteorites even rise to 7-8 g/cm3. Pure stone meteorites are more difficult, having a mass density between 2.2 and 3.8. Often the outer layer show a blackmelted, burned colour.

In 1990-91 data were published, showing that an enormous, 180-300 km wide circular structure below the Yucatan Peninsula in Mexico, could be an impact crater, maybe associated with the extinction of the dinosaurs. As described in the Sky & Telescope article, March 1990, a 10 km wide asteroid, same size as the famous Dooms-day Comet Swift Tuttle, would be large enough to create such a big crater. Estimates tell the implied energy of this particular object would be 10 trillion megaton TNT. (1 megaton TNT is equal to 1.25 TWh).

Huge ocean waves, Tsunamis, would rise following such an impact. Wave- and impact debris have been found around the Caribean/Gulf region. Science, Aug. 14, 1992, as well as Science News, Aug. 15, 1992, published some results on this topic.

According to independent groups, debris from inside the crater, and from the far away Caribbean shores, show exactly the same age, 65 million years, again coincident with the wipe out of the dinosaurs.

Presently, about 150 craters are known worldwide.

Additional literature: Newsweek, "The Science of Doom", Nov. 23, 1992, Nature, January 1994 - giving risk-estimates - as well as the most wonderful 100-year old Science Fiction book by Camille Flammaron, "The End of the World".

Of course meteorites are still rare, but however, if you are lucky, contact the state geological museum.

But so far, the easiest way to approach these objects is of course by watching meteor showers. Below you will find a list of the most frequent meteor showers visible from Europe.

Remember, as in all astronomy, the best results are obtained on moonless nights, far away from any city and house lights. Do not forget warm socks, and hot chocolate.

Meteor Showers:

Date             Name        Maximum         Duration   Velocity   Due to Comet:      
                             (number/hour)   (days)     (km/s)             
                                             (FWHM)                   
                                                              
Jan. 3-5         Quadrantids 100              0.4       42                  
                                                           
April 20-22      Lyrids       20              1         48         1861 I      
                                                        
May 4-5          Eta          60 (difficult)  6         66         Halley      
                 Aquarids    
                                     
June 27-30       Draconids    15              1         20         PonsWinnecke            
     
July 27-31       Delta        30 (difficult)  8         42                  
                 Aquarids                                     
                                                          
Aug. 10-12       Perseids    100              3         59         Swift Tuttle      
                                                                   (Doomsday-Comet)
        
Oct. 15-25       Orionids     30              2         66         Halley      
                                                        
Oct. 30-Nov. 15  Taurids      15             15         28         Encke       
                                                         
Nov. 14-17       Leonids      15             variable   71         Tempel-Tuttle
         
Dec. 9-14        Geminids     90              1.5       34 
                 
Dec. 2           Ursids       15              2         33         Tuttle 1939       

Where do you have to look?

All comet showers have a name due to their "radiant"; the star sign by which they all seem to emerge. For example; if you want to observe Perseids, these all seem to emerge from the area around Perseus.

If you want to perform stereoscopic photography, be sure all students know where to point their camera. Film suggestions: Fujichrome or Kodachrome 400-1000 Asa Slide film. Slide films are best. The picture above was a Kodakchrome 1000 Slide, 15 minutes exposure at 30 mm f 2.8.


What to remember:

1. Camera distance must be set to infinite; do not forget that!

2. Camera needs to be as open as possible e.g. f-ratio = 2.8 or lower.

3. Tell your students to write down the accurate time of any bright fireball, as well as start and stop time of all exposures.

4. Let each exposure last about 10 minutes. All students starting at say 2200, next picture 2210 etc.
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If you apply a rigid mounted camera, you should close the camera immediately after observing any bright fireballs. This makes it more easy to evaluate the meteor trail. More details, good advices on photography etc. may be found on the homepage by the International Meteor Observers (IMO) Commission.

Have a nice hunt!