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Chances of a comet hitting us ?

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Is it possible that an extremely large comet or other such object could hit earth suddenly with little warning ?

 

or is this something we would expect scientists to know about years /decades /centuries in advance?

 

how much advance warning would we expect to have ? And how big and fast would it have to be to be catastrophic for human life ?

 

 

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2 minutes ago, boo said:

Is it possible that an extremely large comet or other such object could hit earth suddenly with little warning ?

It's happened before, it'll happen again.

3 minutes ago, boo said:

how much advance warning would we expect to have ?

how much do you want?

4 minutes ago, boo said:

And how big and fast would it have to be to be catastrophic for human life ?

I don't know, big enough and/or fast enough I guess.

You may as well ask, will we live forever?

 

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1 hour ago, boo said:

Is it possible that an extremely large comet <snip> could hit earth suddenly with little warning ?

If it still has its icy crust, it will reflect light and be more visible, especially as it gets closer to the sun and leaves both an ion tail and a dust tail. We should have quite a bit of warning.

1 hour ago, boo said:

Is it possible that an <snip> object could hit earth suddenly with little warning ?

Certainly, but the larger it is, the more warning we'll have. There are a LOT of folks and equipment watching the skies for such objects.

 

The rest of your questions can't be answered super accurately. The asteroid that seems responsible for wiping out most of the dinosaurs couldn't have hit a worse spot on Earth for triggering an extinction event. Just a few miles further north or south and the Chicxulub strike wouldn't have caused so much soot in the atmosphere. It's not just the size or speed that determines how devastating a big asteroid strike would be.

I'm more worried that small objects will end up hitting a satellite head-on. Every time a pieces break off a satellite, they create debris that threatens everything that moves in those orbits. The US DoD tracks over 19,000 objects bigger than 10 centimeters circling the planet. We need to keep this area clean of the small stuff if we're going to have the best chance of seeing the big stuff from far enough away to do something about them.

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20 minutes ago, Phi for All said:

The rest of your questions can't be answered super accurately.

Correct. There are a number diffdrent of things that will contribute to damage such as impact crater, fireball expansion and thermal radiation, ejecta deposition, seismic shaking, and the propagation of an atmospheric blast wave. Different impacts could be threatening due to different contributions from from such factors.

28 minutes ago, Phi for All said:

It's not just the size or speed that determines how devastating a big asteroid strike would be.

I remember* tests that claimed that impact angle is a major factor for the amount of dust ejected into atmosphere, resulting in climate changes. 

 

*) cant find the reference now, I’ll try again later

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1 hour ago, boo said:

Is it possible that an extremely large comet or other such object could hit earth suddenly with little warning ?

 

or is this something we would expect scientists to know about years /decades /centuries in advance?

 

how much advance warning would we expect to have ? And how big and fast would it have to be to be catastrophic for human life ?

 

 

Extremely large, no.  the larger the object, the easier it is going to be spotted at a further distance.    The only objects that really catch us by surprise are relatively small ones.  For example, in Jan of this year we were buzzed by a small object which we didn't notice until after closest approach, but it was only estimated to be less than 2.5 meters in size.  On the other hand, in 2001, an object was discovered which is predicted to make a close approach in 2028. This object is estimated at 0.6-1.4 km in size.

Generally, the longer you have to observe an object, the better you can nail down its trajectory and any future encounters it may have with the Earth.  But even then, there is an upper limit to how far into the future your prediction in likely to be accurate.  The longer the time between detection and possible impact, the more time that the object's trajectory can be perturbed by outside influences.  With comets, this can be particularly challenging.   A comet is basically snowball of frozen water and some gasses.   As it comes in from the outer solar system, these start to boil and evaporate, and not necessarily evenly.  Pockets can erupt on the surface, which in turn can act like rocket engines, making small adjustments to the comet's trajectory.  This can effectively reduce the accuracy of long term predictions of its orbit. ( and it doesn't take much to turn a miss into a hit or vice-versa,  It only takes the Earth 7 min to travel the width of its diameter,  so an object crossing Earth's orbit just 8 min later or earlier could make all the difference*.)

 

*Actually, a lot depends on the object's relative orbital velocity with respect to the Earth. Both the angle of approach relative to the Earth's orbital direction and the relative speed.  Relative speed plays a role by determining the impact parameter.   Any object approaching the Earth will be affected by its gravity. Thus an object which would have missed the Earth otherwise can have its trajectory curved towards the Earth, making the Earth a "larger target" than just its diameter might suggest.   The faster the object is moving relative to the Earth, the less time the Earth's gravity has to deflect its trajectory, and the "smaller target" the Earth makes. Conversely, it presents a larger target to slower moving objects.

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If a "city-killer" comet comes from a direction from behind the sun, at 20 miles per second, then when it passes the sun it is aligned with the sun, won't it be hard to detect because of the brightness of the sun compared to the dimness of the comet?  How much warning could we have?  Maybe hours at best?

Edited by Airbrush

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2 hours ago, Airbrush said:

If a "city-killer" comet comes from a direction from behind the sun, at 20 miles per second, then when it passes the sun it is aligned with the sun, won't it be hard to detect because of the brightness of the sun compared to the dimness of the comet?  How much warning could we have?  Maybe hours at best?

20 miles/sec 

The sun is 93 million miles away

4.65 million seconds to get from the sun to us. That more than 7 weeks. 14 weeks from the earth’s orbit distance on the other side (ignoring speed changes from gravity)

The earth moves more than a quarter of its orbit in that time. Meaning the comet can’t “hide”

We would see it. It’s not moving fast enough.

 

The greater danger is an asteroid (not shedding mass and creating a tail) coming at us from the other side. Dark, hard to spot, and not much parallax.

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I did not find a reference for the impact angle (yet) but found something else I did not know about. When scientists describe the hazard of a potential impact to the public they combine size, energy and probability into one figure. A really large object with very low probability of colliding with earth is given the same level as a smaller object that is calculated to be more likely to collide.

Quote

The Torino Scale, adopted by the IAU in 1999, is a tool for categorizing potential Earth impact events. An integer scale ranging from 0 to 10 with associated color coding, it is intended primarily to facilitate public communication by the asteroid impact hazard monitoring community. The scale captures the likelihood and consequences of a potential impact event, but does not consider the time remaining until the potential impact. More extraordinary events are indicated by a higher Torino Scale value.

https://cneos.jpl.nasa.gov/sentry/torino_scale.html

 

image.png.42dc8645a4740994503ffe1af94bf687.png

https://en.wikipedia.org/wiki/Torino_scale

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On 9/2/2019 at 9:37 AM, Airbrush said:

If a "city-killer" comet comes from a direction from behind the sun, at 20 miles per second, then when it passes the sun it is aligned with the sun, won't it be hard to detect because of the brightness of the sun compared to the dimness of the comet?  How much warning could we have?  Maybe hours at best?

A comet moving at that speed "coming out of sun" as it were would have to be on the outbound leg of a narrow elliptical orbit, that extends past Earth orbit. We hardly would have missed it on its inbound leg. We would have watched it, and calculated its outbound leg.  While this might not give a perfect solution, we would know approximately where to look for it as it rounded the Sun and found it again fairly easily. 

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