APOD: Tunguska: The Largest Recent Impact... (2011 Oct 02)

[APOD Robot]

Tunguska: The Largest Recent Impact Event

Explanation: Yes, but can your meteor do this? The most powerful natural explosion in recent Earth history occurred on 1908 June 30 when a meteor exploded above the Tunguska River in Siberia, Russia. Detonating with an estimated power 1,000 times greater than the atomic bomb dropped over Hiroshima, the Tunguska event leveled trees over 40 kilometers away and shook the ground in a tremendous earthquake. Eyewitness reports are astounding. The above picture was taken by a Russian expedition to the Tunguska site almost 20 years after the event, finding trees littering the ground like toothpicks. Estimates of the meteor's size range from 60 meters to over 1000 meters in diameter. Recent evidence suggests that nearby Lake Cheko may even have been created by the impact. Although a meteor the size of the Tunguska can level a city, metropolitan areas take up such a small fraction of the Earth's surface that a direct impact on one is relatively unlikely. More likely is an impact in the water near a city that creates a dangerous tsunami. One focus of modern astronomy is to find Solar System objects capable of creating such devastation well before they impact the Earth.

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[jensenr@missouri.edu]

Carl Sagan said on his Cosmos series on PBS that the Tunguska event was a piece of a comet, not a meteor. FYI. Has the conventional wisdom changed since then?

Ron
jeebo@aol.com
jensenr@missouri.edu

[Chris Peterson]

Carl Sagan said on his Cosmos series on PBS that the Tunguska event was a piece of a comet, not a meteor. FYI. Has the conventional wisdom changed since then?

Nobody really knows. By presuming that it was cometary, it is (maybe) easier to explain why no material survived: cometary material is assumed to be considerably more fragile, made either of volatiles or a loose mix of carbonaceous chondrite-like stone. But it is now understood that many asteroids and large meteoroids are not solid bodies, but more like rubble piles. So who knows? Until some good physical evidence is recovered, the question of just what the parent body really was remains open.

[dnov8r]

This is an exact repost from November 14, 2007. I feel gypped.

Also, Nicola Tesla took personal responsibility for the event in letters he sent to the press and military, as well as his autobiography. He claimed he was working on an energy weapon, tried to hit the norther aurora to impress a general and miscalculated. He dismantled his famous tower soon after.

[owlice]

From the APOD FAQ:

Q4: Have some APOD pictures been run more than once?
A4: Yes. Many of our readers have been with us less than a year and are unaware of some really spectacular or important astronomy pictures. New information about old pictures is becoming available over the WWW. The text and links for rerun pictures will make use of this newly available information. So although the picture might be old, some of the text and links of each APOD will be new. Also, more web surfers have larger bandwidth connections, which allows us to post higher-resolution image files that can be transferred conveniently. Software to handle more sophisticated image file formats has also become more common, so the picture's size and/or format might be new. Lastly, rerunning APODs saves us time and helps us update our archive. In general, our rerun policy currently is to only rerun APODs more than one year old to keep the pictures relatively "new" to new APOD viewers. We will almost never rerun more than two pictures in any given week. So when you load the current APOD, it is still, most probably, a new picture.

[dnov8r]

Mmmnn.... yeah... still...

[owlice]

Yeah, still.... what? If you would like new images to look at, mosey on over to the Observation Deck and the Communications Center. Plenty of pics to see!

[Ann]

What I find most amazing is that such a powerful event, causing such destruction over such a very large area - your wikipedia link says that the explosion knocked over 80 million trees covering 2,150 square kilometres (830 sq mi) - nevertheless apparently killed no humans.

To me, this little tidbit carries some profound information about how the Earth has changed in the 103 years that have gone by since then. Mainly, of course, the number of human beings on the Earth has about quadrupled since 1908. According to http://en.wikipedia.org/wiki/Population_growth, the number of human beings on the Earth was about one billion in the year 1800, about two billion in 1927 and about seven billion in 2011.

So it seems likely that an event like the 1908 Tunguska impact would create much greater damage, or at least directly affect many more humans, if it occurred today on our much more crowded Earth.

Ann

[Dick Henry]

How I admire your writing of dates as: 1908 June 30 What can be more confusing than writing dates as e.g. 01/01/01 as so many people still do? Oh, yes, and what about the idea that Tunguska was a tiny-mass black hole?

[Guest]

ok,so there is an asteroid, a big one, heading toward earth, then what ?

[orin stepanek]

Why wouldn't a comet entering the atmosphere be considered a meteor? If I see a streak of fire coming down; I doubt if I would think; hey; is that a comet of an asteroid coming down! I would probably hope I was out of harms way!

[neufer]

Why wouldn't a comet entering the atmosphere be considered a meteor? If I see a streak of fire coming down; I doubt if I would think; hey; is that a comet of an asteroid coming down! I would probably hope I was out of harms way!

Good point.

Perhaps the issue with the APOD explanation should be
not whether the source body of the Tunguska event was originally part of a comet or an asteroid
but rather was this object too large to be considered a meteor/meteoroid:

<<A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteoroid reaches the ground and survives impact, then it is called a meteorite. Many meteors appearing seconds or minutes apart are called a meteor shower. The root word meteor comes from the Greek meteo¯ros, meaning "high in the air". The Minor Planet Center does not use the term "meteoroid".

As of 2011 the International Astronomical Union officially defines a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom". Beech and Steel, writing in Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid is between 100 µm and 10 m across. The NEO definition includes larger objects, up to 50 m in diameter, in this category. Very small meteoroids are known as micrometeoroids (see also interplanetary dust).

The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also give information, especially useful for daytime meteors which are otherwise very difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see Meteor showers) often associated with a parent comet, others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice, to nickel-iron rich dense rocks.

Meteoroids travel around the Sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second) through space in the vicinity of Earth's orbit. The Earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the Earth's atmosphere head-on (which would only occur if the meteors were in a retrograde orbit), the combined speed may reach about 44 miles per second (71 kilometers per second). Meteoroids moving through the earth's orbital space at about 20 km/sec.>>

[twixter]

This is a rerun BTW. See http://apod.nasa.gov/apod/ap071114.html

[owlice]

This is a rerun BTW. See http://apod.nasa.gov/apod/ap071114.html

See http://asterisk.apod.com/viewtopic.php? ... 45#p158894

[Chris Peterson]

Why wouldn't a comet entering the atmosphere be considered a meteor?

It would meet the definition of "meteor", and when the subject of Tunguska comes up in meteoritics publications or conferences, its atmospheric passage is referred to as a meteor (independently of any question about the nature of the parent body).

[Chris Peterson]

Oh, yes, and what about the idea that Tunguska was a tiny-mass black hole?

There are good reasons to think that tiny-mass black holes can't exist. But if they could, there would probably be almost no interaction with the Earth or its atmosphere at all.

[neufer]

Why wouldn't a comet entering the atmosphere be considered a meteor?

It would meet the definition of "meteor", and when the subject of Tunguska comes up in meteoritics publications or conferences, its atmospheric passage is referred to as a meteor (independently of any question about the nature of the parent body).

If it was 60m or larger it was probably too big to officially be considered "a meteor" (; see above).

[Chris Peterson]

It would meet the definition of "meteor", and when the subject of Tunguska comes up in meteoritics publications or conferences, its atmospheric passage is referred to as a meteor (independently of any question about the nature of the parent body).

If it was 60m or larger it was probably too big to officially be considered "a meteor" (; see above).

Not at all. Don't confuse "meteoroid" with "meteor". A comet or asteroid can still produce a meteor: the atmospheric passage of even a massive object like that which occurred at Chicxulub 65 million years ago is still generally called a meteor.

FWIW, the IAU definitions for meteor, meteoroid, and asteroid are quite vague, and nobody I know strictly observes them. Instead, the terms are used as seems appropriate in context.

[orin stepanek]

I always thought that a meteor was when it was burning in the atmosphere and a meteoroid was the pieces that made it to the ground.

[Chris Peterson]

I always thought that a meteor was when it was burning in the atmosphere and a meteoroid was the pieces that made it to the ground. :?

A meteoroid is the parent body (which could be of cometary material), either in space or in the atmosphere. The atmospheric phenomenon is a meteor. If a piece makes it to the ground, it is called a meteorite (although meteorwrongs are much more common <g>).

[orin stepanek]

I always thought that a meteor was when it was burning in the atmosphere and a meteoroid was the pieces that made it to the ground.

A meteoroid is the parent body (which could be of cometary material), either in space or in the atmosphere. The atmospheric phenomenon is a meteor. If a piece makes it to the ground, it is called a meteorite (although meteorwrongs are much more common <g>).

Thanks!

[neufer]

I always thought that a meteor was when it was burning in the atmosphere and a meteoroid was the pieces that made it to the ground.

A meteoroid is the parent body (which could be of cometary material), either in space or in the atmosphere. The atmospheric phenomenon is a meteor. If a piece makes it to the ground, it is called a meteorite (although meteorwrongs are much more common <g>).

<<Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid. The term "asteroid" is ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union from 1853 on. In 2006, the term "Small Solar System Body" was introduced to cover both most minor planets and comets. Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for the larger asteroids. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets, but is not in common use. The three largest objects in the asteroid belt, Ceres, 2 Pallas, and 4 Vesta, grew to the stage of protoplanets. Ceres has been designated as a dwarf planet, the only one in the inner Solar System.

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "Small Solar System Body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.>>

<<A meteor is the visible path of a meteoroid that has entered the Earth's atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km. Millions of meteors occur in the Earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 65 and 120 kilometers above the Earth. They disintegrate at altitudes of 50 to 95 kilometers. Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth. Most meteors are, however, observed at night as low light conditions allow fainter meteors to be observed.

For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres) the visibility is due to the atmospheric ram pressure (not friction) that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second. A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs (for example The Great Daylight 1972 Fireball).

Meteors may occur in showers, which arise when the Earth passes through a trail of debris left by a comet, or as "random" or "sporadic" meteors, not associated with a specific single cause. A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the meteoroids producing the meteors have been calculated. All of the orbits passed through the asteroid belt.>>

[wolf kotenberg]

Too bad water molecules that arrive from space do not carry a barcode that we can read. It would not surprise me if some water in that swamp from the comet is still being drunk by fish.

[islader2]

@ Dick Henry Writing dates in the format "year/month/date" is a very useful tool for accessing files--since only the last digit changes daily. Think about it--the US Army has. ThanX.

[Chris Peterson]

Too bad water molecules that arrive from space do not carry a barcode that we can read. It would not surprise me if some water in that swamp from the comet is still being drunk by fish.

Even if it was a comet, that doesn't mean it had much water- the rocky core of a comet is more likely. And even if the body did have a lot of water, virtually all of it would have ended up in the atmosphere, which means it would have been widely distributed over Asia, and the entire world.

In a sense, water molecules are coded, since there are isotopic forms, and looking at ratios can tell something about the environment the water formed in (or existed in). But there's no useful way to use that information years after an event like this.