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Comment by verisimi | Hacker News Reader

"On October 9, 2009, a two-ton rocket smashed into the moon traveling at 9,000 kilometers per hour. As it exploded in a shower of dust and heated the lunar surface to hundreds of degrees, the jet-black crater into which it plummeted, called Cabeus, briefly filled with light for the first time in billions of years."

Presumably the rocket also came down at 90 degrees to the lunar surface, making another perfectly circular crater.

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at that speed, the angle does not matter, the crater will be circular because the impact vaporized the rocket.

I don't follow. Can you elaborate?

"Why are impact craters always round? Most incoming objects must strike at some angle from vertical, so why don't the majority of impact sites have elongated, teardrop shapes?"

https://www.scientificamerican.com/article/why-are-impact-cr...

9000 km/h is not fast enough to vaporize much rocket.

You can experiment tossing stuff into a tub of talcum powder: craters just really like to be circular, shape of object or angle of impact notwithstanding.

The kinetic energy of 2000 kg at 2500 m/sec is about 6 trillion joules. A ton of TNT is about 4 trillion joules. So the kinetic energy of impact would be the same as 1.5 tons of TNT.

A Nasa document https://ntrs.nasa.gov/api/citations/20090043092/downloads/20... has a slide on the second page about this mission. It estimated about 200 tons of lunar rock and soil were excavated, and that the crater made was about 20-25 meters across and 3 meters deep.

1.5t of TNT certainly has enough energy to vaporize 1.5t of TNT. And, as they say, "then some".

2000 kg of aluminum costs, what, 1/4 billion joules to vaporize? I get 6.25 billion joules of impact energy, not trillion, but still plenty. I am corrected.

You can move a fair bit of material out of the way with that many joules. I guess you would start by vaporizing the 2t of Al plus another several tons of rock, and then dissipate the heat by lofting stuff out of the way as it expands, recycling the heat energy back to kinetic as it cools.

You're right. s/trillion/billion/. The key fact is that the two ton object had the impact of 1.5 tons of TNT. Thanks.

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