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Bright Explosion on the Moon

science.nasa.gov

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Re: Bright Explosion on the Moon

#61

Post by lutorm »

johnpmayer wrote: post #46
lutorm wrote: post #38

Earlier quoted context omitted.

The minimum impact velocity on Earth is 11km/s??? I beg to differ -- bodies can impact the Earth at arbitrarily low velocities. It depends on where they originate. There are clearly some assumptions being made there that aren't spelled out.

Starting from rest at a distance infinity, any object (regardless of it's own mass) necessarily undergoes a minimum increase in kinetic energy due to the gravitational potential of the mass of the target object (in this case the Earth, which has a known mass and so this is calculable). So I think the assumption is that the object isn't propelling itself away from the earth, and the object hasn't experienced any major…

What you say is true but most objects are not starting from rest at infinity. Things thrown up from the Earth, for example.

It also neglects that there is an atmosphere...

Re: Bright Explosion on the Moon

#62

Post by techas »

"Lunar meteor showers have turned out to be more common than anyone expected"

what does that mean? Is there more stuff flying through space than we thought? Does this have some consequence in the odds of hitting something in space?

Re: Bright Explosion on the Moon

#63

Post by teeja »

lutorm wrote: post #38
dmckeon wrote: post #24

Earlier quoted context omitted.

People tend to focus on the size of projectiles from space, but their great speed is often overlooked - from a simulator at: http://impact.ese.ic.ac.uk/ImpactEffects/ > Impact Velocity - This is the velocity of the projectile before it enters the atmosphere. The minimum impact velocity on Earth is 11 km/s. Typical impact velocities are 17 km/s for asteroids and 51 km/s for comets. The maximum Earth impact velocity fo…

The minimum impact velocity on Earth is 11km/s??? I beg to differ -- bodies can impact the Earth at arbitrarily low velocities. It depends on where they originate. There are clearly some assumptions being made there that aren't spelled out.

No kidding. Shoot an 8-foot 2x4 at the Earth at 100km/sec and guess how fast the carbon-fluff will be going when it hits. The pieces of anything that breaks up as it descends will quickly slow to their terminal velocity at that height.

NASA says the February Russian fireball was going about 11.6 miles/sec when it blew up (14 miles up) but there's a lot of denser atmo in that 14 miles.

http://neo.jpl.nasa.gov/news/fireball_130301.html

Re: Bright Explosion on the Moon

#64

Post by halter73 »

lutorm wrote: post #61
johnpmayer wrote: post #46

Earlier quoted context omitted.

Starting from rest at a distance infinity, any object (regardless of it's own mass) necessarily undergoes a minimum increase in kinetic energy due to the gravitational potential of the mass of the target object (in this case the Earth, which has a known mass and so this is calculable). So I think the assumption is that the object isn't propelling itself away from the earth, and the object hasn't experienced any major…

What you say is true but most objects are not starting from rest at infinity. Things thrown up from the Earth, for example. It also neglects that there is an atmosphere...

>Impact Velocity - This is the velocity of the projectile before it enters the atmosphere. The minimum impact velocity on Earth is 11 km/s.

I think it's implied that the impacting objects come from outside the Earth-Moon System. Otherwise there would obviously be no minimum impact velocity.

Re: Bright Explosion on the Moon

#66

Post by InclinedPlane »

mturmon wrote: post #43
InclinedPlane wrote: post #10

Earlier quoted context omitted.

The ejecta aren't tossed about due to recoil they are lifted out of the ground by an expanding sphere of vaporized rock. And that vaporized rock glows because it is superheated to thousands of degrees, that's what creates the bright flash.

Do you have any reference for this? It seems like you're saying the vaporized rock is imparting force to the impacted surface, which is in turn ejected. I had not understood that the vaporized rock played a central role. I thought the impactor imparted force directly to the surface. Basically there is a shock wave in the underlying rock which pushes ejecta out of a region underneath the impact site. Best easy source:…

Let me backtrack a bit and say, more accurately that "the ejecta aren't just tossed about due to recoil", both effects play a role. My point was that the flash is caused by superheated/vaporized rock, which you will pretty much always get at such speeds due to the amount of energy released.
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