As predicted, David Morrison has commented on the new results about the Tunguska impactor:
The main story in this edition of NEO News concerns a proposed downsizing of the energy of the 1908 Tunguska airburst, with associated increase in the expected frequency of such impacts. Mark
Boslough of Sandia has generated supercomputer simulations of the Tunguska atmospheric explosion. In part his models require less energy in the explosion because he includes the substantial downward
momentum of the rocky impactor, rather then modeling it as a stationary explosion. If this revision (down to an estimated energy of 3-5 megatons, and a corresponding diameter of about 50 meters) is correct, the expected frequency of such impacts changes, from once in a couple of millennia to once in a few hundred years. If smaller impactors can do the damage previously associated with larger ones, of course, the total hazard from such impacts is increased.
Remember, this is a statistical frequency. We might not see another 5 megaton blast for a thousand years, or we may get three next month. But over time, the numbers average out. What this all really means is that we’d better figure out a way to push even little rocks aside, which means finding them first. That’s no small task, since they are extremely dim. Most of the time, we don’t see even bigger objects until they have already passed us.
This is a real issue, and one that needs much thought, and much attention.
You can subscribe to David’s newsletter by emailing him at dmorrison at arc dot nasa dot gov.
December 19th, 2007 at 4:41 pm
It’s not just the damage done directly by such events – imagine what would happen of one such event happened just about anywhere over the US, in Europe or Russia, or even in the Middle East – how likely is it that it would be interpreted as a nuclear strike, and cause “retaliation” on a horrendous scale?
Jim D.
December 19th, 2007 at 5:24 pm
What are the odds of an impact over land? Has that been factored in, since earth is 75% ocean. We know that the Tunguska explosion occurred over land, in a relatively unpopulated area, even today. What are the odds of another land impact or over land airburst? *I guess it really doesn’t matter. To Paraphrase an old Bumper Sticker: All it takes is one small chunk of spacerock to ruin your day. At least it would be over relatively quick for those in the blast zone.
December 19th, 2007 at 6:35 pm
why cant they hunt for asteroids with radar?
December 19th, 2007 at 7:50 pm
Well, how many incoming rocks end up detonating above the ground, as opposed to slamming into it? Would actually impacting the Earth significantly change or reduce the effect? I guess what I’m asking is, is he saying the odds of a rock this size entering the atmosphere and exploding above the ground are one in a few hundred years, or simply the odds of a rock this size entering the atmosphere, regardless of whether it impacts the ground or blows up above it?
December 19th, 2007 at 8:00 pm
Radar return from an object is inversely proportional to 4th power of distance. Visual brightness (given constant illumination) varies with inverse square of distance. Nearby earth-crossing asteroids are almost constant in illumination, because they are almost the same distance from the sun. It’s more complicated than that of course, you have crescent illumination and so on. The new moon is somewhat more brightly lit than the full moon but it is not near as bright to us on earth.
On the bright side, the rocks most likely to hit us are those that have perihelion or aphelion near Earth’s orbit, and they are likely to have a number of near misses before being perturbed into a collision, making them more likely to be detected. Not so encouraging is the fact that these are outnumbered by earth-crossing asteroids which are harder to detect – they spend little time near earth and have generally higher crossing velocities. I don’t know how the numbers and probabilities of collision work out, I suspect a fastball from the outer solar system with little warning is more likely.
There are satellites in orbit to detect nuclear explosions, the light curve of a meteor impact would likely be different enough to be distinguished. It would brighten more gradually, have different color temperature, and wouldn’t have the double-pulse emission of a nuclear blast. That’s no guarantee people wouldn’t do something stupid tho.
December 19th, 2007 at 8:40 pm
The odds of an asteroid actually hitting a city are very poor, and we’re not talking destruction of property beyond a major earthquake. Given a bit of warning, simple evacuation is likely to keep the casualties down to acceptable levels, though it’s nowhere near as much fun as blowing it up in deep space.
December 19th, 2007 at 9:40 pm
Steve. Hmmm, I live in Canada. The country with the second or third greatest landmass. I’m considering moving to a very small country. Any suggestions.
December 19th, 2007 at 10:30 pm
Please do Not suggest Vatican City. I really don’t want to take a chance that there is a God, and end up in the line of fire, just in case the Almighty has issues.
December 19th, 2007 at 11:56 pm
Michael Lonergan, can I suggest New Zealand?
December 20th, 2007 at 12:03 am
yeah, that would be ok. Not much chance of an Almighty Deity being peeved with the Kiwi’s
December 20th, 2007 at 1:52 am
Since the Hiroshima bomb was estimated at only 13 kilotons, and Nagasaki 21 kilotonnes, perhaps we should accept that we have little chance of intercepting all damaging meteorite strikes, and concentrate on solving problems that are under our control.
We might also wonder where all these small strikes are- shouldn’t they be proportinately more frequent, say once every 2 years rather than every 200?
December 20th, 2007 at 2:06 am
Michael Lonergan. My homecountry (Denmark) comes to mind as being a small country.
December 20th, 2007 at 4:33 am
So, we need to be able to detect small NEO’s. To do that, we need a good planetary radar system. Hey, what about Arecibo?!
Oh yeah, Congress cuts its funding to save a measly 1 million dollars.
Oh well, maybe we’ll get lucky and the meteors will miss population centers. Let’s give a B-2 a week’s worth of fuel instead.
December 20th, 2007 at 8:33 am
Pat said:
> why cant they hunt for asteroids with radar?
Money. It boils down to money. Money for facilities, money for people to operate the facilities, money to cover 24 hrs a day and 360 deg by 360 deg looking at the sky.
Impium Orexis said:
> Well, how many incoming rocks end up detonating above the ground, as opposed to slamming into it? Would actually impacting the Earth significantly change or reduce the effect?
I’m not a specialist, but my understanding is that you have a certain amount of energy from the meteor – that is determined by the size of the object. Impacting the surface would place most of that energy in the local zone of impact. Airbursts were thought to spread spherically, spreading the energy over a much larger area. Causing more widespread damage, vs. more intense localized damage, and dissipating energy to the sky rather than to the ground. What this appears to be saying is that the airburst energy was directed to a more localized area, intensifying the amount reaching the ground overall. That means a smaller rock can have a more devestating localized strike, but the surrounding zones might get off more luckily. I think this puts airbursts closer to a ground strike rather than further away from a ground strike.
Sprocket said:
> We might also wonder where all these small strikes are- shouldn’t they be proportinately more frequent, say once every 2 years rather than every 200?
Smaller meteor strikes are more frequent than the 50 m diameter objects, but the effects aren’t proportionate. Below a certain size, the meteors tend to burn up on entry rather than airburst, and so that energy is spread over a larger segment of the sky much further up. Thus, no devestating fireballs, impacts, flame blasts, pressure bursts, etc.
December 20th, 2007 at 9:32 am
How many blasts like this are in our known history? While I agree it’s something that needs to be thought about and looked into, if you’re thinking about cost effective ways to save people and make their lives better you might want to be careful of how much you spend worrying about a blast the size of Tunguska. I think it’s the bigger objects we should worry about more.
December 20th, 2007 at 10:01 am
I really do not think a radar the size of Arecibo would detect objects the size we are talking about here. Also, isn’t Arecibo limited by the amount of sky it can scan? I’m no expert but it seems as that if Arecibo was adequate for the task, dozens of them would need to be placed around the world.
December 20th, 2007 at 12:21 pm
Radar isn’t used to find objects – that’s best done in the optical. Where it is useful is getting their orbits nailed accurately and quickly, which is something Arecibo does.
One small note: it’s the NSF that cut Arecibo’s funding, not Congress. Congress may well reinstate it (possibly via NASA, who have a congressional mandate to characterize asteroids).
December 20th, 2007 at 1:20 pm
Crux: New Zealand is out. They just got hit with a big quake. Maybe it’s a sign?
linusrp: Ahhh, Denmark! A thoroughly liberal country with awesome chocolate. That I could handle!
December 20th, 2007 at 2:01 pm
IMO: No more than token funding will ever be directed towards a defense against extra-terrestrial threats of any sort until AFTER an event has already occured that directly impacts one of the G-8 countries. Political will in republics is reactionary. Pro-active thinking only applies to oligarchies, and is rarely for the benefit of the many. (How’s that for cynicism?)