APOD: Almahata Sitta 15 (2009 March 28)

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APOD: Almahata Sitta 15 (2009 March 28)

Post by neufer » Sat Mar 28, 2009 4:50 am

[url]http://apod.nasa.gov/apod/ap090328.html[/url] wrote:
Explanation: Small asteroid 2008 TC3 fell to Earth at dawn on October 7, 2008, tracking through the skies over the Nubian Desert in northern Sudan. That event was remarkable because it was the first time an asteroid was detected in space before crashing into planet Earth's atmosphere. It was generally assumed the asteroid itself had completely disintegrated to dust. But, based on satellite and ground observations of the atmospheric impact event, Dr. Mauwia Shaddad of the University of Khartoum, aided by Dr. Peter Jenniskens of the SETI Institute and NASA Ames Research Center, led an expedition of students and staff to the area, combing the desert for surviving fragments. On December 6, 2008, two hours after their search began, the first meteorite was found. The team ultimately collected some 280 small meteorites, now called Almahata Sitta, with a total mass of about 5 kilograms -- the first material recovered from a known asteroid. In stark contrast to the lighter-colored stones, the black fragment in the picture is Almahata Sitta meteorite number 15. About 4 centimeters in diameter, it is seen as it came to rest on the desert floor.
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Image
http://www.planetary.org/news/2009/0326 ... mains.html

<<Almahata Sitta, it turned out, is an extremely rare type of space rock, known as an F-class "ureilite." A close analysis showed that it is extremely porous and fragile, which is why it had exploded at such a high altitude. All ureilites are thought to have originated from a single primordial source, and the structure of Almahata Sitta indicates that it was a volcanically active body in which gas bubbles were trapped inside the porous rock. "The recovered meteorites," said Jenniskens, "were unlike anything in our meteorite collections." "It was," said Lucy McFadden, professor of astronomy at the University of Maryland, "a very exciting result.">>
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http://en.wikipedia.org/wiki/Black_Stone wrote:
<<The Black Stone (called الحجر الأسود al-Hajar-ul-Aswad in Arabic) is a Muslim object of reverence, which according to Islamic tradition dates back to the time of Adam and Eve. Many consider it to be a Tektite. It is the eastern cornerstone of the Kaaba ("Cube"), the ancient sacred stone building towards which Muslims pray, in the center of the Grand Mosque in Mecca, Saudi Arabia. The Stone is roughly 30 cm (12 in.) in diameter, and 1.5 meters (5 ft.) above the ground.

Image

According to Islamic tradition, the Stone fell from Heaven to show Adam and Eve where to build an altar and offer a sacrifice to God. The Altar became the first temple on Earth. Muslims believe that the stone was originally pure and dazzling white, but has since turned black because of the sins it has absorbed over the years.
  • .......................................................
    Finnegans Wake p. 5.17

    What then agentlike brought about that tragoady thundersday
    this municipal sin business? Our cubehouse still rocks as earwitness
    to the thunder of his arafatas but we hear also through successive
    ages that shebby choruysh of unkalified muzzlenimiissilehims that
    would blackguardise the whitestone ever hurtleturtled out of
    heaven. Stay us wherefore in our search for tighteousness, O
    Sustainer, what time we rise and when we take up to toothmick and
    before we lump down upown our leatherbed and in the night and
    at the fading of the stars ! For a nod to the nabir is better
    than wink to the wabsanti. Otherways wesways like that
    provost scoffing bedoueen the jebel and the jpysian sea.

    .......................................................
The Black Stone has been described variously as basalt lava, an agate, a piece of natural glass or — most popularly — a stony meteorite. It is evidently a hard rock, having survived so much handling. A significant clue to its nature is provided by an account of the Stone's recovery in 951 AD after it had been stolen 21 years earlier; according to a chronicler, the Stone was identified by its ability to float in water.

It has been suggested that the Black Stone may be a glass fragment from the impact of a fragmented meteorite some 6,000 years ago at Wabar, a site in the Rub' al Khali desert some 1,100 km east of Mecca. The craters at Wabar are notable for the presence of blocks of silica glass, fused by the heat of the impact and impregnated by beads of nickel-iron alloy from the meteorite (most of which was destroyed in the impact). Some of the glass blocks are made of shiny black glass with a white or yellow interior and gas-filled hollows, which allow them to float on water. Although scientists did not become aware of the Wabar craters until 1932, they were located near a caravan route from Oman and were very likely known to the inhabitants of the desert. The wider area was certainly well-known; in ancient Arabic poetry, Wabar or Ubar (also known as "Iram of the Pillars") was the site of a fabulous city that was destroyed by fire from the heavens because of the wickedness of its king. If the estimated age of the crater is accurate, it would have been well within the period of human habitation in Arabia and the impact itself may have been witnessed.

When pilgrims circle the Kaaba as part of the Tawaf ritual of the Hajj, many of them try, if possible, to stop and kiss the Black Stone, emulating the kiss that it received from the Islamic prophet Muhammad. If they cannot reach it, they are to point to it on each of their seven circuits around the Kaaba.

The Stone is broken into a number of pieces from damage which was inflicted during the Middle Ages. The pieces are held together by a silver frame, which is fastened by silver nails to the Stone.

A 1315 illustration from the Jami al-Tawarikh, inspired by the Sirah Rasul Allah story of Muhammad and the Meccan clan elders lifting the Black Stone into place.

Islamic tradition holds that Adam's altar and the stone were lost in the process of Noah's Flood and forgotten. It was Abraham who found the Black Stone at the original site of Adam's altar when the Archangel Gabriel revealed it to him. Abraham ordered his son--and the ancestor of Muhammad--Ishmael to build a new temple in which to imbed the Stone. This new temple is the Kaaba in Mecca.

Muhammad is credited with playing a key part in the history of the Black Stone. In 602, before the first of his prophetic revelations, he was present in Mecca during the rebuilding of the Kaaba. The Black Stone had been temporarily removed while a new structure was being constructed. A story found in Ibn Ishaq's Sirah Rasul Allah (as reconstructed and translated by Guillaume) shows Muhammad settling a quarrel between Meccan clans as to which clan should set the Black Stone in place. His solution was to have all the clan elders raise the cornerstone on a cloak, and then Muhammad set the Stone into its final place with his own hands.
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Other views

The reverence of the Black Stone evidently preceded the rise of Islam. The Semitic cultures of the Middle East had a tradition of using unusual stones to mark places of worship, a phenomenon which is reflected in the Hebrew Bible as well as the Qur'an.

Grunebaum, in Classical Islam, says that the Kaaba was a place of pilgrimage even in pre-Islamic times, and was probably the only sanctuary built of stone, but that there are other sources which indicate there were other "Kaaba" structures in other parts of Arabia. A "red stone" was the deity of the south Arabian city of Ghaiman, and there was a "white stone" in the Ka'ba of al-Abalat (near the city of Tabala, south of Mecca). He points out that the experience of divinity of that time period was often associated with stone fetishes, mountains, special rock formations, or "trees of strange growth."

The physical properties of the Black Stone were first described in the 19th and early 20th centuries by European travelers in Arabia who visited the Kaaba in the guise of pilgrims. The Swiss traveler Johann Ludwig Burckhardt, who visited Mecca around 1815 in the guise of a pilgrim, provided a detailed description in his 1829 book Travels in Arabia:

“ It is an irregular oval, about seven inches in diameter, with an undulating surface, composed of about a dozen smaller stones of different sizes and shapes, well joined together with a small quantity of cement, and perfectly well smoothed; it looks as if the whole had been broken into as many pieces by a violent blow, and then united again. It is very difficult to determine accurately the quality of this stone which has been worn to its present surface by the millions of touches and kisses it has received. It appeared to me like a lava, containing several small extraneous particles of a whitish and of a yellow substance. Its colour is now a deep reddish brown approaching to black. It is surrounded on all sides by a border composed of a substance which I took to be a close cement of pitch and gravel of a similar, but not quite the same, brownish colour. This border serves to support its detached pieces; it is two or three inches in breadth, and rises a little above the surface of the stone. Both the border and the stone itself are encircled by a silver band, broader below than above, and on the two sides, with a considerable swelling below, as if a part of the stone were hidden under it. The lower part of the border is studded with silver nails. ”

Visiting the Kaaba in 1853, Sir Richard Francis Burton noted that:
“ The colour appeared to me black and metallic, and the centre of the stone was sunk about two inches below the metallic circle. Round the sides was a reddish brown cement, almost level with the metal, and sloping down to the middle of the stone. The band is now a massive arch of gold or silver gilt. I found the aperture in which the stone is, one span and three fingers broad. ”

The current ritual of the Hajj involves pilgrims attempting to kiss the Black Stone seven times (once for each circumambulation of the Kaaba), emulating the actions of Muhammad. When Umar ibn al-Khattab (580-644), the second Caliph, came to kiss the Stone, he said in front of all assembled: "No doubt, I know that you are a stone and can neither harm anyone nor benefit anyone. Had I not seen Allah's Messenger [Muhammad] kissing you, I would not have kissed you." Many Muslims follow Umar: they pay their respects to the Stone in a spirit of trust in Muhammad, not with any belief in the Stone itself. This, however, does not indicate their disrespect to the Black Stone but their belief that harm and benefit are in the hands of God, and nothing else. In modern times, large crowds no longer make it practically possible for everyone to kiss the stone, so it is currently acceptable for pilgrims to simply point in the direction of the Stone on each of their circuits around the building. Some even say that the Stone is best considered simply as a marker, useful in keeping count of the ritual circumambulations (tawaf) one has performed.

Some Muslims also accept this hadith, from Tirmidhi, which asserts that at the Last Judgement (Qiyamah), the Black Stone will speak for those who kissed it:
  • "It was narrated that Ibn ‘Abbas said: The Messenger of Allah said concerning the Stone: "By Allah, Allah will bring it forth on the Day of Resurrection, and it will have two eyes with which it will see and a tongue with which it will speak, and it will testify in favour of those who touched it in sincerity." "
Apart from the ritual role of the Black Stone, its black colour is deemed to symbolise the essential spiritual virtue of detachment and poverty for God (faqr) and the extinction of ego required to progress towards God (qalb).

The Black Stone is broken into a number of fragments, with varying accounts putting the number at between seven and fifteen, held together by a silver frame. There are differing accounts of how the damage occurred. According to the 1911 Encyclopedia Britannica, the damage occurred during a siege in 638. The editors of Time-Life Books state that the damage occurred during a siege launched by a general of the Umayyad caliph Abd al-Malik (646-705). Other sources, including the 2007 Britannica, state that the damage occurred as the result of a theft in 930 CE, when Qarmatian warriors sacked Mecca and carried the Black Stone away to their base in Ahsa, in medieval Bahrain. According to the historian Al-Juwayni, the Stone was returned twenty-two years later, in 951, under somewhat mysterious circumstances; wrapped in a sack, it was thrown into the Friday Mosque of Kufa accompanied by a note saying "By command we took it, and by command we have brought it back." Its abduction and removal caused further damage, breaking the stone into seven pieces.>>
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by apodman » Sat Mar 28, 2009 2:11 pm

http://news.yahoo.com/s/ap/20090325/ap_on_sc/sci_asteroid_match;_ylt=ApcEXMbBK1i3SpjZ7ED4Gj2p_aF4;_ylu=X3oDMTE5bjFncXY4BHBvcwM1BHNlYwN5bi1tb3N0LXZpZXdlZARzbGsDYXN0cm9ub21lcnNj wrote:It was full of metals, such as iron and nickel, and organics such as graphites, Zolensky said. And most interesting is that it has "nanodiamonds." These diamonds are formed by collisions in space and high pressure and they are all over the rocks, making them glitter like geodes, he said. But they aren't big.

"If bacteria had engagement rings, these would be the right size for them," Zolensky said.

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by neufer » Sat Mar 28, 2009 3:53 pm

apodman wrote:
http://tinyurl.com/cn8emv wrote:It was full of metals, such as iron and nickel, and organics such as graphites, Zolensky said. And most interesting is that it has "nanodiamonds." These diamonds are formed by collisions in space and high pressure and they are all over the rocks, making them glitter like geodes, he said. But they aren't big.

"If bacteria had engagement rings, these would be the right size for them," Zolensky said.
Suddenly something is there on the turnstile,
The girl with kaleidoscope eyes.

Bacilli from the sky with diamonds...
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by jaan » Sat Mar 28, 2009 4:01 pm

The meteorite fragment in the image of March 28 APOD is at rest, perfectly level with the terrain. How could this be? Should there not be impact evidence, even of the fragments? Was it placed there to simply give a contrast to the rest of the surface as to color?
How was it really found? Was it dug from an impact crater?

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by neufer » Sat Mar 28, 2009 4:10 pm

jaan wrote:The meteorite fragment in the image of March 28 APOD is at rest, perfectly level with the terrain. How could this be? Should there not be impact evidence, even of the fragments? Was it placed there to simply give a contrast to the rest of the surface as to color?
How was it really found? Was it dug from an impact crater?
Perhaps, they are standing on the impact crater! :wink:
Image
280 small Almahata meteorites ~ 5 kilograms
Average Almahata meteorite ~ 18 grams


http://www.planetary.org/blog/article/00001896/
Last edited by neufer on Sat Mar 28, 2009 4:16 pm, edited 1 time in total.
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by aristarchusinexile » Sat Mar 28, 2009 4:15 pm

jaan wrote:The meteorite fragment in the image of March 28 APOD is at rest, perfectly level with the terrain. How could this be? Should there not be impact evidence, even of the fragments? Was it placed there to simply give a contrast to the rest of the surface as to color?
How was it really found? Was it dug from an impact crater?
Being volcanic and full of gas bubbles it will be lightweight, not having much impact. Some volcanic rock floats on water. It also may have bounced on first impact, that impact absorbing much of the momentum.
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by BMAONE23 » Sat Mar 28, 2009 4:25 pm

Considering that it is only approx 4cm (about 1-1/2") in diameter and that it is lying in a field of rocks, it likely didn't carry enough kinetic energy to create an impact crater. It likely hit a few feet away and bounced to it's discovered location.
The three rocks to the right of it also appear to be of other origin.
And, the red/yellow rock in the immediate foreground resembles carnelion of which the Red variety is more desireable for jewelry but ranges in color from lemony yellow to bright red and brownish

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Almahata Sitta 15 - Impact crater?

Post by jaan » Sat Mar 28, 2009 6:21 pm

How could the meteorite fragment be resting perfectly level on the surface? Even fragments should leave an impact crater or a trail of motion. Was this fragment placed on the surface to show color contrast only? Where was it actually found?

It would seem to me that even a 1-1/2" size volcanic meteorite fragment would have quite an impact and not just lay flat on a surface. Try dropping anything of that size from even a 6 story building and it will leave some mark of impact. The photograph feels like the metiorite was found elsewhere, placed on a flat area for strickly a photo opp. Sorry.

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by jaan » Sat Mar 28, 2009 6:53 pm

It would seem to me that even a 1-1/2" size volcanic meteorite fragment would have quite an impact and not just lay flat on a surface. Try dropping anything of that size from even a 6 story building and it will leave some mark of impact. The photograph feels like the metiorite was found elsewhere, placed on a flat area for strickly a photo opp. Sorry.

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Re: Almahata Sitta 15 - Impact crater?

Post by BMAONE23 » Sat Mar 28, 2009 7:30 pm

The fragment is sitting asand where it was actually found. It is scarsely 1.5" or 4cm accross. It carried very little kinetic energy with it when it hit. It hit in an area of firmly packed rocky soil and likely bounced some feet away from the initial impact place. Weighing only grams. it wouldn't create an impaact crater. Considering they found 280 pieces for a total of 5000 gms the average piece weighs only 17+grams. Very Small Pieces will not create a crater.

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by BMAONE23 » Sat Mar 28, 2009 7:32 pm

jaan wrote:It would seem to me that even a 1-1/2" size volcanic meteorite fragment would have quite an impact and not just lay flat on a surface. Try dropping anything of that size from even a 6 story building and it will leave some mark of impact. The photograph feels like the metiorite was found elsewhere, placed on a flat area for strickly a photo opp. Sorry.
Yes it will but it will also bounce away from the initial point of impact and come to rest where it leaves little or no mark at all unless you drop it in sand but the desert region in the image is more hard pack stony soil.

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by Chris Peterson » Sat Mar 28, 2009 10:22 pm

jaan wrote:It would seem to me that even a 1-1/2" size volcanic meteorite fragment would have quite an impact and not just lay flat on a surface. Try dropping anything of that size from even a 6 story building and it will leave some mark of impact. The photograph feels like the metiorite was found elsewhere, placed on a flat area for strickly a photo opp. Sorry.
It is very typical for small meteorites (up to a few kilograms) to be found simply sitting on the surface. If they land in soft soil they usually make a dent, but meteorites landing on a hard desert or lake bed surface often don't.
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by aristarchusinexile » Tue Mar 31, 2009 2:59 pm

Plus .. the meteorite's tragectory will effect impact on landing, correct (?) If it flies horizontally it will lose speed (?) I think these things are clear but I feel uncharacteristically unemphatic today, resulting in the (?)
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by Chris Peterson » Tue Mar 31, 2009 3:22 pm

aristarchusinexile wrote:Plus .. the meteorite's tragectory will effect impact on landing, correct (?) If it flies horizontally it will lose speed (?) I think these things are clear but I feel uncharacteristically unemphatic today, resulting in the (?)
Meteorites always hit the ground from very close to vertical- there's no way they can do otherwise unless they still retain some of their cosmic velocity, in which case they form craters- fortunately, a very rare thing!
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by aristarchusinexile » Wed Apr 01, 2009 2:45 pm

Chris Peterson wrote:
aristarchusinexile wrote:Plus .. the meteorite's tragectory will effect impact on landing, correct (?) If it flies horizontally it will lose speed (?) I think these things are clear but I feel uncharacteristically unemphatic today, resulting in the (?)
Meteorites always hit the ground from very close to vertical- there's no way they can do otherwise unless they still retain some of their cosmic velocity, in which case they form craters- fortunately, a very rare thing!
Skipping stone meteorites seem to be common on moons. I saw a fireball flying low and horizontally here on earth. What do you classify as cosmic velocity? Why do you say they have to hit vertically?
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by bystander » Wed Apr 01, 2009 2:59 pm

aristarchusinexile wrote:Skipping stone meteorites seem to be common on moons. I saw a fireball flying low and horizontally here on earth. What do you classify as cosmic velocity? Why do you say they have to hit vertically?
Most meterites have been decelerated by the earth's atmosphere to the point that they are essentially in free fall by the time they reach the ground.

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by Chris Peterson » Wed Apr 01, 2009 3:03 pm

aristarchusinexile wrote:Skipping stone meteorites seem to be common on moons. I saw a fireball flying low and horizontally here on earth. What do you classify as cosmic velocity? Why do you say they have to hit vertically?
On the Moon, or any airless body, meteorites can impact at any angle, and always carry their cosmic velocity. On Earth, however, they are slowed to terminal velocity by atmospheric drag while still miles above the ground. They lose all their cosmic velocity. At impact, the vertical component of their velocity is determined by drag (terminal velocity), and is typically 50-100 m/s. The horizontal component is determined by the wind speed during the last few seconds of the fall, typically no more than a few meters per second. So the impact is close to vertical.

"Cosmic velocity" just refers to the initial velocity of the object; meteorites only carry a component of this when they are large enough to not be completely slowed by drag- fortunately, that happens only very rarely.

You've never seen a fireball close to the ground, because meteors stop burning when they are 10 miles or more (usually much more) above the ground. In addition, there's no way to really tell from a single viewpoint what the actual orientation of a meteor path is with respect to the ground. It is common to see meteors that appear to be horizontal, but those are still high in the atmosphere. Once they slow down enough to stop burning, they will quickly lose all their forward velocity and drop vertically for several minutes before hitting the ground (assuming any material survives, of course). When you see what appears to be a low flying fireball, you're seeing something high in the atmosphere and 100 miles or more away.
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by aristarchusinexile » Wed Apr 01, 2009 4:01 pm

Chris Peterson wrote:The horizontal component is determined by the wind speed during the last few seconds of the fall, typically no more than a few meters per second. So the impact is close to vertical.
I don't think I'm nitpicking when I say 'close to vertical' is not 'vertical'. Like the Dr. of Language said in another thread, 'language is important.'
Chris wrote:"Cosmic velocity" just refers to the initial velocity of the object; meteorites only carry a component of this when they are large enough to not be completely slowed by drag- fortunately, that happens only very rarely.
Thanks.
Chris wrote:You've never seen a fireball close to the ground, because meteors stop burning when they are 10 miles or more (usually much more) above the ground. In addition, there's no way to really tell from a single viewpoint what the actual orientation of a meteor path is with respect to the ground. It is common to see meteors that appear to be horizontal, but those are still high in the atmosphere. Once they slow down enough to stop burning, they will quickly lose all their forward velocity and drop vertically for several minutes before hitting the ground (assuming any material survives, of course). When you see what appears to be a low flying fireball, you're seeing something high in the atmosphere and 100 miles or more away.
Thanks again, Chris. All I can say is the fireball was large, the flames were flames as if in a fire so the velocity must have been low, and to my line of sight it was flying horizontally just above the horizon outside my window, and it was like, Wow! This happened in the black of night, about 30 years ago, in Northern Ontario. I was working on a railroad maintenance gang camped in boarding cars far, far out in the Northwoods. Another interesting thing about the experience was that it confirmed for me personally the existence of fireballs up until then I had seen only in paintings and drawings, much like the Blue Moon .. it is a real blue moon, not just the second full moon of the month. I have seen two genuinely Blue Moons, also in the Northwoods, in the winter. I also saw a fireball high up, seeming to fly through Northern Lights which were strong and vibrant, and the fireball actually 'sizzled' as it flew through the lights. Static Perhaps?
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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by Chris Peterson » Wed Apr 01, 2009 4:42 pm

aristarchusinexile wrote:I don't think I'm nitpicking when I say 'close to vertical' is not 'vertical'.
Exactly. Which is presumably why nobody has used "vertical" without qualification.
Thanks again, Chris. All I can say is the fireball was large, the flames were flames as if in a fire so the velocity must have been low, and to my line of sight it was flying horizontally just above the horizon outside my window, and it was like, Wow!
I collect fireball reports as part of my meteor research, and I have several hundred reporting similar impressions. The apparent nearness of fireballs is a powerful illusion (I've seen it myself). People often report that fireballs dropped behind their neighbor's house, or in the next field, despite the fact that this isn't possible.

In the case of the fireball you saw, the object was traveling at least 5 km/s, since less than that is too slow for ablation and the generation of enough heat to be visible. Its height depends a great deal on its mass- the more mass an object has, the lower it can get into the atmosphere. For a really bright fireball, however, something around 20 miles at the end would be a reasonable estimate. So given your description, it's very likely you were seeing something that was 200 or more miles away.

The fireball you saw that made a sizzling sound is an example of electrophonic noise. This is sometimes produced by auroras as well. It isn't very well understood, and is fairly rare. When we have a large fireball, I may receive 500 reports, and of those, two or three people might report electrophonic noise. It is presumed to be caused by VLF emissions from ionization in the meteor trail, which is transduced to sound by objects near the observer: eyeglass frames, jewelry, barbecue grills, metal garage doors.

If you're interested in meteors, you might want to check my meteor page, which has general information as well as reports on a lot of big fireballs over the last 8 years.
Chris

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by BMAONE23 » Wed Apr 01, 2009 5:02 pm

aristarchusinexile wrote:
Chris Peterson wrote:The horizontal component is determined by the wind speed during the last few seconds of the fall, typically no more than a few meters per second. So the impact is close to vertical.
I don't think I'm nitpicking when I say 'close to vertical' is not 'vertical'. Like the Dr. of Language said in another thread, 'language is important.'
True but 89 degrees, though less than vertical, is still close ehough to it.

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by Chris Peterson » Wed Apr 01, 2009 5:43 pm

BMAONE23 wrote:True but 89 degrees, though less than vertical, is still close ehough to it.
A fairly extreme example (and I don't know of any such in actuality) would be a small stone with a low 50 m/s terminal velocity, caught in a sustained 20 mph wind during the last of its fall. That would be enough to knock it about 10° out of vertical. But looking at cases where meteorites have hit objects, or left dents on the ground, or even been witnessed to fall, they all seem to be no more than a few degrees at most from vertical.

I investigated a witnessed fall a few years ago here in Colorado, the Berthoud meteorite. Its impact was witnessed as it hit packed, but fairly soft ground in a corral. It did not bounce or roll at all, but just lay right in the middle of a little dent that followed the form of the meteorite. Terminal velocity was about 75 m/s, ground level wind was no more than 3 m/s. Typical. There is some limited evidence that the initial trajectory was closer to horizontal than vertical; if so, all of that was lost.
Chris

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Re: Almahata Sitta 15 (APOD 2009 March 28)

Post by aristarchusinexile » Thu Apr 02, 2009 3:39 pm

Chris Peterson wrote: Exactly. Which is presumably why nobody has used "vertical" without qualification.
Still too close to vertical for my love of language. How about providing some angles? Maybe now I'm nitpicking. My apologies if so.
Chris wrote:I collect fireball reports as part of my meteor research, and I have several hundred reporting similar impressions. The apparent nearness of fireballs is a powerful illusion (I've seen it myself). People often report that fireballs dropped behind their neighbor's house, or in the next field, despite the fact that this isn't possible.

In the case of the fireball you saw, the object was traveling at least 5 km/s, since less than that is too slow for ablation and the generation of enough heat to be visible. Its height depends a great deal on its mass- the more mass an object has, the lower it can get into the atmosphere. For a really bright fireball, however, something around 20 miles at the end would be a reasonable estimate. So given your description, it's very likely you were seeing something that was 200 or more miles away.
Your information is comforting, because the other guys in the trailer were not looking out the window and thought the fireball was the result of the many red and orange, white spotted Amanita mushrooms I had been eating over a few days. I had my own doubts at the time, so it's good to know I saw what I saw.
Chris wrote: The fireball you saw that made a sizzling sound is an example of electrophonic noise. This is sometimes produced by auroras as well. It isn't very well understood, and is fairly rare. When we have a large fireball, I may receive 500 reports, and of those, two or three people might report electrophonic noise. It is presumed to be caused by VLF emissions from ionization in the meteor trail, which is transduced to sound by objects near the observer: eyeglass frames, jewelry, barbecue grills, metal garage doors.
Again, thanks for your confirmation, because many people just looked at me like I was a stoned hippy on the brink of too severe an experience of mind expansion. I think I did have my eyeglasses on. At the time, and still today, I had an instinct that I was deeply priviledged to be hearing and seeing what I was experiencing, especially as the Northern Lights were simply grand and awesome. I had taken my sleeping bag down the railroad track for a 'night out' from the railroad gang's sleeping cars' hard roof, and away from camp noise. Mid Autumn. I should write a poem, but few people would read it because I'm not a famous poet writing drivel.
Chris wrote:If you're interested in meteors, you might want to check my meteor page, which has general information as well as reports on a lot of big fireballs over the last 8 years.
Thanks for this also.
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Almahata Sitta 15

Post by Sebastian » Thu Apr 09, 2009 5:30 am

I find it anomalous that fragment #15, as pictured, appears to have made NO dint in the desert surface at all. Surely it should have made at least a small crater ? Or a skid path...

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Re: Almahata Sitta 15

Post by rstevenson » Thu Apr 09, 2009 11:36 am

The fragment shown in that picture is only 4 cm wide -- about the size of a largeish walnut. Surely it could have bounced a little as it was coming to rest, leaving behind whatever crater or skid marks it had created. And from the angle at which the picture was taken, I wouldn't expect to see much of a trail anyway. It looks like the camera is sitting on the ground.

Rob

... And I just realized this question had been asked and answered several times in the other (the first) Almahata Sitta thread, and I believe it is advised that we keep all questions relating to one APOD image in one thread. Sorry about that.

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Different strokes

Post by JohnD » Wed Apr 29, 2009 10:31 pm

All,
A recent APOD, of the Almahata Sita meterorite led to all sorts of allegations here about the veracity of the report, as there was no crater.
See: http://apod.nasa.gov/apod/ap090328.html
and the thread here on this site.

Now Opportunity has encountered the "youngest crater seen on Mars". It may be only 100,000 years old!
See: http://marsrovers.jpl.nasa.gov/spotlight/20090429.html

A meteroid impact on Earth and Mars will be completely different, due to atmosphere and gravity.
But how big would have been the rock that caused Resolution crater?

And how about this as an Apod?
John

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