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Congressional Candidate Brianna Wu Claims Moon-Colonizing Companies Could Destroy Cities By Dropping Rocks (washingtontimes.com)
Applehu Akbar quotes a report from Washington Times: A transgender-issues activist and Democratic candidate for Congress says the advent of the space tourism industry could give private corporations a "frightening amount of power" to destroy the Earth with rocks because of the Moon's military importance. Brianna Wu, a prominent "social justice warrior" in the "Gamergate" controversy who now is running for the House seat in Massachusetts' 8th District, suggested in a since-deleted tweet that companies could drop rocks from the Moon. "The moon is probably the most tactically valuable military ground for earth," the tweet said. "Rocks dropped from there have power of 100s of nuclear bombs." After users on social media questioned her scientific literacy, the congressional candidate clarified that the tweet was "talking about dropping [rocks] into our gravity well." Small space rocks can indeed do nuclear-weapons-scale damage if hitting the Earth at orbital speeds. But launching one from the moon, even setting aside issues of aiming, would still require escaping the satellite's gravitational field, a task that requires the power and thrust contained in a huge rocket.
a.k.a. The Moon Is a Harsh Mistress from Robert A. Heinlein
The lunar module would also burn up in our atmosphere very quickly.
. . . .running against an established Congressman (Stephen Lynch) who has been in Congress for 16 years, who has routinely been winning elections by 70%+ for years.
Wu's only real "in" here, is that Lynch is considered moderate. No idea on how that particular congressional district trends. . .
OK, we get that Slashdot hates Brianna Wu. We know that there are few harsher adjectives around here than the dreaded "Social Justice Warrior". We get that compared to the Slashdot voice, Brianna is a Communist (although compared to the Slashdot voice, Ronald Reagan is one, too).
What makes this front page entry a disappointment though is how far it wandered from reality just to attack one person. All that is being discussed here is the possibility of a kinetic weapon - which has had an entry on wikipedia for over a decade. Wu's statement was then twisted to be used as an attack against her.
And seriously, what does Slashdot have to gain by attacking her, anyways? She wants to represent Massachusetts. Most of Slashdot would see the majority of the voting public in Massachusetts to be total Communists regardless, and if Wu doesn't get the nomination some other person of similar political persuasion will.
Damn_registrars has no butt-hole. Damn_registrars has no use for a butt-hole.
The Moon Escape velocity is 2.38 km/s while on Earth it is 11.186 km/s.
Since energy is proportional to the square of the speed (E=1/2*m*v^2) we can conclude that it is (11.186/ 2.38)^2 = 2 time easier to reach free space from the Moon than from Earth.
However, even if a rock is launched from the Moon at 2.38 km/s, it still inherits the inertia of the Moon. Simply speaking, the rock would not fall to Earth. It would be in an orbit similar to the Moon orbit.
The orbital speed of the Moon is about 1km/s so the rock must be given that additional acceleration to cancel its orbital speed.
At that point, the rock is immobile (from the Earth point of view) and it will start falling toward Earth because of ... gravity.
When it reaches Earth, its speed will be equal to the Earth Escape velocity (a bit less in fact since the rock did not start falling from an infinite distance) so 11.186 km/s.
The kinetic energy is given by the formula 1/2 * m * V^2 so for 1kg the kinetic energy at 11km/s is 1/2 * 1 * 11000^2 = 60 * 10^6 Joules
As a comparison, 1kg of TNT provides 4 * 10^6 Joules so each kg of moon rock would be equivalent to approximatively 15kg of TNT
The Hiroshima bomb was 15 kilotons of TNT = 15 * 10^6 Kg so a similar effect would require a 1000 tons of Moon rock and the ability to accelerate that rock to a speed of 2.38+1 = 3.38 km/s.
Escaping the moon's gravity is the easy part. The moon is in a really high orbit. To get something from the moon to the Earth, you need to either lose enough of your angular momentum to fall
It turns out, however, the higher an orbit is, the easier it is to kill your angular momentum and drop. So the fact that the moon is in a "really high" orbit helps here. You need about 1 km/sec to kill the moon's orbital velocity, actually less than the 2.38 km/sec escape velocity to throw the rock off the surface.
But delta-Vs don't add; energies add. Once your mass driver has gotten your rock to 2.38 km/sec, it only takes another 0.2 km/sec to kill the orbital velocity and make it drop. (Less, if you want to take an indirect trajectory via the "fuzzy boundary", but those take a lot more time).
...and, yes, actually I am a rocket scientist.
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TL;DR: If it were easy for things from the moon to fall to Earth, the moon would have fallen down already.
In fact, rocks splashed off of the moon actually do hit the earth, of course: http://meteorites.wustl.edu/lu...
http://www.geoffreylandis.com