One of the most spectacular sights in the night sky is a fireball; a rock from space impacts the atmosphere and blazes a trail that can last seconds or even minutes. These burn up harmlessly, but when do they turn dangerous? When do asteroids get large enough that they can actually get through the atmosphere and cause some destruction here on the ground?
During an invited talk at the Meteoroids 2007 conference held in Barcelona, Spain, Clark R. Chapman from the Southwest Research Institute delivered a presentation about how to define this line between harmless explosion in the sky and an impact that causes destruction here on the ground. The paper, entitled
Originally, researchers focused their efforts on the largest asteroids: the objects 2 km (1.2 miles) and above. These are the space rocks that could cause wide scale devastation across the planet, affecting the climate and leading the the deaths of hundreds of millions of people. It was calculated that an individual might have a 1-in-25,000 chance of dying in an asteroid impact.
Now that the Spaceguard Survey has discovered 75% of the asteroids 1 km and larger, your chances of dying have dropped to about 1-in-720,000. About the same chance as dying from a fireworks accident or amusement park ride.
According to Chapman, astronomers are now shifting their focus from the largest impacts – like the one that wiped out the dinosaurs 65 million years ago – to the smaller, but still dangerous space rocks. For example, the rock that detonated in the air above Tunguska, Siberia in 1908. That object was probably only between 20-100 metres (65-325 feet) across.
And yet, it leveled the forest for thousands of square kilometres and would have caused immense destruction if it had hit a populated area.
A new survey, informally called the Spaceguard Two Survey, will begin soon with the goal of finding 90% of the near-Earth asteroids larger than 140 metres (460 feet) within the next 15 years.
There are many variables that go into calculating the resulting destruction from an impact. You have to consider the velocity, if it’s a metallic or rocky asteroid, and whether it’s fragmented or not.
What should the response be of national and international emergency management officials to a prediction that a 35 m NEA will strike a populated country a decade in the future? Following current interpretations, we would simply tell people near ground-zero to stay inside and not look directly at the high-altitude explosion. But if objects of that size could cause Tunguska-like damage, we might not only evacuate people for 100 km surrounding ground-zero but we would certainly consider a space mission to move or blow-up the threatening NEA.
Originally, researchers thought that Tunguska level events happened once in 4,000 years, but it might be more common, maybe as often as 1-in-700. And perhaps even smaller, more common, asteroids could still cause destruction on the ground – 1-in-200 years.
If Spaceguard Two Survey gets going, it should locate most of the larger asteroids, but even 50% of the Tunguska-sized impactors. It will even be tracking 1-2 million 30 metre objects.
And if one of those rocks is on a collision course with Earth, governments and space agencies will be able to work out an evacuation or prevention strategy.
Or at least encourage people to avert their eyes.
Original Source: SWRI
What about Comets? I mean, don’t they pose as large a threat as asteroids?
Aren’t comets composed mainly of ice? I was under the impression that they would just burn up in the atmosphere, but I don’t know whether that is entirely true or not.
Ice and dust. But I suppose if it was big enough, it might get through…
Also, what is the critical size needed for an asteriod, meteorite, or comet to be able to get through the atmosphere? If that is known, then any object that might get through the atmosphere can be broken up (with the application of nukes I’d imagine).
Comets are just as dangerous as asteroids. It doesn’t matter if are composed of ice and dust … the kinetic energy is the one that matters. Is like saying that crashing with your airplane in the ocean is better that concrete … A comet will go through atmosphere in a flash, no time for burning up 🙂 . Also, consider that fact that most comets are big, i mean tens of km across?
Good point, but frankly I am not really all that keen on finding out whether or not you are right, I would rather leave it to science, speculation and computers. And not die.
I guess that is true that the kinetic energy is what matters, as well. I suppose regardless of whether or not it burns up in the atmosphere that energy is still going to have to go somewhere, no matter what the size of the object.
‘Ask’ Jupiter if comets are ‘harmless’ – last time it got hit by one – there were some massive explosions…
Believe that scientists have yet to confirm “if” Tanguska was an astroid or comet? But, regardless, of what it was… more importantly what was it made of… that would cause it to explode before stricking the earth?
Years ago I was traveling SR-395 (backside of the Sierra’s) from Reno to Los Angeles at night. Was mesmerized by a bright streaking light that continued getting bright (knowing full well what it was at the moment)… it continued getting brighter, as it slowed, lighting the night, as if it were day, and finally burned out (pooof)… all went dark! No explosion, only it was “lights out”… followed by “did you see that?”
So what causes em’ to explode, what would a foreign object be made of to create another Tunguska?
The problem with comets isn’t that they’re made of ice instead of rock but that they are huge. Levy Shoemaker comet that hit jupiter was in a bunch of chunks – having broken up already. Each chunk was huge. When each chunk hit jupiter, it created a plume about the size of earth and left a dark spot of similar size.
Comets tend to be less of a problem because they are rarer and every one around tends to be discovered quickly – actually there’s an ongoing contest among observers, amateur and professional to be the first to discover new ones. A couple of months back, a minor comet 17pHolmes blew a gasket and brightened up by a massive amount, going from virtually invisible in a telescope to being visible to someone without a telescope. The gas ball surrounding the comet grew to bigger than the diameter of the sun and the apparent size of it grew to be bigger than the apparent size of the moon. What’s more – it’s done that trick in the past.
When you see a meteor shower like the orionides, perseids etc, remember these are cause by dust particles left along the orbit of a comet.
Offhand, it would seem that even small tiny chunk of a comet would be potentially dangerous to people were it to wind up arriving here on earth in a populated area. A substantial chunk could be quite dangerous to civilization.
In July, 2001, a meteorite exploded over the mountains of Northern Pennsylvania, near Williamsport. I was working at a summer camp there, and it felt as if all 4 tires on my truck exploded at the same time with a very sharp short BANG! that shook the truck. People over hundreds of square miles reported hearing and feeling a sharp explosion. Another worker at the camp who had worked maintenance in the Air Force reported a large flare and explosion that reminded him of an F-16 exploding in midair.
I thought the leading explanation to the Tunguska Event was that it was a comet primarily because (1) there was no impact crater at ground zero and (2) there were no traces of meteorite particles to be found in the area. At least that’s what Carl Sagan says in “Cosmos.”
As evidenced by the flattened trees and trees that were stripped of their branches in the Tunguska Event, a comet that was big enough to not burn up in the atmosphere could still produce enough destructive energy upon impact to vaporize itself.
Adrian is right when he comments about the kinetic energy. Comets or asteroids, doesn’t matter, if one hits, we are in trouble. A comet will not burn up in the atmosphere simply because of the speed its travelling at. A comet travelling at say 20,000km an hour will come through the earths atmosphere in less than 6 seconds, which doesn’t leave much time for burning up. The scary part is I haven’t heard of a comet travelling this slow :S So Id 6 say seconds is on the high end estimate 🙂
I think that the Tunguska event was caused by a “rubble pile asteroid”. That is, it was a loosely formed asteroid that allowed the heat and friction to build up on its Earthward side to cause it to explode five miles above the ground.
Comets, especially long period comets with orbits of 200 years or more, are very large, come through our inner solar system infrequently, and would be almost impossible to stop.
Asteroids have smaller orbits, can be tracked, and possible scenarioes can be figured out for the intervention of their impacts with Planet Earth.
According to a documentary I watched recently, the comet Shoemaker-Levy 9 impact on Jupiter, in 1994, created a fireball which was itself the size of planet Earth………..
More generally – If I understood correctly – it was said that the existing and potential ‘community’ of asteroids is far better known and more predictable than that of comets. Comets, it seems, can appear at relatively short notice and, as has been previously observed, they do shift a bit!
My comment is along the lines of Brian’s. The title of the article is “When Do Asteroids Turn Dangerous?”, which I took to mean “How big does an asteroid/comet have to be to survive impact with the atmosphere and remain substantial enough to cause significant damage?” No such lower limit is suggested (I know that speed and make-up are also factors.) Someone estimated the Tunguska asteroid at a minimum of 65 feet in diameter, which, if correct, means that quite a small object can cause a lot of damage. I would like to have seen a lowest-case dangerous diameter. This would give and idea of the smallest objects we need to be able to track to protect against significant possible loss of life. For instance, could a high speed metallic object with a mere 25 foot diameter cause Tunguska-like damage? What is the smallest object that we are capable of tracking?
I think we should pretty much give up on the much-ballyhooed notion of blowing these things up if they threaten us earthlings. It seems to me that doing so would just multiply the effects of a potential earth impact because, instead of one object, we’d create dozens of objects of indeterminate size and orbit(s). “Nudging” a large object into a different orbit would be the way to go.
So, basically, comets aren’t tracked because:
A) we can’t track them cause we can’t see them.
B) if we spot one and it’s due to collide with the Earth, then we’re hooped anyways.
Am I right?
I recall reading that the Tunguska object was a stony asteroid–determined by microscopic fragments found embedded in the tree bark. It exploded high up, with energy roughly equivalent to 1,000 Hiroshima bombs. The thing that put that big hole in the ground near Winslow, Arizona, was about the same size; but it was an iron-nickel asteroid, so it penetrated all the way down instead of exploding in the atmosphere.
So far, I understand the best plan to divert a potential impactor is a gravitational tractor. That involves putting a spacecraft into the same trajectory as the asteroid, just a short distance from it, and affecting its orbit just enough to make it miss us.
The trick is to find enough funding to have the system all developed, tested, and ready to go when we need it. Then, assuming a few years’ warning, we could divert the thing in time.
Bruce
Check out this thorough exploration of what happens to small comets in our atmosphere.
http://smallcomets.physics.uiowa.edu/
i really like comets