<p>It is difficult to see how. Most comets and asteroids would encounter the Earth on a crossing orbit and the encounter velocity would be roughly the vector sum of the Earth's velocity around the Sun (of order 30 km/s) and the individual velocity of the rogue object. </p>

<p>Even if you were to arrange it so that the asteroid/comet was diverted by something else so that it "caught up" with the Earth from behind travelling at an initially low relative velocity, there is the additional influence of the Earth's own gravitational potential. This would accelerate the approaching object to something like the order of the escape velocity from the Earth's surface - about 11 km/s.</p>

<p>This fairly authoritative site on calculating the <a href="http://impact.ese.ic.ac.uk/ImpactEffects/" rel="nofollow noreferrer">effect of asteroid/comet impacts</a> suggests a minimum impact velocity of 11 km/s, which is indeed the Earth's escape velocity.</p>

<p>Edit: (And credit to Keith Thompson for pointing it out). These are the speeds at the top of the atmosphere. If the objects are smaller than 20-30m, then the atmosphere will basically take out most of that kinetic energy, so for <em>small</em> objects, a relatively slow impact is de riguer. But for anything larger than <span class="math-container">$\sim$</span>50 m, it is basically the full 11 km/s or more. <a href="http://www.lsst.org/lsst/science/scientist_near_earth_objects_neoquant" rel="nofollow noreferrer">http://www.lsst.org/lsst/science/scientist_near_earth_objects_neoquant</a></p>

CC BY-SA 3.0
2015-01-30T19:19:17.930Z
Tangential orbit intersection, like after the first burn of a Hohmann transfer can get the velocity at intersection pretty low: http://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Example Ideally you'd have something knock your asteroid from a nice orbit about 0.999AU out from the sun to one at 1.000AU. You might get under 1 km/sec at intercept that way. However, I'm not sure how you accomplish that setup, and am certain the earth's atmosphere would seriously mess with touchdown of even a relatively slow moving extraterrestrial body.
Wayfaring Stranger
CC BY-SA 3.0
2015-01-30T21:01:47.560Z
@WayfaringStranger I'm no expert, but doesn't a transfer orbit require a burn at each end? An asteroid has no engines. What stops it accelerating in the Earth's potential? Even something captured in low Earth orbit is travelling at 7.5 km/s.
ProfRob
CC BY-SA 3.0
2015-01-30T21:36:16.347Z
Yes it does Rob. You can minimize the required size of that second burn by having the initial orbit quite close in size to the final orbit. Without a second impulse the earth will catch up to the asteroid when it's at apogee.
Wayfaring Stranger
CC BY-SA 3.0
2015-01-30T21:52:44.247Z
I must admit to not following the energetics of this. If the Earth and asteroid are on similar orbits with a low "closing speed", what removes the KE that the asteroid gains as it falls into the Earth's gravitational potential?
ProfRob
CC BY-SA 3.0
2015-01-31T00:01:30.667Z
Yeah, agreed, it's still going to fall from 60km up, or wherever the atmosphere takes hold. Even if you work things out so as to account for the atmosphere's rotation speed at the point of entry, terminal velocity from that height could easily be some hundreds of km/h: http://hyperphysics.phy-astr.gsu.edu/hbase/airfri2.html
Wayfaring Stranger
CC BY-SA 3.0
2015-01-31T01:28:33.803Z
@WayfaringStranger the closing speed is also known as Vinfinity. When coming into the earth's sphere of influence it moves along a hyperbolic path and a hyperbola's speed is sort(Vinf^2+Vesc^2). So even if the closing speed is zero, the object would still be moving escape velocity, which is about 11 km/s when entering earth's atmosphere.
HopDavid
CC BY-SA 3.0
2015-01-31T08:57:01.957Z
@HopDavid I thought this is what I'd said; it is certainly what I meant.
ProfRob
CC BY-SA 3.0
2015-01-31T15:39:49.443Z
@RobJeffries After considerable digging around, 11km/s does look to represent a lower limit for closing speed.
Wayfaring Stranger
CC BY-SA 4.0
2015-01-30T12:52:55.363Z
2018-10-05T07:26:32.090Z
2018-10-05T07:26:32.090Z
ProfRob
<p>“Trajectories are reversible” gives a good clue as to the conditions for a low speed impact. Consider two planets in a close, fast orbit such that objects on the far surface of a planet, directly opposite the other planet, are virtually weightless.  At this location acceleration due to gravity of the system is just slightly greater than the tendency to be ‘flung’ away (centripetal acceleration). Under these conditions a rock could be propelled upwards relatively slowly and move away from the planets. Just reverse everything and a rock could land slowly. I admit this is contrived, the atmosphere is likely to be lost, plus I hope Earth won’t be in this situation any time soon!</p>

CC BY-SA 4.0
2020-01-13T22:55:27.470Z
the reversible argument is also stated in [this answer](https://astronomy.stackexchange.com/a/8720/7982)
uhoh
CC BY-SA 4.0
2020-09-03T09:26:34.653Z
The reversible argument is only true in the absence of air resistance.  A small enough micrometeroid can hit the Earth gently.
gerrit
CC BY-SA 4.0
2020-01-13T13:20:43.463Z
2020-01-13T13:20:43.463Z
Nick v
<p>Yes, spacecraft do it all the time by using air resistance (and sometimes rockets) to slow down.</p>
<p>Meteorites enter the atmosphere at high speed, typically 10-70 km/sec, but the smaller ones are slowed by air resistance, so they typically hit the surface at just a few hundred kilometers per hour. Reference: <a href="http://csep10.phys.utk.edu/astr161/lect/meteors/impacts.html" rel="nofollow noreferrer">http://csep10.phys.utk.edu/astr161/lect/meteors/impacts.html</a> (that link is now dead).</p>
<p>As gerrit points out in comments, a sufficiently small micrometeroid, like a dust particle, probably <em>could</em> hit the surface gently.</p>
<p>Larger bodies are not affected nearly as much by the atmosphere. For any such body, the velocity at impact is going to be at least several kilometers per second. A body moving obliquely in the same direction as the Earth's rotation would have a slightly lower speed at impact, but the impact still could not be described as &quot;gentle&quot;; the Earth's rotational speed is still a small fraction of orbital or escape velocity.</p>
<p>Here's one way to think of it. Freefall trajectories are reversible. If you watch a movie of an incoming body in reverse, it still makes physical sense (ignoring air resistance). Any meteorite hitting the surface has to have been in deep space at some point before the impact. If there were a trajectory that allowed such a body to have a &quot;gentle&quot; impact speed, then it would be possible to <em>start</em> with the same body near the surface at the same &quot;gentle&quot; speed, but in the opposite direction, and have it reach deep space. Unless the meteorite has its own propulsion system, that's just not going to happen.</p>
<p>You <em>can</em> get to space with a lower starting speed by moving along with the Earth's rotation - which is why most rockets are launched to the east, to take advantage of that. Reversing such a trajectory can result in a slightly slower impact, but only slightly.</p>

CC BY-SA 4.0
2018-10-05T13:51:56.380Z
Depends on the origin of the asteroid too, interstellar asteroids? No. Trojan asteroids perturbed just right to hit earth at the perfect, low-grade hyperbolic trajectory that dips below the edge of the atmosphere (without going to low)? Maybe.
Magic Octopus Urn
CC BY-SA 4.0
2018-10-05T16:05:16.393Z
@MagicOctopusUrn: No, that's still not possible. For a body big enough that air resistance doesn't have much effect, it can't hit the Earth's surface at low speed. Remember, it still has to fall from the top of the Earth's gravity well to the bottom of it. Try dropping or throwing a rock from the top of the Empire State building and having it hit the ground gently.
Keith Thompson
CC BY-SA 4.0
2018-10-05T17:44:54.177Z
Oh, I took "gently" as "survive impact with discernible features".
Magic Octopus Urn
CC BY-SA 4.0
2020-01-13T13:42:10.950Z
Your reverse trajectory argument is correct, thus the lowest (most gentle) impact speed is escape velocity from earths I.e. 11.2 km/s.
Hartmut Braun
CC BY-SA 4.0
2020-09-03T09:25:10.870Z
You can't ignore air resistance.  Micrometeroids can and do hit the Earth gently.
gerrit
CC BY-SA 4.0
2015-01-30T20:59:28.443Z
2020-09-03T17:57:02.300Z
2020-09-03T17:57:02.300Z
Keith Thompson
<p>Could an object from outer space with the right velocity and orbit come into contact with the surface of our planet in a "slowish" manner? </p>

<impact>
How gently could a comet/asteroid/meteorite "hit" Earth?
Joe
# How gently could a comet/asteroid/meteorite "hit" Earth?

(Asked by: username_0 on 2015-01-29)

Could an object from outer space with the right velocity and orbit come into contact with the surface of our planet in a "slowish" manner?

**Tags:** impact


---

## 3 answers

### Answer by username_7 on 2015-01-30. Score: 8 | Accepted answer

Yes, spacecraft do it all the time by using air resistance (and sometimes rockets) to slow down.

Meteorites enter the atmosphere at high speed, typically 10-70 km/sec, but the smaller ones are slowed by air resistance, so they typically hit the surface at just a few hundred kilometers per hour. Reference: <http://csep10.phys.utk.edu/astr161/lect/meteors/impacts.html> (that link is now dead).

As username_6 points out in comments, a sufficiently small micrometeroid, like a dust particle, probably *could* hit the surface gently.

Larger bodies are not affected nearly as much by the atmosphere. For any such body, the velocity at impact is going to be at least several kilometers per second. A body moving obliquely in the same direction as the Earth's rotation would have a slightly lower speed at impact, but the impact still could not be described as "gentle"; the Earth's rotational speed is still a small fraction of orbital or escape velocity.

Here's one way to think of it. Freefall trajectories are reversible. If you watch a movie of an incoming body in reverse, it still makes physical sense (ignoring air resistance). Any meteorite hitting the surface has to have been in deep space at some point before the impact. If there were a trajectory that allowed such a body to have a "gentle" impact speed, then it would be possible to *start* with the same body near the surface at the same "gentle" speed, but in the opposite direction, and have it reach deep space. Unless the meteorite has its own propulsion system, that's just not going to happen.

You *can* get to space with a lower starting speed by moving along with the Earth's rotation - which is why most rockets are launched to the east, to take advantage of that. Reversing such a trajectory can result in a slightly slower impact, but only slightly.

> **Comments:**
> **username_8**: Depends on the origin of the asteroid too, interstellar asteroids? No. Trojan asteroids perturbed just right to hit earth at the perfect, low-grade hyperbolic trajectory that dips below the edge of the atmosphere (without going to low)? Maybe.  
> **username_7**: @MagicOctopusUrn: No, that's still not possible. For a body big enough that air resistance doesn't have much effect, it can't hit the Earth's surface at low speed. Remember, it still has to fall from the top of the Earth's gravity well to the bottom of it. Try dropping or throwing a rock from the top of the Empire State building and having it hit the ground gently.  
> **username_8**: Oh, I took "gently" as "survive impact with discernible features".  
> **username_9**: Your reverse trajectory argument is correct, thus the lowest (most gentle) impact speed is escape velocity from earths I.e. 11.2 km/s.  
> **username_6**: You can't ignore air resistance.  Micrometeroids can and do hit the Earth gently.  

---

### Answer by username_1 on 2015-01-30. Score: 6

It is difficult to see how. Most comets and asteroids would encounter the Earth on a crossing orbit and the encounter velocity would be roughly the vector sum of the Earth's velocity around the Sun (of order 30 km/s) and the individual velocity of the rogue object.

Even if you were to arrange it so that the asteroid/comet was diverted by something else so that it "caught up" with the Earth from behind travelling at an initially low relative velocity, there is the additional influence of the Earth's own gravitational potential. This would accelerate the approaching object to something like the order of the escape velocity from the Earth's surface - about 11 km/s.

This fairly authoritative site on calculating the [effect of asteroid/comet impacts](http://impact.ese.ic.ac.uk/ImpactEffects/) suggests a minimum impact velocity of 11 km/s, which is indeed the Earth's escape velocity.

Edit: (And credit to username_7 for pointing it out). These are the speeds at the top of the atmosphere. If the objects are smaller than 20-30m, then the atmosphere will basically take out most of that kinetic energy, so for *small* objects, a relatively slow impact is de riguer. But for anything larger than $\sim$50 m, it is basically the full 11 km/s or more. <http://www.lsst.org/lsst/science/scientist_near_earth_objects_neoquant>

> **Comments:**
> **username_2**: Tangential orbit intersection, like after the first burn of a Hohmann transfer can get the velocity at intersection pretty low: http://en.wikipedia.org/wiki/Hohmann_transfer_orbit#Example Ideally you'd have something knock your asteroid from a nice orbit about 0.999AU out from the sun to one at 1.000AU. You might get under 1 km/sec at intercept that way. However, I'm not sure how you accomplish that setup, and am certain the earth's atmosphere would seriously mess with touchdown of even a relatively slow moving extraterrestrial body.  
> **username_1**: @WayfaringStranger I'm no expert, but doesn't a transfer orbit require a burn at each end? An asteroid has no engines. What stops it accelerating in the Earth's potential? Even something captured in low Earth orbit is travelling at 7.5 km/s.  
> **username_2**: Yes it does Rob. You can minimize the required size of that second burn by having the initial orbit quite close in size to the final orbit. Without a second impulse the earth will catch up to the asteroid when it's at apogee.  
> **username_1**: I must admit to not following the energetics of this. If the Earth and asteroid are on similar orbits with a low "closing speed", what removes the KE that the asteroid gains as it falls into the Earth's gravitational potential?  
> **username_2**: Yeah, agreed, it's still going to fall from 60km up, or wherever the atmosphere takes hold. Even if you work things out so as to account for the atmosphere's rotation speed at the point of entry, terminal velocity from that height could easily be some hundreds of km/h: http://hyperphysics.phy-astr.gsu.edu/hbase/airfri2.html  
> **username_3**: @WayfaringStranger the closing speed is also known as Vinfinity. When coming into the earth's sphere of influence it moves along a hyperbolic path and a hyperbola's speed is sort(Vinf^2+Vesc^2). So even if the closing speed is zero, the object would still be moving escape velocity, which is about 11 km/s when entering earth's atmosphere.  
> **username_1**: @username_3 I thought this is what I'd said; it is certainly what I meant.  
> **username_2**: @RobJeffries After considerable digging around, 11km/s does look to represent a lower limit for closing speed.  

---

### Answer by username_4 on 2020-01-13. Score: 3

“Trajectories are reversible” gives a good clue as to the conditions for a low speed impact. Consider two planets in a close, fast orbit such that objects on the far surface of a planet, directly opposite the other planet, are virtually weightless. At this location acceleration due to gravity of the system is just slightly greater than the tendency to be ‘flung’ away (centripetal acceleration). Under these conditions a rock could be propelled upwards relatively slowly and move away from the planets. Just reverse everything and a rock could land slowly. I admit this is contrived, the atmosphere is likely to be lost, plus I hope Earth won’t be in this situation any time soon!

> **Comments:**
> **username_5**: the reversible argument is also stated in [this answer](https://astronomy.stackexchange.com/a/8720/7982)  
> **username_6**: The reversible argument is only true in the absence of air resistance.  A small enough micrometeroid can hit the Earth gently.  

---

