Hacker News Reader

Comment by Avalaxy | Hacker News Reader

The rock would have been there before the dog would be able to hear the boom. More likely explanation: there was a bright flash in the sky, and the dog saw this.

View on HN

It would have been falling at terminal velocity at that point, which would have been subsonic (200 to 400 mph)

https://www.amsmeteors.org/fireballs/faqf/#12

Possibly, but the rock also came to rest on the mattress and not underneath her basement so there’s a possibility that it was already sub-sonic and that the dog had a chance to hear it before it arrived.

It must have slowed down to almost nothing by whatever means, or it wouldn't have landed softly on a pillow, but obliterated the bedroom. Or the entire house. Or the entire neighborhood.

I'm gonna guess it's pretty low-density, and either entered the atmosphere at a very shallow angle, or is a chunk that fell off a larger piece somewhere close to the ground, and most of the energy was absorbed by the roof.

@jcims terminal velocity only applies to something in free fall, not to something with a high magnitude inertial vector relative to the Earth (like a meteorite), or anything with thrust (like a supersonic plane or a bullet).

I think @krona's got the right answer.

It will slow and eventually reach terminal velocity. I don't know if it has enough time/distance to do that before hitting the earth but given the small size I wouldn't be surprised.

> velocity only applies to something in free fall, not to something with a high magnitude inertial vector relative to the Earth (like a meteorite)

It could be conceivably coming at a speed high enough to still be supersonic when it hit the ground. However, were that the case, I doubt there would be still much of a house remaining. Or maybe a block for that matter, depending on how fast it was going.

More likely, it just slowed down due to friction. This is even more likely for a small rock.

> anything with thrust (like a supersonic plane or a bullet).

Plane has thrust, but a bullet doesn't ;)

Initial velocity doesn't matter (outside of relativistic considerations) for whether an object makes it through the atmosphere. Density and size do.

“…an object forced to move faster than its terminal velocity will, upon release, slow down to this constant velocity.”

https://www.britannica.com/science/terminal-velocity

IIUC, the meteorite was forced or “released” into its “high magnitude inertial vector” initially, and would continue to decelerate as it approached to the surface… whether it did much of that on the way down is a different story!

A lot depends on the initial velocity and angle of impact.

Considering that the earths atmosphere isn’t much more than 100km thick, and a meteor could in theory come in head-on with an initial velocity of 70km per second, there isn’t going to much time for it to slow down.

70km/s is interstellar material

Based on some quick googling (not an expert in the related physics like some sibling commenters seem to be), earth’s orbital velocity around the sun is ~30km/s and the sun’s orbital velocity around the center of the galaxy is 250km/s

Air resistance is to first approximation quadratic with velocity. High initial velocities lead to very high braking force, so it wouldn't have 70 km/s for very long. Also see surrounding comments for more related physics.

Actually, Newton's impact depth equation tells you that an unpowered object generally won't sustain high velocity beyond a certain distance when impacting into something (in this case the atmosphere):

https://en.wikipedia.org/wiki/Impact_depth

In this case, the ratio of densities is maybe 2000, so without gravity the meteor would expend all its initial kinetic energy after traveling 2000 times its diameter through the atmosphere. That's less than 1 kilometer. Hence, it will reach terminal velocity long before hitting the surface. (See also the Meteorite part in the wiki article).

Even more likely explanation: the dog was always barking at night, but she was so used to it she stopped noticing it, until now when she connected it to the meteorite.

Reply on news.ycombinator.com