Starship Asterisk*

jensenr@missouri.edu wrote: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?

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.

orin stepanek wrote:
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!

http://en.wikipedia.org/wiki/Meteoroid wrote:
<<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 wrote:This is a rerun BTW. See http://apod.nasa.gov/apod/ap071114.html

orin stepanek wrote:Why wouldn't a comet entering the atmosphere be considered a meteor?

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

Chris Peterson wrote:

orin stepanek wrote:
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).

neufer wrote:

Chris Peterson wrote: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).

orin stepanek wrote: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 wrote:

orin stepanek wrote: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>).

Chris Peterson wrote:

orin stepanek wrote:
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>).

http://en.wikipedia.org/wiki/Asteroid#Terminology wrote:
<<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.>>

http://en.wikipedia.org/wiki/Meteor#Meteor wrote:
<<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 wrote: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.