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Old 01-23-2019, 10:15 PM
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About the lunar meteorite during the eclipse


During the recent total lunar eclipse, a meteorite is seen impacting the darker part of the moon. How is the meteorite visible? Isn't it also in the earth's shadow? Or does the impact somehow ignite?
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Old 01-23-2019, 10:24 PM
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Explosions are very explody.



What you see is many tons of rock instantly transformed into an incandescent gas.
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Old 01-23-2019, 10:40 PM
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Couple of articles with some authoritative guesses about the size of the impactor. Remember, the speed of the acorn/tennis ball/football when it struck would be measured in double-digit miles per second.
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Old 01-23-2019, 10:57 PM
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It's also why craters are round. You would think that because meteors come in from all angles, many craters would be oval. But in fact, the high velocity causes meteors to basically vaporize when they hit, and they blow a nice circular hole in the ground even if they hit at an oblique angle.
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Old 01-23-2019, 11:11 PM
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This is no where close to being the first lunar impact seen, although it has the distinction of being seen more people than any others. NASA has a program to look for them. They always look in the nighttime part pf the Moon, since sunlight makes the impact much more difficult to see. They never actually see the meteorite before it hits; those are way too small.
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Old 01-23-2019, 11:26 PM
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Ok, it's the incandescence I wasn't sure about.
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Old 01-24-2019, 01:18 PM
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Scott Manley of Kerbal and astronomy fame did a quick discussion of the impact and others on his YouTube page. And one of the most interesting bits comes from another impact that was observed and then its crater was found! But it was bigger than the recent one, but still weighed only an estimated 40 kg. So there is no way that a telescope would see it prior to impact.
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Old 01-24-2019, 01:39 PM
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Objects in motion have kinetic energy. It's equal to the mass of the object times the square of the speed.

In a totally elastic collision (think of billiard balls bouncing off each other), kinetic energy is conserved. KE after the collision is the same as KE before the collision. In a totally inelastic collision (think of a snowball hitting a snowman), kinetic energy is dissipated in deforming the objects themselves*, and ultimately is released as heat. In the case of the snowball, the mass is maybe 10 g and the speed is maybe 10 m/s, so that's roughly one Joule of energy. That's about 0.24 gram-calories, enough energy to either raise the temperature of the 10 kg snowman by 0.000024 degrees Celsius or raise the temperature of the snowball by 0.024 degrees Celsius. That's a tiny amount of heat.

Real world collisions are never perfectly elastic. Some kinetic energy always converts to heat.

Let's change the snowman to a mountain and change the snowball to a rock the size of a school bus, with a mass of 200 metric tons. And let's change the speed from 10 m/s to 20 km/s. Now the kinetic energy of the rock is 200,000 kg x (20,000 m/s) ^2 = 80,000,000,000,000 Joules or 19,200,000,000 kilogram-calories. That's enough to raise the temperature of the 200 ton rock by 96,000 degrees Celsius (172,800 degrees Fahrenheit). That's a huge amount of heat.

At this point, you should ask yourself, do you think the rock will bounce off the mountain like a billiard ball? No? Then it's an inelastic collision and most, if not all, of the energy will be released as heat.

Now, some of the heat will go into the mountain and some of it will go into the rock and some of it will go into the surrounding air. Eventually, all the heat will spread out evenly. But in that one second when the rock hits the mountain, it's very likely that the rock will instantly heat up by several thousand degrees. Compare that to glowing red lava which is only 700-1200 degrees C and still a liquid. It will be much hotter than lava, glowing white hot, and turning into a gas which will rapidly expand. In other words, it's a fireball explosion that would resemble a small nuclear weapon being set off. If you were looking straight at it from just a few km away, it might burn your retinas.

But if you watched it from a comfortable distance of 380,000 km, your retinas would not be in danger.

I might have made an arithmetic mistake somewhere here, but this point would remain true: Kinetic energy increases by the square of the speed. When objects collide, most of that energy is converted to heat. If you increase the speed high enough, you get enough energy to melt the object. Increase the speed even higher and you can make the object glow, or turn white hot, or vaporize.


*You can easily demonstrate that deforming objects creates heat by bending a spoon back and forth until it breaks and feel how warm the metal gets.
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Old 01-24-2019, 02:19 PM
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"You hit something hard enough, you strike sparks. We just struck up the biggest sparks in history."
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Old 01-24-2019, 02:38 PM
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Quote:
Originally Posted by Chronos View Post
"You hit something hard enough, you strike sparks. We just struck up the biggest sparks in history."

Close, but close only counts in horseshoes and hypersonic bombardment.
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Old 01-24-2019, 04:49 PM
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Quote:
Originally Posted by dtilque View Post
This is no where close to being the first lunar impact seen, although it has the distinction of being seen more people than any others.
And, from what I've read, the first one to be observed during an eclipse.
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Old 01-24-2019, 05:12 PM
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Note that it's the lunar impact seen by more people than any other, but not the celestial collision seen by the most people. The latter distinction probably goes to Comet Shoemaker-Levy 9.
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Old 01-24-2019, 07:32 PM
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Nobody saw the Shoemaker-Levy 9 collision, except in some extremely low-resolution images sent back from one of the Voyager probes. They happened on the far side of the planet from Earth. But lots of people saw the aftermath a few hours later.
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Old 01-25-2019, 02:29 AM
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Oops, you're right, Chronos. I was just remembering all the S-L9 watching parties (none of which I attended, although they broadcast one on PBS) and forgot that detail.
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Old 01-25-2019, 03:35 AM
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More on SL9. Its Wikipage says

Quote:
Earth-based observers detected the fireball rising over the limb of the planet shortly after the initial impact
As we established upthread, all we see from lunar impacts is the fireball. So it's arguable that seeing the fireball shortly after impact means some people saw the SL9 impacts.


PS The space probe Galileo, which was on its way to Jupiter at the time, also saw SL9.

Last edited by dtilque; 01-25-2019 at 03:38 AM.
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Old 01-25-2019, 09:16 AM
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The What If department of XKCD covered the (figurative) impact of a baseball pitched at 0.9c The results are not good for anyone in the vicinity.
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Old 01-25-2019, 10:19 AM
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Information on the meteor strike on the moon: https://apod.nasa.gov/apod/ap190125.html
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