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The concept in my mind is that an asteroid is on a vector similar to Earth's, but slightly slower (e.g., 50kmh slower). As Earth passes it, it enters the atmosphere at a sharp angle, and since Earth was passing it, it just barely touches down due to Earth's gravity and atmospheric drag.

Given a large asteroid (e.g., 500 meters wide), is there any reason something like this couldn't happen? And, is there any evidence that it has happened?

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    $\begingroup$ Do you count 11 kilometers per second as "slowly"? That's the slowest naive answer. You might get a little discount if you assume some complicated three-body interaction with Luna, but it won't really change the outcome. $\endgroup$
    – dmckee --- ex-moderator kitten
    Mar 21, 2016 at 4:29
  • $\begingroup$ @dmckee well, since a 500 meters wide asteroid moving at 11kms would probably be an extinction level event, and since I mentioned things like "50kmh" and "barely touches down", probably not. Would you mind explaining why you think something like this is impossible? $\endgroup$
    – orokusaki
    Mar 21, 2016 at 4:39
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    $\begingroup$ Eleven kilometers per second is the speed with which on object placed at rest a long distance from the Earth will fall to the surface due to gravity. You can google it as "escape velocity" (because it is also the speed with which you need to launch something from the surface to insure that it never comes back). To simply the understanding. Assume that you get this thing to a height or merely 100 meters and then let it go. How hard does it hit? How hard if you drop it from 100 kilometers? $\endgroup$
    – dmckee --- ex-moderator kitten
    Mar 21, 2016 at 4:45
  • $\begingroup$ @orokusaki 500 meters wide would do a lot of local damage, but it wouldn't be an extinction level event. You need closer to 5,000 meters across for extinction level. The Dino Meteor was at least 10,000 meters across, 5'000 would be 1/8th of that. 500 meters, 1/8000th. 500 meter impacts happen every 500,000 years or so. $\endgroup$
    – userLTK
    Mar 21, 2016 at 6:07
  • $\begingroup$ @userLTK ... and there's already a lot of disputes about the dino asteroid anyway. It was probably one of the contributors to dino extinctions, but not an "extinction level event" on its own. Global event, sure. $\endgroup$
    – Luaan
    Mar 21, 2016 at 8:25

3 Answers 3

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If the asteroid is in parallel to the orbit of the earth and at rest it will feel the gravitational attraction and will fall with velocity growing as $g\cdot t^2.$ This force will be there whatever the angle and velocity of the asteroid, centrifugal forces may make it miss the earth in a parabolic orbit, or be caught in an elliptical as the path of the satellites. To avoid falling on the earth with great velocity it would need not only to have a small velocity relative to earth but also an acceleration equal or larger and opposite to the acceleration of gravity.

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    $\begingroup$ Thanks. So, the inverse squared law of gravitation my slow-motion asteroid fantasy into trouble pretty quickly :( $\endgroup$
    – orokusaki
    Mar 21, 2016 at 4:56
  • $\begingroup$ @orokusaki: You can still fantasize, look at my answer. $\endgroup$
    – kpv
    Mar 21, 2016 at 5:24
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    $\begingroup$ The practical question is if natural passive effects (e.g. repeated aerobraking from an initial elliptic orbit, as we've done with Mars landers etc) can plausibly provide such deceleration if the asteroid "just happens" to arrive at the exact angle that works best. $\endgroup$
    – Peteris
    Mar 21, 2016 at 14:52
  • $\begingroup$ @Peteris well, this will need writing down the equations and doing the calculations . Probability will be very small for shape to be right and angle to be right etc. If it were a mainly gas with a small core higher probabilities $\endgroup$
    – anna v
    Mar 21, 2016 at 15:08
  • $\begingroup$ @Peteris Repeated aero-braking stops working when the atmosphere gets thick enough, and your orbit drops low enough - from that point on, you're basically dropping straight down at full g. The tricky point is that "thick enough" is still not providing enough aero-braking to counteract gravity, while you're no longer able to maintain an orbit - you're losing horizontal speed quite fast, but gaining vertical at the same time. The main problem is that almost all of the atmosphere is pooled very low, while even at very high altitudes, there's enough drag to degrade your orbit (slowly). $\endgroup$
    – Luaan
    Mar 21, 2016 at 17:27
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Well, technically, the answer is no as the other answers and comments also say.

The approach speed can not be less than escape velocity. But in order for such a thing to happen, nature has to be really creative and totally in our favor. For example, the asteroid can have a very very lucky combination of these:

  1. The asteroid has right kind and amount of fluid in it that starts jetting out steam at just the right times and right angles.

  2. The asteroid is parachute shaped with appropriate strength and falls at an appropriate angle.

Again, it would be a miracle, so, please do not hit me.

As we may be lucky due to a three body interaction with moon, this is taking the luck to kind of extreme.

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    $\begingroup$ if the proposal that the large oceans on the earth are due to water asteroids, this fantasy might work because of evaporation of the water due to frictional forces during the fall (small meteors burn up for example, before falling) $\endgroup$
    – anna v
    Mar 21, 2016 at 5:28
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    $\begingroup$ With this level of fantasy you might enjoy the questions and answers at Worldbuilding ;-) $\endgroup$
    – gerrit
    Mar 21, 2016 at 11:15
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    $\begingroup$ Or the asteroid could happen to be Space-Shuttle shaped and happen to fall in just the right attitude :-) $\endgroup$
    – Carl Witthoft
    Mar 21, 2016 at 14:08
  • $\begingroup$ @CarlWitthoft: I was looking forward for this comment. $\endgroup$
    – kpv
    Mar 21, 2016 at 15:48
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The answer is yes. All it takes is for the asteroid to come at a tangential velocity equal to the "orbital" velocity of an object "flying" at a height equal to (radius of the earth + radius of asteroid). Of course, there are other effects that are being ignored to simplify the answer.

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  • $\begingroup$ This cannot happen to earth because of the atmosphere. It could happen on a planet without an atmosphere. Also the trajectory, even if tangential and the same orbital velocity, would be derailed by the gravitational attraction before reaching the surface to rest (mg). $\endgroup$
    – anna v
    Mar 23, 2016 at 19:13
  • $\begingroup$ This is simply wrong. $\endgroup$
    – dmckee --- ex-moderator kitten
    Mar 25, 2016 at 2:34

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