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Yes, I know, c is the limit, but let's say we want to build a shelter that will be meterorite-impact-proof, on the Moon, or some asteroid or anywhere without the atmosphere. We can spot anything bigger than, say, 10cm early enough to destroy or deflect it with remote weaponry, but anything smaller than that will get through and the shield must hold. $Mass = 1{dm}^3 * 2g/{cm}^3$ (average asteroid density), $E=0.5mv^2$. We know how to convert energy to shield thickness, but we still need the v.

What are reasonable expected speeds of "fast" meteorites?

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I kept wondering where this 72 km/s maximum is coming from, and I figured it out! This is the calculation:

30 km/s + (30√2) km/s = 72 km/s

Why? Firstly, it's obvious that the Earth is traveling at 30 km/s in its orbit. But what possible directions can the asteroid hit from? The most logical approach is to hit it moving the exactly opposite direction. That means we demand an asteroid in a retrograde orbit. Normally this won't happen for orbits in the inner solar system around the sun, but this came come from the Ort cloud or somewhere far away.

The idea is that an object very from the sun is disturbed and begins a highly elliptical orbit. These can be retrograde. It would also need to intersect with our orbit at its closest point to the sun (this is why we sum the two velocities).

The kinetic energy of a body in a circular orbit is half of its gravitational potential energy. Since the far point of the object's orbit (formally a comet I think) is nearly infinity, that means its kinetic energy at the close approach will be exactly equal to its potential energy at 1 AU. That means it's specific kinetic energy (just 1/2 v^2) will be twice that of Earth. That means it will be traveling at the square root of 2 times faster than Earth.

Obviously this would be rare, but the principle is that anything moving faster than this when it hits the atmosphere obviously came from somewhere outside our solar system. This is my brief illustration of the concept. Earth is green, sun is yellow, and the object is grey.

object hit

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    $\begingroup$ Just for completeness: the square root of 2 times the circular velocity is simply the escape velocity, which is the velocity everywhere in a parabolic orbit. And indeed, a parabolic orbit is basically an elliptical orbit, with its far point at infinity. $\endgroup$ – Rody Oldenhuis Jul 17 '13 at 21:57
  • $\begingroup$ IIRC, some comets can travel faster than 600km/s when nearing the Sun. $\endgroup$ – coleopterist Jul 19 '13 at 20:21
  • $\begingroup$ @coleopterist Yes, but they can't become a meteor unless they come within Earth's orbit. Those comets that graze the sun pass Earth's path at the same speed of 30 x sqrt(2), but they'll be moving perpendicular, so they only hit Earth's atmosphere at 52 km/s. $\endgroup$ – AlanSE Jul 19 '13 at 20:25
  • $\begingroup$ Or you could just work out that the escape velocity from the solar system (= the velocity of an object arriving from infinity) at the orbit of the Earth is 42 km/s. $\endgroup$ – Rob Jeffries Feb 1 '18 at 8:47
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According to the American Meteor Society, meteorites usually hit the Earth's atmosphere going around 160,000 MPH.

Meteors enter the atmosphere at speeds ranging from 11 km/sec (25,000 mph), to 72 km/sec (160,000 mph!)...

The 70~ish top figure is also repeated in this answers.com answer.

Why such a big range, between 25k and 160k MPH?

The wide range in meteoroid speeds is caused partly by the fact that the Earth itself is traveling at about 30 km/sec (67,000 mph).

Also, there was recently a two to four meter meteorite that hit over California that was calculated to be going around 64k MPH.

researchers have calculated that the parent object of the Sutter’s Mill meteorite entered the atmosphere at 28.6 kilometers per second (64,000 mph).

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The "American Meteor Society" states that meteorites typically enter the Earth's atmosphere at 11 - 72 km/s. This is not cited, but after some Googling it is a figure that is repeated often.

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My understanding from basic physics is that when you calculate the requirements for escape velocity from a body's gravitational pull, the math is also able to point out the maximum accumulated velocity possible due to the pull of that gravity toward an object. In other words, while escape velocity calculates what is necessarily to counteract the body's gravitational force, the terminal impact velocity is the sum of that body's gravitational force as it accumulates over the course of the smaller body's acceleration from the farthest orbital reaches and plunges into the body as an impacting object. For our solar system, we're talking about the attractive force between the Sun's mass, the impacting body's maximum expected mass, plus whatever components of other solar system bodies (i.e. Jupiter, Earth) can be calculated to have a net boosting pull on that body. The latter are probably negligible compared to the Sun's pull across the farthest possible reaches. For asteroids, the upper limit is clearly much smaller than planet size. We can thus calculate a maximum terminal velocity due to gravity, upon impact, and this works out to approximately 160,000 mph. Another poster was incorrect in describing the highest value within a range by saying the speeds are "usually" that high. They are not, at all. In fact, 64,000 mph is the highest velocity we've directly measured from an asteroid/meteor so far, in our contemporary era of space travel and modern astronomy. That was the Sutter's Mill meteorite which was seen across the Western U.S. on November 1, 2016. The fastest possible impact velocity is thankfully seen so rarely on Earth that it probably has not ever occurred within the entire history of human existence. It's an upper limit - a theoretical maximum. I should note that comets must also be included in consideration, not just asteroids. Either one can become an impacting meteorite, and the calculations I've seen are from that perspective--what is the maximum impact velocity we might see from an object in the Solar System? An object can have its trajectory changed through an encounter with Jupiter, but that isn't going to cause more acceleration than the Sun's gravity is able to exert upon far more distant objects that eventually plunge in like cosmic bullets.

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  • $\begingroup$ There's still the matter of initial velocity at which the meteorite travels through interstellar space before entering Sun's gravity well. These would be extremely rare as most meteorites originate from within the solar system, but not impossible. $\endgroup$ – SF. Apr 19 '17 at 21:00
  • $\begingroup$ The formula I've seen only considers solar system objects, rather than interstellar objects. There is that potential for an interstellar object to end up with a higher velocity. Good thing such collisions are extremely rare! :-) $\endgroup$ – MikeS Apr 19 '17 at 21:03
  • $\begingroup$ The sources I studied say that more than 99% of all meteorites involve asteroid impacts. This leaves less than 1% as comets, although I feel that the evidence surrounding Tunguska does favor a cometary interpretation of that 1908 event. A paper was published not too many years ago, noting noctilucent clouds associated with the Tunguska event as evidence of an icy composition most consistent with a cometary explanation. I would guess that the percentage of impacting objects of interstellar origin is either negligibly small, or not even able to be determined yet. :-) $\endgroup$ – MikeS Apr 19 '17 at 21:07
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Clearly, the answer has to be a probability distribution. The actual distribution would necessarily "encode" the past history of the galaxy, neighboring galaxies, etc. Since a fast-moving object has a longer trajectory, it has a greater probability of hitting something. Consequently, the faster-moving, longer-path objects will collide sooner and will be culled from the system sooner, leaving more slower objects. Thus, the probability distribution of number of objects vs speed will shift over time toward slower objects. But, again, it is a probability distribution,, so there is some chance of a very fast-moving object left over from earlier days, or one that was sling-shoted in some less-probable way.

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  • $\begingroup$ s/galaxy/stellar system/, yeah? Neighboring galaxies can't possibly affect meteroid orbits in any meaningful way, never mind contributing bodies. $\endgroup$ – Nathan Tuggy Jan 19 '18 at 2:15

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