Thread: Meteor impacts

From New Scientist 22 February 2014, page 11

The meteor that burst apart above Chelyabinsk, Russia, a year ago, was estimated to be 20 metres in diameter. Further estimates are that similar sized meteors will strike the Earth about once every century, though thousands of smaller meteors burn up every day in the atmosphere. We have no way of detecting meteors of that size range.

The good news is that the last truly massive impact, the one that killed the dinosaurs, was 65 million years ago. It appears unlikely that a civilisation destroyer will hit before humans have managed to set up extraterrestrial and self sufficient colonies.

What is your assessment of impact risk? How much effort should humanity be putting into detecting, and possibly diverting potential impactors?

The US Air Force sponsored a survey of NEOs (Near Earth Objects) and found no large asteroid impacts over the next ten thousand years. There are potential impacts of local significance over the next thousand years but nothing devastating. Past that there could be locally devastating impacts we cannot yet predict. These are relatively small impactors that would probably be reduced to gravel or even vapor with a thermonuclear warhead or two. We will have lots of warning assuming we keep doing astronomy as much as we do today.

I'm not really sure that reducing a big incoming body to gravel is a good idea. Is it better to be shot with a shot gun slug or with a load of buck shot? Deflection is a better goal but it is a doable one.

The weakness in the NEO study is that it only measured the orbits of known earth- orbit- crossing bodies and only ones over a certain size. (I think it was a mile in diameter) Ones considerably smaller than that, while they might not produce extinction level events, could clear a continent or with an ocean hit, depopulate the coastlines.

The biggest issue is that we only observe these bodies during the inner solar-system part of their trajectories. Then they go out into the ortcloud and interact with God knows what.

Sometimes all you can do is place your bets and roll the dice.
I wonder if there is a statistical study of how often Earth takes a major hit and what the odds are of getting hit again before we go extinct or evolve into a new species from other causes.

Originally Posted by Sealeaf

Deflection is a better goal but it is a doable one.

I agree. And several potential methods have been proposed, all of which we should be capable of developing in the time frame of a decade or so - given the political will. Here is an example of one of the simpler methods being considered. As is often the case the Wikipedia article provides some good insights.

Ones considerably smaller than that (1 mile diameter), while they might not produce extinction level events, could clear a continent or with an ocean hit, depopulate the coastlines.

The initial aim was to identify 90% of bodies larger than 1 km by 2008. (See Wikipedia link above). Bodies down to 140m are to be identified by 2020. A mile wide impactor would likely change life as we know it.

The biggest issue is that we only observe these bodies during the inner solar-system part of their trajectories. Then they go out into the ortcloud and interact with God knows what.

You are confusing comets and meteors. NEOs (Near Earth Objects) are all asteroids or meteors with orbits locked into a low eccentricity orbit that periodically approaches Earth. They do not return to the Oort cloud, since the majority of them never came from there.

Comets present a much greater risk, since:

• They are much larger, on average.
• They are traveling at greater velocities.
• Their orbits are subject to change because of activity (boiling gases) as they near the sun.
• They are difficult to detect.
• The lead time for a warning is of the order of months.
• Their size and the time issue means deflecting them is orders of magnitude more difficult.

Originally Posted by dan hunter

I wonder if there is a statistical study of how often Earth takes a major hit and what the odds are of getting hit again before we go extinct or evolve into a new species from other causes.

There is more than one. However, there is uncertainty in the input data and so we get a wide range of output estimates.

Chapman and Morrison (1994) estimated a 1 in 10,000 chance of a 2 km object striking the planet in the next 100 years. This later paper by the pair, and by additional authors, although a decade out of date is still interesting reading. Here is a more recent review, focused on NEOs.

Originally Posted by Sealeaf

I'm not really sure that reducing a big incoming body to gravel is a good idea. Is it better to be shot with a shot gun slug or with a load of buck shot? Deflection is a better goal but it is a doable one.

The weakness in the NEO study is that it only measured the orbits of known earth- orbit- crossing bodies and only ones over a certain size. (I think it was a mile in diameter) Ones considerably smaller than that, while they might not produce extinction level events, could clear a continent or with an ocean hit, depopulate the coastlines.

The biggest issue is that we only observe these bodies during the inner solar-system part of their trajectories. Then they go out into the ortcloud and interact with God knows what.

Gravel won't make it through the atmosphere. Too small; it'll burn up. Even a hundred foot rock would be reduced to harmlessness by a megaton-range weapon. A ground burst from such a weapon vaporizes a hole a thousand feet across in the ground when it goes off. Many of the shots in Operation Castle obliterated entire islands a mile or more across. (Castle was when they started testing thermonuclear designs; Castle Bravo is one of the best known shots, because of its extremely dramatic photos. You've almost certainly seen one.) To withstand even a single hit from a thermonuclear megaton-range nuclear weapon, an asteroid has to be miles across. And we can see those coming more than ten thousand years away. There aren't any.

Originally Posted by dan hunter

I wonder if there is a statistical study of how often Earth takes a major hit and what the odds are of getting hit again before we go extinct or evolve into a new species from other causes.

I've seen guesstimates of how often the Earth is hit by various sized meteors all the way up to the Mt. Everest sized comet (meteor?) that hit the Yucatan peninsula 64M years ago. I'll try to find it . . .

ETA: There's a nice table here: Impact event - Wikipedia, the free encyclopedia

4 m (13 ft)	3 kt	0.75 kt	42.5 km (139,000 ft)	1.3 years
7 m (23 ft)	16 kt	5 kt	36.3 km (119,000 ft)	4.6 years
10 m (33 ft)	47 kt	19 kt	31.9 km (105,000 ft)	10.4 years
15 m (49 ft)	159 kt	82 kt	26.4 km (87,000 ft)	27 years
20 m (66 ft)	376 kt	230 kt	22.4 km (73,000 ft)	60 years
30 m (98 ft)	1.3 Mt	930 kt	16.5 km (54,000 ft)	185 years
50 m (160 ft)	5.9 Mt	5.2 Mt	8.7 km (29,000 ft)	764 years
70 m (230 ft)	16 Mt	15.2 Mt	3.6 km (12,000 ft)	1900 years
85 m (279 ft)	29 Mt	28 Mt	0.58 km (1,900 ft)	3300 years

Stony asteroids that impact sedimentary rock and create a crater

100 m (330 ft)	47 Mt	38 Mt	1.2 km (0.75 mi)	5200 years
130 m (430 ft)	103 Mt	64.8 Mt	2 km (1.2 mi)	11000 years
150 m (490 ft)	159 Mt	71.5 Mt	2.4 km (1.5 mi)	16000 years
200 m (660 ft)	376 Mt	261 Mt	3 km (1.9 mi)	36000 years
250 m (820 ft)	734 Mt	598 Mt	3.8 km (2.4 mi)	59000 years
300 m (980 ft)	1270 Mt	1110 Mt	4.6 km (2.9 mi)	73000 years
400 m (1,300 ft)	3010 Mt	2800 Mt	6 km (3.7 mi)	100000 years
700 m (2,300 ft)	16100 Mt	15700 Mt	10 km (6.2 mi)	190000 years
1,000 m (3,300 ft)	47000 Mt	46300 Mt	13.6 km (8.5 mi)	440000 years

Originally Posted by John Galt

The initial aim was to identify 90% of bodies larger than 1 km by 2008. (See Wikipedia link above). Bodies down to 140m are to be identified by 2020. A mile wide impactor would likely change life as we know it.

Only if we didn't do something about it.

Originally Posted by John Galt

You are confusing comets and meteors. NEOs (Near Earth Objects) are all asteroids or meteors with orbits locked into a low eccentricity orbit that periodically approaches Earth. They do not return to the Oort cloud, since the majority of them never came from there.

Comets present a much greater risk, since:

• They are much larger, on average.
• They are traveling at greater velocities.
• Their orbits are subject to change because of activity (boiling gases) as they near the sun.
• They are difficult to detect.
• The lead time for a warning is of the order of months.
• Their size and the time issue means deflecting them is orders of magnitude more difficult.

All correct as far as I know. However, one more thing to keep in mind: many comets are just iceballs, and they're much less dense than rocks. A couple or three megaton-range blockbusters will vaporize even a few-miles-wide comet.

Originally Posted by John Galt

Originally Posted by dan hunter

I wonder if there is a statistical study of how often Earth takes a major hit and what the odds are of getting hit again before we go extinct or evolve into a new species from other causes.

There is more than one. However, there is uncertainty in the input data and so we get a wide range of output estimates.

Chapman and Morrison (1994) estimated a 1 in 10,000 chance of a 2 km object striking the planet in the next 100 years. This later paper by the pair, and by additional authors, although a decade out of date is still interesting reading. Here is a more recent review, focused on NEOs.

That's your mile-wide impactor, more or less. Also keep in mind that you want to stick with papers that have the NEO search taken into account. We found out a lot doing that search.

Quite correct I was thinking of bodies with long eliptic orbits, comets, not asteroids. I'm still not too happy with the idea of throwing nukes at them, not that I would not do it at need. Gven our track record for getting the poitical will together to do anything about global warming I suspect anything done will be done at the last minute and we do have rockets with nuke warheads available on short notice.

Originally Posted by Sealeaf

Quite correct I was thinking of bodies with long eliptic orbits, comets, not asteroids. I'm still not too happy with the idea of throwing nukes at them, not that I would not do it at need.

There's nothing to worry about. Space is huge; blowing off nuclear weapons outside the atmosphere is safe, at least for the Earth's biosphere. The Sun is a giant continuously exploding hydrogen bomb. It's OK. Space is bigger than big. Way gnarlier things than puny human nuclear weapons happen routinely in space. A supernova is to a hydrogen bomb as a hydrogen bomb is to a kitchen match.

Originally Posted by Sealeaf

Gven our track record for getting the poitical will together to do anything about global warming I suspect anything done will be done at the last minute and we do have rockets with nuke warheads available on short notice.

They're not that hard to make. We all know how. I would be amazed if it took either the US or Russia more than two weeks to put together multi-scores-of-megaton-yield weapons. If it were a big enough rock we could take another few weeks and put together something in the multi-gigaton range if we needed it. We know all the principles now. How to arrange the soda straws. What the correct shapes of the explosive lenses around the pit are. What the total X-ray budget is and how to most effectively utilize it. This is all known. The US may insist on making their own and not proliferating technology. The Russians might do the same. I doubt it will make much difference at this late stage of the game. Tsar Bomba worked. We all know it can be done.

Oh I agree that space has much bigger and nastier things than any human weapon. I don't object to relying on nukes because they are big and nasty. I object because they may not be big and nasty enough. I don't want to trust to an inadequate shield. That thing that hit Jupitor a few years back made a splash bigger than our planet, and it broke up first. I think we may need an answer more sophisticated than "it it with a hammer".