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FAQ - Meteoroids/Meteorites

1. Are asteroids and meteoroids made of similar things?

Meteoroids are bits of solid material of different sizes orbiting the Sun. Some bits come from comets, some bits come from asteroids, and a very few bits from the Moon and other planets. All those bits originally came from the same “pot”—the solar nebula—and so they are mostly made of the same stuff. However, some asteroids (and planets!) became hot enough to melt and differentiate, or separate, into distinct layers of rock and pure iron/nickel. Bits of material blasted off asteroids with those differentiated layers will have those compositions—rock or metal—and are different from most meteoroids.

2. Are most meteorites stony or iron?

Based on the number of falls (objects seen to fall as a meteor), about 5% are irons and 95% are stones. This is based on a little over 1,000 witnessed falls. However, these percentages do not apply for the number of finds (seen on the ground but not seen to fall). About 80% of the finds are irons and only 20% are stones. By weight, stones represent only 2% of the total finds! Why? The falls are probably more representative of what there is out in space (though, being stronger, irons have a better chance of surviving entry). For the finds, irons do not look like Earth rocks while stones may be harder to notice. Also, stones tend to weather faster so do not survive as long. Finally, the biggest finds are primarily irons. Again, weathering may be a big factor.

3. How do we get so many samples of meteorites?

To date, there have been nearly 1,100 recovered falls (meteorites seen to fall) and nearly 40,000 finds (found, but not seen to fall). It is estimated that probably 500 meteorites reach the surface of the Earth each year, but less than 10 are recovered. This is because most fall into the ocean, land in remote areas of the Earth, land in places that are not easily accessible, or are just not seen to fall (fall during the day). From a model animation, it appears that lots of small asteroids/large meteoroids pass close to the Earth each day. Most of these are not detected, but recently, three 5–10 meter “asteroids” have been discovered and have passed well within the orbit of the Moon. Also recently, an asteroid about 500 meters in diameter passed about 2 million km from the Earth (five times the distance to the Moon). It is estimated that each day one or two 5–10 meter objects pass within the Moon’s orbit and that there are probably 30 million near-Earth objects! Most of these are too small to ever cause any damage. Five to ten meters is probably the smallest object that would likely survive passage through the Earth’s atmosphere.

4. Are there any asteroids/meteorites headed toward Earth in the next 100 years? Is there a plan? Is 2012 factual time?

Small asteroids and meteoroids are always coming close to the Earth and sometimes running into the Earth (there are no stop signs or yield signs in space). Surveys such as the Catalina Sky Survey are doing a good job of finding most of the larger asteroids, the ones that can cause severe damage. But, there are more out there, mostly in the size range that could do in a city. However, unlike the movies, there is nothing very large that is predicted, in the near future, to have any chance of hitting the Earth. The largest possible threat is from the asteroid 99942 Apophis. It has a one in 250,000 chance of hitting the Earth in 2029 and about one chance in 300,000 of hitting the Earth in 2036 (due to the uncertainty of its orbit). It has a diameter of about 250 meters, so could create an impact crater about 2 or 3 kilometers in diameter. This would be bad for a city and for a radius of tens of kilometers around it—bad locally, but not beyond that. There is nothing that is big enough to do anything in the near future. Space is big and if there were a new comet heading toward us right now and big enough to cause planet-wide damage, we would already have seen it with our telescopes. However, it is likely that something could in the future hit the Earth. For that reason, there are people who are looking at ways to prevent this from happening and have meetings to discuss plans for such an event.

5. Are meteorites that come from differentiated objects older?

When scientists "date" the age of a rock, they are dating the time when it first crystalized, turned from a liquid to a solid. If one were to date a lava rock, the date you would get would be the one when the lava hardened. Therefore, the oldest meteorites are the ones that have not differentiated since they have never melted. The oldest meteorites are about 4.468 billion years old. Rocks from Vesta, a differentiated asteroid, are about 8 million years younger, implying that Vesta differentiated, started cooling, and had lava flows on its surface all within the first 8 million years of the formation of the Solar System. Rocks from the Moon are as old as 4.5 billion years (when the original crust formed) to about 3.16 billion years old when some of the maria on the Moon filled with lava. On Mars, where there are a number of ancient volcanoes present, the oldest martian meteorites (meteorites from Mars) are about 4.1 billion years old and the youngest less than 200 million years old. Other martian rocks are about 1.4 billion years old.

6. What are the sources of meteor-wrongs? How do these compare to actual meteorites and what gives them their very similar appearance?

"Meteorwrongs" come from a variety of sources. Tektites are pieces of Earth rock that have been melted and thrown into space by a large impact on Earth’s surface. It was still (or became again) liquid as it passed back down through Earth’s atmosphere, so it became aerodynamically shaped. Cooling was so fast that the mineral solidified as a glass, not as a crystalline material. Obsidian is a terrestrial volcanic glass extruded in thick, massive flows at some volcanoes. A good example is Big Glass Mountain in northern California. The volcanic cinder is a broken piece of volcanic rock thrown out of a volcano. Hematite is an oxide of iron (Fe2O3). Rust is a form of hematite. Iron reacts easily with oxygen, so hematite is a natural result of prolonged exposure of iron to the atmosphere. Polished hematite looks like polished iron, but it is not magnetic like iron. Hematite often has a reddish color, unlike the gray of a fresh iron meteorite. Similarly, magnetite is an oxide of iron (Fe3O4) with a somewhat higher iron content than hematite, and so is magnetic. While "meteor-wrongs" are similar in appearance to a meteorite, there are several ways to tell them apart. The polished surface of magnetite looks like a polished iron meteorite, but does not show the pattern of lines when etched. Hematite also shatters like a rock, whereas iron only deforms when struck. Finally, the density of magnetite is about 5.5 g/cm3, significantly less than the density of ~8 g/cm3 for iron meteorites.

7. Do all meteors "vaporize" on impact or do some actually disappear below the surface?

An impact event is similar to an explosion. It involves a lot of energy, but it also takes a lot of energy to vaporize a huge object like an asteroid! Therefore, when an asteroid hit the Earth’s surface at typical velocities (which we know range from about Earth’s escape velocity, 11 km/s, (25,000 mi/hr) to 30 km/s (67,000 mi/hr), only a fraction of the object is vaporized, while most of the object is melted with a fraction staying solid, but completely fractured. Most of the impactor (>99.9%) solid/melt/vapor, is ejected in the impact process and little if any stays in the crater.

This question is important in that it addresses a major misconception about impact cratering. The belief that a big component of the impactor would be buried under the crater drove Daniel Barringer, the original owner or the Barringer crater, or Meteor Crater, to spend years trying to locate the big iron, with the intention of mining it. Eventually, Barringer died of a heart attack after spending his entire fortune looking for the impactor and learning that the impactor was never there. On the other hand, there are cases where meteorites were found buried in the ground under or around craters. This only happens when the impactor is small; in this case the impactor fragments in the atmosphere and individual small pieces, strongly decelerated by the atmosphere, reach the ground but do not have enough energy to melt or disintegrate. In this case we don’t have the classic impact event that resembles an explosion and the object fragments can be buried underground, with or without the formation of a small crater.

8. What is the rate at which stuff burns up when they enter Earth's atmosphere?

The rate at which material is removed from objects passing through the atmosphere depends on the velocity, mass, and surface area of the object, and the strength of the material. As the object moves through the atmosphere, it is decelerated and the lower velocity decreases the amount of drag acting on it. Eventually, the object goes through enough atmosphere that the drag is minimal. This is where the bright path of light of a meteor (a shooting star) ends.

9. About how much material is burned up?

Again, this depends on the speed of entry, the angle it comes in at (does it have time to slow down in the thin atmosphere?), and the strength of the material (fluffy comet material, rocky, or iron). It turns out that comet dust has a good chance of surviving. We find a lot of what are called "interplanetary dust particles" that make it to the surface of the Earth. This is because they are so small and light that they are slowed down very high in the atmosphere (50 to 100 km altitude). Really big objects barely notice the atmosphere and will make it to the surface. For fairly strong objects, good comparisons are: a VW bug outside the atmosphere will give you a microwave oven-sized meteorite or a basketball-sized object will give you a softball-sized meteorite. However, something that is more fragile may lose something like 99% of its mass! Two real-life examples: Tunguska in Russia in 1908 has been estimated to be about 50 meters in diameter and nothing survived because it could not withstand the air pressure and exploded in the atmosphere. Also, the object that was discovered in Tucson, 2008 TC3 is estimated to have been 2 to 5 meters in diameter (10,000 to 100,000 kilograms) and only 4 kilograms (280 pieces) was recovered. So it, too, broke up before landing.

10. If there was a recent meteor shower in the area, how would you know if you came across a meteorite?

First, no meteorites have ever been seen to fall during a meteor shower (at least one related to the shower, i.e., coming from the same region of the sky as the shower). Also, meteor showers cover essentially the entire planet, so are not concentrated in one local area. Now, if we are talking about a possible shower of meteorites from the break-up of a meteorite that was seen to fall, that is different. You can look for rocks that look out of place (usually a dark rock in, say, a sandy area). If they are large enough, they might leave small holes in the ground (nothing like in science fiction movies). Finally, because most of them contain some iron metal, they would be attracted to a magnet or would be picked up by a metal detector.

11. Are there strategies for figuring out if a meteorite or similar space objects are authentic?

Assuming you are talking about items at a show/sale/shop, some dealers are more reliable than others in that they have good knowledge of what a real meteorite is and what it is not. Members of the International Meteorite Collectors Association would also know first-hand if samples are real meteorites or not.

12. What are the chances of any meteor hitting the Earth?

First of all, the chances of any meteor of hitting Earth are… 1! This is because a meteor is the visible display of a meteoroid (a small object, ranging from a dust grain to pebble-size) entering the Earth’s atmosphere and "burning" while traversing it (usually never reaching the Earth’s surface). Therefore, a meteor, by definition, is an object already hitting the Earth! However, if the question is related to any object, small (usually referred to as "meteoroids") or large (usually referred to as "asteroids" or "comets"), then we need to look at the probability of different sized objects hitting the Earth. In a PSI workshop activity "Chances of Impact", this probability is discussed. The chance of any object hitting the Earth varies with the object size: pebble-sized objects hit the Earth everyday; Tunguska-sized objects (equivalent to a small house) hit the Earth every few centuries; Meteor Crater-sized objects (medium house) hit the Earth every millennium or two; civilization-threatening objects (roughly the size of "A" mountain in Tucson, AZ) will hit Earth every million years or so. Obviously, we are not particularly afraid of pebble-sized objects, mainly because they never make it through the Earth’s atmosphere, but even a Tunguska-sized object may create havoc if it hits or explodes over a city.

13. Where do meteorites (meteoroids), asteroids, and comets come from?

When the planets formed, the material left over is what we see today as asteroids and comets. The comets probably have not changed much since the formation of the Solar System. The same is true for the bigger asteroids, but the smaller asteroids probably are the result of collisions of larger asteroids. Asteroids do not come from a destroyed planet. When asteroids break up they make smaller rocky objects called meteoroids. All of these are Solar System objects orbit the Sun like the planets. If a meteoroid runs into the Earth and survives going through the atmosphere, the rock that lands on the Earth is a meteorite.

14. Do asteroids and meteoroids orbit on the same plane as planets or do they have their own plane(s)?

This is where the terminology for asteroids and comets gets fuzzy (pun intended). When a new object is discovered, it is given an asteroid designation if it is star-like or a comet designation if it shows a coma. There have been a number of cases where asteroids have "turned on" and become comet-like (got closer to the Sun) or when comets have ceased to show cometary activity. It is believed that as much as 10% of the Near-Earth Objects are actually extinct comet nuclei. More than 40 asteroids have "unusual" orbits: highly elliptical, highly inclined, and/or retrograde (orbit the Sun opposite the planets and most of the asteroids). Many of these have orbits similar to that of Comet Halley. This means that these are really dead comets that originated far from the Sun. Forty out of several hundred thousand is not many. Other than these, most asteroids have orbital inclinations less than about 30 degrees. Below is a plot of these orbits. The situation for meteoroids is another story. Meteoroids can come from either the debris of asteroid impacts, or from the material that comes off of comets. Therefore, since comets have orbits that are low inclination, high inclination, and retrograde, this is also true for meteoroids.


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