Asteroid 'gives Earth a close shave' on Monday

[Airbrush]

When talking about the ocean surface, a 2D object, the impact would lose energy as roughly [math]1/r[/math] rather than as [math]1/r^2[/math], where r is your distance from the explosion. Even though the waves would lose energy at a far slower rate, at a significant distance you wouldn't even notice it unless your asteroid were truly huge. If you like, consider how much energy you want your tsunami to have, and at what distance. Then you can calculate how much energy an asteroid would need to have to generate that tsunami. I think you'll need a pretty big asteroid to make big waves.

 

Interesting info Mr. Skeptic. Is there a way to calculate approx what size impactor will generate a tsunami comparable to the 12-26-04 Indonesian earthquake tsunami?

 

What earthquakes have over meteor impacts in an ocean is that earthquakes cause movements spread over hundreds of miles. Most meteor impacts displace far less water.

[Mr Skeptic]

Yes, earthquakes release insane amounts of energy, but usually you don't notice because it is spread over such a large area. However, when the area is ocean, it can generate a huge wave that will deliver the energy more or less all at once, and in the form of water rather than shaking.

 

To compare the energies, draw an imaginary line and measure how much wave energy passes over that line. Then, compare the length of your line to the entire circumference surrounding the source (circumference = 2*pi*radius), and multiply so that your line would have measured the energy over the entire circumference. That should tell you how much energy a tsunami released. An asteroid would have to have comparable energy to have a comparable effect.

[Sisyphus]

Well, I'm not an expert (so feel free to correct any flawed assumptions), and obviously there are various factors involved, but:

 

The Tunguska event is estimated to have a seismic effect of about magnitude 5. The 2004 tsunami was about magnitude 9.2, which corresponds to a release of approximately 63 million times as much energy. (The scale is logarithmic - each increase of 1 magnitude corresponds to 31.6 times as much energy. 31.6^5.2=62,859,776.) So, an asteroid 63 million times the size of the one that hit Tunguska? That does seem high...

[Airbrush]

I always thought the Richter scale steps were 10 x each. So from magnitude 5 to 9.2 = Tunguska X 10 X 10 X 10 X 10 X 1.2 (X 10,200). It would take an impactor 10,200 times the mass of the Tunguska object which was about 30 meters across (how do you calculate the size of this hypothetical object which would create tsunamis comparable to the 2004 Indonesian earthquack tsunamis?), and the blast was equivalent to 1,000 Hiroshimas.

 

An object moving at 5 to 10 miles per second will displace many times its own size in water, but the waves radiate out from only a point and dissipate more rapidly than wide-range earthquake-generated and focused tsunamis. What sized impactor could generate that much damage on that same area if the impact occurred at the center of the Indonesian earthquake ground movement.

[cameron marical]

how big of an asteroid would it have been to have killed of the dinosuars?

[Ophiolite]

The KT impactor was around 10km in diameter.

 

All suggestions to 'nudge' asteroids out of the way have failed to take into account the fact that most asteroids are rubble piles that would fly apart if you sneezed at them. Which would you prefer? A single large impact that takes out Berlin, or several small impacts that take out much of Europe?

 

I always thought the Richter scale steps were 10 x each.

The factor of 10x is for the magnitude of the greatest amplitude of horizontal displacement. That, as Sisyphus has pointed out, is different from the energy of the event.

[Sisyphus]

Thinking about it again, it seems like a much bigger impact-related threat than stuff like tsunamis would be climate change. Much like a volcanic eruption, Tunguska caused a detectable decrease in atmospheric transparency for months afterwards all over the world. How much bigger would an impact have to be to seriously screw up the global climate? Obviously something 10km across would (that’s what caused the KT extinction, right?), but what effect would something, say, 100m across (1/millionth KT impact size) cause?

[Airbrush]

A few variables are if it explodes miles above ground, or if it impacts deep water, or shallow water closer to land, or on dry land. Composition of soil that gets vaporized and blasted into the atmosphere could matter, but I don't know how, I'm not an expert. If the Tunguska object impacted deep water a thousand miles from land then the tsunamis may have been unnoticeable but there would be significant water vapor blasted into the atmosphere. If it impacted shallow water, it would kick up dust and water vapor and tsunamis could be devastating locally. If it hit dry land then more atmospheric effects. It would be nice to get expert input on what these possibilities could mean.

 

The 10km dino-killer impacted shallow water.

 

Do most asteroids explode miles above ground? It must depend on steepness of angle of impact, and steeper, more direct, angles are less probable I suppose.

 

A poster had proposed 3 or 4 rockets with a net stretched between them to slow or stop the asteroid. Of course we would rather deflect it, but maybe that is a way to deflect a loose aggregate without breaking it up into pieces. I guess that the more massive the object, the more solid it is. To find out how solid an object really is may require a mission to the asteroid to study it long before it crosses our path.

Edited March 16, 2009 by Airbrush
typos

[cameron marical]

well. just dig a giant hole in your backyard, have it insulated in a shock absorbant material or liquid, and have enough food and space and water and stuff to last you a couple of months. also have seeds to plant after you come out. i think were good.

[Sisyphus]

A poster had proposed 3 or 4 rockets with a net stretched between them to slow or stop the asteroid. Of course we would rather deflect it, but maybe that is a way to deflect a loose aggregate without breaking it up into pieces. I guess that the more massive the object, the more solid it is. To find out how solid an object really is may require a mission to the asteroid to study it long before it crosses our path.

 

If you think about it, that wouldn’t work. To “stop” an asteroid falling towards Earth would mean absorbing all of its kinetic energy. The kinetic energy that is great enough to pose a threat to humanity itself. Stopping something that massive and fast moving just isn’t an option. (And even if you could stop it relative to the Earth, there would be no point, since nothing stays put in space.) Luckily there would never be a need for that. You would only need to push or pull it to the side enough to change its course by a tiny amount in order to make sure it misses the Earth (and the farther away you can get to it, the less its course needs to change).

[D H]

If you think about it, that wouldn’t work.

Thanks for that. There has been a lot of nonsense in this thread.

 

You would only need to push or pull it to the side enough to change its course by a tiny amount in order to make sure it misses the Earth (and the farther away you can get to it, the less its course needs to change).

So long as you do so years in advance.

 

A meteor discovered a day or so before impact is going to impact. Blow it up into lots of little pieces and all (or almost all) of those little pieces would still impact the Earth were it not for impacting Earth's atmosphere first. Being small, all of those little pieces will blow up in the upper atmosphere. Good bye, ozone layer.

 

This is essentially an engineering project rather than a science project. At some point the incoming meteor will cross a "point of no return". Once past this threshold the meteor will hit. "No can defense", to quote Mr. Miyage. The same thing happens to airplanes en-route between cities and ships coming into dock. An airplane can not return to its city of origin once it has consumed more than half the fuel in its tanks. A big ship coming into dock cannot be turned away after it has crossed some threshold distance.

 

Many of you posting in this thread are considering neither the theoretical nor practical points of no return when it comes to meteor defense.

[jackson33]

Isn't current thought for altering a course of an object, to place mass (assume in some spacecraft) near the object and letting that created gravity on the object change its course. Think that's where "years" come in...Last I heard a potential impact 2029 return visit 2036 is being planned in this manner.

 

As for space debris on earths ozone, we are showered daily with tons of the stuff, not all dust, actually cleaning areas for new ozone forming, inside a month.

 

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V3S-450KJGS-16&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=a1affa0a940f09b2e156b9c06414a951

 

If an impact was eminent without time to divert trajectory, say weeks out, wouldn't destruction be more plausible than leaving it alone. If big enough to wipe out Berlin, I hardly think the remnants of a blasted meteor would destroy Europe. Much would be diverted initially, with much of the remaining burning up in Earths atmosphere.

[Airbrush]

If an impact was eminent without time to divert trajectory, say weeks out, wouldn't destruction be more plausible than leaving it alone. If big enough to wipe out Berlin, I hardly think the remnants of a blasted meteor would destroy Europe. Much would be diverted initially, with much of the remaining burning up in Earths atmosphere.

 

Everything depends on how big it is and how far away it gets blown to pieces. For some sized objects it might be better to blow it into pieces to increase the surface area that gets burned up in the atmosphere. Or it could even cause some big pieces to miss the Earth entirely. I think (correct me if I am wrong) that small enough pieces will not explode in the atmosphere but merely burn up. If doing so should create an ozone hole, I think that would be the lesser of evils.

[Ophiolite]

DH has rightly pointed out that there has been much nonsense talked in this thread (as well as some very pertinent points). Here, in no particular order, are some hard facts regarding bolide impact. (I’m using the general term bolide, since it covers comets, asteroids and anything in between.)

 

The energy of the impact is determined by the kinetic energy of the body, which is in turn proportional to 0.5mV ^2. So a small difference in velocity can produce a large difference in the magnitude of the impact. For example: a 5m diameter carbonaceous chondrite impacting at 12 km/sec (close to the slowest possible impact speed) will produce around 1.4 E13 joules. At 24km/sec this rises to 5.6 E13 joules, four times the impact energy. If the impactor was an iron this would rise to 1.4 E14 joules. These values are about the same as that of the Hiroshima atomic bomb – from a 5m diameter object.

 

A 1.5 km diameter asteroid, impacting at 20 km/sec would deliver as much energy as the Earth releases each year through volcanism, heat flow and earthquakes. (1.3 E21 joules) The exceptional Christmas Eve earthquake in Indonesia released 2.0 E18 joules, about what you would get from a 200m diameter chondrite asteroid impacting at 17km/sec. Such impacts (200m) will occur every ten thousand years or so.

 

At the upper size range (50m diameter)for this most recent visitor, an impact would have generated around 5.0 E16 joules, equivalent to ten ‘typical’ hydrogen bombs.

 

A couple of points on impact velocity. This cannot be lower than the escape velocity of the Earth, which is about 11 km/sec. The maximum impact velocity is a combination of the Earth’s velocity around the sun (30km/sec) and the solar system escape velocity at the Earth’s orbit (42 km/sec), giving a maximum of 72 km/sec. Since most asteroids tend to be travelling in much the same direction as the Earth actual impact velocities for these are lower, typically in the range of 15 – 25 km/sec.

 

Anything less than 50m in diameter is unlikely to make it through the atmosphere without exploding. Larger bodies may also break up if they are loose rubble masses, as many asteroids and comets are now thought to be.

 

The frequency of impacts is as topic provoking much debate. Data are sparse and we have only acknowledged the possibility of such events in the last few decades. An impact from an object such as the one we are discussing might be expected every couple of hundred years, so even if this one had hit any damage would be localized. (Which is not in any way intended to understate the devastation that would occur locally, just to point out that the global effect would be minimal to non-existent.)

A Tunguska sized strike is estimated to occur from every 200-300 years (Bryant, E. 2008 Tsunami, The Underrated Hazard. Praxis Publishing p234) to every 1900 years (French, B.M. 1998 Traces of Catastrophe. The Lunar and Planetary Institute Table 2.1)

 

There is a lot more that could be said, but that should do for starters. I have to eat.

[Airbrush]

Good info Ophiolite. Those are the kind of numbers I had been wanting to see. Do you think that angle contributes to which bolides will explode in the atmosphere and which ones will reach the ground? Most will not hit the earth broadside at a 90 degree angle, but at oblique angles and the extended amount of time burning in the upper atmosphere, and slowing down, would help some to explode miles high, the way we think Tunguska did.

 

Hiroshima-sized impacts are thought to happen every year in the high atmosphere. People must have always thought they were just lightning and thunder.

 

http://en.wikipedia.org/wiki/Impact_event

 

"The late Eugene Shoemaker of the U.S. Geological Survey came up with an estimate of the rate of Earth impacts, and suggested that an event about the size of the nuclear weapon that destroyed Hiroshima occurs about once a year. Such events would seem to be spectacularly obvious, but they generally go unnoticed for a number of reasons: the majority of the Earth's surface is covered by water; a good portion of the land surface is uninhabited; and the explosions generally occur at relatively high altitude, resulting in a huge flash and thunderclap but no real damage."

Edited March 17, 2009 by Airbrush

[Ophiolite]

Do you think that angle contributes to which bolides will explode in the atmosphere and which ones will reach the ground? Most will not hit the earth broadside at a 90 degree angle, but at oblique angles and the extended amount of time burning in the upper atmosphere, and slowing down, would help some to explode miles high, the way we think Tunguska did.

Absolutely this is a factor and I know there has been research done on it. I'm trying to locate something solid right now.

 

In a related area, I know that the angle of impact has little effect upon the shape of the resultant crater if the bolide makes it all the way to surface. You wind up with a roughly circular crater whether the object comes in at 90 deg or 20 deg. I think at very low angles (<15 or 20) there is an assymetry to the resultant crater, but it is not a big effect.

[Airbrush]

How does the air burst of a rock explode so energetically? I read somewhere that a meteroroid of up to 10m across will explode in the atmosphere with about the energy of Hiroshima. I just cannot imagine how a rock that small getting instantly vaporized generates that much energy. Does any nuclear reaction occur?

 

BTW while reading about Tunguska on wikipedia it said the explosion was 10 to 15 megatons, then it said it was 1,000 times Hiroshima. Both cannot be true. Hiroshima was less than 0.2 megatons (13-18 kilotons) and 1,000 times 0.2 megatons is only 200 times Hiroshima.

 

"Estimates of the energy of the blast range from 5 megatons to as high as 30 megatons of TNT, with 10–15 megatons the most likely...about 1,000 times as powerful as the bomb dropped on Hiroshima, Japan...."

 

http://en.wikipedia.org/wiki/Tunguska_event

Edited March 18, 2009 by Airbrush

[cameron marical]

either way, 200 hiroshimas would screw us up.

[Ophiolite]

How does the air burst of a rock explode so energetically?

Refer to my earlier post. Kinetic energy, energy of motion, is converted to thermal energy and shock waves. Remember the equation is Energy = 0.5 x mass x Velocity ^2.

The objects are travelling very fast - typically 20km/sec. Square that an d you get a very big number. At that speed, hitting the atmosphere for a small object is like hitting a brick wall. The object explodes violently.

 

You say you can't imagine this. Don't. I'll assume you have Excel or some similar spreadsheet program. Create a small routine where you enter the diameter of the asteroid and its density. (Volume of a sphere = 4/ pi r^2). I used these values for density.

Iron7.5 Stony-Iron4.5 Chondrite3.5 Carbonaceous Chondrite2.95

You can then calculate the mass of the asteroid.

Offere a range of velocities, use the equation I gave above and there you have your energies. You no longer have to imagine, you can see what happens when one or other variable changes.

[Airbrush]

I intend to do exactly that. Thank you that is cool info. Those are the kind of numbers.

 

OK I just calculated an iron rock (7.5) X 20km/sec^2 X 0.5 = 75,000m/sec^2. What does that mean? Is that jouls? How many jouls is a lot of jouls?

 

Did they ever mention which composition of bolides is most common among the bigger ones?

 

I think I heard that asteroid impacts are far more frequent than comet impacts, and comets usually have more speed, especially if they arrive coming the opposite direction of our orbit, but that would be extremely rare.

Edited March 19, 2009 by Airbrush

[Mr Skeptic]

OK I just calculated an iron rock (7.5) X 20km/sec^2 X 0.5 = 75,000m/sec^2. What does that mean? Is that jouls? How many jouls is a lot of jouls?

 

Well, a joule is units of kg m^2/s^2 so no. What you missed was

1) the value of 7.5 for iron meteorites has units of g/cm^3, so you can convert volume into mass by multiplying a given volume by its density. Use the formula for volume of a sphere and density to calculate the mass of a meteorite of a certain radius and composition.

2) keep track of the units, and do proper unit conversions so that you get units of kg m^2/s^2 if you want your answer in joules. Remember in particular that when squaring a velocity you also square the units. From joules you can convert to megatons of TNT if you prefer.

[Airbrush]

That sounds good Mr. Skeptic, I plan to try calculating that soon. In case anyone else is interested, my hypothetical iron meteroid is 50 meters across and impacted at a speed of 12 km/sec. Can you beat me to the answer? The data above are based upon an actual impact in case anyone can figure out which one.

 

My other questions are which composition of large meteoroids impact most frequently? What percentage of impacts are believed to be comets rather than meteoroids/asteroids? Pop quiz on Monday.

[Ophiolite]

IDid they ever mention which composition of bolides is most common among the bigger ones?

 

I think I heard that asteroid impacts are far more frequent than comet impacts, and comets usually have more speed, especially if they arrive coming the opposite direction of our orbit, but that would be extremely rare.

I'm trying to find you solid data on the first question. Around 70-80% of meteorites are stony. I would expect much the same mix amongst asteroids, which they are derived from.

 

To complicate the matter some asteroids are derived from the break up of comets that have lost all or most of their volatiles. There is increasing recognition that the comet/asteroid dichotomy is not necessarily a simple division, but I can't get a good handle yet on what current thinking is. (Give me a few days.)

[Airbrush]

Well, a joule is units of kg m^2/s^2 so no. What you missed was

1) the value of 7.5 for iron meteorites has units of g/cm^3, so you can convert volume into mass by multiplying a given volume by its density. Use the formula for volume of a sphere and density to calculate the mass of a meteorite of a certain radius and composition.

2) keep track of the units, and do proper unit conversions so that you get units of kg m^2/s^2 if you want your answer in joules. Remember in particular that when squaring a velocity you also square the units. From joules you can convert to megatons of TNT if you prefer.

 

7.5 x 3.14 x 125,000m^3 x 144km^2/sec^2 and divide it all by 6 =

 

7.5 x 3.14 x 125,000 x 24 (m^3Km^2/sec^2) =

 

70,650,000 (m^3 x km^2/sec^2) joules

 

How do you combine the meters cubed and km squared?

[NowThatWeKnow]

How do you combine the meters cubed and km squared?

 

Wouldn't [math]125,000m^3 = 125km^3 = 1,953,125km[/math]

There is an energy conversion calculator that will convert Joules to just about anything at:

http://www.cthreepo.com/cp_html/math1.htm