# Why is meteor speed what it appears to be?

Is the speed of a meteor through our sky because of the speed of the earth's axis rotation, or because the meteor is speeding towards us at that speed?

Meteors are essentially bits of rock that are independently in orbit around the Sun and which cross the Earth's orbit. If the Earth happens to be there at the same time then it will enter the Earth's atmosphere and we will see a meteor.

The velocity of meteors is related to how fast they we going in their orbit around the Sun, combined with how fast the Earth is going around the Sun. In addition, the Earth's gravitational field will accelerate the meteors as they approach the Earth.

Objects falling from infinity towards the Earth could reach a free-fall velocity as high as 11.2 km/s. The orbital velocity of the Earth in its orbit around the Sun is around 30 km/s. A meteor initially in orbit around the Sun however could have a velocity as high as 42 km/s at the distance that the Earth is from the Sun and that could be directed with the Earth's motion or against it.

The velocities of meteors as they enter the Earth's atmosphere can vary from $30 + 42 = 72$ km/s for a head-on collision with a meteor in a very long period orbit, to as little as $11$ km/s for something "approaching Earth from behind" with an initially small closing velocity, because it is in a very similar orbit to the Earth to begin with.

The axial rotation of the Earth (0.5 km/s) is a negligible contributor.

All speed is relative. But an object that starts from rest at infinity will reach a velocity of about 11 km/s when it hits Earth, if Earth is the only thing pulling on it. At the same time, Earth is moving with an orbital speed of about 30 km/s. Their relative importance will depend on the direction from which the meteor is approaching - but on the whole Earth's orbital speed is pretty significant. The rotation of the earth's surface (and atmosphere) matters much less - at the equator, it's 465 m/s, orders of magnitude smaller than the other two.

If you take into account that the meteorite is moving in an orbit around the sun, then it can have a velocity up to $\sqrt{2}v_{earth}$ when it crosses Earth's orbit (if it's falling straight towards the Sun) - about 42 km/s. At the kinetic energy associated with that velocity, the additional energy due to the Earth is not so significant.

Depending on their relative directions, these two velocities (30 km/s and 42 km/s) can either add for a blistering 72 km/s head-on collision, or be reduced to a meagre 12 km/s if the meteor is catching up on the Earth from behind. However, in that case the additional attraction by Earth would not be negligible and the final approach velocity would be affected by Earth's gravity - so you'd reach about $\sqrt{11^2 + 12^2}\approx 16~\rm{ km/s}$. In fact, if the meteorite starts out in an orbit close to earth's, there is no telling how slowly it will approach... but I suppose that it could be as low as 11 km/s.

These are just ballpark numbers - it says that the velocity of Earth and the meteorite both play a significant role in determining the velocity of impact, and that the rotation of the Earth does not.

• 11 km/s sounds suspiciously like Earth's escape velocity. I.e., that's how fast the meteor would impact the Earth if it and the Earth were the only two bodies in the Universe. Not necessarily the same as if the rock were falling toward the Sun, and the Earth happened to get in the way. – Solomon Slow Jan 29 '16 at 17:41
• @jameslarge - you make a fair point. I have updated my answer accordingly. – Floris Jan 29 '16 at 19:58
• Min velocity of a meteor is 11 km/s not 16. @JamesLarge This occurs when the meteor is in almost the same orbit as Earth to begin with. It is similar to the escape velocity of the Earth because gravity goes as $1/r^2$ and the Sun doesn't come into it, just think of it in the frame of reference of theEarth. – ProfRob Jan 29 '16 at 22:13