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No Bomb Powerful Enough To Destroy an On-Rushing Asteroid, Sorry Bruce Willis
coondoggie writes "Maybe it's the doom predictions about the end of the Mayan calendar this year, or maybe these guys are obsessed with old Bruce Willis movies. Either way a class of physics students from the University of Leicester decided to evaluate whether or not the premise of Willis' 1998 'Armageddon' movie — where a group of oil drillers is sent by NASA to detonate nuclear devices on an asteroid that threatens to destroy Earth — could actually happen. The students found it would take a bomb about a billion times stronger than the biggest bomb ever detonated on Earth."
The students devised a formula to find the total amount of kinetic energy needed in relation to the volume of the asteroid pieces, their density, the clearance radius (which was taken as the radius of Earth plus 400 miles), the asteroid's pre-detonation velocity, and its distance from Earth at the point of detonation. Using the measurements and properties of the asteroid as stated in the film, the formula revealed that 800 trillion terajoules of energy would be required to split the asteroid in two with both pieces clearing the planet. However, the total energy output of Big Ivan "only comes to 418,000 terajoules. The asteroid is approximated as a spherical object 1000km in diameter
"First they came for the slanderers and i said nothing."
The biggest one ever detonated, the so called "Tsar Bomba", was 50 megatons of TNT. It could have been made 100 MT, but was scaled back to reduce fallout, and was therefore a very clean bomb for its size.
There was however no point in building bombs of this size, so no one has attempted it since, opting instead for clusters of smaller bombs to carpet an area or using modern targeting to accurately take out small targets with great precision,
Bombs that big where shere lunacy and just a demonstration of power.
This is the real paper, coming in at only 2 pages it's a light read: https://physics.le.ac.uk/journals/index.php/pst/article/viewFile/390/243
You weren't going to RTFA anyway, now were you?..
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P1_1 Could Bruce Willis Save the World?
Back A, Brown G, Hall B and Turner S
Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH.
November 1st, 2011
Abstract
The film Armageddon (1998) puts forward the possibility of using a nuclear weapon buried deep within an Earth-bound asteroid to split the asteroid in two, each half clearing opposite sides of the Earth with only relatively minor damage. This article investigates the feasibility of such a plan and shows that even using the largest nuclear weapon made to date, the bomb comes over 9 orders of magnitude short of the yield required.
[...]
The last thing you want is lots of pieces - there's something called gravity which would cause them to re-agglutinate on the rest of the journey. Breaking up an asteroid takes far more energy than deflection, as should be obvious-despite the current illiteracy, it takes far less energy to brake a car than it does to break it up. Of course Hollywood wouldn't want deflection because there's nothing to see on screen - but deflecting it into a safe orbit would be much safer because you only have to predict the track of one object, not millions of small ones with different trajectories.
From scarped cliff or quarried stone she cries "A thousand types are gone, I care for nothing, no not one."
If you'd read the article, you'd know that the calculation was to determine how powerful the explosion would need to be to split the asteroid in half so that the two pieces would pass by the earth. Basically, the same thing that was done in the movie. Only, in their calculation, the explosion occurred when the asteroid was still 8 billion miles away.
Even worse: There are no asteroids with a diameter of 1000km. The largest of them, Ceres, is 950km and at a very safe distance in a very stable orbit. The second largest, Vesta, is less than 600km in diameter.
In fact, the main "danger" nowadays is seen in objects of about 0.1km in diameter, since that is the size at which asteroids are still damaging, but also escape early detection. That takes about 15 orders of magnitude off the energy requirements. But at this point, you wouldn't even need a bomb. Just shoving a few tons of stuff at a few km/s in front of the asteroid is enough to tear it apart. (The kinetic energy of 1t of material at 2.8km/s is equivalent to 1t of TNT.)
One of the simplest ways to slowly nudge an asteroid off course is simply to have your spacecraft hover near it, with its (low thrust/high ISP) jets askew (instead of pointing straight at the asteroid). You don't need to be physically attached to an asteroid to tow it; gravity can be your "cable".
Another even slower but potentially even simpler way proposed to move an asteroid out of an intercept course is to "paint" it (basically, detonate one or more bombs of reflective dust) on particular locations and use the change in solar radiation pressure to do the work for you.
Sometimes I doubt your commitment to Sparkle Motion.
Yes, it will. The details of course depend on the particular asteroid, but even painting the entire surface white will alter its trajectory.
Also, really, pretty much any method proposed for spacecraft acceleration would work for asteroids as well. Laser-pumped? Check. Solar sail? Check. Even some of the less commonly known ones like a magnetic field generator to repel the solar wind would work. It all depends on how big you're willing to go and how quickly you need to move the thing.
Sometimes I doubt your commitment to Sparkle Motion.